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Highly acid-durable carbon coated Co3O4 nanoarrays as efficient oxygen evolution electrocatalysts
... In situ measurements that probe changes in the reaction intermediates provide indications of the OPM pathway. We develop Co 3−x Ba x O 4 catalysts having an overpotential of 278 mV at 10 mA/cm 2 23 but we find that the performance resides below that of noble-metalbased catalysts ( Figures S1 and S2). Since Ba doping has been seen to increase activity and stability in acidic systems, 24 we synthesized Co 3−x Ba x O 4 and investigated the effects of the Ba dopant on the electronic structure, catalytic performance, and mechanism. ...
... 21,35,36 We calculated the surface free energies (Figure 4a Figure S31 with optimized geometries in Figures S32 and S33. This enabled us to contemplate the effect of Ba doping on the most thermodynamically stable surface and its relative stability under acidic water oxidation conditions (pH = 0−2 and U RHE > 1. 23 (Figure 4a). Without affecting the relative stability, surfaces with Badopants result in more negative surface free energies, and their thermodynamic stabilities are thereby enhanced compared with those of pristine ones (Figure 4b), which agrees with the experimental trends. ...
Acidic water electrolysis enables the production of hydrogen for use as a chemical and as a fuel. The acidic environment hinders water electrolysis on non-noble catalysts, a result of the sluggish kinetics associated with the adsorbate evolution mechanism, reliant as it is on four concerted proton-electron transfer steps. Enabling a faster mechanism with non-noble catalysts will help to further advance acidic water electrolysis. Here, we report evidence that doping Ba cations into a Co3O4 framework to form Co3-xBaxO4 promotes the oxide path mechanism and simultaneously improves activity in acidic electrolytes. Co3-xBaxO4 catalysts reported herein exhibit an overpotential of 278 mV at 10 mA/cm2 in 0.5 M H2SO4 electrolyte and are stable over 110 h of continuous water oxidation operation. We find that the incorporation of Ba cations shortens the Co-Co distance and promotes OH adsorption, findings we link to improved water oxidation in acidic electrolyte.
... This work provides a facile, promising and cost-effective strategy to prepare highly active catalysts for various energy storage and conversion applications. Table S1: Comparison of OER performance of Co2P@N-C-900 with other recently reported electrocatalysts in alkaline electrolyte [34][35][36][37][38][39][40][41][42]; Table S2: EIS calculation parameters of Co2P@N-C-900 and other samples for OER. ...
... V) were obtained at different scanning rates. (a) Co 2 P@N-C-800, (b) Co 2 P@N-C-900 and (c) Co 2 P@N-C-1000; Table S1: Comparison of OER performance of Co 2 P@N-C-900 with other recently reported electrocatalysts in alkaline electrolyte [34][35][36][37][38][39][40][41][42]; Table S2: EIS calculation parameters of Co 2 P@N-C-900 and other samples for OER. ...
It is critical and challenging to develop high performance transition metal phosphides (TMPs) electrocatalysts for oxygen evolution reaction (OER) to address fossil energy shortages. Herein, we report the synthesis of Co2P embedded in N-doped porous carbon (Co2P@N-C) via a facile one-step strategy. The obtained catalyst exhibits a lower overpotential of 352 mV for OER at a current density of 10 mA cm−2 and a small Tafel slope of 84.6 mV dec−1, with long-time reliable stability. The excellent electrocatalytic performance of Co2P@N-C can be mainly owed to the synergistic effect between the Co2P and highly conductive N-C substrate, which not only affords rich exposed active sites but also promotes faster charge transfer, thus significantly promoting OER process. This work presents a promising and industrially applicable synthetic strategy for the rational design of high performance nonnoble metal electrocatalysts with enhanced OER performance.
... This approach showed improved stability, demonstrating an overpotential of 570 mV at 10 mA cm − 2 . Another approach involves applying a carbon coating, which Yang et al. [111] used to synthesize Co 3 O 4 nanoarrays deposited on carbon paper. This approach exhibited an excellent overpotential of 370 mV at 10 mA cm − 2 and a stability of 86.8 h at 100 mA cm − 2 . ...
Among water electrolysis methods, proton exchange membrane electrolyzers (PEMWEs) stand out for their potential to generate high-purity hydrogen with remarkable efficiency and dynamic response, making them a cornerstone technology for the sustainable hydrogen economy. However, a key bottleneck lies in the slow reaction rate of the oxygen evolution reaction (OER) at the anode, a four-electron transfer process that significantly throttles the system's full potential. This significantly impacts overall efficiency and calls for unfolding stable, durable, and highly active electrocatalysts that are cost-effective. However, the inherent acidity generated by the OER itself complicates this task. Noble metal catalysts like iridium (Ir) and ruthenium (Rh), pure or combined with other elements, exhibit excellent activity in the acidic OER environment. However, their high cost hinders large-scale PEMWE deployment. Therefore, extensive research has concentrated on non-noble metal alternatives, particularly transition metal oxides (monometallic and polymetallic) and carbon-based materials. This comprehensive review meticulously examines the emerging progress in non-noble metal electrocatalysts designed for low-pH OER conditions within PEMWEs. Following an introductory classification of water elec-trolyzer technologies, it explores how factors such as structure and synthesis route modulate the crucial performance parameters across diverse catalyst groups. Drawing upon these insights, the review also evaluates the current challenges and outlines promising avenues for future research.
The design and synthesis of oxygen evolution reaction (OER) electrocatalysts that operate efficiently and stably under acidic conditions are important for the preparation of green hydrogen energy. The low intrinsic catalytic activity and poor acid resistance of commercial RuO2 limit its further development, and the construction of heterointerface structures is the most promising strategy to break through the intrinsic activity limitation of electrocatalysts. Herein, we synthesized spherical and oxygen vacancy-rich heterointerface MnO2/RuO2 using morphology control, which promoted the kinetics of the oxygen evolution reaction with the interaction between oxygen vacancies and the oxide heterointerface. MnO2/RuO2 was reported to be an acidic OER catalyst with excellent performance and stability, requiring only an ultra-low overpotential of 181 mV in 0.5 M H2SO4 to achieve a current density of 10 mA cm-2. The catalyst activity remained essentially unchanged in a 140 h stability test with an ultra-high mass activity (858.9 A g-1@ 1.5 V), which was far superior to commercial RuO2 and most previously reported noble metal-based acidic OER catalysts. The experimental results showed that the effect of more oxygen vacancies and the heterointerfaces of manganese ruthenium oxides broke the intrinsic activity limitation, provided more active sites for the OER, accelerated reaction kinetics, and improved the stability of the catalyst. The excellent performance of the catalyst suggests that MnO2/RuO2 provides a new idea for the design and study of heterointerfaces in metal oxide nanomaterials.
The development of efficient non-noble metal electrocatalysts for the oxygen evolution reaction (OER) in acidic conditions remains a critical challenge. Here, we reported a N-doped carbonaceous component-engineered Co3O4 (NCEC) catalyst...
The oxygen evolution reaction (OER) electrocatalysts, which can keep active for a long time in acidic media, are of great significance to proton exchange membrane water electrolyzers. Here, Ru-Co3O4 electrocatalysts with transition metal oxide Co3O4 as matrix and the noble metal Ru as doping element have been prepared through an ion exchange–pyrolysis process mediated by metal-organic framework, in which Ru atoms occupy the octahedral sites of Co3O4. Experimental and theoretical studies show that introduced Ru atoms have a passivation effect on lattice oxygen. The strong coupling between Ru and O causes a negative shift in the energy position of the O p-band centers. Therefore, the bonding activity of oxygen in the adsorbed state to the lattice oxygen is greatly passivated during the OER process, thus improving the stability of matrix material. In addition, benefiting from the modulating effect of the introduced Ru atoms on the metal active sites, the thermodynamic and kinetic barriers have been significantly reduced, which greatly enhances both the catalytic stability and reaction efficiency of Co3O4.
Water splitting is an important way to obtain hydrogen applied in clean energy, which mainly consists of two half-reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, the kinetics of the OER of water splitting, which occurs at the anode, is slow and inefficient, especially in acid. Currently, the main OER catalysts are still based on noble metals, such as Ir and Ru, which are the main active components. Hence, the exploration of new OER catalysts with low cost, high activity, and stability has become a key issue in the research of electrolytic water hydrogen production technology. In this paper, the reaction mechanism of OER in acid was discussed and summarized, and the main methods to improve the activity and stability of non-noble metal OER catalysts were summarized and categorized. Finally, the future prospects of OER catalysts in acid were made to provide a little reference idea for the development of advanced OER catalysts in acid in the future.
