Single‐Atom Electrocatalysts

Recent years have witnessed a dramatic increase in the production of sustainable and renewable energy. However, the electrochemical performances of the various systems are limited, and there is an intensive search for highly efficient electrocatalysts by more rational control over the size, shape, composition, and structure. Of particular interest are the studies on single‐atom catalysts (SACs), which have sparked new interests in electrocatalysis because of their high catalytic activity, stability, selectivity, and 100 % atom utilization. In this Review, we introduce innovative syntheses and characterization techniques for SACs, with a focus on their electrochemical applications in the oxygen reduction/evolution reaction, hydrogen evolution reaction, and hydrocarbon conversion reactions for fuel cells (electrooxidation of methanol, ethanol, and formic acid). The electrocatalytic performance is further considered at an atomic level and the underlying mechanisms are discussed. The ultimate goal is the tailoring of single atoms for electrochemical applications.     

A Layer-Structured Metal-Organic Framework-Derived Mesoporous Carbon for Efficient Oxygen Reduction Reaction

Developing highly active and durable electrocatalysts for the oxygen reduction reaction (ORR) is crucial to large-scale commercialization of fuel cells and metal-air batteries. Here we report a facile approach for the synthesis of nitrogen and oxygen dual-doped mesoporous layer-structured carbon electrocatalyst embedded with graphitic carbon coated cobalt nanoparticles by direct pyrolysis of a layer-structured metal-organic framework. The electrocatalyst prepared at 800℃ exhibits comparable ORR performance to Pt/C catalysts but possesses superior stability to Pt/C catalysts. This synthetic approach provides new prospects in developing sustainable carbon-based electrocatalysts for electrochemical energy conversion devices.     

石墨烯衍生物负载的纳米催化剂用于氧还原反应

综述了用于燃料电池中氧还原反应(ORR)的石墨烯衍生物负载的各种纳米催化剂的最新进展。介绍了用于表征石墨烯基电催化剂的常规电化学技术以及石墨烯基电催化剂最新的研究进展。负载于还原氧化石墨烯(RGO)上的Pt催化剂的电化学活性和稳定性均得到显著提高。其它贵金属催化剂,如Pd, Au和Ag也表现出较高的催化活性。当以RGO或少层石墨烯为载体时, Pd催化剂的稳定性提高。讨论了氧化石墨烯负载Au或Ag催化剂的合成方法。另外,以N4螯合络合物形式存在的非贵过渡金属可降低氧的电化学性能。 Fe和Co是可替代的廉价ORR催化剂。在大多数情况下,这些催化剂稳定性和耐受性的问题均可得到解决,但其整体性能还很难超越Pt/C催化剂。     

Three-dimensional nitrogen and phosphorous Co-doped graphene aerogel electrocatalysts for efficient oxygen reduction reaction

The development of efficient electrocatalysts for oxygen reduction reaction (ORR) is of importance for fuel cells and metal-air batteries. Herein, three-dimensional nitrogen and phosphorous co-doped graphene aerogel (NPGA) was prepared via the pyrolysis of polyaniline (PANi) coated graphene oxide aerogel synthesized by oxidative polymerization of aniline on graphene oxide (GO) sheets in the presence of phytic acid. The uniform coating of PANi thin layer on the surface of GO sheets enables the formation of highly porous composite aerogel of PANi and GO. The subsequent thermal treatment is able to prepare the porous NPGA due to the carbonization of PANi and phytic acid as nitrogen and phosphorous resources. When used as electrocatalysts, the as-prepared NPGA electrocatalysts exhibited good catalytic activity to ORR via an efficient four-electron pathway with good stability, benefiting from the highly porous structure and the heteroatom co-doping. More importantly, Zn-air batteries operated in ambient air have been fabricated by coupling a Zn plate with the NPGA electrocatalyst in an air electrode, demonstrating the maximal power density as high as ～260 W/g and a good long-term stability with slightly potential decay for over 450 h. The facile method for preparing efficient carbon based ORR electrocatalysts would generate other potential applications including fuel cells and others.     

