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Non-noble Metal Electrocatalysts for the Hydrogen Evolution Reaction in Water Electrolysis
Abstract
Water electrolysis is a sustainable approach for hydrogen production by using electricity from clean energy sources. However, both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) associated with water electrolysis are kinetically sluggish, leading to low efficiency in corresponding electrolysis devices. In addition, current electrocatalysts that can catalyze both HER and OER to practical rates require noble metals such as platinum that are low in abundance and high in price, severely limiting commercialization. As a result, the development of high-performance and cost-effective non-noble metal electrocatalysts to replace noble ones has intensified. Based on this, this review will comprehensively present recent research in the design, synthesis, characterization and performance validation/optimization of non-noble metal HER electrocatalysts and analyze corresponding catalytic mechanisms. Moreover, several important types of non-noble metal electrocatalysts including zero-dimensional, one-dimensional, two-dimensional and three-dimensional materials are presented with an emphasis on morphology/structure, synergetic interaction between metal and support, catalytic property and HER activity/stability. Furthermore, existing technical challenges are summarized and corresponding research directions are proposed toward practical application. Water electrolysis is a sustainable approach for hydrogen production by using electricity from clean energy sources. However, both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are kinetically sluggish, causing low efficiency of the electrolysis devices. The currently used noble metals, such as Pt-based electrocatalysts for catalyzing both HER and OER to practical rates, have low abundances and high price, limiting their commercialization. In this regard, developing high-performance and cost-effective non-noble metal electrocatalysts to replace noble ones has become a hot research topic.
... However, the scarcity, high cost, and environmental concerns related to Pt limit their widespread use in large-scale hydrogen production. As a result, researchers have been exploring alternative materials, such as non-noble metal-based catalysts, to overcome these limitations [106,107]. Non-noble metal-based catalysts have attracted significant attention as promising alternatives to noble metal catalysts in HERs. These materials include transition metal chalcogenides (e.g., MoS2, WS2), nitrides (e.g., MoN, VN), and phosphides (e.g., Ni2P, CoP) [107]. ...
... Non-noble metal-based catalysts have attracted significant attention as promising alternatives to noble metal catalysts in HERs. These materials include transition metal chalcogenides (e.g., MoS2, WS2), nitrides (e.g., MoN, VN), and phosphides (e.g., Ni2P, CoP) [107]. These catalysts have shown promising activity and stability, making them attractive candidates for HERs. ...
... Numerous studies have demonstrated the advantages of Non-noble metal-based catalysts have attracted significant attention as promising alternatives to noble metal catalysts in HERs. These materials include transition metal chalcogenides (e.g., MoS 2 , WS 2 ), nitrides (e.g., MoN, VN), and phosphides (e.g., Ni 2 P, CoP) [107]. These catalysts have shown promising activity and stability, making them attractive candidates for HERs. ...
Artificial photosynthesis is a technology with immense potential that aims to emulate the natural photosynthetic process. The process of natural photosynthesis involves the conversion of solar energy into chemical energy, which is stored in organic compounds. Catalysis is an essential aspect of artificial photosynthesis, as it facilitates the reactions that convert solar energy into chemical energy. In this review, we aim to provide an extensive overview of recent developments in the field of artificial photosynthesis by catalysis. We will discuss the various catalyst types used in artificial photosynthesis, including homogeneous catalysts, heterogeneous catalysts, and biocatalysts. Additionally, we will explore the different strategies employed to enhance the efficiency and selectivity of catalytic reactions, such as the utilization of nanomaterials, photoelectrochemical cells, and molecular engineering. Lastly, we will examine the challenges and opportunities of this technology as well as its potential applications in areas such as renewable energy, carbon capture and utilization, and sustainable agriculture. This review aims to provide a comprehensive and critical analysis of state-of-the-art methods in artificial photosynthesis by catalysis, as well as to identify key research directions for future advancements in this field.
... Energy is always needed for the survival and civilization of human beings, and at the same time, is also the driving force of sustainable development of society [1]. As is well-known, the traditional fossil fuel energy sources (such as coal, oil and natural gas, etc.) are non-renewable and their usage can cause environmentally polluting [2,3]. Therefore, there is an urgent need to develop clean, efficient and safe renewable energy sources as the alternatives, commonly including wind, solar, water fall, geothermal, etc. ...
... In acidic media, the main difference between the two-step 2-electron process and the one-step 4-electron process is whether or not the intermediate of hydrogen peroxide (H 2 O 2 ) is produced [Eqs. (1)- (3)]. While in alkaline media, the main difference between the two is whether HO 2 is produced [Eqs. ...
... It is critical to develop countermeasures to alleviate heavy pollution in the environment and foodstuffs. Among existing control and mitigation strategies, advanced oxidation processes have been well developed for reducing pollutants, such as ozonation, UV/H 2 O 2 , Fenton-like reaction, persulfate activation, heterogeneous photocatalysis, and electrochemical advanced oxidation process [5]. However, the preceding methods may be limited in a real-world application due to several drawbacks. ...
... Finally, contaminants adsorbed on the catalyst surfaces can be degraded directly by photocatalyst surface carriers or indirectly via reactive substances, yielding CO 2 and H 2 O (equations (5) and (6)). For example, highly toxic Cr(VI) can be directly reduced to less toxic Cr(III) in reacting electrons. ...
... However, the generated electricity energy is generally weather-dependent and intermittent, requiring smoothing and storage for daily applications. In this regard, electrochemical energy storage and conversion technologies 1-4 such as fuel cells, [5][6][7] batteries, [8][9][10][11] supercapacitors, 12,13 water electrolysis for hydrogen generation, 14 and so forth have been recognized as reliable, efficient, and practical options. To further increase the energy density/ energy efficiency, lifetime, and safety of these devices, online monitoring, diagnosis, and performance analysis during the operation of these devices are definitely needed. ...
... Nowadays, joint time-frequency analysis cases are emerging; for details, refer to Wu and colleagues. 14,15,47,48 Therefore, joint time-frequency analysis can offer indepth insight into electrochemical energy devices. A thorough model reduction of fractional impedance spectra is required to implement a joint time-frequency analysis for electrochemical energy devices. ...
Joint time–frequency analysis is an emerging method for interpreting the underlying physics in fuel cells, batteries, and supercapacitors. To increase the reliability of time–frequency analysis, a theoretical correlation between frequency-domain stationary analysis and time-domain transient analysis is urgently required. The present work formularizes a thorough model reduction of fractional impedance spectra for electrochemical energy devices involving not only the model reduction from fractional-order models to integer-order models and from high- to low-order RC circuits but also insight into the evolution of the characteristic time constants during the whole reduction process. The following work has been carried out: (i) the model-reduction theory is addressed for typical Warburg elements and RC circuits based on the continued fraction expansion theory and the response error minimization technique, respectively; (ii) the order effect on the model reduction of typical Warburg elements is quantitatively evaluated by time–frequency analysis; (iii) the results of time–frequency analysis are confirmed to be useful to determine the reduction order in terms of the kinetic information needed to be captured; and (iv) the results of time–frequency analysis are validated for the model reduction of fractional impedance spectra for lithium-ion batteries, supercapacitors, and solid oxide fuel cells. In turn, the numerical validation has demonstrated the powerful function of the joint time–frequency analysis. The thorough model reduction of fractional impedance spectra addressed in the present work not only clarifies the relationship between time-domain transient analysis and frequency-domain stationary analysis but also enhances the reliability of the joint time–frequency analysis for electrochemical energy devices. © 2023 The Authors. Carbon Energy published by Wenzhou University and John Wiley & Sons Australia, Ltd.
... The inventive development and preparation of non-noble metal HER electrocatalysts are crucial because of the strong correlation between material characteristics, such as morphology and structure [142]. Despite considerable advancements of high-performance HER electrocatalysts, further endeavors are required to enable their practical use in commercial settings for sustainable hydrogen production. ...
Hydrogen energy is regarded as an auspicious future substitute to replace fossil fuels, due to its environmentally friendly characteristics and high energy density. In the pursuit of clean hydrogen production, there has been a significant focus on the advancement of effective electrocatalysts for the process of water splitting. Although noble metals like Pt, Ru, Pd and Ir are superb electrocatalysts for the hydrogen evolution reaction (HER), they have limitations for large-scale applications, mainly high cost and low abundance. As a result, non-precious transition metals have emerged as promising candidates to replace their more expensive counterparts in various applications. This review focuses on recently developed transition metal phosphides (TMPs) electrocatalysts for the HER in alkaline media due to the cooperative effect between the phosphorus and transition metals. Finally, we discuss the challenges of TMPs for HER.
... Currently, the H 2 production can be achieved through numerous technologies including thermal [6], electrolytic [7], and photolytic processes [8]. This diversity of energy sources nominates hydrogen as the most promising energy carrier [9]. ...
The development of highly active and precious metal free catalysts is of great interest for electrocatalytic hydrogen evolution reaction. A lot of efforts have been devoted for catalyzing HER process through single atom catalysis (SAC), but search for robust electrocatalytic system remains a massive challenge. In this work, transition metal (TM) doped on C24 complexes have been investigated simultaneously for thermodynamic and kinetic aspects to describe the HER activity. The interaction energies of TM@C24 catalysts suggested that all the designed catalysts are thermodynamically stable (− 0.15 to − 5.60 eV). The best catalytic performance for HER is observed for Mn@C24 catalyst with Gibbs's free energy (ΔG H) value of 0.07 eV. FMO analysis reveals that significant change in HOMO-LUMO gap is observed in case of HTM@C24 and validated through density of states analysis. Exchange current is obtained through volcano plot as a function Gibbs free energy for hydrogen evolution over designed SAC. The volcano plot indicates the near thermoneutral activity of Mn@C24 catalyst for HER. The kinetic aspect of the study reveals that Mn@C24 catalyst has a smaller energy barrier than reported for Pt(111) surface for both elementary reactions of HER. The key findings nominate that Mn@C24 catalyst can act as potential electrocatalyst for hydrogen evolution with low overpotential.
... However, optimal use of g-C3N4 for electrochemical applications requires the improvement of its poor conductivity, which can be increased in several ways: physically mixing g-C3N4 with conductive carbon materials, immobilizing g-C3N4 on carbon bases (carriers) or depositing metal nanoparticles using microwave-assisted processes, hydrothermal and solvothermal syntheses routes, sol-gel processes, chemical reduction, etc. The application of earth-abundant transition/noble metalsfree (TMs, where M=Co, Ni, Fe, Mn, Mo) and TMs-based alloys or non-metallic (TMXs, where X=N, O, S, C, P, etc.) compounds as active electrocatalysts for HER/OER has been reported [14][15][16][17][18]. TMXs have received much attention due to their distinctive structural features, abundant active sites, tunable electronic properties, compositions, and ease of employment for large-scale production. ...
