No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Heteroatom-doped carbon-based oxygen reduction electrocatalysts with tailored four-electron and two-electron selectivity
Abstract
Oxygen reduction reaction (ORR) plays a pivotal role in electrochemical energy conversion and commodity chemical production. Oxygen reduction involving a complete four-electron (4e-) transfer is important for the efficient operation of polymer electrolyte fuel cells, whereas the ORR with a partial 2e- transfer can serve as a versatile method for producing industrially important hydrogen peroxide (H2O2). For both the 4e- and 2e- pathway ORR, platinum-group metals (PGMs) have been materials of prevalent choice owing to their high intrinsic activity, but they are costly and scarce. Hence, the development of highly active and selective non-precious metal catalysts is of crucial importance for advancing electrocatalysis of the ORR. Heteroatom-doped carbon-based electrocatalysts have emerged as promising alternatives to PGM catalysts owing to their appreciable activity, tunable selectivity, and facile preparation. This review provides an overview of the design of heteroatom-doped carbon ORR catalysts with tailored 4e- or 2e- selectivities. We highlight catalyst design strategies that promote 4e- or 2e- ORR activity. We also summarise the major active sites and activity descriptors of the respective ORR pathways and describe the catalyst properties controlling the ORR mechanisms. We conclude the review with a summary and suggestions for future research.
... [13][14][15][16][17][18][19] Among the other non-platinum catalysts heteroatom doping of nanocarbons has been found to be an easy and effective way to improve the ORR and OER kinetics. [20][21][22][23][24][25] In fact, doped carbon materials are attractive due to their low cost, good tolerance to fuel impurities, long-term stability and good electrocatalytic activity towards the ORR. [22,26] Mostly N, P, S and B doping have been investigated owing to their reasonable atomic size in order to enter the carbon network. ...
... [20][21][22][23][24][25] In fact, doped carbon materials are attractive due to their low cost, good tolerance to fuel impurities, long-term stability and good electrocatalytic activity towards the ORR. [22,26] Mostly N, P, S and B doping have been investigated owing to their reasonable atomic size in order to enter the carbon network. [26][27][28][29][30][31][32] Fluorine-doped carbon materials have lately attracted much attention mostly because of the high electronegativity of this element. ...
Functionalization of nanocarbon materials with heteroatoms is of paramount interest as doping of carbon with electron withdrawing groups results in change of electrochemical properties of the potential catalyst. Adding fluorine, as the most electronegative element into the doping process next to boron is expected to have significant effect on the design of novel nanocarbon‐based electrocatalysts. In this paper boron and fluorine co‐doped reduced graphene oxide/few‐walled carbon nanotube (BF‐rGO/FWCNT) catalyst are synthesized via simple and low‐cost direct pyrolysis method using boron trifluoride diethyl etherate (BTDE). Composition analysis confirmed that boron and fluorine have been grafted onto the carbon support. Rotating disk electrode (RDE) measurements revealed that BF‐rGO/FWCNT has remarkable electrocatalytic activity toward the oxygen reduction reaction (ORR) both in alkaline and acid media. The onset potential of the best BF‐rGO/FWCNT catalyst was 50 mV more positive in alkaline and 600 mV more positive in acidic media compared with un‐doped rGO/FWCNT. The half‐wave potential was 100 mV more positive in alkaline media and 700 mV more positive in acidic media in comparison with un‐doped rGO/FWCNT.
... However, pure and metal-free CNTs without any modification show rather poor electrocatalytic activity for ORR [48,49]. Therefore, heteroatom doping is one opportunity to improve the electrocatalytic properties of the CNT material [50][51][52][53]. For example, introducing nitrogen into CNTs impacts π-electrons and causes changes in electronic structure, which could enhance electrocatalytic activity and electronic conductivity [1]. ...
... The N,S co-doped carbon materials have been used previously as cathodes for AEMFCs showing lower fuel cell performance (N-S-MPC, P max = 17.2 mW cm − 2 [79], N-S/Gr-1000, P max = 20 mW cm − 2 [80], SNBC12, P max = 217 mW cm − 2 [81]) as compared to the AEMFC results obtained in this work. However, it should be noted that operating conditions (e.g., cell temperature, gas flow rate) and materials (AEM and ionomer) are different, which makes a comparison with literature difficult [52,[82][83][84][85]. As we have compared the N,S-doped nanocarbon composites with Pt/C in similar AEMFC conditions and N,S-SiCDC/MPC exhibited the highest P max value, we could consider that our synthesised composite catalysts are promising cathode materials for the AEMFC application. ...
... Polycyclic π-conjugated carbon-based molecules have numerous potential applications in organic electronics; therefore, a detailed understanding of their fundamental properties is crucial. 1 Doping heteroatoms into carbon-based molecules is an effective strategy to adjust their physical and chemical properties of the material, thereby improving their performance in electronic, photonic, optoelectronic, and spintronic applications. [2][3][4][5][6][7][8][9][10][11] Nitrogen is the most commonly used dopant as it has a similar covalent radius as carbon while providing one extra electron. A fundamental property of heteroatom-doped carbon-based molecules is their stability with respect to the control of energy levels, which can be measured by the energy gap (E gap ). ...
... Pauling established a well-known formula relating the bond length R(p) to its bond order p as 26 (7) where c is an empirical constant and R(1) is the standard single bond length. The value of c can be calculated by taking the bond lengths for the typical single (p = l) and double (p = 2) bonds as follows: (8) Using Equation (7), it is also possible to calculate the bond number, p, for any bond length, R(p), as follows: (9) Focusing to Huckel theory that deals with π-electron system, the bond length can be calculated from the π-electron bond order following Gordy's formula: 39,40 ...
Hückel theory is a simple and powerful method for predicting the molecular orbital and the energy of conjugated molecules. However, the presence of nitrogen atoms in aza aromatic molecules alters the Coulomb and resonance integrals owing to the difference in electronegativity between nitrogen and carbon atoms. In this study, we focus on acridine and phenazine. Further correction is implemented based on the ring current model, thus revealing the change in resonance integral for the carbon–carbon bond along the bridge of the molecule. The Hamiltonian of the π–electron system in the Hückel method is solved using the HuLiS software. Various geometry-based aromaticity indices are used to obtain the aromaticity indices of the two non-equivalent rings. For further evaluation, the results for bond lengths are used to calculate the associated bond energy. Considering the carbon–hydrogen (CH) bonds, the total molecular energy is compared with the experimental heats of formation for a number of benzenoid hydrocarbons and aza aromatics, in addition to the two studied molecules. Finally, the correlation between the nitrogen atom on the aromaticity index and the ring energy content is evaluated to determine to which extent the Hückel model agrees with previous experimental and advanced computational studies.
... These limitations can be mitigated by employing a conductive support, such as carbon materials [14]. Moreover, heteroatom doping of carbon materials confers additional active sites for ORR [15,16]. Reportedly, N-doping enhances ORR performance of carbon materials by changing its charge distribution, as nitrogen atoms have higher electronegativity than carbon [17]. ...
The development of sustainable and high-performance oxygen reduction reaction (ORR) electrocatalysts is fundamental to fuel cell implementation. Non-precious transition metal oxides present interesting electrocatalytic behavior, and their incorporation into N-doped carbon supports leads to excellent ORR performance. Herein, we prepared a shrimp shell-derived biochar (CC), which was doped with nitrogen via a ball milling approach (N-CC), and then used as support for Co3O4 nanoparticles growth (N-CC@Co3O4). Co3O4 loading was optimized using three different amounts of cobalt precursor: 1.56, 2.33 and 3.11 mmol in N-CC@Co3O4_1, N-CC@Co3O4_2 and N-CC@Co3O4_3, respectively. Interestingly, all prepared electrocatalysts, including the initial biochar CC, presented electrocatalytic activity towards ORR. Both N-doping and the introduction of Co3O4 NPs had a significant positive effect on ORR performance. Meanwhile, the three composites showed distinct ORR behavior, demonstrating that it is possible to tune their electrocatalytic performance by changing the Co3O4 loading. Overall, N-CC@Co3O4_2 achieved the most promising ORR results, displaying an Eonset of 0.84 V vs. RHE, jL of −3.45 mA cm−2 and excellent selectivity for the 4-electron reduction (n = 3.50), besides good long-term stability. These results were explained by a combination of high content of pyridinic-N and graphitic-N, high ratio of pyridinic-N/graphitic-N, and optimized Co3O4 loading interacting synergistically with the porous N-CC support.
... 2 The performance of PEMFCs is posed by the intrinsically sluggish kinetics of the oxygen reduction reaction (ORR). 3 The ORR process has two pathways: 4 in the 4e − ORR pathway, O 2 is directly reduced to H 2 O and in the 2e − ORR pathway, O 2 is partially reduced to H 2 O 2 or further reduced to H 2 O. The 4e − selective pathway is regarded as favorable for ORR. 5 While Pt-based catalysts have been commonly utilized to accelerate ORR kinetics, 6 the expensive price and shortages of Pt-based catalysts present barriers to the large-scale commercialization of PEMFCs. ...
Zeolitic Imidazolate Frameworks-8 (ZIF-8) is commonly used as an ideal precursor for non-noble metal catalysts because of its high specific surface area, ultra-high porosity, and N-rich content. Upon pyrolyzing ZIF-8 at 900 °C in Ar, the resulting material, referred to as Z8, displayed good activity toward the oxygen reduction reaction (ORR). Then the ZIF-8 was mixed with various conductive carbon materials, such as multiwall carbon nanotubes (MWCNTs), Acetylene black (ACET), Vulcan XC-72R (XC-72R), and Ketjenblack EC-600JD (EC-600JD), to form Z8 composites. The Z8/MWCNTs composite exhibited enhanced ORR activity owing to its network structure, meso-/microporous hierarchical porous structure, improved electrical conductivity, and graphitization. Subsequently, iron and nitrogen co-doping is achieved through the pyrolysis of a mixture comprising Fe, N precursor, and ZIF-8/MWCNTs, which is denoted as FeN-Z8/MWCNTs. The intrinsically high electrical conductivity of MWCNTs facilitated efficient electron transfer during the ORR, while the meso-/microporous hierarchical porous structure and network structure of Fe, N co-doped ZIF-8/MWCNTs promoted oxygen transport. The presence of Fe-containing species in the catalyst acted as activity centers for ORR. This strategy of preparing Z8 composites and modifying them with Fe, N co-doping offers an insightful approach to designing cost-effective electrocatalysts.