The development of non-precious metal electrocatalysts for acidic oxygen evolution reaction (OER) that are highly durable, cost-effective, and efficient is crucial to advancing the use of proton exchange membrane water electrolyzers (PEMWEs).
Earth-abundant, acid-stable catalysts for the oxygen evolution reaction are essential for terawatt-scale hydrogen production using proton exchange membrane (PEM) electrolysers. Here we report that optimizing the lattice oxygen structure of manganese oxide allows it to sustain the oxygen evolution reaction for over one month at 1,000 mA cm⁻² in 1 M H2SO4. The lifetime enhancement was achieved by substituting pyramidal oxygen with planar oxygen, which has a stronger Mn–O bond and thus suppresses the dissolution of manganese ions. Calculations show that the lattice oxygen dissolution is the bottleneck of deactivation, and this process is less favourable by over 0.2 eV on planar oxygen compared with pyramidal oxygen. Our material shows excellent performance even in a PEM electrolyser, reaching 2,000 mA cm⁻² at 2 V with durability exceeding 1,000 h at 200 mA cm⁻². This study expands the potential of Earth-abundant catalysts for PEM electrolysis, which may mitigate the reliance on iridium.
A monolithic chainmail electrode is developed by embedding self-phosphorized carbon-coated Co 2 P nanoparticles within a porous carbon membrane, exhibiting excellent activity and durability for both alkaline HER and OER at high current densities.
High-purity hydrogen produced by water electrolysis has become a sustainable energy carrier. Due to the corrosive environments and strong oxidizing working conditions, the main challenge faced by acidic water oxidation is the decrease in the activity and stability of anodic electrocatalysts. To address this issue, efficient strategies have been developed to design electrocatalysts toward acidic OER with excellent intrinsic performance. Electronic structure modification achieved through defect engineering, doping, alloying, atomic arrangement, surface reconstruction, and constructing metal-support interactions provides an effective means to boost OER. Based on introducing OER mechanism commonly present in acidic environments, this review comprehensively summarizes the effective strategies for regulating the electronic structure to boost the activity and stability of catalytic materials. Finally, several promising research directions are discussed to inspire the design and synthesis of high-performance acidic OER electrocatalysts.
The challenge of the practical application of a water electrolyzer system lies in the development of low‐manufacturing cost, highly active, and stable electrocatalysts to replace the noble metal ones, in order to enable environmentally friendly hydrogen production on a large scale. Herein, a facile method is proposed for boosting the performance of Co 3 O 4 through the incorporation of large‐sized single atoms. Due to the larger ionic radius of rare earth metals than that of Co, the incorporation elongates the bond length of Co─O, resulting in the narrowed d‐p band centers and the high spin configuration, which is favorable for the interaction and charge transfer with absorbent (*OH). As a result, the Ce‐incorporated Co 3 O 4 with the longest Co─O bond length exhibits the best oxygen evolution reaction (OER) performance, specifically, the turnover frequency is over 17 times higher than that of pristine Co 3 O 4 nanosheet under an overpotential of 400 mV. Powered by a commercial Si solar cell, a two‐electrode solar water‐splitting device combining Ce‐incorporated Co 3 O 4 and Pt delivers a solar‐to‐hydrogen conversion efficiency of 13.53%. The strategy could provide a new insight for improving the performance of OER electrocatalysts in acid toward practical applications.
Proton exchange membrane (PEM) electrolyzer play a vital role in sustainable energy conversion and storage. However, the oxygen evolution reaction (OER) has been considered as the major bottleneck, which limits...
Implementation of proton‐exchange membrane water electrolyzers for large‐scale sustainable hydrogen production requires the replacement of scarce noble‐metal anode electrocatalysts with low‐cost alternatives. However, such earth‐abundant materials often exhibit inadequate stability and/or catalytic activity at low pH, especially at high rates of the anodic oxygen evolution reaction (OER). Here, the authors explore the influence of a dielectric nanoscale‐thin oxide layer, namely Al 2 O 3 , SiO 2 , TiO 2 , SnO 2 , and HfO 2 , prepared by atomic layer deposition, on the stability and catalytic activity of low‐cost and active but insufficiently stable Co 3 O 4 anodes. It is demonstrated that the ALD layers improve both the stability and activity of Co 3 O 4 following the order of HfO 2 > SnO 2 > TiO 2 > Al 2 O 3 , SiO 2 . An optimal HfO 2 layer thickness of 12 nm enhances the Co 3 O 4 anode durability by more than threefold, achieving over 42 h of continuous electrolysis at 10 mA cm ⁻² in 1 m H 2 SO 4 electrolyte. Density functional theory is used to investigate the superior performance of HfO 2 , revealing a major role of the HfO 2 |Co 3 O 4 interlayer forces in the stabilization mechanism. These insights offer a potential strategy to engineer earth‐abundant materials for low‐pH OER catalysts with improved performance from earth‐abundant materials for efficient hydrogen production.
The oxygen evolution kinetics of industrial copper electrodeposition is slow, resulting in low electrocatalytic activity and high energy consumption. In this work, a quaternary composite of carbon coated active particles containing Mn, Co and Ce were prepared (Mn-Co3O4/CeO2@C), and Ti/Sb-SnO2/PbO2 electrode doped with these active particles was prepared by co-electrodeposition. The microstructure and chemical composition of the electrode was characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffractometry (XRD). Linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and Tafel polarization curve (Tafel) were used to study the electrochemical properties of anode materials. The results showed that the doping of Mn-Co3O4/CeO2@C active particles promoted the crystal transition of PbO2, decreased the average grain size, and the doping of Ce increases the average valence state of Co. The modified titanium electrode showed excellent catalytic activity of the oxygen evolution reaction (OER) characteristics. The overpotential of the doped Ti/Sb-SnO2/PbO2 anode was only 453 mV when the current density was 20 mA cm−2 in 0.5 M H2SO4 solution, which is 508 mV lower than that of the undoped Ti/Sb-SnO2/PbO2 anode. In simulated copper electro-deposition experiments, the cell voltage was reduced by about 400 mV, compared to the undoped Ti/Sb-SnO2/PbO2 electrode.
This article reveals the promising potential of MnCoOx solid solutions as an effective catalyst for the oxygen evolution reaction (OER). The solid oxide solution was synthesized by utilizing redox deposition...
The proton exchange membrane water electrolyzer (PEM-WE) is a well-known green technology for hydrogen production. The main obstacle to its development, on a large scale, is the sluggish kinetics of the oxygen evolution reaction (OER). At present, the design of acid-stable electrocatalysts with low overpotential and excellent stability for the OER constitutes an important activity in electrocatalysis. This review presents an analysis of the fundamentals and strategies for the design of advanced electrocatalysts for oxygen evolution, reaction mechanisms, and OER descriptors. The scrutiny of OER electrocatalysts, with elemental composition from single-to multielemental, are presented. In addition, the purpose of high-entropy alloys (HEAs), a recent research strategy, for the design of advanced materials is summarized. Briefly, the effect of support materials, which are beneficial for modulating the electronic properties of catalysts, is presented. Finally, the prospects for the development of acidic OER electrocatalysts are given.
The development of proton exchange membrane water electrolyzers (PEMWEs) is primarily challenged by the slow and unstable oxygen evolution reaction (OER) at the anode. Thus, the pursuit of low‐cost, highly efficient, and durable electrocatalysts is the foundation for the widespread adoption of PEMWEs. In the past few decades, various types of electrocatalysts are proposed to serve as anode materials for acidic OER, but only a few demonstrate catalytic activity and stability comparable to iridium‐ and ruthenium‐based electrocatalysts. In recent years, some transition‐metal oxides are studied as potential candidates for acidic OER. For instance, spinel‐type oxides, e.g., Co 3 O 4 , and corresponding compounds demonstrate promise due to the rich coordination structure of metallic ions and moderate adoption energy for intermedia. In this review, recent advances in spinel‐type oxides for acidic OER are summarized. First, a fundamental understanding of reaction mechanisms for OER in acidic media is introduced. Thereafter, recent progress in rational design principles and optimization strategies for spinel oxides is systematically summarized. Finally, challenges and perspectives for the development of spinel‐based acidic OER electrocatalysts are discussed.
Rational surface engineering of non-noble metal electrocatalysts supported on carbon materials (metal@C) provides potential opportunities to gain sustainable energy by electrocatalytic water splitting. Here, the plastic wastes-based electrode materials by...
