Noble‐Metal‐Free Electrocatalysts for Oxygen Evolution

Oxygen evolution reaction (OER) plays a vital role in many energy conversion and storage processes including electrochemical water splitting for the production of hydrogen and carbon dioxide reduction to value‐added chemicals. IrO₂ and RuO₂, known as the state‐of‐the‐art OER electrocatalysts, are severely limited by the high cost and low earth abundance of these noble metals. Developing noble‐metal‐free OER electrocatalysts with high performance has been in great demand. In this review, recent advances in the design and synthesis of noble‐metal‐free OER electrocatalysts including Ni, Co, Fe, Mn‐based hydroxides/oxyhydroxides, oxides, chalcogenides, nitrides, phosphides, and metal‐free compounds in alkaline, neutral as well as acidic electrolytes are summarized. Perspectives are also provided on the fabrication, evaluation of OER electrocatalysts and correlations between the structures of the electrocatalysts and their OER activities.     

Noble Metal‐Free Nanocatalysts with Vacancies for Electrochemical Water Splitting

The fast development of nanoscience and nanotechnology has significantly advanced the fabrication of nanocatalysts and the in‐depth study of the structural‐activity characteristics of materials at the atomic level. Vacancies, as typical atomic defects or imperfections that widely exist in solid materials, are demonstrated to effectively modulate the physicochemical, electronic, and catalytic properties of nanomaterials, which is a key concept and hot research topic in nanochemistry and nanocatalysis. The recent experimental and theoretical progresses achieved in the preparation and application of vacancy‐rich nanocatalysts for electrochemical water splitting are explored. Engineering of vacancies has shown to open up a new avenue beyond the traditional morphology, size, and composition modifications for the development of nonprecious electrocatalysts toward efficient energy conversion. First, an introduction followed by discussions of different types of vacancies, the approaches to create vacancies, and the advanced techniques widely used to characterize these vacancies are presented. Importantly, the correlations between the vacancies and activities of the vacancy‐rich electrocatalysts via tuning the electronic states, active sites, and kinetic energy barriers are reviewed. Finally, perspectives on the existing challenges along with some opportunities for the further development of vacancy‐rich noble metal‐free electrocatalysts with high performance are discussed.     

Carbon‐Rich Nanomaterials: Fascinating Hydrogen and Oxygen Electrocatalysts

Hydrogen energy is commonly considered as a clean and sustainable alternative to the traditional fossil fuels. Toward universal utilization of hydrogen energy, developing high‐efficiency, low‐cost, and sustainable energy conversion technologies, especially water‐splitting electrolyzers and fuel cells, is of paramount significance. In order to enhance the energy conversion efficiency of the water‐splitting electrolyzers and fuel cells, earth‐abundant and stable electrocatalysts are essential for accelerating the sluggish kinetics of hydrogen and oxygen reactions. In the past decade, carbon‐rich nanomaterials have emerged as a promising class of hydrogen and oxygen electrocatalysts. Here, the development and electrocatalytic activity of various carbon‐rich materials, including metal‐free carbon, conjugated porous polymers, graphdiyne, covalent organic frameworks (COFs), atomic‐metal‐doped carbon, as well as metal–organic frameworks (MOFs), are demonstrated. In particular, the correlations between their porous nanostructures/electronic structures of active centers and electrocatalytic performances are emphatically discussed. Therefore, this review article guides the rational design and synthesis of high‐performance, metal‐free, and noble‐metal‐free carbon‐rich electrocatalysts and eventually advances the rapid development of water‐splitting electrolyzers and fuel cells toward practical applications.     

Transition‐Metal‐Based Electrocatalysts as Cocatalysts for Photoelectrochemical Water Splitting: A Mini Review

Converting solar energy into hydrogen via photoelectrochemical (PEC) water splitting is one of the most promising approaches for a sustainable energy supply. Highly active, cost‐effective, and robust photoelectrodes are undoubtedly crucial for the PEC technology. To achieve this goal, transition‐metal‐based electrocatalysts have been widely used as cocatalysts to improve the performance of PEC cells for water splitting. Herein, this Review summarizes the recent progresses of the design, synthesis, and application of transition‐metal‐based electrocatalysts as cocatalysts for PEC water splitting. Mo, Ni, Co‐based electrocatalysts for the hydrogen evolution reaction (HER) and Co, Ni, Fe‐based electrocatalysts for the oxygen evolution reaction (OER) are emphasized as cocatalysts for efficient PEC HER and OER, respectively. Particularly, some most efficient and robust photoelectrode systems with record photocurrent density or durability for the half reactions of HER and OER are highlighted and discussed. In addition, the self‐biased PEC devices with high solar‐to‐hydrogen efficiency based on earth‐abundant materials are also addressed. Finally, this Review is concluded with a summary and remarks on some challenges and opportunities for the further development of transition‐metal‐based electrocatalysts as cocatalysts for PEC water splitting.     