This study presents the synthesis of graphitic carbon nitride (g−C3N4) and its nanostructures with cobalt ferrite oxide (CoFe2O4) and silver nanocubes (Ag) using the combined pyrolysis of melamine and polyol method. The resulted nanostructures were tested as electrocatalysts for hydrogen and oxygen evolution reactions in alkaline media. It was found that the Ag/CoFe2O4/g−C3N4 shows the highest current density and gives the lowest overpotential of −259 mV for HER to reach a current density of 10 mA cm−2 in 1 M KOH. Overpotentials to reach the current density of 10 mA·cm−2 for OER are 370.2 mV and 382.7 mV for Ag/CoFe2O4/g−C3N4 and CoFe2O4/g−C3N4, respectively. The above results demonstrate that CoFe2O4/g−C3N4 and Ag/CoFe2O4/g−C3N4 materials could act as a bifunctional catalyst due to the notable performance towards HER and OER and for total water splitting in practical applications is a promising alternative to noble metal-based electrocatalysts.
... The inventive development and preparation of non-noble metal HER electrocatalysts are crucial because of the strong correlation between material characteristics, such as morphology and structure [130]. Despite considerable advancements of high-performance HER electrocatalysts, further endeavors are required to enable their practical use in commercial settings for sustainable hydrogen production. ...
Hydrogen energy is regarded as an auspicious future substitute to replace fossil fuels, because of its environmentally friendly characteristics and high energy density. In the pursuit of clean hydrogen production, there has been a significant focus on the advancement of effective electrocatalysts for the process of water splitting. Although noble metals like Pt, Ru, Pd and Ir are superb electrocatalysts for the hydrogen evolution reaction (HER), they have limitations for large-scale applications, mainly high cost and low abundance. As a result, non-precious transition metals have emerged as promising candidates to replace their more expensive counterparts in various applications. This review focuses on recently developed transition metal phosphides (TMPs) electrocatalysts for the HER in alkaline media due to the cooperative effect between the phosphorus and transition metals. Finally, we discuss the challenges of TMPs for HER.
... However, the optimal use of g-C 3 N 4 for electrochemical applications requires an improvement of its poor conduc-Crystals 2023, 13, 1342 3 of 17 tivity, which can be increased in several ways: physically mixing g-C 3 N 4 with conductive carbon materials; immobilizing g-C 3 N 4 on carbon bases (carriers) or depositing metal nanoparticles via microwave-assisted processes; hydrothermal and solvothermal syntheses routes; sol-gel processes; chemical reduction, etc. The application of earth-abundant transition/noble metal-free (TMs, where M = Co, Ni, Fe, Mn, Mo) and TM-based alloys or non-metallic (TMXs, where X = N, O, S, C, P, etc.) compounds as active electrocatalysts for HER/OER has been reported [45][46][47][48][49]. TMXs have received much attention due to their distinctive structural features, abundant active sites, tunable electronic properties, compositions, and ease of employment for large-scale production. ...
This study presents the synthesis of graphitic carbon nitride (g-C3N4) and its nanostructures with cobalt ferrite (CoFe2O4) and silver nanocubes (Ag) when using the combined pyrolysis of melamine and the polyol method. The resulted nanostructures were tested as electrocatalysts for hydrogen and oxygen evolution reactions in alkaline media. It was found that Ag@CoFe2O4/g-C3N4 showed the highest current density and gave the lowest overpotential of −259 mV for HER to reach a current density of 10 mA cm−2 in a 1 M KOH. The overpotentials for reaching the current density of 10 mA·cm−2 for OER were 370.2 mV and 382.7 mV for Ag@CoFe2O4/g-C3N4 and CoFe2O4/g-C3N4, respectively. The above results demonstrated that CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4 materials could act as bifunctional catalysts due to their notable performances and high stabilities toward hydrogen and oxygen evolution reactions (HER and OER). Total water splitting in practical applications is a promising alternative to noble-metal-based electrocatalysts.
... Given the impressively high cost of precious metals and their scarcity in nature, significant progress has been made in the search for alternative, costeffective, and innovative substitutes. As evidence of this, a rapidly increasing number of publications has been reported on earth-abundant transition/noble metal-free (TMs, M=Co, Ni, Fe, Mn, and Mo) and TMs-based alloys or non-metallic (TMXs, X=N, O, S, C, P, etc.) compounds, aimed to function as active HER/OER electrocatalysts [17][18][19][20][21]. TMXs have received much attention due to their compositions, ease of application for large-scale production, distinctive structural features, abundant active sites, and tunable electronic properties. ...
This study focuses on fabricating cobalt particles deposited on graphitic carbon nitride (Co/gCN) using annealing, microwave-assisted and hydrothermal syntheses, and their employment in hydrogen and oxygen evolution (HER and OER) reactions. Composition, surface morphology, and structure were examined using inductively coupled plasma optical emission spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The performance of Co-modified gCN composites for the HER and OER were investigated in an alkaline media (1 M KOH). Compared to the metal-free gCN, the modification of gCN with Co enhances the electrocatalytic activity towards the HER and OER. Additionally, thermal annealing of both Co(NO3)2 and melamine at 520 °C for 4 h results in the preparation of an effective bifunctional Co3O4/gCN catalyst for the HER with the lower Eonset of −0.24 V, a small overpotential of −294.1 mV at 10 mA cm−2, and a low Tafel slope of −29.6 mV dec−1 in a 1.0 M KOH solution and for the OER with the onset overpotential of 286.2 mV and overpotential of 422.3 mV to achieve a current density of 10 mA cm−2 with the Tafel slope of 72.8 mV dec−1.
... In our case, the Tafel slope value of more than 120 mV dec -1 indicates a dominance of the Volmer reaction in hydrogen desorption from the catalyst surface. Generally, this dominance of the Volmer reaction indicates HER prefers to proceed via the Tafel mechanism [80]. It reveals the improved catalytic behavior and synergistic performance of MTIembedded composite with graphene and g-C 3 N 4 for these metal-free heterostructures. ...
Tri-s-triazine heterostructures have been prepared using different mass ratios of g-C3N4, melem, mellitic trianhydride (MTA), and graphene as precursors. The heterostructures were characterized by elemental analysis (CHN), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), specific surface area, thermogravimetric analysis (TGA/DTA), and diffuse reflection spectroscopy (DRS). The formation of reactive species under light irradiation was verified by electron paramagnetic resonance (EPR). Hydrogen evolution reactions (HER) were conducted in a potentiostat. According to the results, the heterostructures produced with melem and MTA are formed by mellitic triimide (MTI) and tri-s-triazine species (g-C3N4 and melem) layers. EPR spin-trapping experiments showed hydroxyl radicals formation when the materials were irradiated, which, however, are produced indirectly by superoxide anion radicals. The increased amount of MTI is detrimental to photocatalytic activity. On the other hand, we proved that the increase in the MTA content increased the HER. The results showed that modulation of MTI content in intramolecular tri-s-triazine heterostructures is a promising approach for producing materials for photo- and electrocatalysis.
Graphical abstract
Metal-free hybrid photocatalysts of tri-s-triazine species (melem and g-C3N4), graphene, and mellitic trianhydride (MTA) were synthesized: g-g, g-m, and g-g-m. The mellitic triimide (melem and MTA) ratio and graphene as a cocatalyst in heterostructures for generating reactive oxygen species (in visible light) and hydrogen evolution reactions were evaluated.
... Under neutral and alkaline environments, the mechanisms of HER involve two additional water dissociation processes as compared with those in an acidic environment, which is unfavorable to fast reaction kinetics [14,15]. However, the relatively moderate characteristics of neutral and alkaline electrolytes allow the use of non-noblemetal catalysts and are thus beneficial to cost reduction in, and large-scale application of, water electrolysis [16,17]. ...
NiMo-based nanostructures are among the most active hydrogen evolution reaction (HER) catalysts under an alkaline environment due to their strong water dissociation ability. However, these nanostructures are vulnerable to the destructive effects of H2 production, especially at industry-standard current densities. Therefore, developing a strategy to improve their mechanical strength while maintaining or even further increasing the activity of these nanocatalysts is of great interest to both the research and industrial communities. Here, a hierarchical interconnected NiMoN (HW-NiMoN-2h) with a nanorod-nanowire morphology was synthesized based on a rational combination of hydrothermal and water bath processes. HW-NiMoN-2h is found to exhibit excellent HER activity due to the accomodation of abundant active sites on its hierarchical morphology, in which nanowires connect free-standing nanorods, concurrently strengthening its structural stability to withstand H2 production at 1 A cm⁻². Seawater is an attractive feedstock for water electrolysis since H2 generation and water desalination can be addressed simultaneously in a single process. The HER performance of HW-NiMoN-2h in alkaline seawater suggests that the presence of Na⁺ ions interferes with the reation kinetics, thus lowering its activity slightly. However, benefiting from its hierarchical and interconnected characteristics, HW-NiMoN-2h is found to deliver outstanding HER activity of 1 A cm⁻² at 130 mV overpotential and to exhibit excellent stability at 1 A cm⁻² over 70 h in 1 M KOH seawater.
Supplementary Information
The online version contains supplementary material available at 10.1007/s40820-023-01129-y.
... Particularly, noble metal single atom catalysts (SACs) have shown perfect HER performances [8], however, the sophisticated, complex and expensive loading process of the catalysts and the high price of noble metal limits their commercial applications. On the other hand, transition metals, such as Fe, Co, Ni, Cu, Mo and W, are regarded as promising substitutes of noble metal catalysts and widely used in catalysts for HER [9][10][11][12][13][14][15]. However, the activity and working life of these catalysts are far not enough to ensure the scale demand. ...
Metallic glasses (MGs, also known as amorphous alloys) have shown excellent hydrogen evolution performances due to the unique long-distance disorder atomic structures. Here, we demonstrate that the hydrogen evolution performances of MG can be greatly improved by introducing nanoscale heterogeneous structures. A nanoscale homogeneous Fe75B25 MG (Homo-MG) is prepared by melt-spinning, and then it is processed by severe plastic deformation to form a nanoscale heterogenous Fe75B25 MG (Hetero-MG). The hetero-MG shows superior activity and stability of hydrogen evolution compared with those of the homo-MG. The overpotential at 10 mA cm−2 of the hetero-MG is only 116 mV in 1 M KOH, which is much smaller than that of the homo-MG of 402 mV. Moreover, the hetero-MG shows very high and stable catalytic performances even at industrial level current densities (e.g., 300 mA cm−2) which could not be obtained in the homo-MG. This study clearly shows that fabrication of nanoscale heterogeneous structures is an effective strategy to improve the catalytic performances of MGs, and it also has a very positive guiding significance for further promoting the application of MG in energy conversion and storage, sewage degradation, catalysis, and other fields.