Recent advances of layered double hydroxides–based bifunctional electrocatalysts for ORR and OER

The oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) have attracted increasing attention for the sake of clean, renewable, and efficient energy technologies in recent years. The design of ORR/OER bifunctional electrocatalysts is a challenging task in the promotion of highly efficient rechargeable metal-air batteries as well as regenerative fuel cells. Owing to the wide adaptability of different types and ratios of metals in the interlayer space as well as the adjustable interlayer distance, composite materials with layered double hydroxides (LDHs) and their derivatives have recently been registered as electrode materials and catalysts supports for various electrochemical reactions. This study examines the recent development of bifunctional electrocatalysts based on LDHs for ORR/OER to expand the application of LDHs in the field of energy storage and conversion. Various bifunctional electrocatalysts associated with LDHs are discussed in detail to improve their performance. Finally, existing problems and future prospects for improving the performance of LDHs bifunctional electrocatalysts are proposed.     

Functional Electrospun Nanocomposites for Efficient Oxygen Reduction Reaction

Electrospinning with a simple and controllable process has extremely received considerable concerns by virtue of the fabrication and development of nanofibers. Moreover, nanofibers are playing an increasing impact on energy conversion and storage devices, especially for fuel cells based on oxygen reduction reaction(ORR), in view of the rich porosity, large surface area, excellent mass transportation and simply tunable composition, as well as good mechanical strength. In this review, we mainly introduce the primary principle of electrospinning technique, electrochemical reaction mechanism of ORR and synthetic strategies, and summarize the recent advances of unique non-noble-metal nanofibers on the basis of metal-organic framework(MOF) derivatives, single-atom catalysts(SACs) and transition metal oxides. More importantly, we emphasize on the influences of the components, morphology and architecture of advanced electrospun catalysts on their correspon-ding electrochemical performances towards ORR. Finally, the remaining puzzles and perspectives for further development of the electrospinning nanofibers involving electrocatalysis are presented. It is envisioned that this review would offer an important direction in designing novel electrocatalysts based on electrospinning nanofibrous structures and developing their potential.     

单原子催化剂在氧还原反应中的分子级调控

氧还原反应(ORR)在电化学能量存储和转换系统以及精细化学制剂的清洁合成中发挥着重要作用. 然而, ORR过程的动力学极其缓慢, 需要使用铂族贵金属催化剂加快其反应动力学速率. 铂基催化剂的高成本严重阻碍了其大规模的商业化. 由于单原子催化剂(SACs)具有结构明确、 本征活性高和原子效率高的特点, 有望取代昂贵的铂族贵金属催化剂. 迄今, 在进一步提高SACs的ORR活性方面已有大量的研究报道, 包括定制金属中心的配位结构、 丰富金属中心的浓度以及设计衬底的电子结构和孔隙率等. 本文综合评述了近年来SACs在ORR性能以及与ORR相关的H₂O₂生产、 金属-空气电池和低温燃料电池等方面的应用研究进展. 总结了通过引入其它金属或配体来调整孤立金属中心的配位结构、 通过增加金属负载来增加单原子位点的浓度以及通过优化载体的孔隙度来优化催化性能和电子传输等方面的研究进展, 并对SCAs的未来发展方向和面临的挑战提出了展望.     