Functionalized Carbon Dots on Graphene as Outstanding Non‐Metal Bifunctional Oxygen Electrocatalyst

Carbon‐based bifunctional electrocatalysts for both oxygen reduction and evolution reactions are potentially cost‐effective to replace noble metals in energy devices such as fuel cells, metal–air batteries, and photoelectrochemical converters, but enrichment of active sites holds the key to efficiency. Here, graphene frameworks with heteroatom‐doped carbon dots (CDs) are developed via a hydrothermal route followed by pyrolysis. The CDs are rationally prepared with careful selection of heteroatoms, embedded on the substrate to provide enriched active sites. Structural characterizations (e.g., transmission electron microscopy and X‐ray photoelectron spectroscopy) reveal the successful addition of CDs with nitrogen and sulfur species. Especially, a heat‐treated N,S codoped sample, NS‐CD@gf_a900, exhibits the optimum oxygen electrocatalysis, even closer to noble‐metal counterparts, as a result of the effect of active sites of the CDs and the synergistic behavior of N and S. Considering the importance of size and dopants of the material, this approach not only suggests a straightforward preparation route of nanocarbons, but also appoints the utilization of a new class of non‐metal species as efficient oxygen electrocatalysts.     

Heteroatom‐Doped Carbon Materials for Hydrazine Oxidation

The key in designing efficient direct liquid fuel cells (DLFCs), which can offer some solutions to society's grand challenges associated with sustainability and energy future, currently lies in the development of cost‐effective electrocatalysts. Among the many types of fuel cells, direct hydrazine fuel cells (DHFCs) are of particular interest, especially due to their high theoretical cell voltages and clean emission. However, DHFCs currently use noble‐metal‐based electrocatalysts, and the scarcity and high cost of noble metals are hindering these fuel cells from finding large‐scale practical applications. In order to replace noble‐metal‐based electrocatalysts with sustainable ones and help DHFCs become widely usable, great efforts are being made to develop stable heteroatom (e.g., B, N, O, P and S)‐doped carbon electrocatalysts, the activities of which are comparable to, or better than, those of noble metals. Here, the recent research progress and the advancements made on the development of heteroatom‐doped carbon materials, their general properties, their electrocatalytic activities toward the HzOR, and their dopant‐ and structure‐related electrocatalytic properties for the HzOR are summarized. Perspectives on the different directions that the research endeavors in this field need to take in the future and the challenges associated with DHFCs are included.     

Electronic Structure Evolution in Tricomponent Metal Phosphides with Reduced Activation Energy for Efficient Electrocatalytic Oxygen Evolution

Non‐noble metal catalysts for high‐active electrocatalytic oxygen evolution reaction (OER) are essential in large‐scale application for water splitting. Herein, tricomponent metal phosphides with hollow structures are synthesized from cobalt‐contained metal organic frameworks (MOFs), i.e., ZIF‐67, by tailoring the feeding ratios of Ni and Fe, followed by a high‐temperature reduction and a subsequent phosphidation process. Excellent OER activity and long‐time stability are achieved in 1 m NaOH aqueous solution, with an overpotential of 329 mV at 10 mA cm^?2 and Tafel slope of 48.2 mV dec^?1, even superior to the noble metal‐based catalyst. It is evidenced that the formed (oxyhydr)oxide/phosphate species by in situ electrochemical surface oxidation are responsible for active OER. Accordingly, the simultaneous introduction of external Ni and Fe elements significantly influences the electronic structures of the parent metal phosphides, leading to the in situ electrochemical formation of surface active layer with decreased OER activation energy for greatly improved water oxidation performance. This electronic structure tuning strategy by introducing multicomponent metals demonstrates a versatile method to use MOFs as precursors for synthesizing high‐efficient water splitting electrocatalysts.     