... Many studies have shown that catalysts can significantly improve the performance of the battery. Common noble metal catalysts include Pt, Pd, Ru, Ir, etc. [9]. According to M-H bond energy [10,11], Pt-based catalysts only show good catalytic activity for ORR, and have been widely reported in the fields of photocatalysis [12], electrochemical catalytic decomposition of water to produce hydrogen Sirong Li and Mengyao Zhao are contributed equally to this work. ...
The development of new energy storage and conversion pathways has gradually freed people from dependence on oil, and improving conversion efficiency and safety is still the mainstream trend in the development of new energy sources, and electrode catalysts have become crucial in solving these problems. At present, precious metal catalysts have good performance, but the scarcity of resources and the high price limit commercialization. Therefore, it is very important to develop a catalyst for commercialization. In this paper, the Pt nanoparticles were successfully loaded on the mesoporous FeCo2O4 nanosheet catalyst by the sodium borohydride (NaBH4) reduction method. The formation of the Pt-O bond results in covalent coupling between Pt and FeCo2O4. The increase in defect sites also facilitates improved catalytic activity. According to the test results, the ORR process is mainly carried out through the "4-electron reaction" pathway, and the main product is water, and the Tafel slope of Pt-FeCo2O4 (-73 mV dec⁻¹) is smaller than that of Pt/C (-84 mV dec⁻¹) and FeCo2O4 (-76 mV dec⁻¹). The stability of Pt-FeCo2O4 to ORR (After 12 h, only 15.5% current density loss) is significantly higher than that of commercial Pt/C. Meanwhile, in the OER process, Pt-FeCo2O4 (28.1 mA cm⁻²) has a higher current density than RuO2 (24.4 mA cm⁻²) and FeCo2O4 (15.1 mA cm⁻²). Therefore, Pt-FeCo2O4 can be used as a bifunctional catalyst to accelerate the ORR and OER processes, which has important theoretical research significance and commercial value.
... Meanwhile, the intrinsic mechanism of the photocatalytic water splitting for hydrogen production is relatively consistent with the photocatalytic degradation of pollutants [114][115][116][117]. When the photon energy absorbed by TiO 2 aerogels is greater than the bandgap, electronhole pairs can be excited, and then the uncombined electrons and holes participate in different redox reactions to achieve the purpose of hydrogen production, as shown in equations (14)-(17) (Figure 9(a)). ...
Photocatalysis is a new type of renewable energy technology used in environmental treatment and hydrogen energy production. In this regard, a new class of photocatalysts, TiO2 aerogels, are attractive for having the chemical characteristics of TiO2 nanomaterials such as high catalytic activity, good stability, non-toxic, and non-polluting, and the structural characteristics of aerogels such as large specific surface area, high porosity, the 3-dimensional interconnected network structure composed of relatively uniform nanoparticles, and high light transmittance. Here we review the recent progress in TiO2 aerogels for photocatalysis, focusing on preparation techniques, the crystalline phases’ influence on photocatalytic properties, the modification of photocatalytic properties, and the analysis and discussion of future development. In particular, we first summarize various preparation techniques, including sol–gel method, nanoparticles self-assembly synthesis, and high-temperature aerosol technique, then detail the structure and composition of TiO2 crystalline phases that affect the photocatalytic properties. Subsequently, we discuss strategies to further enhance the photocatalytic properties of TiO2 aerogels by the composite of SiO2 aerogel semiconductors, the doping of metal dopants, and the doping or composite of non-metallic substances, and elaborate the modification mechanism and the modification effect achieved. Finally, combined with the research status of TiO2 aerogels and the development experience of other aerogels, we conduct a reasonable analysis and discussion on their further research directions and industrialization roads.
... The reaction is only kinetically viable when its overpotential is lowered by a suitable electrocatalyst (i.e., a hydrogen-evolving catalyst or HEC), serving to decrease the activation barriers involved in the various chemical steps. The best solid-state HEC is platinum [90], which displays several favorable properties, such as an almost-zero hydrogen adsorption free energy, durability, and high electrical conductivity. However, it is a rare and expensive material, and this makes its wide adoption unfeasible. ...
The extraordinary potential of hydrogen as a clean and sustainable fuel has sparked the interest of the scientific community to find environmentally friendly methods for its production. Biological catalysts are the most attractive solution, as they usually operate under mild conditions and do not produce carbon-containing byproducts. Hydrogenases promote reversible proton reduction to hydrogen in a variety of anoxic bacteria and algae, displaying unparallel catalytic performances. Attempts to use these sophisticated enzymes in scalable hydrogen production have been hampered by limitations associated with their production and stability. Inspired by nature, significant efforts have been made in the development of artificial systems able to promote the hydrogen evolution reaction, via either electrochemical or light-driven catalysis. Starting from small-molecule coordination compounds, peptide- and protein-based architectures have been constructed around the catalytic center with the aim of reproducing hydrogenase function into robust, efficient, and cost-effective catalysts. In this review, we first provide an overview of the structural and functional properties of hydrogenases, along with their integration in devices for hydrogen and energy production. Then, we describe the most recent advances in the development of homogeneous hydrogen evolution catalysts envisioned to mimic hydrogenases.
... Because it has cleaner combustion byproducts and a greater energy density than fossil fuels, hydrogen is well known to be a suitable energy source to replace them [53]. Water electrolysis, coal gasification, electrocatalysis, and photocatalysis are now the most widely utilized processes for producing hydrogen [54][55][56][57][58][59][60][61][62][63][64][65][66]. However, due to its sustainability and absence of secondary emissions, photocatalytic hydrogen generation is the most promising of these preparation techniques. ...
MXenes (Ti3C2Tx) have gotten a lot of interest since their discovery in 2011 because of their distinctive two-dimensional layered structure, high conductivity, and rich surface functional groups. According to the findings, MXenes (Ti3C2Tx) may block photogenerated electron-hole recombination in the photocatalytic system and offer many activation reaction sites, enhancing the photocatalytic performance and demonstrating tremendous promise in the field of photocatalysis. This review discusses current Ti3C2Tx-based photocatalyst preparation techniques, such as ultrasonic mixing, electrostatic self-assembly, hydrothermal preparation, and calcination techniques. We also summarised the advancements in photocatalytic CO2 reduction, photocatalytic nitrogen fixation, photocatalytic hydrogen evolution, and Ti3C2Tx-based photocatalysts in photocatalytic degradation of pollutants. Lastly, the challenges and prospects of Ti3C2Tx in photocatalysis are discussed based on the practical application of Ti3C2Tx.
Developing high‐efficiency and stable oxygen evolution reaction (OER) electrocatalysts is an imperative requirement to produce green and clean hydrogen energy. In this work, the FeCoS y /NCDs composite with nitrogen‐doped carbon dots (NCDs) modified Fe–Co sulfide (FeCoS y ) nanosheets is prepared by using a facile and mild one‐pot solvothermal method. Benefiting from the low crystallinity and the synergistic effect between FeCoS y and NCDs, the optimal FeCoS y /NCDs‐3 composite exhibits an overpotential of only 284 mV at 10 mA cm ⁻² , a small Tafel value of 52.1 mV dec ⁻¹ , and excellent electrochemical durability in alkaline solution. Remarkably, unlike ordinary metal sulfide electrocatalysts, the morphology, components, and structure of the FeCoS y /NCDs composite can be well retained after OER test. The NCDs modified FeCoS y composite with excellent electrocatalytic performance provides an effective approach to boost metal sulfide electrocatalysts for practical application.
We provide a comprehensive overview of the recent developments in the field of two-dimensional (2D) materials for energy conversion and storage applications. The focus of this review is the electronic...
The research on high-efficiency two-dimensional (2D) catalytic materials for the small-molecule oxidation-assisted hydrogen evolution reaction (HER) is prospective for efficient hydrogen production. Herein, we report heterostructured Pt/Rh metallene with Pt nanoparticles (NPs) uniformly anchored on Rh metallene for the HER and ethylene glycol oxidation reaction (EGOR). The ultrathin sheet structure of the Pt/Rh metallene offers high surface areas and sufficient active sites. More importantly, the Pt/Rh heterostructure can optimize catalytic active centers and adjust electronic structure. Thus, Pt/Rh metallene exhibits superior electrocatalytic HER activity with a low overpotential of 28 mV in 1 M KOH at 10 mA cm-2 and EGOR activity with a specific activity of 8.39 mA cm-2 in 1 M KOH with 3 M EG, along with outstanding CO tolerance. In a two-electrode system, Pt/Rh metallene requires a low potential of 0.51 V for stable and efficient hydrogen production at 10 mA cm-2 in 1 M KOH + 3 M EG, with the simultaneous production of high-value-added products. The job proposes an attractive strategy for the synthesis of 0D/2D metallene toward simultaneous energy-saving hydrogen production and chemical update.
Hydrogen is a carbon-free alternative energy source for use in future energy frameworks with the advantages of environment-friendliness and high energy density. Among the numerous hydrogen production techniques, sustainable and high purity of hydrogen can be achieved by water electrolysis. Therefore, developing electrocatalysts for water electrolysis is an emerging field with great importance to the scientific community. On one hand, precious metals are typically used to study the two-half cell reactions, i.e., hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, precious metals (i.e., Pt, Au, Ru, Ag, etc.) as electrocatalysts are expensive and with low availability, which inhibits their practical application. Non-precious metal-based electrocatalysts on the other hand are abundant with low-cost and eco-friendliness and exhibit high electrical conductivity and electrocatalytic performance equivalent to those for noble metals. Thus, these electrocatalysts can replace precious materials in the water electrolysis process. However, considerable research effort must be devoted to the development of these cost-effective and efficient non-precious electrocatalysts. In this chapter, we provide key fundamental knowledge of water electrolysis, progress, and challenges of the development of most-studied electrocatalysts in the most desirable electrolytic solutions: alkaline water electrolysis (AWE), solid-oxide electrolysis (SOE), and proton exchange membrane electrolysis (PEME). Lastly, we discuss remaining grand challenges, prospect, and future work with key recommendations that must be done prior to the full commercialization of water electrolysis systems. Noble-metal-based electrocatalysts (Pt, RuO2, IrO2, etc.) have shown superior activity towards these reactions. However, their lower natural abundance and inferior stability make the cost to performance ratio of water electrolysis too high. Thus, huge amount of research efforts is being carried out to develop electrocatalysts consisting of earth abundant elements (transition metals, carbon etc.) as the replacement of these noble-metal-based materials. Transition metal compounds, carbonaceous and hybrid materials have shown promise as efficient electrocatalysts but there is still huge gap between the activities of these materials and the noble-metal-based electrocatalysts. Several strategies like morphology modulation, elemental doping, defect engineering etc. are being deployed to enhance the activity of these noble-metal-free electrocatalysts.KeywordsHydrogen productionHEROERCatalystsElectrolysisCatalytic activityNoble metalsNon-noble metalsH2O adsorptionEnergeticsCarbon-based materials
Discovering highly efficient and stable non-precious metal catalysts for the oxygen evolution reaction (OER) is crucial for energy conversion in water splitting. However, preparing high-performance OER catalysts and elucidating the structural changes in the process are still challenging. Herein, we synthesize the NiTe/Ni2P heterostructure and demonstrate the strain engineering of NiTe/Ni2P via the lattice incompatibility between the phosphide and the telluride. The strain engineering of the NiTe/Ni2P heterostructure not only significantly boosts the OER activity but also effectively stabilizes the intrinsic structure of the catalyst after the OER process by using the in situ-produced metal salt as a protection layer. After the OER stability test, no oxyhydroxide phase is observed, and in situ Raman spectroscopy reveals that a voltage-dependent phase transition appears during the OER, which is different from most previously reported Ni-based catalysts, for which the generation of irreversible NiOOH occurs after the OER. Density functional theory calculations further reveal that the tensile strain of Ni2P will inhibit the presence of irreversible phase transitions of Ni2P into NiOOH due to the weak adsorption ability of the oxygen species caused by strain engineering. In short, this work opens a new gate for using strain nanotechnology to design high-performance OER catalysts.