... Over the last decades, carbon-based materials (nanotubes, graphene, and other allotropes) have been extensively studied as future catalytic electrodes for oxygen reduction reaction (ORR) since they are thought to be efficient, inexpensive, and ecologically acceptable alternatives to Pt-based catalysts. [1][2][3] To increase the competitiveness of carbon-based catalysts, two factors should be considered thoroughly to increase the potential use of carbon-based materials in catalytic electrodes: high heteroatom contents and large specific areas. First, heteroatoms are capable of substituting carbon atoms in carbon-based materials, thereby enhancing the conversion efficiency per dopant atom and altering the physicochemical characteristics and electronic activities of such materials. ...
Graphynes (GYs) are a novel set of carbon allotropes with high potential as future catalytic electrodes for oxygen reduction reactions because of their unique physical and chemical properties. In recent years, a number of heteroatom‐doped graphdiyne (GDYs) based electrocatalysts have been developed. However, the development of GYs has made slow progress due to their limited synthetic strategies. Here, the first case of nitrogen and sulfur co‐doped graphynes (NS‐GYs) synthesized through the copolymerization between hydrogen‐deficient heterocyclic aromatic monomers via the Sonogashira–Hagihara cross‐coupling reaction is reported. The NS‐GYs exhibit abundant porosity after heat treatment with large specific area and high heteroatom content for use as potential electrocatalysts. In addition, NS‐GY‐3‐800 with the best electrocatalytic performance shows excellent power density and stability in Zn‐air batteries.
... Currently, the Oxygen Reduction Reaction (ORR) is a challenge for fuel cells due to its reaction rate being six or more orders of magnitude smaller than that of anode hydrogen oxidation [2]. Hence, many strategies are applied to cathode ORR for enhancing electrocatalyst efficient [3]: finding catalyst with 4 electron transfer number (n) [4], tuning the nanostructure Yangjing Jiao and Chunxin Yu have contributed equally to this work. of the catalyst with high active sites [5] and large active surface area [6], etc. ...
Hollow carbon spheres (HCS) have been employed as supporting materials for Pt, Ag, and Au nanoparticles (NPs) in the oxygen reduction reaction (ORR). The NPs Pt, Ag, and Au have been hydrothermally coated on HCS uniformly. The diameter of the Pt, Ag and Au NPs ranges between 11 and 32 nm, with the loading of 4.58, 4.73, and 4.07 wt.%, respectively. It is found that Pt, Ag, and Au/HCS exhibit stable catalytic activity after 5000 CV scanning and follow a four-electron route in the ORR. Among them, Tafel plots show that Ag/HCS has the fastest kinetic rates and Pt/HCS has the largest effective active area from CV curve. Hence HCS provides a stable supportive material for Pt, Ag, Au nanoparticle catalysts in the ORR due to its nanopores structure and large surface area.
... Recently many studies have proposed and fabricated non-platinum group metal (Non-PGM) catalysts. Most of these electrocatalysts are based on iron transition metals or heteroatom doped carbon alloys, which are synthesized via pyrolysis under high temperatures to prepare atomically, dispersed iron in the carbon support [Zhao et al., 2005, Wu et al., 2016, Rojas-Carbonell et al., 2017, de Oliveira et al., 2020, Woo et al., 2021, De Oliveira et al., 2021, Goto et al., 2021. ...
... Currently, many noble and non-noble metal NPs and SAs have been supported on NC materials and applied in catalytic, electrocatalytic, or photocatalytic reactions. Some excellent reviews have covered the use of noble and non-noble metals supported on NC for applications in electrocatalysis or photocatalysis [4,7,10,11,[28][29][30][31][32][33][34][35][36]. So far, the catalytic and photocatalytic applications photocatalytic of NC based materials composed of nonnoble and noble metal nanoparticles have been discussed in a previous review by He et al. [4]. ...
Noble metals nanoparticles (NPs) and single atoms (SAs) supported on nitrogen-doped carbon (NC) materials display remarkable activity and selectivity in a wide variety of reactions, spanning hydrogenations, oxidations, Fischer-Tropsch synthesis, and Suzuki coupling. Due to the unique interaction between the NC structure and the anchored metal center, both physical and chemical properties of the catalysts can be finely tuned. Moreover, the precise control of the coordination environment in the host support can pave the way to designing efficient noble metal catalysts with optimized active centers. This approach opens avenues for improving stability, selectivity, and catalytic activity. This review covers the recent progress in the field of catalysis by noble metals supported on N-doped carbon materials. An overview of various catalytic systems based on Au, Ag, Pd, Pt, Ru, Rh is discussed, and structure-performance relations in catalysis are described based on theoretical and experimental investigations for different classes of metals and reactions. Finally, challenges and perspectives for engineering heterogeneous catalysts based on noble metals embedded in N-doped carbon materials are described to tackle challenges regarding activity and selectivity.
... Table 3 Structural parameters of the GNP-Cu 3 N, GNP-Cu 3 N/ Ag, and raw GNP materials The oxygen reduction reaction has been studied intensively and its mechanism described in many papers [29,39,41]. In general, the ORR process can proceed through a one-step four-electron pathway resulting in a complete reduction of oxygen to water [42]: or through a two-step two-electron pathway, where oxygen is partially reduced to hydrogen peroxide: An efficient catalyst for oxygen reduction reaction should favor the 4e − ORR pathway, similar to commercial electrodes. For metal-free catalysts, such as graphene-based materials, the ORR electron transfer number is usually about 2 [30,43]. ...
This work presents attempts to synthesize silver-doped copper nitride nanostructures using chemical solution methods. Copper(II) nitrate and silver(I) nitrate were used as precursors and the oleylamine as a reducing and capping agent. Homogeneous Cu 3 N/Ag nanostructures with a diameter of ~ 20 nm were obtained in a one-pot synthesis by the addition of the copper(II) salt precursor to the already-synthesized silver nanoparticles (Ag NPs). Synthesis in a two-pot procedure performed by adding Ag NPs to the reaction medium of the Cu 3 N synthesis resulted in the formation of a Cu 3 N@Ag nanocomposite, in which Ag NPs are uniformly distributed in the Cu 3 N matrix. The morphology, structure, and chemical composition of the obtained specimens were studied by TEM, XRD, XPS, and FT-IR methods, while optical properties using UV–Vis spectroscopy and spectrofluorimetry. The band gap energy decreased for Cu 3 N/Ag ( E g = 2.1 eV), in relation to pure Cu 3 N ( E g = 2.4. eV), suggesting the insertion of Ag atoms into the Cu 3 N crystal lattice. Additionally, Cu 3 N and Cu 3 N/Ag nanostructures were loaded on graphene (GNP) and tested as a catalyst in the oxygen reduction reaction (ORR) by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The Cu 3 N/Ag-modified GNP hybrid material revealed catalytic activity superior to that of Cu 3 N-based GNP hybrid material and pure GNP, comparable to that of a commercial Pt/C electrode.
Graphical Abstract
... The use of biomass waste for the preparation of catalytic materials is a remarkable strategy, either employing biomass residues as a sacrificial template to control the textural properties of the samples or as a carbon source which could serve as support for the immobilization of, for instance, metal nanoparticles. Furthermore, the design of heteroatomdoped carbonaceous materials is also an interesting opportunity which has taken advantage of biomass waste composition for the low-cost synthesis of advanced materials [19,20]. In this regard, N-doped carbons have emerged as good candidates for the development of metal-free catalytic processes-where nitrogen sites could play a critical role due to their basic properties-or for employment as supports for metal/metal oxide nanoparticles. ...
Chitin, the second most abundant biopolymer in the planet after cellulose, represents a renewable carbon and nitrogen source. A thrilling opportunity for the valorization of chitin is focused on the preparation of biomass-derived N-doped carbonaceous materials. In this contribution, chitin-derived N-doped carbons were successfully prepared and functionalized with palladium metal nanoparticles. The physicochemical properties of these nanocomposites were investigated following a multi-technique strategy and their catalytic activity in reductive amination reactions was explored. In particular, a biomass-derived platform molecule, namely furfural, was upgraded to valuable bi-cyclic compounds under continuous flow conditions.
Electrocatalytic two-electron oxygen reduction reaction (2e⁻ ORR) in seawater offers a sustainable route for hydrogen peroxide (H2O2) production. However, due to the high concentration of Cl⁻ ions and competitive 4e⁻ ORR, there is a lack of efficient and long-term stable seawater electrocatalysts. Here we report a high-performance electrocatalyst design based on NiPS3 nanosheets enabling efficient H2O2 production from seawater. Specifically, the NiPS3 nanosheets deliver a 2e⁻ ORR selectivity of ∼98%, a H2O2 yield of 6.0 mol gcat⁻¹ h⁻¹ and robust stability for over 1,000 h in simulated seawater. Underlying the exciting performance is the synergy of the S²⁻, Ni²⁺ and P⁴⁺ sites where the octahedral S²⁻ skeleton repels Cl⁻ ions, the Ni²⁺ sites enable the modest binding strength of *OOH intermediate, and the P⁴⁺ sites interact with H2O to trigger the protonation of proximal O atom of *OOH. The seawater electrocatalysis system also allows for scalable synthesis of solid H2O2, tandem oxidation reaction of biomass to organic acid and direct use of the produced H2O2 as a sterilizing agent. Once integrated with photovoltaics, the solar-powered electrolysis device can operate in real seawater. Our findings pave the way for sustainable conversion of seawater into value-added products.