It is an appealing avenue for electrosyntheis of high-valued chemicals at both anode and cathode by coupling 5-hydroxymethylfurfural (HMF) oxidation and nitrate reduction reactions simultaneously, while the development such bifunctional electrocatalysts is still in its infancy with dissatisfied selectivity and low yield rate. Here, we first report that Zn-doped Co3O4 nanowires array can be served as an efficient and robust dual-functional catalyst for HMF oxidation and nitrate reduction at ambient conditions. Specifically, the catalyst shows a faradaic efficiency of 91 % and a yield rate of 241.2 μmol h-1 cm-2 for 2,5-furandicarboxylic acid formation together with a high conversion of nearly 100 % at a potential of 1.40 V. It also displays good cycling stability. Besides, the catalyst is capable of catalyzing the reduction of nitrate to NH3, giving a maximal faradaic efficiency of 92 % and a peak NH3 yield rate of 4.65 mg h-1 cm-2 at a potential of -0.70 V. These results surpass those obtained using pristine Co3O4 and are comparable to those of state-of-the-art electrocatalysts. Moreover, the catalyst is further employed as the cathode catalyst to assemble a Zn-nitrate battery, giving a peak power density of 5.24 mW cm-2 and a high yield rate of 0.72 mg h-1 cm-2. Theoretical simulations further reveal that Zn-doping favors the adsorption and dissociation of nitrate and HMF species and reduces the energy barrier as well. Our work demonstrates the potential interest of Co3O4-based materials for the highly selective production of valuable feedstocks via ambient electrolysis.
Oxygen evolution reaction (OER) electrocatalysts in acidic media, except for precious IrO2, have difficulty realizing good electrocatalytic activity and high electrochemical stability simultaneously. However, the scarcity of IrO2 as an acidic OER electrocatalyst impedes its large-scale application in hydrogen generation, organic synthesis, nonferrous metal production and sewage disposal. Herein, we report the design and fabrication of a nanoporous TiMnCoCN compound based on the nanoscale Kirkendall effect, possessing an intriguing self-adjusting capability for the oxygen evolution reaction (OER) in a 0.5 M H2SO4 solution. The nanoporous TiMnCoCN compound electrode for the acidic OER displays a low overpotential of 143 mV for 10 mA cm-2 and exhibits no increase in potential over 50,000 s, which is ascribed to the self-adjusting ability, Carbon/nitrogen (C/N) incorporation and nanoporous architecture. The concentration of inert TiO2 on the reconstructed surface of the compound can self-adjust with the change in OER potential via a cobalt-dissolved vacancy approach according to the stabilization requirement. In this work, the self-reconstruction law of surface structure was discovered, providing a novel strategy for designing and fabricating nonnoble OER electrocatalysts with superior catalytic performance and robust stability in acidic media.
The acidic oxygen evolution reaction (OER) has long been the bottleneck of proton exchange membrane water electrolyzers given its harsh oxidative and corrosive environments. Herein, we suggest an effective strategy to greatly enhance both the acidic OER activity and stability of Co3O4 spinel by atomic Ru selective substitution on the octahedral Co sites. The resulting highly symmetrical octahedral Ru-O-Co collaborative coordination with strong electron coupling effect enables the direct dioxygen radical coupling OER pathway. Indeed, both experiments and theoretical calculations reveal a thermodynamically breakthrough heterogeneous diatomic oxygen mechanism. Additionally, the active Ru-O-Co units are well-maintained upon the acidic OER thanks to the electron transfer from surrounding electron-enriched tetrahedral Co atoms via bridging oxygen bonds that suppresses the overoxidation and thus dissolution of active Ru and Co species. Consequently, the prepared catalyst, even with a low Ru mass loading of ca. 42.8 μg cm-2, exhibits an attractive acidic OER performance with a low overpotential of 200 mV and a low potential decay rate of 0.45 mV h-1 at 10 mA cm-2. Our work suggests an effective strategy to significantly enhance both the acidic OER activity and stability of low-cost electrocatalysts.
The proton exchange membrane (PEM) water electrolyzer has been considered a versatile approach for practical H2 production. However, the oxygen evolution reaction (OER) in acid media with complicated proton‐coupled electron transfer steps possesses sluggish kinetics and high reaction barriers, severely hindering the development of PEM water electrolyzers. Consequently, high‐efficient Ru‐ and Ir‐based catalysts have always been essential to accelerate the OER rate and lower the reaction barrier in PEM water electrolyzer. Therefore, it is very necessary to construct low‐cost catalysts with excellent electrocatalytic performances to replace these noble metal‐based OER electrocatalysts. In this review paper, a detailed discussion towards fundamentally comprehending the reaction mechanisms of OER was conducted. Accordingly, we proposed the principles of designing advanced OER electrocatalysts with enhanced performances and lowered costs. After that, recent developments in designing various acidic OER electrocatalysts were summarized. Meanwhile, the available regulation strategies about noble metals, nonprecious metals, and metal‐free nanomaterials were presented, which are promising for tuning the electronic structures, boosting the electrocatalytic performances, and reducing the costs of electrocatalysts. We also provided the existing challenges and perspectives of various OER electrocatalysts, hoping to promote the development of PEM water electrolyzers.
Few earth-abundant catalysts can catalyze oxygen evolution reaction (OER) in acid with satisfactory durability, due to the significant dissolution of transition metals. Disclosing the deactivation mechanism is the prerequisite for...
Discovery of earth-abundant electrocatalysts to replace iridium for the oxygen evolution reaction (OER) in a proton exchange membrane water electrolyzer (PEMWE) represents a critical step in reducing the cost for green hydrogen production. We report a nanofibrous cobalt spinel catalyst codoped with lanthanum (La) and manganese (Mn) prepared from a zeolitic imidazolate framework embedded in electrospun polymer fiber. The catalyst demonstrated a low overpotential of 353 millivolts at 10 milliamperes per square centimeter and a low degradation for OER over 360 hours in acidic electrolyte. A PEMWE containing this catalyst at the anode demonstrated a current density of 2000 milliamperes per square centimeter at 2.47 volts (Nafion 115 membrane) or 4000 milliamperes per square centimeter at 3.00 volts (Nafion 212 membrane) and low degradation in an accelerated stress test.
Developing cost-efficient and noble metal free electrocatalysts is vastly anticipated for the oxygen evolution reaction (OER). Therefore, in this study, to lift the thermodynamic and kinetic activity of the OER, we attempted to synthesize a bimetallic nickel and manganese-based zeolite imidazolate framework system in a fiber form. For this synthesis, a bottom-up approach has been followed through wet chemical analysis, and electrospinning was utilized for fiber formation. The resultant fiber has shown a lesser overpotential of 256 mV at a benchmarking current density of 10 mA cm-2 under 1 M KOH conditions. As expected, the attained Tafel slope and charge transfer resistance values are lesser. The observed results reveal that the synergism between the Ni and Mn nodes on the imidazolate framework successfully promotes the thermodynamic formation of *O and *OOH intermediates, which significantly helps to improve the faster OER kinetics at the electrode-electrolyte interface.
It is challenging to develop high-efficient and stable nonprecious metal-based electrocatalyst for oxygen evolution reaction (OER) in acid for proton exchange membrane (PEM) water splitting. Herein, P atoms were introduced into the lattice of spinel Co3O4 (P-Co3O4) to replace with octahedral coordinated Co3+ via a hydrothermal process following a thermal treatment. The formation of PO6 geometric configuration unit in Co3O4 can trigger electron rearrangement around Co ions, which resulted in the high-active Co2+ site on the surface, significantly decreasing the energy barrier of rate-determining step for OER. Moreover, the weaker covalency of the Co 3d-O 2p bond and higher formation energy of oxygen vacancy around Co2+ in P-Co3O4 inhibited the participation of lattice oxygen during OER process, enabling that P-Co3O4 can work stably in acidic media. The obtained P-Co3O4 afforded satisfying stability over 30 h in a PEM electrolysis device with an overpotential of 400 mV@10 mA/cm2 in 0.1 M HClO4.
Microwave-induced catalytic is a potential organic wastewater treatment technology. In this work, a new type of microwave-induced catalyst Co3O4 was synthesized and used to remove tetracycline (TC) under microwave (MW). The results have shown that the spherical Co3O4 exhibited impressively high catalytic activity with 100 mg·L⁻¹ TC degradation efficiency of 100 % under MW at 800 W within 30 min. Importantly, Co3O4 catalyst displayed excellent stability with the degradation efficiency. The electron-hole pairs were generated under MW in the presence of Co3O4, which was confirmed by electron paramagnetic resonance and active species scavenging experiments. Part of the electrons (e⁻) was captured by oxygen vacancies produced under MW, and holes (h⁺) acted as the main active species for participating in the degradation process of TC. This work indicated the effect of MW for the separation of electrons and holes and provides new ideas for the mechanism of microwave-induced catalytic degradation.