Hierarchically Porous W‐Doped CoP Nanoflake Arrays as Highly Efficient and Stable Electrocatalyst for pH‐Universal Hydrogen Evolution

It is still challenging to develop high‐efficiency and low‐cost non‐noble metal‐based electrocatalysts for hydrogen evolution reaction (HER) in pH‐universal electrolytes. Herein, hierarchically porous W‐doped CoP nanoflake arrays on carbon cloth (W‐CoP NAs/CC) are synthesized via facile liquid‐phase reactions and a subsequent phosphorization process. The W‐CoP NAs/CC hybrid can be directly employed as a binder‐free electrocatalyst and delivers superior HER performance in pH‐universal electrolytes. Especially, it delivers very low overpotentials of 89, 94, and 102 mV to reach a current density of 10 mA cm^–2 in acidic, alkaline, and neutral electrolytes, respectively. Furthermore, it shows a nearly 100% Faradaic efficiency as well as superior long‐term stability with no decreasing up to 36 h in pH‐universal electrolytes. The outstanding electrocatalytic performance of W‐CoP NAs/CC can be mainly attributed to the porous W‐doped nanoflake arrays, which not only afford rich exposed active sites, but also accelerate the access of electrolytes and the diffusion of H₂ bubbles, thus efficiently promoting the HER performance. This work provides a new horizon to rationally design and synthesize highly effective and stable non‐noble metal phosphide‐based pH‐universal electrocatalysts for HER.     

Anion‐Regulated Selective Generation of Cobalt Sites in Carbon: Toward Superior Bifunctional Electrocatalysis

The introduction of active transition metal sites (TMSs) in carbon enables the synthesis of noble‐metal‐free electrocatalysts for clean energy conversion applications; however, there are often multiple existing forms of TMSs, which are of different natures and catalytic models. Regulating the evolution of distinctive TMSs is highly desirable but remains challenging to date. Anions, as essential elements involved in the synthesis, have been totally neglected previously in the construction of TMSs. Herein, the effects of anions on the creation of different types of TMSs are investigated for the first time. It is found that the active cobalt–nitrogen sites tend to be selectively constructed on the surface of N‐doped carbon by using chloride, while metallic cobalt nanoparticles encased in protective graphite layers are the dominant forms of cobalt species with nitrate ions. The obtained catalysts demonstrate cobalt‐sites‐dependent activity for oxygen reduction reaction and hydrogen evolution reaction in acidic media. The remarkably enhanced catalytic activities approaching that of benchmark Pt/C in an acidic medium have been obtained on the catalyst dominated with cobalt–nitrogen sites, confirmed by the advanced spectroscopic characterization. This finding demonstrates a general paradigm of anion‐regulated evolution of distinctive TMSs, providing a new pathway for enhancing performances of various targeted reactions related with TMSs.     

Electronic and Structural Engineering of Carbon‐Based Metal‐Free Electrocatalysts for Water Splitting

Since first being reported as possible electrocatalysts to substitute platinum for the oxygen reduction reaction (ORR), carbon‐based metal‐free nanomaterials have been considered a class of promising low‐cost materials for clean and sustainable energy‐conversion reactions. However, beyond the ORR, the development of carbon‐based catalysts for other electrocatalytic reactions is still limited. More importantly, the intrinsic activity of most carbon‐based metal‐free catalysts is inadequate compared to their metal‐based counterparts. To address this challenge, more design strategies are needed in order to improve the overall performance of carbon‐based materials. Herein, using water splitting as an example, some state‐of‐the‐art strategies in promoting carbon‐based nanomaterials are summarized, including graphene, carbon nanotubes, and graphitic‐carbon nitride, as highly active electrocatalysts for hydrogen evolution and oxygen evolution reactions. It is shown that by rationally tuning the electronic and/or physical structure of the carbon nanomaterials, adsorption of reaction intermediates is optimized, consequently improving the apparent electrocatalytic performance. These strategies may facilitate the development in this area and lead to the discovery of advanced carbon‐based nanomaterials for various applications in energy‐conversion processes.     

Recent Progress in Cobalt‐Based Heterogeneous Catalysts for Electrochemical Water Splitting

Water electrolysis is considered as the most promising technology for hydrogen production. Much research has been devoted to developing efficient electrocatalysts for hydrogen production via the hydrogen evolution reaction (HER) and oxygen production via the oxygen evolution reaction (OER). The optimum electrocatalysts can drive down the energy costs needed for water splitting via lowering the overpotential. A number of cobalt (Co)‐based materials have been developed over past years as non‐noble‐metal heterogeneous electrocatalysts for HER and OER. Recent progress in this field is summarized here, especially highlighting several important bifunctional catalysts. Various approaches to improve or optimize the electrocatalysts are introduced. Finally, the current existing challenges and the future working directions for enhancing the performance of Co‐implicated electrocatalysts are proposed.     