Microbial fuel cells (MFCs) have been considered an eco-friendly avenue for electricity production and concurrent water purificationtechnology. In MFCs, electrodes as the building blocks of MFCs play a significant role in determining the performances, thus an optimized selection of the suitable electrodes becomes crucial. Over the last decade, two-dimensional (2D) materials have emerged as potential electrodes to substitute precious metals such as platinum owing to their exceptional geometric structure, surface area, electrical conductivity, electronic properties. In this review, the potential benefits of 2D materials as cathodes to facilitate the reduction reaction in MFCs is discussed starting with biological redox reaction, physical chemistry of diverse 2D materials, and the respective MFCs performances. We cover from conventional graphene and layered double hydroxides (LDHs) as prototypical active material to emerging compounds such as, metal organic frameworks (MOFs), transition metal dichaldogenides (TMDs), and transition metal carbides (MXenes). The power density of 2D-based cathodes yields in the range of 50-2000 mW/m 2 , which stimulates further development. Additionally, some recent advances in the oxygen reduction reaction mechanism of 2D materials are also summarized. This insight may provide a deeper understanding for further designing highly catalytic materials and consequently improving the performances of MFCs. Finally, the future outlook and research direction are highlighted in further exploiting the uniqueness of 2D materials as cathodes for enhanced performances and reliability of MFCs to accelerate market adoption, paving the way for sustainable and renewable energy production.
Biowaste-derived heteroatom-doped porous carbons have garnered substantial attention as nonmetal catalysts for electrocatalysis. However, various heteroatom species and/or diverse coordination states within the carbon framework complicate the understanding of their...
A still unexplored class of heterostructured bifunctional catalysts for oxygen evolution (OER) and reduction (ORR) reactions is investigated: Ni−Fe sulfides deposited onto a N,S‐co‐doped zeolite‐templated carbon (ZTC) substrate achieved upon the impregnation of a ZTC with thiourea and Ni and Fe precursors (Ni/Fe atomic ratio of 1). By heat‐treating the impregnated ZTC substrate at 700 °C under N2 atmosphere the efficient bifunctional catalyst is generated. A difference of only 0.76 V is measured between the potential required to drive an OER current density of 10 mA cm⁻² and the ORR half‐wave potential. Using electrochemical measurements and physico‐chemical characterizations, it is evidenced that OER activity results from the presence of a sulfide containing both Fe and Ni whereas the ORR activity originates from the N,S‐doped ZTC substrate. The selectivity of the ORR process is improved through the presence of sulfide phases. Compared to more conventional carbon substrate such as N,S‐co‐doped reduced graphene oxide, high specific surface area ZTC favors high ORR performances. The functional ZTC material was further successfully implemented as bifunctional air electrode in a coin‐type Zn‐air battery.
A topotactic molecular doping strategy is developed via in-situ coupling the dechlorinated carbon with ringed heterocyclic molecules at mild conditions. Specifically, pyridine and pyrrole-converted carbon obtain pyridinic N to pyrrolic...
Herein, we have utilized agri-waste and amalgamating low Fe³⁺, to develop an economic iron oxide–carbon hybrid-based electrocatalyst for oxygen reduction reaction (ORR) with water as a main product following close to 4e⁻ transfer process. The electrocatalytic activity is justified by electrochemical active surface area, synergetic effect, and density functional theory calculations.
Iron phthalocyanine (FePc) has been integrated on boron-doped reduced graphene oxide (B-RGO) resulting in the composite, FePc@B-RGO. Boron alters the electronic structure around FePc and shows a higher selectivity than that of the benchmark catalyst, Pt/C, for four-electron oxygen reduction. FePc@B-RGO exhibits high oxygen reduction reaction activity with a high onset potential and half-wave potential (0.95 and 0.85 V vs. RHE respectively). FePc@B-RGO also shows a low Tafel slope of 39 mV dec⁻¹ and high efficiency, stability, and methanol crossover for oxygen reduction in basic media.
The lack of selectivity toward the oxygen reduction reaction (ORR) in metal nanoparticles can be linked to the generation of intermediates. This constitutes a crucial constraint on the performance of specific electrochemical devices, such as fuel cells and metal–air batteries. To boost selectivity of metal nanoparticles, a novel methodology that harnesses the unique electrocatalytic properties of polyoxometalates (POM) to scavenge undesired intermediates of the ORR (such as HO2⁻) promoting selectivity is proposed. It involves the covalent functionalization of metal nanoparticle's surface with an electrochemically active capping layer containing a new sulfur‐functionalized vanadium‐based POM (AuNP@POM). To demonstrate this approach, preformed thiolate Au(111) nanoparticles with a relatively poor ORR selectivity are chosen. The dispersion of AuNP@POM on the surface of carbon nanofibers (CNF) enhances oxygen diffusion, and therefore the ORR activity. The resulting electrocatalyst (AuNP@POM/CNF) exhibits superior stability against impurities like methanol and a higher pH tolerance range compared to the standard commercial Pt/C. The work demonstrates for the first time, the use of a POM‐based electrochemically active capping layer to switch on the selectivity of poorly selective gold nanoparticles, offering a promising avenue for the preparation of electrocatalyst materials with improved selectivity, performance, and stability for ORR‐based devices.
N‐doped carbon‐based materials have been regarded as promising alternatives to Pt‐based electrocatalysts for the four‐electron (4e‐) oxygen reduction reaction (ORR), which is an important electrochemical reaction for the polymer electrolyte fuel cells. Here, we report a N‐doped graphene and N‐doped carbon nanoparticles integrated composite electrocatalyst by a multi‐step acid etching plus annealing method. Despite the low N‐doping level, the material exhibits efficient 4e‐ ORR activity with an onset potential of 0.932 V, a half‐wave potential of 0.814 V, and a limiting current density of 5.3 mA cm‐2 in 0.1 M KOH solution. We demonstrate that the promoted 4e‐ ORR activity is attributed to the special 2D‐0D integrated structure for exposing massive active sites, the favorable porous structure facilitating the H2O transfer dynamics, and the high content of oxygen‐containing C‐O‐C species and the increased intrinsic carbon defects for additional active sites. A “decomposition and recrystallization” mechanism is proposed for the formation of N‐doped graphene.
The advancement of economical and readily available electrocatalysts for the oxygen reduction reaction (ORR) holds paramount importance in the advancement of fuel cells and metal‐air batteries. Recently, 2D non‐metallic materials have obtained substantial attention as viable alternatives for ORR catalysts due to their manifold advantages, encompassing low cost, ample availability, substantial surface‐to‐volume ratio, high conductivity, exceptional durability, and competitive activity. The augmented ORR performances observed in metal‐free 2D materials typically arise from heteroatom doping, defects, or the formation of heterostructures. Here, the authors delve into the realm of electrocatalysts for the ORR, pivoting around metal‐free 2D materials. Initially, the merits of metal‐free 2D materials are explored and the reaction mechanism of the ORR is dissected. Subsequently, a comprehensive survey of diverse metal‐free 2D materials is presented, tracing their evolutionary journey from fundamental concepts to pragmatic applications in the context of ORR. Substantial importance is given on the exploration of various strategies for enhancing metal‐free 2D materials and assessing their impact on inherent material performance, including electronic properties. Finally, the challenges and future prospects that lie ahead for metal‐free 2D materials are underscored, as they aspire to serve as efficient ORR electrocatalysts.
The process of oxygen evolution reaction (OER) is crucial for energy storage and conversion, and the spin electronic structure of catalyst significantly influences its catalytic activity. Precisely regulating the spin electronic structures of metal active centers with intermediate spin (IS) states is challenging but important. This study presents a general method for achieving spin‐state precise modulation by altering the secondary coordination sphere (SCS) in Fe‐substituted LaCo1‐xFexO3 perovskites, denoted as Co6‐y−[Co]−Fey (y = 0–6). The concentration‐dependent SCSs can precisely regulate the spin state of Co³⁺ from high‐spin (HS) to IS and low‐spin (LS) state by tuning the Co─O binding energy of primary coordination sphere (PCS) to ≈567 KJ mol⁻¹. The binding energy demonstrates a strong negative correlation with the spin state of Co³⁺, serving as a quantitative descriptor for precise spin‐state modulation. Furthermore, a universal optimal doping concentration is proposed for generating IS‐state Co³⁺ with the best OER activity, ranging from 1/(m+1) to 2/(m+1) in M‐doped ACo1‐xMxOy system with the coordination number of m. As a proof‐of‐concept, the LaCo7/9Fe2/9O3 with IS Co³⁺ exhibits significantly enhanced OER activity, almost six times higher than the control samples (without IS Co³⁺). These findings provide new insights into spin‐state modulation for effective OER catalysts.
The use of platinum-free (Pt) cathode electrocatalysts for oxygen reduction reactions (ORRs) has been significantly studied over the past decade, improving slow reaction mechanisms. For many significant energy conversion and storage technologies, including fuel cells and metal–air batteries, the ORR is a crucial process. These have motivated the development of highly active and long-lasting platinum-free electrocatalysts, which cost less than proton exchange membrane fuel cells (PEMFCs). Researchers have identified a novel, non-precious carbon-based electrocatalyst material as the most effective substitute for platinum (Pt) electrocatalysts. Rich sources, outstanding electrical conductivity, adaptable molecular structures, and environmental compatibility are just a few of its benefits. Additionally, the increased surface area and the simplicity of regulating its structure can significantly improve the electrocatalyst’s reactive sites and mass transport. Other benefits include the use of heteroatoms and single or multiple metal atoms, which are capable of acting as extremely effective ORR electrocatalysts. The rapid innovations in non-precious carbon-based nanomaterials in the ORR electrocatalyst field are the main topics of this review. As a result, this review provides an overview of the basic ORR reaction and the mechanism of the active sites in non-precious carbon-based electrocatalysts. Further analysis of the development, performance, and evaluation of these systems is provided in more detail. Furthermore, the significance of doping is highlighted and discussed, which shows how researchers can enhance the properties of electrocatalysts. Finally, this review discusses the existing challenges and expectations for the development of highly efficient and inexpensive electrocatalysts that are linked to crucial technologies in this expanding field.