Exploiting an affordable, durable, and high-performance electrocatalyst for the oxygen evolution reaction (OER) under lower pH condition (acidic) is highly challengeable and much attractive toward the hydrogen-based energy technologies. A spinel CoCr2O4 is observed as a potential noble-metal-free candidate for OER in alkaline medium. The presence of Cr further leads to electronic structure modulation of Co3O4 and thereby greatly increases the corrosive resistance toward OER in acidic environment. Herein, a typical CoCr2O4 with three different morphologies was synthesized for the very first time and employed as an electrocatalyst for OER in alkaline (1 M KOH) and acidic (0.5 M H2SO4) medium. Moreover, different morphologies display a different intrinsic exposed active site and thereby display different electrocatalytic activities. Likewise, the CoCr2O4 Mic (synthesized by the microwave heating method) displays a higher catalytic activity toward OER and delivers a low overpotential of 293 and 290 mV to attain 10 mA/cm2 current density and smaller Tafel slope values of 40 and 151 mV/dec, respectively, in alkaline and acidic environment than the synthesized CoCr2O4 Wet (wet-chemically synthesized) and CoCr2O4 Hyd (hydrothermally synthesized). Moreover, CoCr2O4 Mic exhibits a long-term durability of 24 h (1 M KOH) and 10.5 h (0.5 M H2SO4). The optimized Co-O bond energy in OER condition makes the CoCr2O4 Mic superior than the CoCr2O4 Hyd and CoCr2O4 Wet. Moreover, the substitution of Cr induces the electron delocalization around the Co active species and thereby, positive shifting of the redox potential leads to providing an optimal binding energy for OER intermediates. Also, interestingly, this work represents a catalytic activity trend by a simple experimental result without any complex theoretical calculation. The morphology-dependent electrocatalytic activity obtained in this work will provide a new strategy in the field of electrochemical conversion and energy storage application.
The state-of-the-art transition-based electrocatalysts in alkaline media generally suffer from unavoidable surface reconstruction during oxygen evolution reaction measurements, leading to the collapse and loss of the crystalline matrix. Low potential discharge offers a gentle way for surface reconstruction and thus realizes the manipulation of the real active site. Nevertheless, the absence of a fundamental understanding focus on this discharge region renders the functional phase, either the crystalline or amorphous matrix, for the controllable reconstruction still undecidable. Herein, we report a scenario to employ different crystalline matrices as electrocatalysts for discharge region reconstruction. The representative low crystalline Ni2P (LC-Ni2P) possesses a relatively weak surface structure compared with highly crystalline or amorphous Ni2P (HC-Ni2P or A-Ni2P), which contributes abundant oxygen vacancies after the discharge process. The fast discharge behavior of LC-Ni2P leads to the uniform distribution of these vacancies and thus endows the inner interface with reactant activating functionality. A high increase in current density of 36.7% is achieved at 2.32 V (vs RHE) for the LC-Ni2P electrode. The understanding of the discharge behavior in this study, on different crystalline matrices, presents insights into the establishment of controllable surface reconstruction for an effective oxygen evolution reaction.
The design and maintenance of high active sites in acidic environment is vital and challenging for oxygen evolution reaction (OER). Here, we find that the obtained CoO2 under high applied potential can be stable on MnO2 host in acidic environment, which may act as an effective means to solve the instability of cobalt-based electrocatalyst. The significant improvement of acidic OER activity (6.9 times) and stability (46.4 times) of 90-Co-MnO2 (treated by molten salt with more Co deposition sites) demonstrates the advantages of this approach. In-situ Raman and Pourbaix diagram suggest that the enhanced performance derives from the stable presence of CoO2 at the voltage greater than 1.8 V versus RHE. However, when the potential is less than 1.8 V, the corresponding other cobalt specie is too unstable to facilitate OER. Density functional theory (DFT) calculations reveal that the deposited cobalt oxides can act as active sites, thus, effectively reducing the reaction energy barrier of the rate-determining step (RDS). This work provides a new perspective for enhancing the stability of cobalt-based electrocatalyst. In the future, the dual consideration of applied potential and stable species of active element in Pourbaix diagram may be a new direction for developing acid-stable electrocatalysts. This article is protected by copyright. All rights reserved.
Transition metal phosphide (TMP) is an attractive candidate for asymmetric supercapacitors (ASCs), but their low electric conductivity, limited redox active sites and sluggish charge transfer kinetics are still challenging. Herein, various phosphorus vacancy modified CP/FP heterostructures are successfully acquired via general three-step approach. Benefiting from their natural hierarchical heterostructure, these materials can access more active sites ions for redox reaction and boost their charge transfer kinetics. The formed phosphorus vacancy in lattice structure can provide plentiful lone pair electrons to facilitate electric conductivity. As a result, the pV-CP/FP2 electrode material achieves the best electrochemical performances, which delivers a specific capacitance (1028.8F/g at 5 mV s⁻¹), outstanding rate capability and good cycling performance. Furthermore, the ASCs matched with pV-CP/FP2 and FeOOH electrode yields an exceptional cycling stability (85.7 % retention after 10,000 cycles), good Coulombic efficiency of 73.9 % from 1 to 8 A/g, a high energy density of 45.5 Wh kg⁻¹ at a power density of 700 W kg⁻¹, and as well maintains 36.0 Wh kg⁻¹ at 5600 W kg⁻¹. This work could supply original insights into the defect regulation and interfacial coupling of diatomite derivate heterojunctions for ASCs, and the Coscinodiscus-like pV-CP/FP2 electrode shows great potential for energy storage devices.
Hydrogen (H2 ) is one of the most important clean and renewable energy sources for future energy sustainability. Nowadays, photocatalytic and electrocatalytic hydrogen evolution reactions (HERs) from water splitting are considered as two of the most efficient methods to convert sustainable energy to the clean energy carrier, H2 . Catalysts based on transition metal dichalcogenides (TMDs) are recognized as greatly promising substitutes for noble-metal-based catalysts for HER. The photocatalytic and electrocatalytic activities of TMD nanosheets for the HER can be further improved after hybridization with many kinds of nanomaterials, such as metals, oxides, sulfides, and carbon materials, through different methods including the in situ reduction method, the hot-injection method, the heating-up method, the hydro(solvo)thermal method, chemical vapor deposition (CVD), and thermal annealing. Here, recent progress in photocatalytic and electrocatalytic HERs using 2D TMD-based composites as catalysts is discussed.
A unique functional electrode made of hierarchal Ni-Mo-S nanosheets with abundant exposed edges anchored on conductive and flexible carbon fiber cloth, referred to as Ni-Mo-S/C, has been developed through a facile biomolecule-assisted hydrothermal method. The incorporation of Ni atoms in Mo-S plays a crucial role in tuning its intrinsic catalytic property by creating substantial defect sites as well as modifying the morphology of Ni-Mo-S network at atomic scale, resulting in an impressive enhancement in the catalytic activity. The Ni-Mo-S/C electrode exhibits a large cathodic current and a low onset potential for hydrogen evolution reaction in neutral electrolyte (pH ~7), for example, current density of 10 mA/cm(2) at a very small overpotential of 200 mV. Furthermore, the Ni-Mo-S/C electrode has excellent electrocatalytic stability over an extended period, much better than those of MoS2/C and Pt plate electrodes. Scanning and transmission electron microscopy, Raman spectroscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and x-ray absorption spectroscopy were used to understand the formation process and electrocatalytic properties of Ni-Mo-S/C. The intuitive comparison test was designed to reveal the superior gas-evolving profile of Ni-Mo-S/C over that of MoS2/C, and a laboratory-scale hydrogen generator was further assembled to demonstrate its potential application in practical appliances.
Layered LiCo0.8 Fe0.2 O2 demonstrates dramatically enhanced OER activity and durability in an alkaline solution than LiCoO2 and other reported state-of-the-art catalysts, including benchmark IrO2 . This superior performance is attributed to Fe-doping-induced synergistic effects.
Herein, we report the preparation of Pongam seed shells-derived activated carbon and cobalt oxide (~2-10 nm) nanocomposite (PSAC/Co3O4) by using a general and facile synthesis strategy. The as-synthesized PSAC/Co3O4 samples were characterized by a variety of physicochemical techniques. The PSAC/Co3O4-modified electrode is employed in two different applications such as high performance non-enzymatic glucose sensor and supercapacitor. Remarkably, the fabricated glucose sensor is exhibited an ultra-high sensitivity of 34.2 mA mM-1 cm-2 with a very low detection limit (21 nM) and long-term durability. The PSAC/Co3O4 modified stainless steel electrode possesses an appreciable specific capacitance and remarkable long-term cycling stability. The obtained results suggest the as-synthesized PSAC/Co3O4 is more suitable for the non-enzymatic glucose sensor and supercapacitor applications outperforming the related carbon based modified electrodes, rendering practical industrial applications.