Phosphorus-rich compounds have emerged as a promising class of energy storage and conversion materials due to their interesting structures and electrochemical properties. Herein, we propose that a metallic CrP2 monolayer, isomorphic to 1H-phase MoS2, is a good prospect as an anode for K-ion batteries and a catalyst for hydrogen evolution through first-principles calculations. The CrP2 monolayer demonstrates not only a desirable high K storage capacity (940 mA h g-1) but also a low K-ion diffusion barrier (0.10 eV) and average open circuit voltage (0.40 V). On the other hand, its Gibbs free energy (0.02 eV)/active site density is superior/comparable to that of commercial Pt, resulting from the contribution of the lone pair electrons of the P atom. Its high structural stability and intrinsic metallicity can ensure high safety and performance during the cyclic process. These interesting properties make the CrP2 monolayer a promising multifunctional material for energy storage and conversion devices.
The exploration of non-noble-metal-based catalysts for the hydrogen evolution reaction (HER) is important for the green synthesis of hydrogen via electrochemical water splitting. Graphitic carbon nitride (g-C3N4) is a widely studied photocatalyst for the HER; however, its intrinsic catalytic activity is poor. In the present study, bare and S-doped g-C3N4 were combined with the less explored NiV layered double hydroxide (LDH) and their performance for electrochemical HER catalysis was studied. The fused layers of the composites result in a higher specific surface area and expose a higher number of active centres for better catalytic activity. The composite of sulphur-doped g-C3N4 and the NiV LDH showed the least overpotential in comparison with its counterparts, with commendable stability during continuous operation for 8 h. The kinetics of the HER are enhanced by compositing g-C3N4 with the NiV LDH, as indicated by the Tafel slope of 79 mV dec⁻¹ and impedance spectroscopy analysis. This study proposes a novel g-C3N4/NiV LDH-based composite for the HER and gives a comprehensive analysis of the catalytic mechanism in the prepared catalysts.
In this study, a Pd@MXene catalyst was synthesized to enhance the electrocatalytic hydrodehalogenation (ECH) of emerging halogenated organic pollutants (HOPs) by improving the dispersibility, catalytic activity, and stability of palladium (Pd). The average size of Pd nanoparticles (NPs) was reduced to 3.62 ± 0.34 nm with a more intensive peak of Pd (111), which facilitated atomic hydrogen (H*) production. The Pd@MX/CC electrode demonstrated superior ECH activity for diclofenac (DCF) degradation, with a reaction rate constant (kobs) 2.48 times higher than that of Pd/CC (without MXene). The satisfactory ECH performance of Pd@MX/CC remained consistent within a wide range of initial DCF concentrations (5-100 mg/L), and no significant ECH attenuation was observed even after up to 10 batches. Furthermore, the high activity of Pd@MX/CC was also observed in the ECH of other halogenated organic pollutants (levofloxacin, tetrabromobisphenol A, and diatrizoate). Density functional theory (DFT) calculations revealed that electronic configuration modulation of the Pd@MXene catalyst optimized binging energies to H* , DCF, and dechlorinated products, thereby enhancing the ECH efficiency of DCF.
Water splitting is a promising technique in the sustainable "green hydrogen" generation to meet energy demands of modern society. Its industrial application is heavily dependent on the development of novel catalysts with high performance and low cost for hydrogen evolution reaction (HER). As a typical non-precious metal, cobalt-based catalysts have gained tremendous attention in recent years and shown a great prospect of commercialization. However, the complexity of the composition and structure of newly-developed Co-based catalysts make it urgent to comprehensively retrospect and summarize their advance and design strategies. Hence, in this review, the reaction mechanism of HER is first introduced and the possible role of the Co component during electrocatalysis is discussed. Then, various design strategies that could effectively enhance the intrinsic activity are summarized, including surface vacancy engineering, heteroatom doping, phase engineering, facet regulation, heterostructure construction, and the support effect. The recent progress of the advanced Co-based HER electrocatalysts is discussed, emphasizing that the application of the above design strategies can significantly improve performance by regulating the electronic structure and optimizing the binding energy to the crucial intermediates. At last, the prospects and challenges of Co-based catalysts are shown according to the viewpoint from fundamental explorations to industrial applications.
Dealing with overcoming the sluggishness of the oxygen evolution reaction (OER), the urea oxidation reaction (UOR) existed. ZnCo2O4, an excellent OER electrocatalyst, its application towards UOR is studied with surface-grown polydopamine (PDA). ZnCo2O4@PDA is produced over the surface of nickel foam by hydrothermal method followed by self-polymerization of dopamine hydrochloride. Dopamine hydrochloride was varied in solution mixture to study the optimal growth of PDA necessary to enhance electrochemical activity. Hence, prepared ZnCo2O4@PDA is characterized by X-ray diffraction, electronic structural, and morphology/microstructure studies for confirmation. With successful confirmation, the developed electrode material is applied towards UOR and ZnCo2O4@ PDA-1.5, delivering an excellent low overpotential of 80 mV @ 20 mA/cm2 in the electrolyte mixture of 1 M potassium hydroxide + 0.33 M urea. Also, to support excellent UOR activity, other electrochemical properties such as tafel slope, electrochemical surface active sites, and electrochemical impedance spectroscopy are studied. Further, a schematic illustration explaining the UOR mechanism is shown to clearly understand the obtained electrochemical activity. Finally, urea water electrolysis is carried out in a two-electrode symmetrical cell and compared with water electrolysis. This clearly shows the potential of the developed material for efficient electrochemical hydrogen production.
The synergetic effect is an effective approach to promote green hydrogen evolution reaction (HER), however not yet reported for ternary metal doped porous g-C3N4 fiber-like nanostructures. Herein we synthesized ternary...
Engineering competent electrocatalysts to accelerate the production of sustainable hydrogen fuel is an important criterion in electrochemical water splitting. Hydrogen evolution reaction (HER) as an equally crucial step in water...
Precious metals and related compounds typically exhibit superb catalytic activity for electrocatalytic water splitting. However, further enhancement of catalytic performance encounters a bottleneck that originates from limited regulation strategies. As an electrocatalyst, rhenium (Re)-based materials have become high-potential catalytic materials in recent years due to their unique 1 T (1T′) phase structure, weak interlayer coupling, direct bandgap semiconductivity, and in-plane anisotropy. These materials have elicited considerable attention in the field of electrocatalytic water splitting. In this paper, we highlighted recent progress in regulation strategies for Re-based electrocatalysts to provide references for the study of Re-based electrocatalytic materials. First, we introduce the basic mechanism of the Re-based electrocatalytic process. Then, we provide an in-depth discussion of the regulatory strategies used to enhance the catalytic activity of Re-based electrocatalysts, including edge site, defect/vacancy engineering, interface engineering, and phase engineering strategies. We also summarize current challenges in Re-based electrocatalysts and present possible prospects for the design of highly efficient Re-based electrocatalysts in the future.
The development of sustainable energy conversion technologies accounts for the availability of efficient and abundant electrocatalysts for the oxygen evolution reaction (OER). These catalysts play a vital role in various energy conversion technologies, i.e., metal-air batteries, fuel cells, and water electrolysis. Therefore, it is necessary to identify and develop electrocatalysts that can perform the OER with high performance while being composed of abundant and inexpensive elements. In present work, we describe a straightforward method to produce a series of CuO/cobalt manganese oxide (CMO) spinal nanocomposites serving as an electrocatalyst for OER reactions. The CuO content can be easily changed to improve the activity of CuO-CMOX (X = 12%, 16%, 20%) catalysts. The obtained results show that as compared to pure CMO spinel, the inclusion of CuO improved the OER activity, and CuO-CMO-16% of them demonstrated the best OER performance. The optimized electrocatalyst CuO-CMO-16% is characterized by different physical analyses. All electrochemical tests were performed via cyclic voltammetry (CV) and linear sweep voltammetry (LSV) techniques. This fabricated CuO-CMO-16% shows 277 mV overpotential at 10-mA cm⁻² current density which is smaller as compared to pristine CMO. Its higher conductivity is also indicated by smaller value of charge transfer resistance, obtained by Nyquist plot. The larger Co³⁺/Co²⁺ and Mn⁴⁺/Mn³⁺ ratio has a wider specific surface area of the CuO-CMO-16% catalyst improving the oxygen evolution due to exposed more active sites, which benefited the OER activity. The contact between both individuals can be simplified by the greater electrical conductivity, which considerably improved its OER capabilities.
Water electro-splitting driven by renewable energy is significant in energy conversion for the development of hydrogen energy sources. The hydrogen evolution reaction (HER) directly generating hydrogen products occurs in cathode catalysis. Over the years, significant progress has been made to boost the HER efficiency by exploratively designing highly active and economical Pt-based electrocatalysts. However, there are still some urgent problems to be solved for Pt-based HER catalysts in more economical alkaline electrolytes, such as the slow kinetics caused by additional hydrolysis dissociation steps, which greatly hinders the practical application. This review systematically summarizes several strategies for optimizing alkaline HER kinetics and provides direct guidelines for the design of highly active Pt-based electrocatalysts. Specifically, the intrinsic HER activity in alkaline water electrolysis can be boosted by accelerating the water dissociation, optimizing the hydrogen binding energy or modulating the spatial dimensions of the electrocatalyst based on the HER mechanism. Finally, we prospect the challenges for the alkaline HER on novel Pt-based electrocatalysts, including the active site study, the HER mechanism exploration and the extensible catalyst preparation technologies.