An original strategy was reported to manipulate the ORR selectivity of N-doped hollow mesoporous carbon spheres (N-HMCS) via heteroatom (P and S) doping. Heteroatom doping cannot only modulate electronic structure of N-HMCS and proportion/configuration of doped N, but also regulate ORR pathway. Specifically, rich-graphitic N sites and electron-rich C–P bond domains in N,P-HMCS act as electron donors, making *OOH more unstable and easier to desorb into H2O2. This severely distorts the sp2 lattice of graphene, catalyzing two-electron ORR synergistically. Conversely, a strong interaction between electron-deficient C–S bond domain and *OOH intermediate due to the introduction of highly electronegative S hinders the desorption of *OOH, leading to breakage of the O–O bond and promoting four-electron ORR along with the rich-pyridinic N receptor site. The reaction activity and kinetics mechanism present a positive correlation with the proportion of graphitic N, revealing graphitic N is the primary active site with a much faster electron transfer capacity.
This work reports an atomic-scale carbon layer configuration tuning strategy induced by a boron dopant. Through regulating the doping level of boron, it was found that the boron dopant not only favors carbon layer growth by strengthening the metallic state of the Ni core, but also enhances the abundance of pyrrolic N species and graphitization degree of carbon by tailoring the carbon/nitrogen atom configuration, thereby contributing to more active pyrrolic N/carbon sites and accelerated interface reaction dynamics. Consequently, the developed Ni@B,N-C catalyst achieves remarkable electrochemical H2O2 production performances with a high selectivity of 95.5% and a yield of 795 mmol g-1 h-1. In comparison with previous reports in which the boron dopant mainly acts as an electronic structure regulator, this study reveals the tuning effect of boron dopants on the atomic-scale carbon layer configuration, opening up a new avenue for the development of advanced catalysts.
Exploitation of the efficient and cost-effective electrode materials is urgently desirable for the development of advanced energy devices. With the unique features of good electronic conductivity, structure flexibility, and desirable physicochemical property, carbon-based nanomaterials have attracted enormous research attention as efficient electrode materials. Electronic and microstructure engineering of carbon-based nanomaterials are the keys to regulate the electrocatalytic properties for the specific redox reactions of advanced metal-based batteries. However, the critical roles of carbon-based electrocatalysts for rechargeable metal batteries have not been comprehensively discussed. With the basic introduction on the electronic and microstructure engineering strategies, we summarize the recent advances on the rational design of carbon-based electrocatalysts for the important redox reactions in various metal-air batteries and metal-halogen batteries. The relationships between the composition, structure, and the electrocatalytic properties of carbon-based materials were well-addressed to enhance the battery performance. The overview of present challenges and opportunities of the carbon-based active materials for future energy-related applications was also discussed.
Constructing nonprecious-metal catalysts for oxygen reduction/evolution reactions (ORR/OER) in Zinc-air battery (ZAB) by structural regulation is crucial, but balance between stable structure and efficient mass transfer is still ambiguous. Here, hollow bimetallic sulfide nanocages with anchored N-doped carbon-quantum-dots are synthesized using a selective-etching method (Ni-Fe-S/NCQDs). The marked Ni-Fe-S/3NCQDs exhibits a promising half-wave potential of 0.85 V (E1/2, ORR) and an excellent overpotential of 0.295 V at 10 mA cm⁻² (OER). Ni-Fe-S/3NCQDs has a negative E1/2 shift of only 12.8 mV after 5000 cycles (ORR) and a current-density decline of only 7.05% after 20 h tests (OER). Ni-Fe-S/3NCQDs with porous-hollow structure (478.35 m² g⁻¹) facilitates mass transfer and exposure of active-sites. Ni/Fe oxyhydroxides (in-situ X-ray diffraction) contributes to excellent OER activity/stability. ZAB with Ni-Fe-S/3NCQDs can be repeatedly charged and discharged for 240 h at 10 mA cm⁻². It provides a new strategy for constructing open-hollow structure to improve ORR/OER performances.
A novel two-dimensional (2D) single-atom Fe/N/C catalyst (Fe1Zn30NCPDI) has been prepared upon pyrolyzing a well-designed quasi-Fe-/Zn-phthalocyanine-based polymer (qFePc1@qZnPc30@PDI). In the qFePc1@qZnPc30@PDI precursor, qFePc moieties containing Fe-N4 sites are efficiently segregated by lots of qZnPc moieties connected together by perylene linkers, which not only leads to the larger specific area in the resulting Fe1Zn30NCPDI but more importantly ensures the in-situ formation of Fe-N4 sites on the self-supporting 2D N-doped carbon backbone. Impressively, Fe1Zn30NCPDI exhibites its superior oxygen reduction reaction (ORR) electro-catalytic activity and peroxidase (POD)-like activity. For ORR in an alkaline medium, it exhibites a much higher onset potential of 0.97 V and half-wave potential of 0.89 V than 20 wt% Pt/C catalyst with good durability of 92% retention rate after 20000 s. And, Fe1Zn30NCPDI also shows better peak power density (86 mW cm⁻²) compared with 20 wt% Pt/C in Zn-air battery. Benefiting from the specific POD-like activity of Fe-N4, the Fe1Zn30NCPDI also shows excellent and selective performance in colorimetric H2O2 sensing, with the ultrafast response (30 s), a low limit of detection (10 nM), and good robustness. This work sheds light on a promising design concept for synthesizing 2D polymer-derived single-atom multifunctional catalysts.
Nanocarbon materials are considered to be active for electrochemical oxygen reduction reaction (ORR) for hydrogen peroxide (H2O2) synthesis. In the present work, a new type of fullerene 60 (C60)-carbon nanotubes (CNTs) hybrid with covalently attached C60 onto outer surface of CNTs was synthesized. The structure of C60-CNT hybrid was confirmed by physical and chemical characterizations and its conformation is proposed featuring the covalent incorporation of CNTs and C60 derivative. C60-CNT hybrid showed high efficiencies on electro-generating H2O2, owing to huge surface area and intermolecular electron-transfer in the hybrid structure. A high H2O2 production rate of 4834.57 mg L−1 h−1 (426.58 mmol L−1) was achieved at − 0.2 V vs saturated calomel electrode (SCE).
H₂O₂ is important in large-scale industrial processes and smaller on-site activities. The present industrial route to H₂O₂ involves hydrogenation of an anthraquinone and O₂ oxidation of the resulting dihydroanthraquinone — a costly method and one that is impractical
for routine on-site use. Electrosynthesis of H₂O₂ is cost-effective and applicable on both large and small scales. This Review describes methods to design and assess electrode materials for H₂O₂ electrosynthesis. H₂O₂ can be prepared by oxidizing H₂O at efficient anodic catalysts such as those based on BiVO₄. Alternatively, H₂O₂ forms by partially reducing O₂ at cathodes featuring either noble metal alloys or doped carbon. In addition to the catalyst materials used, one must also consider the form and geometry of the electrodes and the type of reactor in order to strike a balance between properties such as mass transport and electroactive area, both of which substantially affect both the selectivity and rate of reaction. Research into catalyst materials and reactor designs is arguably quite mature, such that the future of H₂O₂ electrosynthesis will instead depend on the design of complete and efficient electrosynthesis systems, in which the complementary properties of the catalysts and the reactor lead to optimal selectivity and overall yield.
Owing to the difficulty in controlling the dopant or defect types and their homogeneity in carbon materials, it is still a controversial issue to identify the active sites of carbon-based metal-free catalysts. Here we report a proof-of-concept study on the active-site evaluation for a highly oriented pyrolytic graphite catalyst with specific pentagon carbon defective patterns (D-HOPG). It is demonstrated that specific carbon defect types (an edged pentagon in this work) could be selectively created via controllable nitrogen doping. Work-function analyses coupled with macro and micro-electrochemical performance measurements suggest that the pentagon defects in D-HOPG served as major active sites for the acidic oxygen reduction reaction, even much superior to the pyridinic nitrogen sites in nitrogen-doped highly oriented pyrolytic graphite. This work enables us to elucidate the relative importance of the specific carbon defects versus nitrogen-dopant species and their respective contributions to the observed overall acidic oxygen reduction reaction activity. © 2019, The Author(s), under exclusive licence to Springer Nature Limited.
Replacing precious platinum with earth‐abundant materials for the oxygen reduction reaction (ORR) in fuel cells has been the objective worldwide for several decades. In the last 10 years, the fastest‐growing branch in this area has been carbon‐based metal‐free ORR electrocatalysts. Great progress has been made in promoting the performance and understanding the underlying fundamentals. Here, a comprehensive review of this field is presented by emphasizing the emerging issues including the predictive design and controllable construction of porous structures and doping configurations, mechanistic understanding from the model catalysts, integrated experimental and theoretical studies, and performance evaluation in full cells. Centering on these topics, the most up‐to‐date results are presented, along with remarks and perspectives for the future development of carbon‐based metal‐free ORR electrocatalysts.
A highly efficient, metal‐free carbon nanocatalyst is presented that possesses abundant active, oxygenated graphitic edge sites. The edge site‐rich nanocarbon catalyst exhibits about 28 times higher activity for H2O2 production than a basal plane‐rich carbon nanotube with a H2O2 selectivity over 90 %. The oxidative treatment further promotes the H2O2 generation activity to reach close to the thermodynamic limit. The optimized nanocarbon catalyst shows a very high H2O2 production activity, surpassing previously reported catalysts in alkaline media. Moreover, it can stably produce H2O2 for 16 h with Faradaic efficiency reaching 99 % and accumulated H2O2 concentration of 24±2 mm. Importantly, we find that the heterogeneous electron transfer kinetics of the carbon‐based catalyst is closely related to the electrocatalytic activity, suggesting that first outer‐sphere electron transfer to O2 is an important step governing the H2O2 production rate.