A Chemical Dealloying Approach for Pt Surface-enriched Pt3Co Alloy Nanoparticles as Oxygen Reduction Reaction Electrocatalysts

Pt-based alloys are the optimal electrocatalysts for oxygen reduction reaction(ORR) currently. Dealloying of Pt-based alloys has shown to be an effective approach to improving ORR activity. Electrochemical dealloying is controllable for morphology by changing electrochemical parameters but is difficult to scale up due to complex operation and energy consumption. Chemical dealloying is suitable for a large scale but it is not easy to control the morphology because highly corrosive acids(HNO₃ or H₂SO₄) are commonly used. In this work, a facile chemical dealloying method for Pt₃Co/C has been employed to synthesize elec-trocatalysts for ORR using weak acids and buffer solutions of different pH, which could slow down the dissolution rate for Co atoms and increase the diffusion time for Pt atoms to improve ORR activity. It can be observed that the mass activities(MA) of the Pt₃Co/C alloy after dealloying with H₃PO₄ and NaH₂PO₄/Na₂HPO₄ buffer solution of pH=6 are close to that after electrochemical dealloying process, and are more than two times that of commercial Pt/C. In addition, Pt₃Co/C after dealloying with a buffer solution of pH=6 only showed a slight degradation in the half-wave potential and electrochemical surface area(ECSA) after stability test for 5000 cycles, which is more stable than commercial Pt/C. It shows that by controlling pH of the solvent, the ORR activity can be further increased. This facile approach provides a new strategy to control morphology of Pt-based electrocatalysts by chemical dealloying, which can contribute to promising application for cathodic electrocatalysts design of proton exchange membrane fuel cells (PEMFCs).     

Stable Thiophene-sulfur Covalent Organic Frameworks for Oxygen Reduction Reaction(ORR)

Exploring novel materials deriving from earth resources to substitute for platinum(Pt) electrocatalyst to promote oxygen reduction reaction(ORR) of fuel cell cathode is very important. Herein, we have exploited two crystallographic thiophene-sulfur covalent organic frameworks(COFs), termed JUC-607 and JUC-608, as electrocatalysts that exhibited good ORR performances. These thiophene-sulfur COFs exhibited high stability, and their functional groups acting as active centers in the ORR can be precisely determined. Notably, due to a larger aperture for mass transfer and electrons transport, JUC-608 displayed a growing electrochemical performance, leading to a better ORR activity. Thus, this study will provide a new strategy for designing heteroatom-based COF materials for high-performance electrochemical catalysis.     

Low‐Pt Loaded on a Vanadium Nitride/Graphitic Carbon Composite as an Efficient Electrocatalyst for the Oxygen Reduction Reaction

The high cost of platinum electrocatalysts for the oxygen reduction reaction (ORR) has hindered the commercialization of fuel cells. An effective support can reduce the usage of Pt and improve the reactivity of Pt through synergistic effects. Herein, the vanadium nitride/graphitic carbon (VN/GC) nanocomposites, which act as an enhanced carrier of Pt nanoparticles (NPs) towards ORR, have been synthesized for the first time. In the synthesis, the VN/GC composite could be obtained by introducing VO₃^? and [Fe(CN)₆]^4? ions into the polyacrylic weak‐acid anion‐exchanged resin (PWAR) through an in‐situ anion‐exchanged route, followed by carbonization and a subsequent nitridation process. After loading only 10 % Pt NPs, the resulting Pt‐VN/GC catalyst demonstrates a more positive onset potential (1.01 V), higher mass activity (137.2 mA mg^?1), and better cyclic stability (99 % electrochemical active surface area (ECSA) retention after 2000 cycles) towards ORR than the commercial 20 % Pt/C. Importantly, the Pt‐VN/GC catalyst mainly exhibits a 4 e^?‐transfer mechanism and a low yield of peroxide species, suggesting its potential application as a low‐cost and highly efficient ORR catalyst in fuel cells.     