Monodisperse cobalt (Co) nanoparticles (NPs) were synthesized and stabilized against oxidation via reductive annealing at 600C. The stable Co NPs are active for catalyzing the oxygen evolution reaction (OER) in 0.1 M KOH, producing a current density of 10 mA/cm2 at an overpotential of 0.39 V (1.62 V vs. RHE, no iR-correction). Their catalysis is superior to the commercial Ir catalyst in both activity and stability. These Co NPs are also assembled into a monolayer array on the working electrode, allowing the detailed study of their intrinsic OER activity. The Co NPs in the monolayer array show 15 times higher turnover frequency (TOF) (2.13 s-1) and mass activity (1949 A/g) than the NPs deposited on conventional carbon black (0.14 s-1 and 126 A/g respectively) at an overpotential of 0.4 V. These stable Co NPs are a promising new class of noble-metal-free catalyst for water-splitting.
A facile electrochemical co-deposition method has been developed for the fabrication of graphene-cobalt nanocomposite modified electrodes that achieve exceptionally efficient water oxidation in highly alkaline media. In the method reported, a graphene-cobalt nanocomposite film was deposited electrochemically from a medium containing 1 mg ml(-1) graphene oxide, 0.8 mM cobalt nitrate and 0.05 M phytic acid (pH 7). The formation of the nanocomposite film was confirmed using electrochemical, Raman spectroscopic and scanning electron microscopic techniques. The nanocomposite film exhibits excellent activity and stability towards water oxidation to generate oxygen in 1 M NaOH aqueous electrolyte media. A turn over frequency of 34 s(-1) at an overpotential of 0.59 V and a faradaic efficiency of 97.7% were deduced from analysis of data obtained by rotating ring disk electrode voltammetry. Controlled potential electrolysis data suggests that the graphene supported catalyst exhibits excellent stability under these harsh conditions. Phytate anion acts as stabilizer for the electrochemical formation of cobalt nanoparticles. Fourier transformed ac voltammetry allowed the redox chemistry associated with catalysis to be detected directly under catalytic turnover conditions. Estimates of formal reversible potentials obtained from this method and derived from the overall reactions 3Co(OH)2 + 2OH(-) ⇌ Co3O4 + 4H2O + 2e(-), Co3O4 + OH(-) ⇌ 3CoOOH + e(-) and CoOOH + OH(-) ⇌ CoO2 + H2O + e(-) are 0.10, 0.44 and 0.59 V vs. Ag/AgCl, respectively.
While electrochemical water splitting is one of the most promising methods to store light/electrical energy in chemical bonds, a key challenge remains in the realization of an efficient oxygen evolution reaction catalyst with large surface area, good electrical conductivity, high catalytic properties, and low fabrication cost. Here, a facile solution reduction method is demonstrated for mesoporous Co3O4 nanowires treated with NaBH4. The high-surface-area mesopore feature leads to efficient surface reduction in solution at room temperature, which allows for retention of the nanowire morphology and 1D charge transport behavior, while at the same time substantially increasing the oxygen vacancies on the nanowire surface. Compared to pristine Co3O4 nanowires, the reduced Co3O4 nanowires exhibit a much larger current of 13.1 mA cm-2 at 1.65 V vs reversible hydrogen electrode (RHE) and a much lower onset potential of 1.52 V vs RHE. Electrochemical supercapacitors based on the reduced Co3O4 nanowires also show a much improved capacitance of 978 F g-1 and reduced charge transfer resistance. Density-functional theory calculations reveal that the existence of oxygen vacancies leads to the formation of new gap states in which the electrons previously associated with the Co-O bonds tend to be delocalized, resulting in the much higher electrical conductivity and electrocatalytic activity.
A facile, one-pot solvothermal method is developed to synthesize MoS2 nanoflowers (MoS2NFs) coated on reduced graphene oxide (rGO) paper. The resulting MoS2NF/rGO paper serves as a freestanding, flexible and durable working electrode for hydrogen evolution reaction (HER), exhibiting an overpotential lowered to -0.19 V with a Tafel slope of ∼95 mV per decade.
In the last decade, molybdenum disulphide (MoS2), one of the most well-known two-dimensional (2D) layered transition- metal chalcogenides (TMDs), has received considerable attention due to its unique mechanical, electronic and optical properties as well as its various applications. [1] In particular, single- or few-layer MoS2 nanosheets have been widely used for homogeneous bio-probes, [2] phototransistors, [3] field-effect transistors, [4] gas sensors, [4b,5] memory devices, [6] electro-catalysts, [7] templates for epitaxial growth of noble metal nanostructures, [8] and other applications. [1c,9]
Electroactive MoSx catalysts on porous 3D sponges synthezied by a simple and scalable thermolysis process are proposed. Although no conducting materials are used to host the MoSx catalysts, they still serve as efficient electrodes for hydrogen evolution. The high current density of the MoSx-coated sponges are attributed to the large electrochemical surface area and their S-rich chemical structure.
Cited By :32, Export Date: 10 October 2017
Self-assembled IrO2 nanoparticles (NPs) of two distinct chain-like morphologies had been successfully synthesized on DNA scaffold at room temperature by the reduction of hydrated iridium salt precursor under continuous stirring. Different morphologies of IrO2 NPs were formed by tuning the concentration ratio of DNA to iridium salt solution. The probable growth mechanisms of the IrO2 NPs on DNA were elaborated. The potentiality of the DNA@IrO2 NPs were tested in two important applications, one as a catalyst for the oxidation of 2-propanol to acetone and other as an electrocatalyst for oxygen evolution reaction (OER). Catalysis study revealed that the reaction completed in a short time with higher product yield. The self-assembled, chain-like IrO2 NPs were screened as a potential electrocatalyst for the OER study that required an overpotential of 312 mV, to produce anodic current densities of 10 mA cm−2 (0.1 M NaOH) with turnover frequency (TOF) of 7.88 s-1. This is one among the lowest oxygen overpotential reported for IrO2 alone. The presence of phosphorous on DNA-phosphate backbone on the IrO2 NPs surface might be the key factor for the enhancement of OER activity. The overall syntheses and application processes are simple, less time consuming, reproducible and occur at room temperature. The process can be extended for the synthesis of other important nano-catalysts at a short time scale for their applications in different interdisciplinary fields like organic catalysis, electrocatalytic methanol and ethanol oxidations, and in oxygen reduction reactions (ORR) studies.
It is attractive but still remains a big challenge to develop non-noble metal bifunctional electrocatalysts efficient for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) under alkaline conditions. Herein, an amorphous CoSe film electrodeposited on a Ti mesh (a-CoSe/Ti) is demonstrated to exhibit high electrocatalytic activity and stability for both reactions in 1.0 M KOH. It needs overpotentials of 292 and 121 mV to drive 10 mA cm(-2) for OER and HER, respectively. The two-electrode alkaline water electrolyzer affords a water-splitting current of 10 mA cm(-2) at a cell voltage of 1.65 V. This work offers an attractive cost-effective catalytic material toward full water splitting applications.
Iridium (Ir) is widely used as a catalyst in polymer electrolyte membrane water electrolyzers (PEMWEs). However, high cost and limited catalytic performance of Ir hamper its large-scale industrial application. Here, based on a modified galvanic replacement, we introduce Cu nanoparticles as a template to prepare single-crystalline Cu–Ir polyhedral nanocages (NCs). Alloying Ir with 3d transition metal Cu not only significantly reduces the loading of Ir but also remarkably enhances its catalytic activity by forming a unique NC structure and tuning the d-band structure of Ir. The as-prepared single-crystalline Cu1.11Ir NCs exhibit enhanced catalytic activity toward the oxygen evolution reaction (OER) in 0.05 M H2SO4, with a smaller overpotential (286 mV) required for a current density of 10 mA cm−2 and a Tafel slope of 43.8 mV per decade. The mass activity can reach 73 mA mgIr−1 at an overpotential of 0.28 V for Cu1.11Ir NCs. Hence, the obtained Cu1.11Ir NCs would be a promising electrocatalyst for practical electrocatalytic water splitting systems.