Electrocatalysis is dominated by reaction at the solid–liquid–gas interface; surface properties of electrocatalysts determine the electrochemical behavior. The surface charge of active sites on catalysts modulate adsorption and desorption of intermediates. However, there is no direct evidence to bridge surface charge and catalytic activity of active sites. Defects (active sites) were created on a HOPG (highly oriented pyrolytic graphite) surface that broke the intrinsic sp²‐hybridization of graphite by plasma, inducing localization of surface charge onto defective active sites, as shown by scanning ion conductance microscopy (SICM) and Kelvin probe force microscopy (KPFM). An electrochemical test revealed enhanced intrinsic activity by the localized surface charge. DFT calculations confirmed the relationship between surface charge and catalytic activity. This work correlates surface charge and catalytic activity, providing insights into electrocatalytic behavior and guiding the design of advanced electrocatalysts.
The oxygen reduction reaction (ORR) is a core reaction for electrochemical energy technologies such as fuel cells and metal–air batteries. ORR catalysts have been limited to platinum, which meets the requirements of high activity and durability. Over the last few decades, a variety of materials have been tested as non‐Pt catalysts, from metal–organic complex molecules to metal‐free catalysts. In particular, nitrogen‐doped graphitic carbon materials, including N‐doped graphene and N‐doped carbon nanotubes, have been extensively studied. However, due to the lack of understanding of the reaction mechanism and conflicting knowledge of the catalytic active sites, carbon‐based catalysts are still under the development stage of achieving a performance similar to Pt‐based catalysts. In addition to the catalytic viewpoint, designing mass transport pathways is required for O2. Recently, the importance of pyridinic N for the creation of active sites for ORR and the requirement of hydrophobicity near the active sites have been reported. Based on the increased knowledge in controlling ORR performances, bottom‐up preparation of N‐doped carbon catalysts, using N‐containing conjugative molecules as the assemblies of the catalysts, is promising. Here, the recent understanding of the active sites and the mechanism of ORRs on N‐doped carbon catalysts are reviewed.
Non-precious-metal or metal-free catalysts with stability are desirable but challenging for proton exchange membrane fuel cells. Here we partially unzip a multiwall carbon nanotube to synthesize zigzag-edged graphene nanoribbons with a carbon nanotube backbone for electrocatalysis of oxygen reduction in proton exchange membrane fuel cells. Zigzag carbon exhibits a peak areal power density of 0.161 W cm-2 and a peak mass power density of 520 W g-1, superior to most non-precious-metal electrocatalysts. Notably, the stability of zigzag carbon is improved in comparison with a representative iron-nitrogen-carbon catalyst in a fuel cell with hydrogen/oxygen gases at 0.5 V. Density functional theory calculation coupled with experimentation reveal that a zigzag carbon atom is the most active site for oxygen reduction among several types of carbon defects on graphene nanoribbons in acid electrolyte. This work demonstrates that zigzag carbon is a promising electrocatalyst for low-cost and durable proton exchange membrane fuel cells.
Doping with pyridinic nitrogen atoms is known as an effective strategy to improve the activity of carbon-based catalysts for the oxygen reduction reaction. However, pyridinic nitrogen atoms prefer to occupy at the edge or defect sites of carbon materials. Here, a carbon framework named as hydrogen-substituted graphdiyne provides a suitable carbon matrix for pyridinic nitrogen doping. In hydrogen-substituted graphdiyne, three of the carbon atoms in a benzene ring are bonded to hydrogen and serve as active sites, like the edge or defect positions of conventional carbon materials, on which pyridinic nitrogen can be selectively doped. The as-synthesized pyridinic nitrogen-doped hydrogen-substituted graphdiyne shows much better electrocatalytic performance for the oxygen reduction reaction than that of the commercial platinum-based catalyst in alkaline media and comparable activity in acidic media. Density functional theory calculations demonstrate that the pyridinic nitrogen-doped hydrogen-substituted graphdiyne is more effective than pyridinic nitrogen-doped graphene for oxygen reduction.
One of the recent trends in electrocatalytic reactions involves the oxygen reduction reaction (ORR), where a new paradigm has been shaped to exploit this reaction for the synthesis of hydrogen peroxide (H2O2). H2O2 is a very versatile chemical of high commercial value, prepared currently through poorly sustainable processes. The emergence of metal‐free carbon catalysts for the selective synthesis of H2O2 is expected to revolutionize ORR research, beckoning at the development of new industrial schemes. The complexities of the mechanism and the factors dominating the selectivity of the process have been unveiled through a combination of theoretical and experimental studies. The key aspects of the electrocatalytic synthesis of hydrogen peroxide from oxygen reduction by metal‐free catalysts, an emerging field of research, are presented. The majority of catalysts for this purpose feature heteroatom‐doped carbon materials, where the porosity and the distribution of heteroatoms are the main actors in driving the activity and, importantly, the selectivity of the oxygen reduction process.
The oxygen reduction reaction (ORR) is a fundamental reaction for energy storage and conversion. It has mainly relied on platinum-based electrocatalysts, but the chemical doping of carbon-based materials has proven to be a promising strategy for preparing metal-free alternatives. Nitrogen doping in particular provides a diverse range of nitrogen forms. Here, we introduce a new form of nitrogen doping moieties -sp-hybridized nitrogen (sp-N) atoms into chemically defined sites of ultrathin graphdiyne, through pericyclic replacement of the acetylene groups. The as-prepared sp-N-doped graphdiyne catalyst exhibits overall good ORR performance, in particular with regards to peak potential, half-wave potential and current density. Under alkaline conditions it was comparable to commercial Pt/C, and showed more rapid kinetics. And although its performances are a bit lower than those of Pt/C in acidic media they surpass those of other metal-free materials. Taken together, experimental data and density functional theory calculations suggest that the high catalytic activity originates from the sp-N dopant, which facilitates O2 adsorption and electron transfer on the surface of the catalyst. This incorporation of chemically defined sp-N atoms provides a new synthetic route to high-performance carbon-based and other metal-free catalysts.
Electrochemical oxygen reduction has garnered attention as an emerging alternative to the traditional anthraquinone oxidation process to enable the distributed production of hydrogen peroxide. Here, we demonstrate a selective and efficient non-precious electrocatalyst, prepared through an easily scalable mild thermal reduction of graphene oxide, to form hydrogen peroxide from oxygen. During oxygen reduction, certain variants of the mildly reduced graphene oxide electrocatalyst exhibit highly selective and stable peroxide formation activity at low overpotentials (<10 mV) under basic conditions, exceeding the performance of current state-of-the-art alkaline catalysts. Spectroscopic structural characterization and in situ Raman spectroelectrochemistry provide strong evidence that sp2-hybridized carbon near-ring ether defects along sheet edges are the most active sites for peroxide production, providing new insight into the electrocatalytic design of carbon-based materials for effective peroxide production. Electrochemical routes for the production of hydrogen peroxide would reduce the waste inherent in the current anthraquinone process, and also make distributed and on-site production more feasible. Here, inexpensive reduced graphene oxide is proven to be a stable and selective catalyst for oxygen reduction at remarkably low overpotentials.
This review focuses on the type A cytochrome c oxidases (CcO), which are found in all mitochondria and also in several aerobic bacteria. CcO catalyzes the respiratory reduction of dioxygen (O2) to water by an intriguing mechanism, the details of which are fairly well understood today as a result of research for over four decades. Perhaps even more intriguingly, the membrane-bound CcO couples the O2 reduction chemistry to translocation of protons across the membrane, thus contributing to generation of the electrochemical proton gradient that is used to drive the synthesis of ATP as catalyzed by the rotary ATP synthase in the same membrane. After reviewing the structure of the core subunits of CcO, the active site, and the transfer paths of electrons, protons, oxygen, and water, we describe the states of the catalytic cycle and point out the few remaining uncertainties. Finally, we discuss the mechanism of proton translocation and the controversies in that area that still prevail.
Hydrogen peroxide (H2O2) is a valuable chemical with a wide range of applications, but the current industrial synthesis of H2O2 involves an energy-intensive anthraquinone process. The electrochemical synthesis of H2O2 from oxygen reduction offers an alternative route for on-site applications; the efficiency of this process depends greatly on identifying cost-effective catalysts with high activity and selectivity. Here, we demonstrate a facile and general approach to catalyst development via the surface oxidation of abundant carbon materials to significantly enhance both the activity and selectivity (~90%) for H2O2 production by electrochemical oxygen reduction. We find that both the activity and selectivity are positively correlated with the oxygen content of the catalysts. The density functional theory calculations demonstrate that the carbon atoms adjacent to several oxygen functional groups (–COOH and C–O–C) are the active sites for oxygen reduction reaction via the two-electron pathway, which are further supported by a series of control experiments. The direct synthesis of hydrogen peroxide via oxygen reduction is an attractive alternative to the anthraquinone process. Here, a general trend linking oxygenation of carbon surfaces with electrocatalytic performance in peroxide synthesis is demonstrated, and computational studies provide further insight into the nature of the active sites.
Electrochemical oxygen reduction (ORR) is a challenging approach for the sustainable production of hydrogen peroxide (H2O2) and is also a reaction of relevance in fuel-cell applications. Here, we propose an outstanding metal-free electrocatalyst for the unexpectedly selective ORR to H2O2, consisting of graphitized N-doped single-wall carbon nanohorns (CNHs). The catalyst can operate at acidic pH to a faradic efficiency as high as 98%, but it also shows excellent performance at either physiological or alkaline pH. Moreover, the very positive onset potentials observed at all pH values investigated (+0.40 V, +0.53 V, and +0.71 V at pH 1.0, 7.4, and 13.0, respectively), good stability, and excellent reproducibility make this material a benchmark catalyst for ORR to H2O2. The outstanding activity arises from a combination of several factors, such as CNH-dependent facilitation of electron delivery, suitable porosity, and a favorable distribution of the types of N atoms.