To produce materials with enhanced catalytic activity toward hydrogen evolution reaction we combined MoS2 as transition metal dichalcogenide and Bi2Se3 as topological insulator. The composites were produced by three methods: mechanical milling, high power sonication and spin-coating. MoS2 and Bi2Se3 as precursors in composites preparation were synthesized by hydrothermal method. The structure and morphology of various composites were correlated with their electrochemical properties obtained by impedance spectroscopy, linear sweep and cyclic voltammetry. Mechanical milling provided composites with the most pronounced activity improvement as a result of the largest damage and amount of introduced defects into the materials structure. The potential required to achieve the current density of 10 mA/cm2 in these samples is lowered up to 50 mV compared to as-synthesized material. Bi2Se3 in composite materials promotes the electron transfer to MoS2 which leads to the decrease of charge transfer resistance by 25 Ω.
The use of metal single-atom catalysts (SACs) has recently received a lot of attention for efficient oxygen evolution reaction (OER) water splitting because of their unique catalytic properties due to...
Electrocatalytic oxidation of urea (UOR) is a potential energy-saving hydrogen production technology that can replace oxygen evolution reaction (OER). Therefore, CoSeP/CoP interface catalyst is synthesized on nickel foam using hydrothermal, solvothermal, and in situ template methods. The strong interaction of tailored CoSeP/CoP interface promotes the hydrogen production performance of electrolytic urea. During the hydrogen evolution reaction (HER), the overpotential can reach 33.7 mV at 10 mA cm-2 . The cell voltage can reach 1.36 V at 10 mA cm-2 in the overall urea electrolytic process. Notably, the overall urine electrolysis performance of the catalyst in the human urine medium can reach 1.40 V at 10 mA cm-2 and can exhibit durable cycle stability at 100 mA cm-2 . Density functional theory (DFT) proves that the CoSeP/CoP interface catalyst can better adsorb and stabilize reaction intermediates CO* and NH* on its surface through a strong synergistic effect, thus enhancing the catalytic activity.
Developing highly efficient, low cost, and stable electrocatalysts that work at a large current density is crucial for upgrading the current industrial electrochemical water splitting to produce H 2 . Molybdenum selenide (MoSe 2 ) is a promising 2D transition metal dichalcogenide (TMD), however, its reported output is inadequate due to its inert basal plane. Herein, the catalytic activity of MoSe 2 nanosheet arrays is activated by a novel and controllable method of He ⁺ ion irradiation to introduce multiple vacancies simultaneously into their inert basal planes. The vacancies activated MoSe 2 have improved electrocatalytic performance and stability with a minimum overpotential of 90 mV at 10 mA cm ⁻² , a Tafel slope of 49 mV dec ⁻¹ and high stability of 650 h at the industry‐level large current density of 1000 mA cm ⁻² compared to several hours for the pristine sample. The DFT results reveal that single Se and single Mo vacancies on the MoSe 2 basal plane can efficiently increase the electrical conductivity and reduce energy barriers for water dissociation and subsequent proton adsorption, thus improving the electrocatalytic capability. This finding proves the application of ion beam in defect engineering for effective hydrogen evolution in TMDs‐based catalysts.
Herein, we synthesized porous one-dimensional graphitic carbon nitride (g-C3N4) doped atomically with metal atoms (M/g-C3N4) (M = Cu, Mn, and Fe) for the electrochemical and photo-electrochemical hydrogen evolution reaction (HER). This is driven by the direct acidification of an aqueous solution of metal precursors and melamine followed by pyrolysis at 550 °C under N2. The as-obtained M/g-C3N4 had well-defined pore sizes (5–10 nm), nanofibers (90 ± 5 nm in width and 5 ± 1 μm in length) morphology, high surface area, and M atomic contents (1.7 ± 0.2 wt.%). The HER performance is in the order of Cu/g-C3N4 > Fe/g-C3N4 > Mn/g-C3N4 in terms of the overpotential, onset potential, H2 production rate, and mass/specific activity. Notably, Cu/g-C3N4 achieved turnover frequency (TOF) close to that of commercial 10 wt.% Pt/C, but higher mass/specific activity and great H2 production rate of (222.15 μmol· g⁻¹·h⁻¹). This work open the doorway for the utilization of g-C3N4 doped metal-atoms at low content in electrocatalytic and photocatalytic HER.
The development of cheap, abundant, and highly efficient electrocatalysts for the oxygen evolution reaction (OER) is urgently needed for hydrogen production from water splitting. Herein, we demonstrate a novel OER electrocatalyst (NiFe(CN)5NO/Ni3S2) prepared by coupling Ni3S2 and a bimetallic metal-organic framework (MOF) of NiFe(CN)5NO on nickel foam (NF) via a simple two-step route. The NiFe(CN)5NO/Ni3S2 electrocatalyst displays an interesting rod-like hierarchical architecture assembled by ultrathin nanosheets. The combination of NiFe(CN)5NO and Ni3S2 optimizes the electronic structure of the metal active sites and increases the electron transfer capability. Benefitting from the synergistic effect between Ni3S2 and the NiFe-MOF as well as the unique hierarchical architecture, the NiFe(CN)5NO/Ni3S2/NF electrode exhibits excellent electrocatalytic OER activity with ultralow overpotentials of 162/197 mV at 10/100 mA cm-2 and an ultrasmall Tafel slope of 26 mV dec-1 in 1.0 M KOH, which are far superior to those of the individual NiFe(CN)5NO, Ni3S2 and commercial IrO2 catalysts. In particular, unlike common metal sulfide-based electrocatalysts, the composition, morphology and microstructure of the NiFe-MOF/Ni3S2 composite electrocatalyst can be well retained after the OER, which endows it with fantastic long-term durability. This work offers a new strategy for the construction of novel and high-efficiency MOF-based composite electrocatalysts for energy applications.
Manufacturing low-cost, high-performance and earth-rich catalysts for hydrogen evolution (HER) and oxygen evolution reactions (OER) is critical to achieving sustainable green hydrogen production. Herein, we utilize lacunary Keggin-structure [PW9O34]9- (PW9) as a molecular pre-assembly platform to anchor Ni within a single PW9 molecule by vacancy-directed and nucleophile-induced effects for the uniform dispersion of Ni at the atomic level. The chemical coordination of Ni with PW9 can avoid the aggregation of Ni and favor the exposure of active sites. The Ni3S2 confined by WO3 prepared from controlled sulfidation of Ni6PW9/Nickel Foam (Ni6PW9/NF) exhibited excellent catalytic activity in both 0.5 M H2SO4 and 1 M KOH solutions, which required only 86 mV and 107 mV overpotentials for HER at a current density of 10 mA∙cm-2 and 370 mV for OER at 200 mA∙cm-2. This is attributed to the good dispersion of Ni at the atomic level induced by trivacant PW9 and the enhanced intrinsic activity by synergistic effect of Ni and W. Therefore, the construction of active phase from the atomic level is insightful to the rational design of dispersed and efficient electrolytic catalysts.
Catalysts with a single atom site allow highly tuning of the activity, stability, and reactivity of heterogeneous catalysts. Therefore, atomistic understanding of the pertinent mechanism is essential to simultaneously boost the intrinsic activity, site density, electron transport, and stability. Here, we report that atomically dispersed nickel (Ni) in zincblende cadmium–zinc sulfide quantum dots (ZCS QDs) delivers an efficient and durable photocatalytic performance for water splitting under sunlight. The finely tuned Ni atoms dispersed in ZCS QDs exhibit an ultrahigh photocatalytic H 2 production activity of 18.87 mmol hour ⁻¹ g ⁻¹ . It could be ascribed to the favorable surface engineering to achieve highly active sites of monovalent Ni(I) and the surface heterojunctions to reinforce the carrier separation owing to the suitable energy band structures, built-in electric field, and optimized surface H 2 adsorption thermodynamics. This work demonstrates a synergistic regulation of the physicochemical properties of QDs for high-efficiency photocatalytic H 2 production.
Hydrogen fuel is considered as the cleanest renewable resource and the primary alternative to fossil fuels for future energy supply. Sustainable hydrogen generation is the major prerequisite to realize future hydrogen economy. The electrocatalytic hydrogen evolution reaction (HER), as the vital step of water electrolysis to H2 production, has been the subject of extensive study over the past decades. In this comprehensive review, we first summarize the fundamentals of HER and review the recent state-of-the-art advances in the low-cost and high-performance catalysts based on noble and non-noble metals, as well as metal-free HER electrocatalysts. We systemically discuss the insights into the relationship among the catalytic activity, morphology, structure, composition, and synthetic method. Strategies for developing an effective catalyst, including increasing the intrinsic activity of active sites and/or increasing the number of active sites, are summarized and highlighted. Finally, the challenges, perspectives, and research directions of HER electrocatalysis are featured.
Hydrogen production by electrochemical water splitting is a technology with the potential to meet the growing worldwide demand for sustainable and clean energy. However, the development of cost-effective catalysts to replace noble metals, such as platinum or ruthenium, remains crucial for large-scale hydrogen production. This study presents the synthesis of a transition non-noble metal-based ternary NiMoCo hybrid nanowire array as an efficient bifunctional electrocatalyst for overall water splitting in 1.0 M KOH electrolyte. The catalyst exhibits a low cell voltage of 1.56 V to achieve a water-splitting current density of 10 mA cm⁻² together with long-term stability with only 5% of the initial current lost after 100 hours. X-ray absorption spectroscopy confirms that the addition of Co to the binary Ni-Mo system results in a highly mixed chemical binding state with modulated electronic structures. Density functional theory (DFT) calculations reveal that the Co atoms on the ternary alloy become catalytically active sites and facilitate adsorption of intermediates by ensuring preferable interactions between the reactants and the catalyst surface in comparison to the binary counterpart. This work provides a new direction along which to activate binary alloys to further enhance their catalytic abilities in overall water splitting.
Single-atom catalysts offer a pathway to cost-efficient catalysis using the minimal amount of precious metals. However, preparing and keeping them stable during operation remains a challenge. Here we report the synthesis of double transition metal MXene nanosheets—Mo2TiC2Tx, with abundant exposed basal planes and Mo vacancies in the outer layers—by electrochemical exfoliation, enabled by the interaction between protons and the surface functional groups of Mo2TiC2Tx. The as-formed Mo vacancies are used to immobilize single Pt atoms, enhancing the MXene’s catalytic activity for the hydrogen evolution reaction. The developed catalyst exhibits a high catalytic ability with low overpotentials of 30 and 77 mV to achieve 10 and 100 mA cm⁻² and a mass activity about 40 times greater than the commercial platinum-on-carbon catalyst. The strong covalent interactions between positively charged Pt single atoms and the MXene contribute to the exceptional catalytic performance and stability. © 2018, The Author(s), under exclusive licence to Springer Nature Limited.