Water splitting for the production of hydrogen and oxygen is an appealing solution to advance many sustainable and renewable energy conversion and storage systems, while the key fact depends on the innovative exploration regarding the design of efficient electrocatalysts. Reported herein is the growth of CoP mesoporous nanorod arrays on conductive Ni foam through an electrodeposition strategy. The resulting material of well-defined mesoporosity and a high specific surface area (148 m2 g-1) can be directly employed as a bifunctional and flexible working electrode for both hydrogen and oxygen evolution reactions, showing superior activities as compared with noble metal benchmarks and state-of-the-art transition-metal-based catalysts. This is intimately related to the unique nanorod array electrode configuration, leading to excellent electric interconnection and improved mass transport. A further step is taken toward an alkaline electrolyzer that can achieve a current density of 10 mA cm-2 at a voltage around 1.62 V over a long-term operation, better than the combination of Pt and IrO2. This development is suggested to be readily extended to obtain other electrocatalysis systems for scale-up water-splitting technology.
Mixed bimetallic oxides offer great opportunities for a systematic tuning of electrocatalytic activity and stability. Here, we demonstrate the power of this strategy using well-defined thermally prepared Ir-Ni mixed oxide thin film catalysts for the electrochemical oxygen evolution reaction (OER) under highly corrosive conditions such as in acidic proton exchange membrane (PEM) electrolyzers and photo-electrochemical cell (PEC) anodes. Variation of the Ir to Ni ratio resulted in a volcano type OER activity curve with an unprecedented 20-fold improvement in Ir mass-based activity over Ir oxide. In-situ spectroscopic probing of metal dissolution indicated that, against common views, activity and stability are not directly anti-correlated. To uncover activity and stability controlling parameters, the Ir-Ni mixed thin oxide film catalysts were characterized by a wide array of spectroscopic, microscopic, scattering, and electrochemical techniques in conjunction with DFT theoretical computations. By means of an intuitive model for the formation of the catalytically active state of the bimetallic Ir-Ni oxide surface we identify the coverage of reactive surface hydroxyl groups as a suitable descriptor for activity and stability and relate it to controllable synthetic parameters. Overall, our study highlights a novel, highly active oxygen evolution catalyst; moreover, it provides novel important insight in the structure and performance of bimetallic oxide OER electrocatalysts in corrosive acid environments.
Electrochemical water splitting into hydrogen and oxygen is a promising method for solar energy storage. The development of efficient electrocatalysts for water splitting has drawn much attention. However, catalysts that are active for both the hydrogen evolution and oxygen evolution reactions are rare. Herein, we show for the first time that nickel phosphide (Ni2P), an excellent hydrogen evolving catalyst, is also highly active for oxygen evolution. A current density of 10 mA cm-2 is generated at an overpotential of only 290 mV in 1 M KOH. The high activity is attributed to the core-shell (Ni2P/NiOX) structure that the material adopts under catalytic conditions. The Ni2P nanoparticles can serve as both cathode and anode catalysts for an alkaline electrolyzer, which generates 10 mA cm-2 at 1.63 V.
The investigation of nickel phosphide (Ni5 P4 ) as a catalyst for the hydrogen (HER) and oxygen evolution reaction (OER) in strong acidic and alkaline environment is described. The catalyst can be grown in a 3D hierarchical structure directly on a nickel substrate, thus making it an ideal candidate for practical water splitting devices. The activity of the catalyst towards the HER, together with its high stability especially in acidic solution, makes it one of the best non-noble materials described to date. Furthermore, Ni5 P4 was investigated in the OER and showed activity superior to pristine nickel or platinum. The practical relevance of Ni5 P4 as a bifunctional catalyst for the overall water splitting reaction was demonstrated, with 10 mA cm(-2) achieved below 1.7 V.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Reported herein is elucidation of a novel Co-based oxygen evolution catalyst generated in-situ from cobalt phosphide (CoP) nanoparticles. The present CoP nanoparticles, efficient alkaline hydrogen-evolving materials at cathode, are revealed to experience unique metamorphosis upon anodic potential cycling in alkaline electrolyte, engendering efficient and robust catalytic environments toward oxygen evolution reaction (OER). Our extensive ex-situ characterization shows that the transformed catalyst bears porous and nanoweb-like dispersed morphologies along with unique microscopic environments mainly comprised of discrete cobalt-oxo/hydroxo molecular units within phosphate-enriched amorphous network. Outstanding OER efficiency is achievable with the activated catalyst, which is favorably comparable to even a precious iridium catalyst. More remarkable feature is its outstanding long-term stability, superior to iridium and conventional cobalt oxide based materials. 12 hr bulk electrolysis continuously operating at high current density is completely tolerable with the present catalyst.
There exists a strong demand to replace expensive noble metal catalysts with cheap metal sulfides or phosphides for hydrogen evolution reaction (HER). Recently metal phosphides such as NiP, FeP and CoP have been considered as promising candidates to replace Pt cathodes. Here we report that the nanocrystalline CoP nanosheet assembly on carbon cloth can be formed by a two-step process: electrochemical deposition of Co species followed by gas phase phosphidation. The CoP catalyst in this report exhibits a Tafel slope of 30.1 mV/dec in 0.5 M H2SO4 and 42.6 mV/dec in 1 M KOH. The high HER performance of our CoP catalysts is attributed to the rugae-like morphology which results in a high double-layer capacitance and high density of active sites, estimated as 7.77×1017 sites/cm2.
There is a strong demand to replace expensive Pt catalysts with cheap metal sulfides or phosphides for hydrogen generation in water electrolysis. The earth-abundant Fe can be electroplated on carbon cloth (CC) to form high surface area rague-like FeOOH assembly. Subsequent gas phase phosphidation converts the FeOOH to FeP or FeP2 and the morphology of the crystal assembly is controlled by the phosphidation temperature. The FeP prepared at 250 oC presents lower crystallinity and those prepared at higher temperatures 400 oC and 500 oC possess higher crystallinity but lower surface area. The phosphidation at 300 oC produces nanocrystalline FeP and preserves the high-surface area morphology; thus it exhibits the highest HER efficiency in 0.5 M H2SO4; i.e. the required overpotential to reach 10 and 20 mA/cm2 is 34 and 43 mV respectively. These values are lowest among the reported non-precious metal phosphides on CC. The Tafel slope for the FeP prepared at 300 oC is around 29.2 mV/dec comparable to that of Pt/CC, indicating that the hydrogen evolution for our best FeP is limited by Tafel reaction (same as Pt). Importantly, the FeP/CC catalyst exhibits much better stability in a wide range working current density (up to 1 V/cm2), suggesting that it is a promising replacement of Pt for HER.
We report a high-performance bi-functional electrocatalyst composed of 3D crumpled graphene (CG)-cobalt oxide nanohybrids. This is the first report on using CG coupled with nanocrystals as both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts. The nitrogen-doped CG-CoO hybrid exhibits excellent catalytic activity and durability, making it a high-performance non-precious metal-based bi-functional catalyst for both ORR and OER.
Iridium oxide is the catalytic material with the highest stability in the oxygen evolution reaction (OER) performed under acidic conditions. However, its high cost and limited availability demand that IrO2 is utilized as efficiently as possible. We report the synthesis and OER performance of highly active mesoporous IrO2 catalysts with optimized surface area, intrinsic activity, and pore accessibility. Catalytic layers with controlled pore size were obtained by soft-templating with micelles formed from amphiphilic block copolymers poly(ethylene oxide)-b-poly(butadiene)-b-poly(ethylene oxide). A systematic study on the influence of the calcination temperature and film thickness on the morphology, phase composition, accessible surface area, and OER activity reveals that the catalytic performance is controlled by at least two independent factors, that is, accessible surface area and intrinsic activity per accessible site. Catalysts with lower crystallinity show higher intrinsic activity. The catalyst surface area increases linearly with film thickness. As a result of the templated mesopores, the pore surface remains fully active and accessible even for thick IrO2 films. Even the most active multilayer catalyst does not show signs of transport limitations at current densities as high as 75 mA cm(-2) .
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
A mixed-phased Co-based catalyst composed of Co phosphide and Co phosphate is successfully fabricated for bifunctional water electrolysis. The highly porous morphology in this anodized film enables efficient catalytic activity toward water splitting in an extremely low loading mass. The mixed phases in the porous film afford an ability to generate both H2 and O2 in a single electrolyzer.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
A large-area, self-supported Co3O4 nanocrystal/carbon fiber electrode for oxygen and hydrogen evolution reaction was fabricated via thermal decomposition of the [Co(NH3)n](2+)-oleic acid complex and subsequent spray deposition. Due to the exposed active sites and good electrical conductivity, its operate voltage for overall water splitting is nearly the same as commercial Pt/C.