Developing the low-cost, highly active carbonaceous materials for oxygen reduction reaction (ORR) catalysts has been a high-priority research direction for durable fuel cells. In this paper, two novel N-doped carbonaceous materials with flaky and rod-like morphology using the natural halloysite as template are obtained from urea nitrogen source as well as glucose (denoted as GU) and furfural (denoted as FU) carbon precursors, respectively, which can be directly applied as metal-free electrocatalysts for ORR in alkaline electrolyte. Importantly, compared with a benchmark Pt/C (20wt%) catalyst, the as-prepared carbon catalysts demonstrate higher retention in diffusion limiting current density (after 3000 cycles) and enhanced methanol tolerances with only 50-60mV negative shift in half-wave potentials. In addition, electrocatalytic activity, durability and methanol tolerant capability of the two N-doped carbon catalysts are systematically evaluated, and the underneath reasons of the outperformance of rod-like catalysts over the flaky are revealed. At last, the produced carbonaceous catalysts are also used as cathodes in the single cell H2/O2 anion exchange membrane fuel cell (AEMFC), in which the rod-like FU delivers a peak power density as high as 703 mW cm⁻² (vs. 1106 mW cm⁻² with a Pt/C benchmark cathode catalyst).
Maximum atom efficiency as well as distinct chemoselectivity is expected for electrocatalysis on atomically dispersed (or single site) metal centres, but its realization remains challenging so far, because carbon, as the most widely used electrocatalyst support, cannot effectively stabilize them. Here we report that a sulfur-doped zeolite-templated carbon, simultaneously exhibiting large sulfur content (17 wt% S), as well as a unique carbon structure (that is, highly curved three-dimensional networks of graphene nanoribbons), can stabilize a relatively high loading of platinum (5 wt%) in the form of highly dispersed species including site isolated atoms. In the oxygen reduction reaction, this catalyst does not follow a conventional four-electron pathway producing H2O, but selectively produces H2O2 even over extended times without significant degradation of the activity. Thus, this approach constitutes a potentially promising route for producing important fine chemical H2O2, and also offers opportunities for tuning the selectivity of other electrochemical reactions on various metal catalysts.
Replacing the rare and precious platinum (Pt) electrocatalysts with earth-abundant materials for promoting the oxygen reduction reaction (ORR) at the cathode of fuel cells is of great interest in developing high-performance sustainable energy devices. However, the challenging issues associated with non-Pt materials are still their low intrinsic catalytic activity, limited active sites, and the poor mass transport properties. Recent advances in material sciences and nanotechnology enable rational design of new earth-abundant materials with optimized composition and fine nanostructure, providing new opportunities for enhancing ORR performance at the molecular level. This Review highlights recent breakthroughs in engineering nanocatalysts based on the earth-abundant materials for boosting ORR.
The availability of low-cost, efficient, and durable catalysts for oxygen reduction reaction (ORR) is a prerequisite for commercialization of the fuel cell technology. Along with intensive research efforts of more than half a century in developing nonprecious metal catalysts (NPMCs) to replace the expensive and scarce platinum-based catalysts, a new class of carbon-based, low-cost, metal-free ORR catalysts was demonstrated to show superior ORR performance to commercial platinum catalysts, particularly in alkaline electrolytes. However, their large-scale practical application in more popular acidic polymer electrolyte membrane (PEM) fuel cells remained elusive because they are often found to be less effective in acidic electrolytes, and no attempt has been made for a single PEM cell test. We demonstrated that rationally designed, metal-free, nitrogen-doped carbon nanotubes and their graphene composites exhibited significantly better long-term operational stabilities and comparable gravimetric power densities with respect to the best NPMC in acidic PEM cells. This work represents a major breakthrough in removing the bottlenecks to translate low-cost, metal-free, carbon-based ORR catalysts to commercial reality, and opens avenues for clean energy generation from affordable and durable fuel cells.
The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are traditionally carried out with noble metals (such as Pt) and metal oxides (such as RuO2 and MnO2) as catalysts, respectively. However, these metal-based catalysts often suffer from multiple disadvantages, including high cost, low selectivity, poor stability and detrimental environmental effects. Here, we describe a mesoporous carbon foam co-doped with nitrogen and phosphorus that has a large surface area of ∼1,663 m(2) g(-1) and good electrocatalytic properties for both ORR and OER. This material was fabricated using a scalable, one-step process involving the pyrolysis of a polyaniline aerogel synthesized in the presence of phytic acid. We then tested the suitability of this N,P-doped carbon foam as an air electrode for primary and rechargeable Zn-air batteries. Primary batteries demonstrated an open-circuit potential of 1.48 V, a specific capacity of 735 mAh gZn(-1) (corresponding to an energy density of 835 Wh kgZn(-1)), a peak power density of 55 mW cm(-2), and stable operation for 240 h after mechanical recharging. Two-electrode rechargeable batteries could be cycled stably for 180 cycles at 2 mA cm(-2). We also examine the activity of our carbon foam for both OER and ORR independently, in a three-electrode configuration, and discuss ways in which the Zn-air battery can be further improved. Finally, our density functional theory calculations reveal that the N,P co-doping and graphene edge effects are essential for the bifunctional electrocatalytic activity of our material.
Although numerous reports on nonprecious metal catalysts for replacing expensive Pt-based catalysts have been published, few of these studies have demonstrated their practical application in fuel cells. In this work, we report graphitic carbon nitride and carbon nanofiber hybrid materials synthesized by a facile and gram-scale method via liquid-based reactions, without the use of toxic materials or a high pressure-high temperature reactor, for use as fuel cell cathodes. The resulting materials exhibited remarkable methanol tolerance, selectivity, and stability even without a metal dopant. Furthermore, these completely metal-free catalysts exhibited outstanding performance as cathode materials in an actual fuel cell device: a membrane electrode assembly with both acidic and alkaline polymer electrolytes. The fabrication method and remarkable performance of the single cell produced in this study represent progressive steps toward the realistic application of metal-free cathode electrocatalysts in fuel cells.
The basicity of the graphitic, pyridine, and pyrrole nitrogen groups on the graphene and single-walled carbon nanotubes is evaluated and compared in terms of both Brønsted base and Lewis base. It turns out that the pyridine group is the most strong basic site, while the graphitic nitrogen does not bring any improvements over the undoped one.
Oxygen reduction and evolution reactions constitute the core process of many vital energy storage or conversion techniques. However, the kinetic sluggishness of the oxygen redox reactions and heavy reliance on noble-metal-based electrocatalysts strongly limit the energy efficiency of the related devices. Developing high-performance noble-metal-free bifunctional ORR and OER electrocatalysts has gained worldwide attention, where much important progress has been made during the last decade. This review systematically addresses the design principles to obtain high-performance noble-metal-free bifunctional oxygen electrocatalysts by emphasizing strategies of both intrinsic activity regulation and active site integration. A statistical analysis of the reported bifunctional electrocatalysts is further carried out to reveal the composition-performance relationship and guide further exploration of emerging candidates. Finally, perspectives for developing advanced bifunctional oxygen electrocatalysts and aqueous rechargeable metal-air batteries are proposed.
Hydrogen peroxide is a widely used and important chemical in industry. Two-electron electrochemical oxygen reduction reaction (2e⁻ ORR) is a clean and on-site method for H2O2 production. Here, we report metal-free catalysts (mesoporous carbon hollow spheres, MCHS) for high efficiency H2O2 production in neutral electrolytes (0.1M PBS). The selectivity of H2O2 on MCHS catalysts is higher than 90% under a wide range of potential (0.35-0.62 V), and it can reach to 99.9% at a potential of 0.57 V. It shows one of the best performances for H2O2 production in neutral electrolytes. It is preferred to develop H2O2 catalysts in a neutral environment as the pH of the stabilizers used for H2O2 is also close to neutral. The outstanding activity of our catalyst arises comes from a combination of factors such as suitable porosity, the content of oxygen functional groups and the type of different species of oxygen functional group. First-principles simulations show that the catalyst with suitable mixed oxygen and COOH functional groups plays an important role in the catalytic formation of H2O2. The reported metal free catalysts are promising catalysts for high efficiency production of H2O2 in the future.
Selective two-electron (2 e⁻) pathway oxygen reduction reaction (ORR) has gained prominence for enabling small-scale, on-site electrochemical H2O2 production and has emerged as a promising alternative to the conventional anthraquinone process. The rational design of catalysts that can suppress the competing four-electron pathway ORR is critical. This review highlights catalyst design strategies for promoting the selective 2 e⁻ pathway ORR, including alloying with inert metals, partial surface poisoning, and generating atomically dispersed sites. The major results and advances, as well as unresolved challenges are summarized.
Atomically dispersed precious metal catalysts have emerged as a frontier in catalysis. However, a robust, generic synthetic strategy toward atomically dispersed catalysts is still lacking, which has limited systematic studies unveiling their general catalytic trends distinct from conventional nanoparticle (NP)-based catalysts. Herein, we report a general synthetic strategy to atomically dispersed precious metal catalysts, which consists of “trapping” precious metal precursors on a heteroatom-doped carbonaceous layer coated on a carbon support and “immobilizing” them with a SiO2 layer during thermal activation. Through “trapping-and-immobilizing” method, five atomically dispersed precious metal catalysts (Os, Ru, Rh, Ir, and Pt) could be obtained and served as model catalysts for unravelling catalytic trends for the oxygen reduction reaction (ORR). Owing to their isolated geometry, the atomically dispersed precious metal catalysts generally showed higher selectivity for H2O2 production than their NP counterparts for the ORR. Among the atomically dispersed catalysts, the H2O2 selectivity was changed by the types of metals, with atomically dispersed Pt catalyst showing the highest selectivity. Combining experimental results and density functional theory calculations, it was found that the selectivity trend of atomically dispersed catalysts could be correlated to the binding energy difference between *OOH and *O species. In terms of 2 e⁻ ORR activity, atomically dispersed Rh catalyst showed the best activity. Our general approach to atomically dispersed precious metal catalysts may help in understanding their unique catalytic behaviors for the ORR.