Designing highly active and robust platinum-free catalysts for hydrogen evolution reaction is of vital importance for clean energy applications yet challenging. Here we report highly active and stable cobalt-substituted ruthenium nanosheets for hydrogen evolution, in which cobalt atoms are isolated in ruthenium lattice as revealed by aberration-corrected high-resolution transmission electron microscopy and X-ray absorption fine structure measurement. Impressively, the cobalt-substituted ruthenium nanosheets only need an extremely low overpotential of 13 mV to achieve a current density of 10 mA cm⁻² in 1 M KOH media and an ultralow Tafel slope of 29 mV dec⁻¹, which exhibit top-level catalytic activity among all reported platinum-free electrocatalysts. The theoretical calculations reveal that the energy barrier of water dissociation can greatly reduce after single cobalt atom substitution, leading to its superior hydrogen evolution performance. This study provides a new insight into the development of highly efficient platinum-free hydrogen evolution catalysts.
Highly active, stable, and cheap Pt‐free catalysts for the hydrogen evolution reaction (HER) are facing increasing demand as a result of their potential use in future energy‐conversion systems. However, the development of HER electrocatalysts with Pt‐like or even superior activity, in particular ones that can function under alkaline conditions, remains a significant challenge. Here, the synthesis of a novel carbon‐loaded ruthenium nanoparticle electrocatalyst (Ru@CQDs) for the HER, using carbon quantum dots (CQDs), is reported. Electrochemical tests reveal that, even under extremely alkaline conditions (1 m KOH), the as‐formed Ru@CQDs exhibits excellent catalytic behavior with an onset overpotential of 0 mV, a Tafel slope of 47 mV decade⁻¹, and good durability. Most importantly, it only requires an overpotential of 10 mV to achieve the current density of 10 mA cm⁻². Such catalytic characteristics are superior to the current commercial Pt/C and most noble metals, non‐noble metals, and nonmetallic catalysts under basic conditions. These findings open a new field for the application of CQDs and add to the growing family of metal@CQDs with high HER performance.
Carbon-based metal-free catalysts possess desirable properties such as high earth abundance, low cost, high electrical conductivity, structural tunability, good selectivity, strong stability in acidic/alkaline conditions, and environmental friendliness. Because of these properties, these catalysts have recently received increasing attention in energy and environmental applications. Subsequently, various carbon-based electrocatalysts have been developed to replace noble metal catalysts for low-cost renewable generation and storage of clean energy and environmental protection through metal-free electrocatalysis. This article provides an up-to-date review of this rapidly developing field by critically assessing recent advances in the mechanistic understanding, structure design, and material/device fabrication of metal-free carbon-based electrocatalysts for clean energy conversion/storage and environmental protection, along with discussions on current challenges and perspectives.
Graphical Abstract
The electrocatalytic oxidation of water is a challenging step towards the production of hydrogen as an alternative fuel. In nature, water oxidation is catalysed by a high oxidation state Mn4CaOx cluster. The corresponding industrial development of manganese catalysts for water oxidation is very attractive due to the low cost of this metal. A few manganese complexes have been previously explored as water oxidation catalysts using various chemical oxidants in homogeneous and heterogeneous systems. Efficient electrochemical water oxidation catalysed by a soluble manganese-oxo cluster, however, has not been achieved. Here, we report the synthesis and characterization of [Mn12O12(O2CC6H3(OH)2)16(H2O)4] (Mn12DH), a unique example within this class of compounds in being both highly soluble and stable in water. We demonstrate that Mn12DH, which is readily prepared from cheap and environmentally benign starting materials, is a stable homogeneous water oxidation electrocatalyst operating at pH 6 with an exceptionally low overpotential of only 334 mV. During photosynthesis, nature uses an enzyme with a manganese–calcium core for water oxidation. Here, the authors report the synthesis of a stable, water-soluble manganese cluster that acts as a homogeneous water oxidation electrocatalyst, displaying low overpotential and high Faradaic efficiency.
Low-cost, layered transition-metal dichalcogenides (MX2) based on molybdenum and tungsten have attracted substantial interest as alternative catalysts for the hydrogen evolution reaction (HER). These materials have high intrinsic per-site HER activity; however, a significant challenge is the limited density of active sites, which are concentrated at the layer edges. Here we unravel electronic factors underlying catalytic activity on MX2 surfaces, and leverage the understanding to report group-5 MX2 (H-TaS2 and H-NbS2) electrocatalysts whose performance instead mainly derives from highly active basal-plane sites, as suggested by our first-principles calculations and performance comparisons with edge-active counterparts. Beyond high catalytic activity, they are found to exhibit an unusual ability to optimize their morphology for enhanced charge transfer and accessibility of active sites as the HER proceeds, offering a practical advantage for scalable processing. The catalysts reach 10 mA cm−2 current density at an overpotential of ∼50–60 mV with a loading of 10–55 μg cm−2, surpassing other reported MX2 candidates without any performance-enhancing additives.
Graphene-based nanomaterials are promising bifunctional electrocatalysts for overall water splitting (OWS) to produce hydrogen and oxygen as sustainable fuel sources because graphene-based bifunctional electrocatalysts can provide distinct features such as large surface areas, more active sites and facile synthesis of multiple co-doped nanomaterials. Based on this, this review will present recent advancements in the development of various bifunctional graphene-based electrocatalysts for OWS reactions and discuss advancements in the tuning of electronic surface-active sites for the electrolytic splitting of water. In addition, this review will evaluate perspectives and challenges to provide a deep understanding of this emerging field.
Water splitting is limited by the high theoretical potential of oxygen evolution reaction (OER) at anode, which can reduce the energy efficiency. Nevertheless, replacing sluggish oxygen evolution by ammonia oxidation reaction (AOR), urea oxidation reaction (UOR), hydrazine oxidation reaction (HzOR)) of low theoretical potential is an energy-saving approach. In this paper, a new bifunctional catalyst of N doped NiZnCu-layered double hydroxides with reduced graphene oxide on nickel foam (N–NiZnCu LDH/rGO) is synthesized. To validate its electrochemical performance and stability, a two-electrode electrolyzer is constructed (N–NiZnCu LDH/rGO||N–NiZnCu LDH/rGO). Experiments show that at a current density of 10 mA cm⁻², the voltages of AOR, UOR, and HzOR are 0.489 V, 1.305 V, and 0.010 V with a high stability (over 3000 CV cycles), which are much better than those of Pt/C||IrO2. This study demonstrates N–NiZnCu LDH/rGO can replace precious metals for commercial hydrogen energy production in the hybrid-water electrolysis, and can be employed for treatment of industrial wastewater.
Sulfur (S) vacancies in MoS2 have been found to act as a new active center, which shows an unprecedented intrinsic HER activity under elastic strain. However, such S-vacancies are unstable and the activities are very sensitive to the vacancy concentration. A strategy to stabilize these abundant active sites is thus highly desirable. Herein, we rationally design a catalyst system to stabilize S-vacancies in the basal plane of 2H-MoS2 supported on defective vertical graphene network (VGN). The energetically favorable line-shaped S-vacancies in MoS2 show a consistently high HER activity that is insensitive to S-vacancy concentration. Moreover, the defective graphene support effectively stabilizes these S-vacancies. The optimized catalyst exhibits a superior HER activity with overpotential of 128 mV at 10 mA cm⁻² and Tafel slope of 50 mV dec⁻¹. Most importantly, the catalyst shows greatly increased stability over 500 h; benchmarking the most stable nonprecious HER catalyst in acidic media to date.
Water splitting is a sustainable approach for production of hydrogen to fuel some clean energy technologies. This process, unfortunately, has been significantly impeded by the puzzles in either the efficient but economically-unaffordable noble metal-based catalysts or the low-cost but kinetically sluggish abundant-element based catalysts. Particularly, the discovery of efficient bifunctional catalysts that can simultaneously trigger the reactions of both anode and cathode for overall water splitting still remains as a grand challenge. Herein, a novel low-cost bifunctional Ni2P/Ni0.96S heterostructured electrocatalyst, which is active for both urea oxidation reaction (UOR) at anode and hydrogen evolution reaction (HER) at cathode, is innovated for high-efficient overall splitting of urea-rich wastewater. A systematic configuration of Ni-foam (NF) supporting Ni2P/Ni0.96S catalyst electrode exhibits superior catalytic activity and stability. The Ni2P/Ni0.96S/NF||Ni2P/Ni0.96S/NF cell needs only 1.453V to reach a current density of 100 mA•cm-2 in basic urea-containing water, while it is 1.693 V for a referential noble-based Pt/C/NF||IrO2/NF electrolysis cell. This work therefore not only contributes to develop a low-cost, high-efficient, bifunctional electrocatalyst, but also provides a practically-feasible approach for urea-rich wastewater treatment.
Kleiner Anteil, große Wirkung: Eine skalierbare und allgemeine Synthesemethode zur Herstellung übergangsmetalldotierter RuM/Kohlenstoff‐Quantenpunkte (CQDs; M=Ni, Mn, Cu) durch Metall‐vermittelte CQD‐Kondensation und ‐Karbonisierung wurde entwickelt. RuM/CQD‐Katalysatoren mit niedrigem Rutheniumanteil zeigen eine herausragende Aktivität und Stabilität in der Katalyse der Wasserstoffentwicklung bei allen pH‐Werten.
Abstract
A challenging but pressing task to design and synthesize novel, efficient, and robust pH‐universal hydrogen evolution reaction (HER) electrocatalysts for scalable and sustainable hydrogen production through electrochemical water splitting. Herein, we report a facile method to prepare an efficient and robust Ru‐M (M=Ni, Mn, Cu) bimetal nanoparticle and carbon quantum dot hybrid (RuM/CQDs) for pH‐universal HER. The RuNi/CQDs catalysts exhibit outstanding HER performance at all pH levels. The unexpected low overpotentials of 13, 58, and 18 mV shown by RuNi/CQDs allow a current density of 10 mA cm⁻² in 1 m KOH, 0.5 m H2SO4, and 1 m PBS, respectively, for Ru loading at 5.93 μgRu cm⁻². This performance is among the best catalytic activities reported for any platinum‐free electrocatalyst. Theoretical studies reveal that Ni doping results in a moderate weakening of the hydrogen bonding energy of nearby surface Ru atoms, which plays a critical role in improving the HER activity.
In this study, uniform CoMoS4 nanorods array with an average nanorod diameter of ca. 80 nm have been successfully synthesized on the carbon rod via in-situ sulfurizing the precursor of uniform CoMoO4 nanorods array and applied for hydrogen evolution reaction (HER) in water splitting. The optimum sulfurization temperature was explored, and it is found that the electrode sulfurized at 400 °C (CoMoS4-400 °C) exhibited the best activity for HER in the acid solution with a low overpotential of 143 mV for reaching 10 mA/cm² current density, a small Tafel slope of 64 mV/dec, and a stable catalytic property for 1000 cycles. In addition, the CoMoS4-400 °C also performed excellent HER electrocatalytic activity in the basic solution. It is expected that such an in-situ sulfurization of one dimensional metal oxide precursor could be a promising way for the preparation of efficient electrocatalyts for HER.