Efficient electrochemical water splitting to hydrogen and oxygen is considered a promising technology to overcome our dependency on fossil fuels. Searching for novel catalytic materials for electrochemical oxygen generation is essential for improving the total efficiency of water splitting processes. We report the synthesis, structural characterization and electrochemical performance in the oxygen evolution reaction (OER) of Fe doped NiO nanocrystals. The facile solvothermal synthesis in tert-butanol leads to the formation of ultrasmall crystalline and highly dispersible FexNi1-xO nanoparticles with dopant concentrations of up to 20 %. The increase in Fe content is accompanied by a decrease in particle size resulting in non-agglomerated nanocrystals of 1.5-3.8 nm in size. The Fe content and composition of the nanoparticles is determined by X-ray photoelectron spectroscopy (XPS) and Energy-dispersive X-ray spectroscopy (EDX) measurements while Mössbauer and Extended X-ray Absorption Fine Structure (EXAFS) analyses reveal a substitutional incorporation of Fe(III) into the NiO rock salt structure. The excellent dispersibility of the nanoparticles in ethanol allows for the preparation of homogeneous ca. 8 nm thin films with a smooth surface on various substrates. The turnover frequencies (TOF) of these films could be precisely calculated using a quartz crystal microbalance (QCM). Fe0.1Ni0.9O was found to have the highest electrocatalytic water oxidation activity in basic media with a TOF of 1.9 1/s at the overpotential of 300 mV. The current density of 10 mA/cm2 is reached at the overpotential of 297 mV with a Tafel slope of 37 mV/dec. The extremely high catalytic activity, facile preparation and low cost of the monocrystalline FexNi1-xO nanoparticles make them very promising catalysts for the oxygen evolution reaction.
Solar generation of fuel is a promising future energy technology, and strong acidic conditions are highly desirable for integrated solar hydrogen generators. In particular, water splitting near pH 0 is attractive due to the availability of high theoretical efficiency, high performance hydrogen evolution catalysts, and robust ion exchange membranes. The lack of a stable, earth-abundant oxygen evolution catalyst inhibits deployment of this technology, and development of such a material is hampered by the strong anti-correlation between electrochemical stability and catalytic activity of non-precious metal oxides. High-throughput screening of mixed metal oxides offers a promising route to the identification of new stable catalysts and requires careful design of experiments to combine the concepts of rapid experimentation and long-term stability. By combining serial and parallel measurement techniques, we have created a high-throughput platform to assess the catalytic activity of material libraries in the as-prepared state and after 2 h of operation. By screening the entire (Mn–Co–Ta–Sb)O
x
composition space, we observe that the compositions with highest initial activity comprised cobalt and manganese oxides, but combinations with antimony and tantalum offer improved stability. By combining the desired properties of catalytic activity and stability, the optimal composition regions are readily identified, demonstrating the success and fidelity of this novel high-throughput screening platform.
Ni-Co amorphous double hydroxides nanomaterials with a hollow structure and tunable Ni/Co molar ratio are synthesized using a template method. The amorphous NiCo2.7(OH)x nanocages demonstrate high surface reactivity, comparable catalytic activity, and excellent stability for efficient water oxidation. Density functional theory simulations suggest that the component-dependent electrocatalytic activities are connected to the binding energies of oxygen radical on diverse hydroxides.
Replacing Pt by earth abundant catalysts is one of the most important tasks toward potential large-scale HER applications. Among many potential candidates, low cost and earth abundant transition metal dichalcogenides such as MoS2 and WS2 have been promising as good H-2 evolution electrocatalysts when they are engineered into the structures with active sites. In this work, we have performed systematic studies on the catalytic reactivity of both MoS2 and WS2 materials produced by one-step and scalable thermolysis from (NH4)(2)WS4 and (NH4)(2)MoS4 precursors respectively. Structural analysis shows that these materials prepared at a higher thermolysis temperature exhibit higher crystallinity. The H-2 evolution electrocatalysts efficiency for the MoS2 prepared at a lower temperature is higher than those at higher temperatures, where amorphous MoS2 or S-2(2-) species instead of crystalline MoS2 is the main active site. By contrast, crystalline WS2 prepared at high temperature is identified to be the key reaction site. Both catalysts display excellent efficiency and durability as an electrocatalyst operating in acidic electrolytes. This work provides fundamental insights for further design and preparation of emergent metal dichalcogenide catalysts, beneficial for the development in clean energy. Copyright
A Ni3S2 nanorods/Ni foam composite electrode is prepared as a high-performance catalyst for the oxygen evolution reaction (OER), which exhibits excellent OER activity with a small overpotential of 157 mV based on the onset of catalytic current.
The preparation of single-layer TiS2 and TaS2 nanosheets is realized by optimizing the electrochemical lithium interaction and exfoliation method. As a proof of concept, Pt and Au nanoparticles are grown on the aforementioned ultra-thin nanosheets to form functional composites. Notably, the Pt-TiS2 hybrid presents good electrocatalytic activity in the hydrogen evolution reaction.
Active and highly stable oxide-supported IrNiOx core–shell catalysts for electrochemical water splitting are presented. IrNix@IrOx nanoparticles supported on high-surface-area mesoporous antimony-doped tin oxide (IrNiOx /Meso-ATO) were synthesized from bimetallic IrNix precursor alloys (PA-IrNix /Meso-ATO) using electrochemical Ni leaching and concomitant Ir oxidation. Special emphasis was placed on Ni/NiO surface segregation under thermal treatment of the PA-IrNix /Meso-ATO as well as on the surface chemical state of the particle/oxide support interface. Combining a wide array of characterization methods, we uncovered the detrimental effect of segregated NiO phases on the water splitting activity of core–shell particles. The core–shell IrNiOx /Meso-ATO catalyst displayed high water-splitting activity and unprecedented stability in acidic electrolyte providing substantial progress in the development of PEM electrolyzer anode catalysts with drastically reduced Ir loading and significantly enhanced durability.
Cost-effective electrocatalysts based on nonprecious metals for efficient water splitting are crucial for various technological applications represented by fuel cell. Here, 3d transition metal layered double hydroxides (LDH) with varied contents of Ni and Fe were successfully synthesized through a homogeneous precipitation. The exfoliated Ni-Fe LDH nanosheets were hetero-assembled with graphene oxide (GO) as well as reduced graphene oxide (rGO) into superlattice-like hybrids, in which two kinds of oppositely charged nanosheets are stacked face-to-face in alternating sequence. Hetero-structured composites of Ni2/3Fe1/3 LDH nanosheets and GO (Ni2/3Fe1/3-GO) exhibited excellent oxygen evolution reaction (OER) efficiency with a small overpotential of about 0.23 V and Tafel slope of 42 mV/decade. The activity was further improved via the combination of Ni2/3Fe1/3 LDH nanosheets with more conductive rGO (Ni2/3Fe1/3-rGO) to achieve an overpotential as low as 0.21 V and Tafel plot of 40 mV/decade. The catalytic activity was enhanced with an increased Fe content in the bimetallic Ni-Fe system. Moreover, the composite catalysts were found to be effective for hydrogen evolution reaction (HER). An electrolyzer cell powered by a single AA battery of 1.5 V was demonstrated by using the bi-functional catalysts.
Herein, we report a simple and scalable synthesis of Co3O4 nanocubes possessing exposed low-surface energy planes supported on nitrogen-doped graphene (Co3O4-NC/NGr) by a hydrothermal method as an efficient electrocatalyst for water oxidation. Three different types of morphologies of Co3O4 (i.e. nanocubes, blunt edge nanocubes and spherical particles) have been synthesized by systematically varying the reaction time. Subsequently, their catalytic activity towards oxygen evolution reaction (OER) has been screened in alkaline medium. Among the three different morphologies, the intermediate architecture, i.e. the blunt edged nanocubes designated as Co3O4-NC/NGr-12h, has shown the highest OER activity. The catalyst displayed an overpotential (η) of ~280 mV at 10 mA/cm2 in 1 M KOH solution, which is lower than that of the other prepared samples such as Co3O4-NC/NGr-3h (~348 mV), Co3O4-NC/NGr-9h (~356 mV), Co3O4-NC/NGr-24h (~320 mV), Co3O4-NC/Gr-12h (~300 mV) and Co3O4 (~310 mV). Along with that, the electrochemical stability of the catalyst is also found to be remarkably good. The role of the low index planes of Co3O4 nanocubes (Co3O4-NC) and the importance of the doped nitrogen in the carbon framework for the uniform dispersion and direct coupling with Co3O4-NC have been examined. The controlled interplay of the exposed crystal planes of Co3O4 and its dispersion and synergistic interaction with the nitrogen doped graphene are found to be the decisive factors in bringing in the modulated OER activity of the system.