Catalyzing Sustainable Water Splitting with Single Atom Catalysts: Recent Advances

Electrochemical water splitting for sustainable hydrogen and oxygen production have shown enormous potentials. However, this method needs low-cost and highly active catalysts. Traditional nano catalysts, while effective, have limits since their active sites are mostly restricted to the surface and edges, leaving interior surfaces unexposed in redox reactions. Single atom catalysts (SACs), which take advantage of high atom utilization and quantum size effects, have recently become appealing electrocatalysts. Strong interaction between active sites and support in SACs have considerably improved the catalytic efficiency and long-term stability, outperforming their nano-counterparts. This review‘s first section examines the Hydrogen Evolution Reaction (HER) and the Oxygen Evolution Reaction (OER). In the next section, SACs are categorized as noble metal, non-noble metal, and bimetallic synergistic SACs. In addition, this review emphasizes developing methodologies for effective SAC design, such as mass loading optimization, electrical structure modulation, and the critical role of support materials. Finally, Carbon-based materials and metal oxides are being explored as possible supports for SACs. Importantly, for the first time, this review opens a discussion on waste-derived supports for single atom catalysts used in electrochemical reactions, providing a cost-effective dimension to this vibrant research field. The well-known design techniques discussed here may help in development of electrocatalysts for effective water splitting.     

Copper‐Modified Covalent Triazine Frameworks as Non‐Noble‐Metal Electrocatalysts for Oxygen Reduction

The electrochemical oxygen reduction reaction (ORR) is an important cathode reaction of various types of fuel cells. The development of electrocatalysts composed only of abundant elements is a key goal because currently only platinum is a suitable catalyst for ORR. Herein, we synthesized copper‐modified covalent triazine frameworks (CTF) hybridized with carbon nanoparticles (Cu‐CTF/CPs) as efficient electrocatalysts for the ORR in neutral solutions. The ORR onset potential of the synthesized Cu‐CTF/CP was 810 mV versus the reversible hydrogen electrode (RHE; pH 7), the highest reported value at neutral pH for synthetic Cu‐based electrocatalysts. Cu‐CTF/CP also displayed higher stability than a Cu‐based molecular complex at neutral pH during the ORR, a property that was likely as a result of the covalently cross‐linked structure of CTF. This work may provide a new platform for the synthesis of durable non‐noble‐metal electrocatalysts for various target reactions.     

Recent Progress in Precious Metal-Free Carbon-Based Materials towards the Oxygen Reduction Reaction: Activity,Stability, and Anti-Poisoning

The large-scale application of polymer electrolyte membrane fuel cells (PEMFCs) depends heavily on the development of high-performance and cost-effective oxygen reduction reaction (ORR) electrocatalysts to replace the traditional Pt-based materials. Among the numerous candidates of ORR electrocatalysts, precious metal-free carbon-based materials have received ever-increasing attention for their unique electronic features, tunable nanostructures, and robustness. Although tremendous efforts have been devoted to raising the intrinsic properties of precious metal-free carbon-based materials, there exists a lot of room for them to be improved in activity, stability, and poison-tolerance. In this Minireview, the latest strategies for enhancing activity by increasing the accessible active sites and promoting the intrinsic activity have been summarized. In addition, with special emphasis on devastating catalyst poisoning, up-to-data tactics for elevating the stability and poison-tolerance are introduced. Finally, conclusions and perspectives are also presented.     

A comparison study on single metal atoms (Fe,Co, Ni) within nitrogen-doped graphene for oxygen electrocatalysis and rechargeable Zn-air batteries

Single atom catalysts (SACs) with atomically dispersed transition metals on nitrogen-doped carbon supports have recently emerged as highly active non-noble metal electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), showing great application potential in Zn-air batteries. However, because of the complex structure-performance relationships of carbon-based SACs in the oxygen electrocatalytic reactions, the contribution of different metal atoms to the catalytic activity of SACs in Zn-air batteries still remains ambiguous. In this study, SACs with atomically dispersed transition metals on nitrogen-doped graphene sheets (M-N@Gs, M = Co, Fe and Ni), featured with similar physicochemical properties and M-N@C configurations, are obtained. By comparing the on-set potentials and the maximum current, we observed that the ORR activity is in the order of Co-N@G > Fe-N@G > Ni-N@G, while the OER activity is in the order of Co-N@G > Ni-N@G > Fe-N@G. The Zn-air batteries with Co-N@G as the air cathode catalysts outperform those with the Fe-N@G and Ni-N@G. This is due to the accelerated charge transfer between Co-N@C active sites and the oxygen-containing reactants. This study could improve our understanding of the design of more efficient bifunctional electrocatalysts for Zn-air batteries at the atomic level.     