Low cost and highly efficient bifuctional catalysts for overall water electrolysis have drawn considerable interests over the past several decades. Here, rationally synthesized mesoporous nanorods of nickel–cobalt–iron–sulfur–phosphorus composites are tightly self‐supported on Ni foam as a high‐performance, low cost, and stable bifunctional electrocatalyst for water electrolysis. The targeted designing and rational fabrication give rise to the nanorod‐like morphology with large surface area and excellent conductivity. The NiCoFe‐PS nanorod/NF can reach 10 mA cm−2 at a small overpotential of 195 mV with a Tafel slope of 40.3 mV dec−1 for the oxygen evolution reaction and 97.8 mV with 51.8 mV dec−1 for the hydrogen evolution reaction. Thus, this bifunctional catalyst shows low potentials of 1.52 and 1.76 V at 10 and 50 mA cm−2 toward overall water splitting with excellent stability for over 200 h, which are superior to most non‐noble metal‐based bifunctional electrocatalysts recently. This work provides a new strategy to fabricate multiple metal‐P/S composites with the mesoporous nanorod‐like structure as bifunctional catalysts for overall water splitting. Rationally synthesized mesoporous nanorods of Ni/Co/Fe phosphosulfide are tightly held on Ni foam as a high‐performance and stable bifunctional electrocatalyst for water electrolysis. This targeted designing and rational fabrication gives rise to the nanorod‐like morphology with large surface area and excellent conductivity. The as‐prepared catalyst shows low potential of 1.52 V at 10 mA cm−2 for overall water splitting.
Half-reaction of water splitting is hydrogen evolution reaction (HER), which requires low cost and high activity catalysts. Two-dimensional (2D) transition metal carbon/nitrides (MXene) materials have shown great potential as highly efficient catalysts due to their excellent properties. In this work, the heteroatom X (X=N, B, P, S) doping effect on the HER of M2C MXene (M=Ti, Mo) with or without oxygen functional groups have been performed by well-defined density functional calculations (DFT). The X doped M2CT2 (M=Mo, Ti; T=O) exhibited better HER catalytic activity than the X doped pristine M2C (M=Mo and Ti). Moreover, the calculated Gibbs free energies of hydrogen adsorption (ΔGH) indicate that the N-doped Ti2CO2 has improved electrocatalytic activity than that of Pt(111). Additionally, based on the electronic structure of X-doped Ti2CO2, the electrical conductivity of N-doped Ti2CO2 is higher than that of pristine Ti2CO2.
Herein, a hydrothermal electrodeposition method was developed to synthesize MnO2 interlinked nanowires on carbon fabric (CF). Facile chemical vapour deposition (CVD)-polymerization was subsequently employed to deposit polypyrrole (PPy) films on MnO2 nanowires to construct a hierarchical [email protected]2 structure on CF ([email protected]2@CF). This unique 1D structure is beneficial due to its high electrical conductivity, structure stabilization and fast electrochemical kinetics. The [email protected]2@CF showed a maximum areal capacitance of 1.24 F cm⁻² at a current density of 1 mA cm⁻². A flexible solid-state asymmetric supercapacitor (FSASC) was assembled using [email protected]2@CF and Fe2O3@CF as positive and negative electrodes, respectively. The FSASC delivered a maximal energy density of 1.93 mWh cm⁻³ at a power density of 9.8 mW cm⁻³ and a long term life with 92.6% of initial capacitance after 10,000 cycles. In addition, the device exhibited exceptional flexibility and excellent mechanical stability after a 200-bending-cycle test, suggesting that it is close to practical application in flexible electronics.
Single‐atom catalysts (SACs) are attractive for various reactions because of their unique properties. However, common metal‐based SACs are quite difficult to design due to the complex synthesis processes required. Here, we report a single atom nickel iodide (SANi‐I) electrocatalyst with atomically dispersed non‐metal iodine atoms. The SANi‐I is prepared via a simple calcination step in a vacuum‐sealed ampoule and subsequent cyclic voltammetry activation. Various advanced characterizations, including aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy and synchrotron‐based X‐ray absorption spectroscopy are applied to confirm the atomic‐level dispersion of iodine atoms and detailed structure of SANi‐I. Single iodine atoms are found to be isolated by oxygen atoms. The SANi‐I exhibits robust structural stability and exceptional electrocatalytic activity for the hydrogen evolution reaction. In‐situ Raman spectroscopy reveals that the hydrogen adatom (Hads) is adsorbed by a single iodine atom, forming the I‐Hads intermediate, which promotes the HER process.
Nonprecious metal single‐atom materials have attracted extensive attention in the field of electrocatalysis due to their low cost, high reactivity, high selectivity, and high atomic utilization. However, the high surface energy of a single atom causes agglomeration during preparation and catalytic measurement, resulting in damage to the catalytic sites. The strong interaction between substrate and monoatoms is the key factor to prevent the aggregation of individual metal atoms, and the geometry and electronic structure of the catalysts can be adjusted to optimize the catalytic activity. Due to the hierarchically pores, high specific surface area, and defect effect, nitrogen‐doped porous carbon (NPC) has been widely studied as an ideal nonprecious metal single‐atom support, which synergistically enhance the electrocatalytic performance toward oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, carbon dioxide reduction reaction, and nitrogen reduction reaction with non‐noble metal single atoms. This review summarizes the controllable synthesis, characterization, theoretical calculation, and application of M (M = Co, Fe, Ni, Cu, Zn, Mo, etc.) single atoms on nitrogen‐doped porous carbon. Finally, the future development and challenges of nitrogen‐doped porous carbon supported nonprecious metal single‐atom electrocatalysts for practical commercialization are concluded.
Generate hydrogen fuel from electrochemical water splitting has been considered as a promising approach. However, to obtain the low-cost and high performance catalysts towards hydrogen evolution reaction (HER)which can be applied in both alkaline and acid solution remains a challenge. Herein, we synthesized an active and stable HER catalyst composed of Mo 2 C nanocrystals embedded in the nanocarbon layers (Mo 2[email protected])by using MoO 3 nanorods as precursor. Benefiting from the porous one dimensional structure and ultrafine Mo 2 C nanocrystals, Mo 2[email protected] exhibits high HER catalytic activity for 10 mA cm ⁻² with the overpotential of 119 mV in 1 M KOH solution and 170 mV in 0.5 M H 2 SO 4 solution, respectively. Moreover, Mo 2[email protected] displays long durability during the HER process with almost no decay and maintains the porous one dimensional architecture after the HER stability test. This study offers the guideline for the further design and fabrication of the nanostructured HER electrocatalysts in wide pH range.
The design of highly efficient non-noble metal electrocatalysts for large-scale hydrogen production remains an ongoing challenge. We report here a Ni2P nanoarray catalyst grown on a commercial Ni foam substrate, which demonstrates an outstanding electrocatalytic activity and stability in basic electrolyte. The high catalytic activity can be attributed to the favorable electron transfer, superior intrinsic activity and the intimate connection between the nanoarrays and their substrate. Moreover, the unique “superaerophobic” surface feature of the Ni2P nanoarrays enables a remarkable capability to withstand internal and external forces and release the in-situ generated H2 bubbles in a timely manner at large current densities (such as > 1000 mA cm-2) where the hydrogen evolution becomes vigorous. Our results highlight that an aerophobic structure is essential to catalyze gas evolution for large-scale practical applications.
Hierarchical bimetallic Ni-Co phosphides hollow spheres were encapsulated in P-doped reduced graphene oxide (PrGO)to provide superior electrochemistry performances. The hollow spheres with controlling Co/Ni ratios were obtained via a hydrothermal process and following phosphorization reaction using Ni-Co based glycerate microsphere as a self-sacrificing template. The hollow shell with vertical branches was formed via a morphology evolution of solid, yolk-shell to hollow. The hollow spheres were encapsulated in PrGO networks by a hydrothermal synthesis and phosphorization process to fabricate PrGO/Ni-Co phosphide composite (PrGO/NiCoP). Because of unique morphology and composition, the composite with a Ni/Co molar ratio of 1:1 exhibits enhanced supercapacitor and electrocatalysis properties. As a supercapacitor electrode, PrGO/NiCoP provides high discharge specific capacity, excellent rate performance and cycle stability. An asymmetric supercapacitor assembled with PrGO/NiCoP and activated carbon possesses an energy density of 49.7 Wh/Kg with power density of 0.366 kW/kg, which can successfully illuminate red-light light emitting diode. As an electrocatalyst, PrGO/NiCoP exhibits high hydrogen and oxygen evolution reaction activities in 1 M KOH electrolyte. PrGO/NiCoP electrolyzer shows excellent overall water splitting performance and durability, which required a cell voltage of 1.56 V to achieve a current density of 10 mA cm ⁻² .
Two-dimensional (2D) molybdenum sulfide (MoS 2 ) is considered as a promising catalyst for hydrogen evolution reaction (HER), originated from its abundant hydrogen evolution active sites. However, the HER performance of MoS 2 is currently hindered by the limited exposed density of the active sites and low conductivity. Herein, we report a facile and scalable electrospinning technique to fabricate 2D MoS 2 nanoplates doped with phosphorus within one-dimensional nitrogen doped-carbon nanofibers (NCNFs-MoS 2 |P) as a highly efficient HER catalyst. The space-confined growth with the presence of NCNFs avoided the stacking and aggregation of the MoS 2 nanoplates, resulting in more exposed edge sites. The introduction of phosphorus atoms further activated the surface of MoS 2 and enhanced the electron transfer. The overpotential of NCNFs-MoS 2 |P reached 98 mV at 10 mA cm ⁻² , exhibiting excellent HER catalytic activity. Besides, almost no decay was observed after the stability test (5000 cycles or 20 h). The density functional calculations (DFT) elucidated that the incorporation of phosphorus atoms significantly improved the electrical conductivity and decreased the H adsorption energy barrier on MoS 2 , leading to a high catalytic performance of NCNFs-MoS 2 |P.
Oxygen evolution reaction (OER) has a high overpotential, which can significantly reduce the energy efficiency in water decomposition. Using urea oxidation reaction (UOR) to replace OER has been a feasible and energy-saving approach because of its lower electrode potential. Furthermore, UOR is also an important process in wastewater treatment. This paper successfully synthesizes a high-performance bifunctional catalyst for urea electrolysis. The catalyst is nickel nitride bead-like nanospheres array supported on Ni foam (Ni3N/NF). Several characterization methods are used to analyze the catalyst’s morphology, structure and composition as well as catalytic activity/stability, including XRD, SEM, TEM, XPS, and electrochemical methods (CV, LSV, EIS, and CAM). A concurrent two-electrode electrolyser (Ni3N/NF||Ni3N/NF) is constructed and used to validate the catalyst performance, and the results show that the cell performs 100 mA•cm−2 at 1.42 V. While that of Pt/C||IrO2 is 1.60 V, indicating Ni3N/NF catalyst is superior to precious metals.