Exploration of low-cost and earth-abundant photocatalysts for highly efficient solar photocatalytic water splitting is of great importance. Although transition-metal dichalcogenides (TMDs) showed outstanding performance as co-catalysts for the hydrogen evolution reaction (HER), designing TMD-hybridized photocatalysts with abundant active sites for the HER still remains challenge. Here, a facile one-pot wet-chemical method is developed to prepare MS2 -CdS (M=W or Mo) nanohybrids. Surprisedly, in the obtained nanohybrids, single-layer MS2 nanosheets with lateral size of 4-10 nm selectively grow on the Cd-rich (0001) surface of wurtzite CdS nanocrystals. These MS2 -CdS nanohybrids possess a large number of edge sites in the MS2 layers, which are active sites for the HER. The photocatalytic performances of WS2 -CdS and MoS2 -CdS nanohybrids towards the HER under visible light irradiation (>420 nm) are about 16 and 12 times that of pure CdS, respectively. Importantly, the MS2 -CdS nanohybrids showed enhanced stability after a long-time test (16 h), and 70 % of catalytic activity still remained.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Cost-effective production of solar fuels requires robust and earth-abundant oxygen evolution reaction (OER) catalysts. Herein, we report that ultrathin nanoplates of cobalt-manganese layered double hydroxide (CoMn LDH) are a highly active and stable oxygen evolution catalyst. The catalyst was fabricated by a one-pot co-precipitation method at room temperature and its turnover frequency (TOF) is more than 20 times higher than the TOFs of Co and Mn oxides and hydroxides, and 9 times higher than the TOF of a precious IrO2 catalyst. The activity of the catalyst was promoted by anodic conditioning which was proposed to form amorphous regions and reactive Co(IV) species on the surface. The stability of the catalyst was demonstrated by continued electrolysis.
Iron is the cheapest and one of the most abundant transition metals. Natural [FeFe]-hydrogenases exhibit remarkably high activity in hydrogen evolution, but they suffer from high oxygen sensitivity and difficulty in scale-up. Herein, an FeP nanowire array was developed on Ti plate (FeP NA/Ti) from its β-FeOOH NA/Ti precursor through a low-temperature phosphidation reaction. When applied as self-supported 3D hydrogen evolution cathode, the FeP NA/Ti electrode shows exceptionally high catalytic activity and good durability, and it only requires overpotentials of 55 and 127 mV to afford current densities of 10 and 100 mA cm2, respectively. The excellent electrocatalytic performance is promising for applications as non-noble-metal HER catalyst with a high performance–price ratio in electrochemical water splitting for large-scale hydrogen fuel production.
Two-dimensional materials have attracted increasing research interest owing to their unique electronic, physical, optical, and mechanical properties. We thus developed a general strategy for the fabrication of ultralong hybrid microfibers from a mixture of reduced graphene oxide and transition-metal dichalcogenides (TMDs), including MoS2, TiS2, TaS2, and NbSe2. Furthermore, we prepared fiber-based solid-state supercapacitors as a proof-of-concept application. The performance of thus-prepared supercapacitors was greatly improved by the introduction of the TMDs.
Fluorine doped iridium oxide (IrO2:F) powders with varying F content ranging from 0 to 20 wt.% has been synthesized by using a modification of the Adams fusion method. The precursors (IrCl4 and NH4F) are mixed with NaNO3 and heated to elevated temperatures to form high surface area nanomaterials as electro-catalysts for PEM based water electrolysis. The catalysts were then coated on a porous Ti substrate and have been studied for the oxygen evolution reaction in PEM based water electrolysis. The IrO2:F with an optimum composition of IrO2:10 wt.% F shows remarkably superior electrochemical activity and chemical stability compared to pure IrO2. The results have also been supported via kinetic studies by conducting rotating disk electrode (RDE) experiments. The RDE studies confirm that the electro-catalysts follow the two electron transfer reaction for electrolysis with calculated activation energy of ∼25 kJ mol−1. Single full cell tests conducted also validate the superior electrochemical activity of the 10 wt.% F doped IrO2.
MoSx is grown on crumpled graphene particles supported on carbon cloth substrates for the hydrogen evolution reaction. Modifying carbon cloth with crumpled graphene allows for higher loading levels of MoSx and thus significantly enhances its electrocatalytic activity. Measurements yield a current density of –220 mA cm−2 at an overpotential of 0.3 V (before iR correction) for the crumpled-graphene-modified carbon cloth.
The development of effective and inexpensive hydrogen evolution reaction (HER) electrocatalysts for future renewable energy systems is highly desired. The strongly acidic conditions in proton exchange membranes create a need for acid-stable HER catalysts. A nanohybrid that consists of carbon nanotubes decorated with CoP nanocrystals (CoP/CNT) was prepared by the low-temperature phosphidation of a Co3 O4 /CNT precursor. As a novel non-noble-metal HER catalyst operating in acidic electrolytes, the nanohybrid exhibits an onset overpotential of as low as 40 mV, a Tafel slope of 54 mV dec(-1) , an exchange current density of 0.13 mA cm(-2) , and a Faradaic efficiency of nearly 100 %. This catalyst maintains its catalytic activity for at least 18 hours and only requires overpotentials of 70 and 122 mV to attain current densities of 2 and 10 mA cm(-2) , respectively.
Novel OER catalysts - monodisperse Au@Co3 O4 core-shell nanocrystals - have been prepared by synthesizing Au nanocrystals, followed by deposition of Co shells and their conversion to Co3 O4 shells. Owing to the synergistic effect, Au@Co3 O4 nanocrystals have an OER activity 7 times as high as a Au and Co3 O4 nanocrystals mixture or Co3 O4 nanocrystals alone, and 55 times as high as Au nanocrystals alone.
The slow kinetics of the oxygen evolution reaction (OER) greatly hinders the large-scale production of hydrogen fuel from water splitting. Although many OER electrocatalysts have been developed to negotiate this difficult reaction, substantial progresses in the design of cheap, robust and efficient catalysts are still required and have been considered a huge challenge. Here, we report a composite material consisting of CoSe2 nanobelts anchored on reduced graphene oxides (denoted as NG-CoSe2) as a highly efficient OER electrocatalyst. In 0.1 M KOH, the new NG-CoSe2 catalyst afforded a current density of 10 mA cm-2 at a small overpotential of mere 0.366 V and a small Tafel slope of ~40 mV/decade, comparing favorably with the state-of-the-art RuO2 catalyst. This NG-CoSe2 catalyst also presents better stability than that of RuO2 under harsh OER cycling conditions. Such good OER performance is comparable to the best literature results and the synergistic effect was found to boost the OER performance. These results raise the possibility for the development of effective and robust OER electrodes by using cheap and easily prepared NG-CoSe2 to replace the expensive commercial catalysts such as RuO2 and IrO2.
A three-dimensional (3D) electrode composed of nitrogen, oxygen-dual doped graphene-carbon nanotube hydrogel film has been fabricated which greatly favors the transport and access of gas and reaction intermediates, and shows a remarkable oxygen evolution catalytic performance in both alkaline and acidic solutions.
Composite materials: Tungsten disulfide and WS2 /reduced graphene oxide (WS2 /rGO) nanosheets were fabricated by hydrothermal synthesis using tungsten chloride, thioacetamide, and graphene oxide (GO) as starting materials. The WS2 nanosheets are efficiently templated on the rGO layer. The WS2 /rGO hybrid nanosheets show much better electrocatalytic activity for the hydrogen evolution reaction than WS2 nanosheets alone.
Over the past several decades, tremendous effort has been put into developing cost-effective, highly active and durable electrocatalysts for oxygen evolution reaction (OER) in the proton exchange membrane water electrolyzer. This report explores an advanced and effective "soft" material-assistant method to fabricate Ir0.6Sn0.4O2 electrocatalysts with a 0.6/0.4 ratio of Ir/Sn in precursors. Adopting a series of characterization methods, the collective results suggest that the surfactant-material F127 content, as an important factor, can efficiently control the formation of Ir-Sn oxides with varying surface properties and morphologies, such as the grainy and rod-shaped structures. Associating with the half-cell and single electrolyzer, it is affirmed that the optimal ratio of (It + Sn)/F127 is 100 for the preparation of S100-Ir0.6Sn0.4O2 with obviously enhanced activity and sufficient durability under the electrolysis circumstances. The lowest cell voltages obtained at 80 degrees C are 1.631 V at 1000 mA cm(-2), and 1.820 V at 2000 mA cm(-2), when applying S100-Ir0.6Sn0.4O2 OER catalyst and Ti-material diffusion layer on the anode side and Nafion (R) 115 membrane. Furthermore, the noble-metal It loading in the same cell decreases to 0.77 mg cm(-2). These results highlight that Ir-Sn oxide synthesized by the soft-material method is a promising OER electrocatalyst. (C) 2013 Published by Elsevier B.V.