Selective two-electron oxygen reduction reaction (ORR) offers a promising route for hydrogen peroxide synthesis, and defective sp2 carbon-based materials are attractive, low-cost electrocatalysts for this process. However, due to a wide range of possible defect structures formed during material synthesis, identification and fabrication of precise active sites remain a challenge. Here, we report a graphene edge-based electrocatalyst for two-electron ORR – nanowire-templated three-dimensional fuzzy graphene (NT-3DFG). NT-3DFG exhibits notable efficiency (onset potential of 0.79 ± 0.01 V versus RHE), high selectivity (94 ± 2 % H2O2), and tunable ORR activity as a function of graphene edge site density. Using spectroscopic surface characterization and density functional theory calculations, we find that NT-3DFG edge sites are readily functionalized by carbonyl (C=O) and hydroxyl (C–OH) groups under alkaline ORR conditions. Our calculations indicate that multiple functionalized configurations at both armchair and zigzag edges may achieve a local coordination environment that allows selective, two-electron ORR. We derive a general geometric descriptor based on the local coordination environment that provides activity predictions of graphene surface sites within ~ 0.1 V of computed values. We combine synthesis, spectroscopy, and simulations to improve active site characterization and accelerate carbon-based electrocatalyst discovery.
The electrochemical direct synthesis of hydrogen peroxide (H2O2) is significant but still challenging because of the lack of highly selective and active catalysts. Here, we report the synthesis of hollow nanospheres constructed by atomically dispersing platinum in amorphous CuSx support (h-Pt1-CuSx) with a high concentration of single atomic Pt sites (24.8 at%), and this catalyst can consistently reduce O2 into H2O2 with selectivity of 92%–96% over a wide potential range of 0.05–0.7 V versus RHE in HClO4 electrolyte. Scanning transmission electron microscopy and X-ray absorption fine structure spectroscopy confirmed the atomically isolated form of Pt with a low valance of +0.75. An electrochemical device that can synthesize H2O2 directly from H2 and O2 is fabricated with H2O2 productivity as high as 546 ± 30 mol kgcat⁻¹ h⁻¹. The well-defined and high-concentration single atomic Pt sites result in ultrahigh productivity of H2O2.
Proton exchange membrane fuel cells (PEMFC) have attracted much attention because of their high energy conversion efficiency, high power density and zero emission of pollutants. However, the high cost of the cathode platinum group metal (PGM) catalysts creates a barrier for the large-scale application of PEMFC. Tremendous efforts have been devoted to the development of low-cost PGM-free catalysts, especially the Fe-N-C catalysts, to replace the expensive PGM catalysts. However, the characterization methods and evaluation standards of the catalysts varies, which is not conducive to the comparison of PGM-free catalysts. U.S. Department of energy (DOE) is the only authority that specifies the testing standards and activity targets for PGM-free catalysts. In this review, the major breakthroughs of Fe-N-C catalysts are outlined with the reference of DOE standards and targets. The preparation and characteristics of these highly active Fe-N-C catalysts are briefly introduced. Moreover, the efforts on improving the mass transfer and the durability issue of Fe-N-C fuel cell are discussed. Finally, the prospective directions concerning the comprehensive evaluation of the Fe-N-C catalysts are proposed. © 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Direct electrochemical production of hydrogen peroxide (H2O2) through two‐electron oxygen electrochemistry, for example, the oxygen reduction in fuel cells or water oxidation in water electrolyzers, could provide an attractive alternative to locally produce this chemical on demand. The efficiency of these processes depends greatly on the availability of cost‐effective catalysts with high selectivity, activity, and stability. In recent years, various novel nanostructured materials have been reported to selectively produce H2O2. Through combined experimental and theoretical approaches, underlying mechanisms in the electrochemical synthesis of H2O2 via oxygen electrochemistry have been unveiled. Considering the remarkable progress in this area, the authors summarize recent developments regarding the direct production of H2O2 through two‐electron electrochemical oxygen reactions. The fundamental aspects of electrochemical oxygen reactions are first introduced. Various types of catalysts that can effectively produce H2O2 via two‐electron oxygen electrochemistry are then presented. In parallel, the unique structure‐, component‐, and composition‐dependent electrochemical performance together with the underlying catalytic mechanisms are discussed. Finally, a brief conclusion about the recent progress achieved in electrochemical generation of H2O2 and an outlook on future research challenges are given.
H2O2 is a valuable, environmentally friendly oxidizing agent, with a wide range of uses, from the provision of clean water to the synthesis of valuable chemicals. The on-site electrolytic production of H2O2 would bring the chemical to applications beyond its present reach. The successful commercialization of electrochemical H2O2 production requires cathode catalysts with high activity, selectivity and stability. In this Perspective, we highlight our current understanding of the factors that control the cathode performance. We review the influence of catalyst material, electrolyte and the structure of the interface at the mesoscopic scale. We provide original theoretical data on the role of the geometry of the active site and its influence on activity and selectivity. We have also conducted a series of original experiments on (i) the effect of pH on H2O2 production on glassy carbon, pure metals, and metal-mercury alloys, and (ii) the influence of cell geometry and mass transport in liquid half-cells in comparison to membrane electrode assemblies.
Electrochemical hydrogen peroxide (H2O2) production by two-electron oxygen reduction is a promising alternative process to the established industrial anthraquinone process. Current challenges relate to finding cost-effective electrocatalysts with high electrocatalytic activity, stability and product selectivity. Here, we explore the catalytic H2O2 activity and selectivity of a number of distinct nitrogen-doped mesoporous carbon catalysts and report a previously unachieved H2O2 selectivity of ~95-98 % in acidic solution. To explain our observations, we correlate their structural, compositional and other physico-chemical properties with their electrocatalytic H2O2 performance and uncover a close correlation between the H2O2 product yield and the surface area and interfacial Zeta potential. Nitrogen doping was found to sharply boost H2O2 activity and selectivity. Chronoamperometric H2O2 electrolysis confirms the exceptionally high H2O2 production rate and large H2O2 faradaic selectivity for the optimal nitrogen-doped CMK-3 sample in acidic, neutral and alkaline solutions. In alkaline solution, the catalytic H2O2 yield increases further, where production rates of the HO2- anion reaches a value as high as 561.7 mmol h-1 g-1 catalyst with H2O2 faradaic selectivity above 70%. Our work provides new insight for the design, synthesis, and mechanistic investigation of advanced carbon-based electrocatalysts for H2O2 production
Despite the dedicated search for novel catalysts for fuel cell applications, the intrinsic oxygen reduction reaction (ORR) activity of materials has not improved significantly over the past decade. Here, we review the role of theory in understanding the ORR mechanism and highlight the descriptor-based approaches that have been used to identify catalysts with increased activity. Specifically, by showing that the performance of the commonly studied materials (e.g., metals, alloys, carbons, etc.) is limited by unfavorable scaling relationships (for binding energies of reaction intermediates), we present a number of alternative strategies that may lead to the design and discovery of more promising materials for ORR.
A comprehensive review of recent advances in the field of oxygen reduction electrocatalysis utilizing nonprecious metal (NPM) catalysts is presented. Progress in the synthesis and characterization of pyrolyzed catalysts, based primarily on the transition metals Fe and Co with sources of N and C, is summarized. Several synthetic strategies to improve the catalytic activity for the oxygen reduction reaction (ORR) are highlighted. Recent work to explain the active-site structures and the ORR mechanism on pyrolyzed NPM catalysts is discussed. Additionally, the recent application of Cu-based catalysts for the ORR is reviewed. Suggestions and direction for future research to develop and understand NPM catalysts with enhanced ORR activity are provided.
Electrochemical synthesis of hydrogen peroxide (H2O2) via two-electron pathway of oxygen reduction reaction is a promising alternative to the current anthraquinone process. The H2O2 production from O2 is a competing reaction with four-electron O2 reduction to H2O, and the selectivity is related to the adsorption energy of the OOH intermediate on electrocatalysts surface. Generally, the properties for binding of OOH intermediate on catalysts can be controlled by changing its electronic structure. Herein, the electronic structure of porous carbon materials was tuned by doping different types and contents of fluorine species. The yield of H2O2 generation depended on the F content and the best catalytic activity toward H2O2 electrosynthesis was obtained with F content of 3.41 at.%. The resultant F-doped porous carbon (FPC) catalysts exhibited good H2O2 selectivity of 97.5–83.0% and the H2O2 production rate could reach 112.6–792.6 mmol h⁻¹ g⁻¹ over the potential range of 0.2 V to −0.3 vs. RHE (pH 1). The density functional theory (DFT) calculations and experiments revealed that the incorporation of CF2, 3 into carbon plane promotes the activation of O2 molecule and facilitates desorption of OOH intermediate, which was crucial to H2O2 synthesis.
The development of small-scale, decentralized reactors for H2O2 production that can couple to renewable energy sources would be of great benefit, particularly for water purification in the developing world. Herein, we describe our efforts to develop electrochemical reactors for H2O2 generation with high Faradaic efficiencies of >90%, requiring cell voltages of only ∼1.6 V. The reactor employs a carbon-based catalyst that demonstrates excellent performance for H2O2 production under alkaline conditions, as demonstrated by fundamental studies involving rotating-ring disk electrode methods. The low-cost, membrane-free reactor design represents a step towards a continuous, modular-scale, de-centralized production of H2O2.
The development of high-performance and low-cost oxygen reduction and evolution catalysts that can be easily integrated into existing devices is crucial for the wide deployment of energy storage systems that utilize O2-H2O chemistries, such as regenerative fuel cells and metal-air batteries. Herein, we report an NH3-activated N-doped hierarchical carbon (NHC) catalyst synthesized via a scalable route, and demonstrate its device integration. The NHC catalyst exhibited good performance for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), as demonstrated by means of electrochemical studies and evaluation when integrated into the oxygen electrode of a regenerative fuel cell. The activities observed for both the ORR and the OER were comparable to those achieved by state-of-the-art Pt and Ir catalysts in alkaline environments. We have further identified the critical role of carbon defects as active sites for electrochemical activity through density functional theory calculations and high-resolution TEM visualization. This work highlights the potential of NHC to replace commercial precious metals in regenerative fuel cells and possibly metal-air batteries for cost-effective storage of intermittent renewable energy.