Bioinspired Transition-Metal Complexes as Electrocatalysts for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is one of the most important reactions in life processes and energy conversion systems. To alleviate global warming and the energy crisis, the development of high-performance electrocatalysts for the ORR for application in energy conversion and storage devices such as metal–air batteries and fuel cells is highly desirable. Inspired by the biological oxygen activation/reduction process associated with heme- and multicopper-containing metalloenzymes, iron and copper-based transition-metal complexes have been extensively explored as ORR electrocatalysts. Herein, an outline into recent progress on non-precious-metal electrocatalysts for the ORR is provided; these electrocatalysts do not require pyrolysis treatment, which is regarded as desirable from the viewpoint of bioinspired molecular catalyst design, focusing on iron/cobalt macrocycles (porphyrins, phthalocyanines, and corroles) and copper complexes in which the ORR activity is tuned by ligand variation/substitution, the method of catalyst immobilization, and the underlying supporting materials. Current challenges and exciting imminent developments in bioinspired ORR electrocatalysts are summarized and proposed.     

Facile synthesis of highly active and stable Pt-Ir/C electrocatalysts for oxygen reduction and liquid fuel oxidation reaction

A facile room temperature synthesis technique has been developed for Pt-Ir/C electrocatalysts for applications to low-temperature fuel cells. The prepared Pt(x)Ir(y) electrocatalyst was highly stable and active toward the oxygen reduction reaction (ORR), as well as liquid fuel oxidation reaction with high CO tolerance.     

Electrochemical Preparation of N‐Doped Cobalt Oxide Nanoparticles with High Electrocatalytic Activity for the Oxygen‐Reduction Reaction

Nitrogen‐doped CoO (N‐CoO) nanoparticles with high electrocatalytic activity for the oxygen‐reduction reaction (ORR) were fabricated by electrochemical reduction of CoCl₂ in acetonitrile solution at cathodic potentials. The initially generated, highly reactive nitrogen‐doped Co nanoparticles were readily oxidized to N‐CoO nanoparticles in air. In contrast to their N‐free counterparts (CoO or Co₃O₄), N‐CoO nanoparticles with a N content of about 4.6 % exhibit remarkable ORR electrocatalytic activity, stability, and immunity to methanol crossover in an alkaline medium. The Co?N_x active sites in the CoO nanoparticles are held responsible for the high ORR activity. This work opens a new path for the preparation of nitrogen‐doped transition metal oxide nanomaterials, which are promising electrocatalysts for fuel cells.     

Carbon-Based Electrocatalysts for Acidic Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) is vital for clean and renewable energy technologies, which require no fossil fuel but catalysts. Platinum (Pt) is the best-known catalyst for ORR. However, its high cost and scarcity have severely hindered renewable energy devices (e.g., fuel cells) for large-scale applications. Recent breakthroughs in carbon-based metal-free electrochemical catalysts (C-MFECs) show great potential for earth-abundant carbon materials as low-cost metal-free electrocatalysts towards ORR in acidic media. This article provides a focused, but critical review on C-MFECs for ORR in acidic media with an emphasis on advances in the structure design and synthesis, fundamental understanding of the structure-property relationship and electrocatalytic mechanisms, and their applications in proton exchange membrane fuel cells. Current challenges and future perspectives in this emerging field are also discussed.     

Transition metal coordinated framework porphyrin for electrocatalytic oxygen reduction

A series of transition metal coordinated framework porphyrin was evaluated regarding the electrocatalytic oxygen reduction reactivity for an optimized selection of the coordinated metal ion.