Water electrolysis is the cleanest method for hydrogen production, and can be 100% green when renewable energy is used as electricity source. When the hydrogen evolution reaction (HER) is carried out in alkaline media, nickel (Ni) is a low cost catalyst and an interesting alternative to platinum. Still, its performance has to be enhanced to meet the high efficiency of the nobler metals, an objective that requires further tailoring of the surface area and morphology of Ni-based electrode materials. Unlike commercially available porous Ni, these features can be easily controlled via electrodeposition, a one-step process, taking advantage of the dynamic hydrogen bubble template (DHBT). Generally, changes in surface porosity and morphology have been mainly achieved by altering the main parameters, such as the current density or the deposition time. However, very scarce work has been done on the role of supporting electrolyte (i.e., its concentration and composition) in tailoring the foam features and consequently their catalytic activity. Hence, this approach paves the way to optimum design of metallic foam structures that can be obtained only with modifications in the electrolytic bath. In this work, 3D Ni foams are obtained from different composition baths by galvanostatic electrodeposition in the hydrogen evolution regime on stainless steel current collectors. Their porosity and morphology are analysed by optical microscopy and SEM. The electrochemical performance is evaluated by cyclic voltammetry, while catalytic activity towards HER and materials’ stability in 8 M KOH are tested using polarisation curves and chronoamperometry measurements, respectively. The recorded high currents and extended stability of the Ni foams with dendritic morphology demonstrate its outstanding performance, making it an attractive cathode material for HER in highly alkaline media.
Water electrolysis is a promising approach for large-scale and sustainable hydrogen production; however, its kinetics is slow and requires precious metal electrocatalysts to efficiently operate. Therefore, great efforts are being undertaken to design and prepare low-cost and highly efficient electrocatalysts to boost the hydrogen evolution reaction (HER). This is because traditional transition-metal electrocatalysts and corresponding hybrids with nonmetal atoms rely mainly on the interaction of metal–H bonds for the HER, which inevitably suffers from corrosion in extreme acidic and alkaline solutions. And as a result of all this effort, novel nanostructured electrocatalysts, such as carbon-encapsulated precious metals and non-precious metals including single metals or their alloys, transition-metal carbides, phosphides, oxides, sulfides, and selenides have all been recently reported to exhibit good catalytic activities and stabilities for hydrogen evolution. Here, the catalytic activity is thought to originate from the electron penetration effect of the inner metals to the surface carbon, which can alter the Gibbs free energy of hydrogen adsorption on the surface of materials. In this review, recent progresses of carbon-encapsulated materials for the HER are summarized, with a focus on the unique effects of carbon shells. In addition, perspectives on the future development of carbon-coated electrocatalysts for the HER are provided.
Graphical Abstract
Carbon-encapsulated electrocatalysts, such as carbon-encapsulated precious metals and non-precious metals (single metals or their alloys, metal carbides, phosphides, oxides, sulfides, and selenides), are emerging as promising candidates for water splitting. In this review, recent progresses in carbon-encapsulated electrocatalysts for hydrogen evolution are reviewed, especially the unique effects of carbon shells. Open image in new window
In this paper, novel cobalt-doped Ni0.85Se chalcogenides (CoxNi0.85-xSe, x=0.05, 0.1, 0.2, 0.3, 0.4) are successfully synthesized and studied as high active and stable electrocatalysts for hydrogen evolution reaction (HER) in electrolysis water splitting. The morphologies, structures and composition of these as-prepared catalysts are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and transmission electron microscopy. The electrochemical tests, such as linear sweep voltammetry, cyclic voltammetry, electrochemical impedance spectroscopy and chronoamperometry testing, are performed to evaluate these catalysts’ HER catalytic performance including activity and stability. The results indicate that a suitable doping can result in synergetic effect for increasing the catalytic performance. Among different catalysts, Co0.1Ni0.75Se shows the highest HER performance. After introducing the reduced graphene oxide into this catalyst as the support, the resulted Co0.1Ni0.75Se/rGO shows even better performance than unsupported Co0.1Ni0.75Se, which are confirmed by the reduction of HER overpotential of Co0.1Ni0.75Se/rGO to 103 mV compared to 153 mV of Co0.1Ni0.75Se at a current density of 10 mA/cm2, and the smaller Tafel slope (43 mV/dec) and kinetic resistance (21.34 Ω) than those of Co0.1Ni0.75Se (47 mV/dec, 30.23 Ω). Furthermore, the large electrochemical active surface area and high conductivity of such a Co0.1Ni0.75Se/rGO catalyst, induced by r-GO introduction, are confirmed to be responsible for the high HER performance.
Transition metal-doped nickel phosphide nanoparticles with metallic properties are prepared by a simple and facile wet-chemical method. It is shown for the first time that these transition metals: iron, cobalt, manganese, and molybdenum, can atomically substitute nickel in the parent hcp phosphide lattice as a single phase without significant change in its metallic structure and morphology. They are employed as electro- and photocatalysts for hydrogen evolution reaction, which show highly tunable activities dependent on electron filling of their metallic bands and H coverage according to our experimental and theoretical rationalizations. Molybdenum-doped nickel phosphide nanoparticle with lower H coverage exhibits the best hydrogen evolution performance in electrocatalytic hydrogen evolution reaction, which also shows excellent photocatalytic hydrogen production with organic photosensitizer. In addition, cobalt-doped nickel phosphide nanoparticle with higher H coverage with aqueous photosensitizer gives more superior hydrogen evolution rate.
Developing active and stable electrocatalysts from earth-abundant elements are key to water splitting for hydrogen production through electrolysis. Here, we report a strategy to turn non-electrocatalytic Ti2CTx into an active electrocatalyst by the nitridation of 2D titanium carbide MXene (Ti2CTx) nanosheets using sodium amide (NaNH2). The addition of NaNH2 results in chemical bonding of Ti-Nx at 500 °C on the surface of Ti2CTx, which was designed as an efficient electrocatalytic material toward hydrogen evolution reactions (HER). When used as electrocatalytic materials for HER, the nitrided-Ti2CTx (N-Ti2CTx) exhibited high activity with an overpotential of -215 mV vs NHE for the hydrogen evolution reaction (HER) at 10 mA cm⁻². Those values are over three times smaller than for pristine-Ti2CTx (-645 mV vs NHE for the HER). The as-synthesized sample showed excellent durability in acidic (0.5 M H2SO4) condition, indicating the robust catalytic activity towards the HER. The nitridation strategy implemented here could be extended to other 2-dimensional transition metal carbide electrocatalysts to improve catalytic performance.
The study of hydrogen evolution reaction and oxygen evolution reaction electrocatalysts for water electrolysis is a developing field in which noble metal-based materials are commonly used. However, the associated high cost and low abundance of noble metals limit their practical application. Non-noble metal catalysts, aside from being inexpensive, highly abundant and environmental friendly, can possess high electrical conductivity, good structural tunability and comparable electrocatalytic performances to state-of-the-art noble metals, particularly in alkaline media, making them desirable candidates to reduce or replace noble metals as promising electrocatalysts for water electrolysis. This article will review and provide an overview of the fundamental knowledge related to water electrolysis with a focus on the development and progress of non-noble metal-based electrocatalysts in alkaline, polymer exchange membrane and solid oxide electrolysis. A critical analysis of the various catalysts currently available is also provided with discussions on current challenges and future perspectives. In addition, to facilitate future research and development, several possible research directions to overcome these challenges are provided in this article.
Graphical Abstract
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Sustainable hydrogen production through water splitting provides an appealing solution for hydrogen energy to achieve a carbon neutral future. However, the realization and scalability of this technology require efficient electrocatalysts to promote cathodic hydrogen evolution and anodic oxygen evolution. The rapid evolution of metal/covalent-organic frameworks (MOFs/COFs) provides new opportunities for the development of water splitting. This review summarizes recent advances on MOF- and COF-based electrocatalysts for water electrolysis and highlights the advantages of these electrocatalysts over those conventional used. Particular attentions are paid to the design principles, construction strategies and structure-property relationship for MOF/COF-based electrocatalysts. Finally, the current challenges and future perspectives of MOF/COF-based electrocatalysts for water splitting are also presented. We hope this review will provide a timely reference for advanced MOF/COF-based electrode materials towards practicable water splitting and useful insights for future energy technologies beyond water electrolysis.
The novel hybrid Ni/Ni2P nanoparticles with graphitic carbon coating supported by 3D N, S dual modified binder‐free macroporous carbon framework (Ni/Ni2P@3DNSC) are reported as an efficient bifunctional electrocatalysts for the production of hydrogen and oxygen. The obtained Ni/Ni2P@3DNSC electrode exhibits an excellent catalytic activity with a low overpotential of 231 mV for oxygen evolution reaction (OER) and 92 mV for hydrogen evolution reaction (HER) due to the strong coupled interface of Ni and Ni2P, synergistic effects between the hybrid Ni/Ni2P core and the graphitic carbon shell, the unique porous structure, and the heteroatoms doped effect. When both are used as cathode and anode, only a low cell voltage of 1.55 V is needed to achieve the current density of 10 mA cm⁻². This work demonstrates a new avenue for the development of bifunctional nonprecious electrocatalysts for the sustainable energy utilization.
The development of abundant and cheap electrocatalysts for the hydrogen evolution reaction (HER) has attracted increasing attention over recent years. However, to achieve low-cost HER electrocatalysis, especially in alkaline media, is still a big challenge due to the sluggish water dissociation kinetics as well as the poor long-term stability of catalysts. In this paper we report the design and synthesis of a two-dimensional (2D) MoS2 confined Co(OH)2 nanoparticle electrocatalyst, which accelerates water dissociation and exhibits good durability in alkaline solutions, leading to significant improvement in HER performance. A two-step method was used to synthesize the electrocatalyst, starting with the lithium intercalation of exfoliated MoS2 nanosheets followed by Co²⁺ exchange in alkaline media to form MoS2 intercalated with Co(OH)2 nanoparticles (denoted Co-Ex-MoS2), which was fully characterized by spectroscopic studies. Electrochemical tests indicated that the electrocatalyst exhibits superior HER activity and excellent stability, with an onset overpotential and Tafel slope as low as 15 mV and 53 mV dec–1, respectively, which are among the best values reported so far for the Pt-free HER in alkaline media. Furthermore, density functional theory calculations show that the cojoint roles of Co(OH)2 nanoparticles and MoS2 nanosheets result in the excellent activity of the Co-Ex-MoS2 electrocatalyst, and the good stability is attributed to the confinement of the Co(OH)2 nanoparticles. This work provides an imporant strategy for designing HER electrocatalysts in alkaline solutions, and can, in principle, be expanded to other materials besides the Co(OH)2 and MoS2 used here.