Understanding exact electronic configurations of carbon atoms bonded by nitrogen (N) functionalities at atomic-level may literally open a door to advance metal-free carbon materials as efficient catalysts for the oxygen reduction reaction (ORR). In this paper, a set of well-defined carbon nanotubes with controlled doping of various N species, such as pyrrolic, pyridinic and graphitic N, have been achieved by in-situ pyrolysis of polyaniline (PANI) nanotubes at different temperatures. Among these synthesized samples, the carbon nanotubes fabricated at 700 °C exhibit the highest electro-catalytic ORR activity, long-standing stability and good tolerance against methanol in alkaline medium. The improved activity is mainly attributed to the high nitrogen level of the active pyridinic and graphitic N. But, the pyridinic N possesses higher activity than the graphitic N because of their different sp² electronic structures. Pyridinic N, after bonding with two carbon(C) atoms, has two distorting N−C orbitals and one dangling orbital occupied by a lone electron pairs which are exposed, as the N sits at the edge of the carbon planes. Such unique electronic configuration makes the nitrogen and surrounding carbon atoms, bonded in the CN bonds, can serve host of active sites or work as active sites for the ORR.
Nitrogen-doped graphene is favored as a catalyst for oxygen reduction reaction (ORR) over rare metals. However, the effects of bonding state, nitrogen doped site and defects on catalytic conversion are still unclear. Here, we investigate oxygen reduction reaction using nitrogen-doped graphene with selective bonding state through pyridinic and graphitic nitrogen selective approaches. Both types show ORR activity and the catalytic reaction is clarified to be a four electron reaction path. Graphitic nitrogen with a low level of defects is found superior from the viewpoint of using single graphene sheet for the ORR application. Our investigation provides useful information for various applications using doped graphene.
To probe the active sites of nitrogen-doped carbon nanostructures (CNx), the effect of dihydrogen phosphate (H2PO4-) anion on their ORR performance was investigated by adding increasing concentrations of phosphoric acid in half-cell measurements. A linear decrease in specific kinetic current at 0.7 V was noted with increasing phosphate anion concentration. It was also found that the adsorption of phosphate species on CNx was strong and the corresponding ORR activity was not recovered when the catalyst was re-introduced to a fresh HClO4 solution. Trends similar to those noted upon addition of H3PO4 in half-cell were observed when CNx catalysts were soaked in phosphoric acid. Adsorption of dihydrogen phosphate ions on the surface of CNx exposed to phosphoric acid was verified by transmission infrared (IR) and Raman spectroscopy as well as X-ray photoelectron spectroscopy (XPS). XPS results also showed a decrease in the surface concentration of pyridinic-N species accompanied by an increase of equal magnitude in the surface fraction of quaternary-N species, which would include the pyridinic-NH sites. A linear correlation was observed between the loss in pyridinic-N site density and that in ORR activity. The observed poisoning phenomenon is consistent with the two possible active site models, i.e., pyridinic-N sites, which would be rendered inactive by protonation or the C sites neighboring pyridinic-N species. These latter species would be poisoned by a site blocking effect if they strongly adsorb the phosphate ions. Strong adsorption of negatively charged phosphate ions on neighboring C atoms would also stabilize the pyridinic-NH sites. By identifying a poison that can be used as a probe, this study provides a first step towards identification and quantification of active sites in CNx catalysts.
A bifunctional graphene catalyst with abundant topological defects is achieved via the carbonization of natural gelatinized sticky rice to probe the underlying oxygen electrocatalytic mechanism. A nitrogen-free configuration with adjacent pentagon and heptagon carbon rings is revealed to exhibit the lowest overpotential for both oxygen reduction and evolution catalysis. The versatile synthetic strategy and novel insights on the activity origin facilitate the development of advanced metal-free carbocatalysts for a wide range of electrocatalytic applications.
Fenton oxidation using an aqueous mixture of Fe(2+) and H2 O2 is a promising environmental remediation strategy. However, the difficulty of storage and shipment of concentrated H2 O2 and the generation of iron sludge limit its broad application. Therefore, highly efficient and cost-effective electrocatalysts are in great need. Herein, a graphene catalyst is proposed for the electro-Fenton process, in which H2 O2 is generated in situ by the two-electron reduction of the dissolved O2 on the cathode and then decomposes to generate (.) OH in acidic solution with Fe(2+) . The π bond of the oxygen is broken whereas the σ bond is generally preserved on the metal-free reduced graphene oxide owing to the high free energy change. Consequently, the oxygen is reduced to H2 O2 through a two-electron pathway. The thermally reduced graphene with a high specific surface area (308.8 m(2) g(-1) ) and a large oxygen content (10.3 at %) exhibits excellent reactivity for the two-electron oxygen reduction reaction to H2 O2 . A highly efficient peroxide yield (64.2 %) and a remarkable decolorization of methylene blue (12 mg L(-1) ) of over 97 % in 160 min are obtained. The degradation of methylene blue with hydroxyl radicals generated in situ is described by a pseudo first-order kinetics model. This provides a proof-of-concept of an environmentally friendly electro-Fenton process using graphene for the oxygen reduction reaction in an acidic solution to generate H2 O2 .
A novel carbon nanotube-graphene hybrid nanostructure is developed using an aerosol-assisted assembly approach. After doping with nitrogen and phosphorus, the prepared hybrid nanomaterials exhibit excellent electrocatalytic performance for oxygen reduction in both alkaline and acidic media. This research presents a continuous and low-cost route to prepare high-performance metal-free electrocatalysts while replacing Pt-based materials.
The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core-shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.
The right kind of dopant
The oxygen reduction reaction is an important step in fuel cells and other electrochemical processes but is still largely dependent on precious metal-containing catalysts. Recently explored alternatives include carbon materials that are doped with different, preferably non-precious metal, atoms. Guo et al. studied model graphite catalysts to try to understand the role of nitrogen doping and to elucidate the active catalytic sites. A nitrogen atom bound to two carbons formed an active catalyst site with an activity rivaling that of N-doped graphene catalysts.
Science , this issue p. 361
As a catalyst, single-atom platinum may provide an ideal structure for platinum minimization. Herein, a single-atom catalyst of platinum supported on titanium nitride nanoparticles were successfully prepared with the aid of chlorine ligands. Unlike platinum nanoparticles, the single-atom active sites predominantly produced hydrogen peroxide in the electrochemical oxygen reduction with the highest mass activity reported so far. The electrocatalytic oxidation of small organic molecules, such as formic acid and methanol, also exhibited unique selectivity on the single-atom platinum catalyst. A lack of platinum ensemble sites changed the reaction pathway for the oxygen-reduction reaction toward a two-electron pathway and formic acid oxidation toward direct dehydrogenation, and also induced no activity for the methanol oxidation. This work demonstrates that single-atom platinum can be an efficient electrocatalyst with high mass activity and unique selectivity.
To promote the oxygen reduction reaction of metal-free catalysts, the introduction of porous structure is considered as a desirable approach because the structure can enhance mass transport and host many catalytic active sites. However, most of the previous studies reported only half-cell characterization; therefore, studies on membrane electrode assembly (MEA) are still insufficient. Furthermore, the effect of doping-site position in the structure has not been investigated. Here, we report the synthesis of highly active metal-free catalysts in MEAs by controlling pore size and doping-site position. Both influence the accessibility of reactants to doping sites, which affects utilization of doping sites and mass-transport properties. Finally, an N,P-codoped ordered mesoporous carbon with a large pore size and precisely controlled doping-site position showed a remarkable on-set potential and produced 70 % of the maximum power density obtained using Pt/C.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
In this work, the effect of pH is extensively investigated on the nitrogen-doped ordered mesoporous carbon catalyst for the oxygen reduction reaction (ORR). Electrochemical methods, including the cyclic voltammetry (CV), rotating ring-disk electrode (RRDE) and cathodic stripping voltammetry, are applied to investigate the electrochemical behavior in the electrolyte solutions of different pHs (0-2, 7, 12-14). CV result reveals that nitrogen-doped carbon has a variety of enriched reversible redox couples on the surface and pH has a significant effect. Whether these redox couples are electrochemically active or inactive to the ORR depends on the electrolyte used. In acid media, oxygen molecule directly interacts with the redox couple, and its reduction proceeds by the surface-confined redox-mediation mechanism, yielding water as the product. Similarly, the first electron transfer in alkaline media is achieved by the surface-confined redox-mediation mechanism at the higher potentials. With decreasing the potential, another parallel charge transfer process by the outer-sphere electron transfer (OSET) mechanism gets pronounced, followed by parallel 2-e and 4-e reduction of oxygen. The proposed mechanisms are well supported by the following electrochemical results. At high potentials, the Tafel slope remains unchanged (60-70 mV dec-1) at all investigated pHs, and the reaction order of proton and hydroxyl ions is found to be 1 and -0.5, respectively, in acid and alkaline media. The electron transfer number is approximate to 4 at high potentials in both acid and alkaline media; however, at higher pHs, it shows a considerable decrease as the potential decreases, indicating the change in the reaction pathway. Finally, the nitrogen-doped carbon catalyst shows a superior performance in alkaline media than does in acid media. Such a gap in performance is rationalized by considering the chemical change in the surface at different pH values.
In this Review, we discuss the state-of-the-art understanding of non-precious transition metal oxides that catalyze the oxygen reduction and evolution reactions. Understanding and mastering the kinetics of oxygen electrocatalysis is instrumental to making use of photosynthesis, advancing solar fuels, fuel cells, electrolyzers, and metal-air batteries. We first present key insights, assumptions and limitations of well-known activity descriptors and reaction mechanisms in the past four decades. The turnover frequency of crystalline oxides as promising catalysts is also put into perspective with amorphous oxides and photosystem II. Particular attention is paid to electronic structure parameters that can potentially govern the adsorbate binding strength and thus provide simple rationales and design principles to predict new catalyst chemistries with enhanced activity. We share new perspective synthesizing mechanism and electronic descriptors developed from both molecular orbital and solid state band struct