In Situ Exfoliated,Edge‐Rich,Oxygen‐Functionalized Graphene from Carbon Fibers for Oxygen Electrocatalysis

Metal‐free electrocatalysts have been extensively developed to replace noble metal Pt and RuO₂ catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in fuel cells or metal–air batteries. These electrocatalysts are usually deposited on a 3D conductive support (e.g., carbon paper or carbon cloth (CC)) to facilitate mass and electron transport. For practical applications, it is desirable to create in situ catalysts on the carbon fiber support to simplify the fabrication process for catalytic electrodes. In this study, the first example of in situ exfoliated, edge‐rich, oxygen‐functionalized graphene on the surface of carbon fibers using Ar plasma treatment is successfully prepared. Compared to pristine CC, the plasma‐etched carbon cloth (P‐CC) has a higher specific surface area and an increased number of active sites for OER and ORR. P‐CC also displays good intrinsic electron conductivity and excellent mass transport. Theoretical studies show that P‐CC has a low overpotential that is comparable to Pt‐based catalysts, as a result of both defects and oxygen doping. This study provides a simple and effective approach for producing highly active in situ catalysts on a carbon support for OER and ORR.     

Support and Interface Effects in Water‐Splitting Electrocatalysts

Water‐splitting electrolyzers that can convert electricity into storable hydrogen are a fascinating and scalable energy conversion technology for the utilization of renewable energies. To speed up the sluggish hydrogen and oxygen evolution reactions (HER and OER), electrocatalysts are essential for reducing their kinetic energy barriers and eventually improving the energy conversion efficiency. As efficient strategies for modulating the binding ability of water‐splitting intermediates on electrocatalyst surface, the support effect and interface effect are drawing growing attention. Herein, some of the recent research progress on the support and interface effects in HER, OER, and overall water‐splitting electrocatalysts is highlighted. Specifically, the correlation between the electronic interaction of the constituent components and the electrocatalytic water‐splitting performance of electrocatalysts is profoundly discussed, with the aim of advancing the development of highly efficient water‐splitting electrocatalysts, which may eventually replace the noble‐metal‐based electrocatalysts and bring the practically widespread utilization of water‐splitting electrolyzers into a reality.     

Ultrafine Dual‐Phased Carbide Nanocrystals Confined in Porous Nitrogen‐Doped Carbon Dodecahedrons for Efficient Hydrogen Evolution Reaction

Designing novel non‐noble electrocatalysts with controlled structures and composition remains a great challenge for efficient hydrogen evolution reaction (HER). Herein, a rational synthesis of ultrafine carbide nanocrystals confined in porous nitrogen‐doped carbon dodecahedrons (PNCDs) by annealing functional zeolitic imidazolate framework (ZIF‐8) with molybdate or tungstate is reported. By controlling the substitution amount of MO₄ units (M = Mo or W) in the ZIF‐8 framework, dual‐phase carbide nanocrystals confined in PNCDs (denoted as MC‐M₂C/PNCDs) can be obtained, which exhibit superior activity toward the HER to the single‐phased MC/PNCDs and M₂C/PNCDs. The evenly distributed ultrafine nanocrystals favor the exposure of active sites. PNCDs as the support facilitate charge transfer and protect the nanocrystals from aggregation during the HER process. Moreover, the strong coupling interactions between MC and M₂C provide beneficial sites for both water dissociation and hydrogen desorption. This work highlights a new feasible strategy to explore efficient electrocatalysts via engineering on nanostructure and composition.     

Progress in the Development of Fe‐Based PGM‐Free Electrocatalysts for the Oxygen Reduction Reaction

Development of alternative energy sources is crucial to tackle challenges encountered by the growing global energy demand. Hydrogen fuel, a promising way to store energy produced from renewable power sources, can be converted into electrical energy at high efficiency via direct electrochemical conversion in fuel cells, releasing water as the sole byproduct. One important drawback to current fuel‐cell technology is the high content of platinum‐group‐metal (PGM) electrocatalysts required to perform the sluggish oxygen reduction reaction (ORR). Addressing this challenge, remarkable progress has been made in the development of low‐cost PGM‐free electrocatalysts synthesized from inexpensive, earth‐abundant, and easily sourced materials such as iron, nitrogen, and carbon (Fe–N–C). PGM‐free Fe–N–C electrocatalysts now exhibit ORR activities approaching that of PGM electrocatalysts but at a fraction of the cost, promising to significantly reduce overall fuel‐cell technology costs. Herein, recent developments in PGM‐free electrocatalysis, demonstrating increased fuel‐cell performance, as well as efforts aimed at understanding the key limiting factor, i.e., the nature of the PGM‐free active site, are summarized. Further improvements will be accomplished through the controlled and/or rationally designed synthesis of materials with higher active‐site densities, while at the same time establishing methods to mitigate catalyst degradation.     

Recent Progress in Graphene‐Based Noble‐Metal Nanocomposites for Electrocatalytic Applications

The fast industrialization process has led to global challenges in the energy crisis and environmental pollution, which might be solved with clean and renewable energy. Highly efficient electrochemical systems for clean‐energy collection require high‐performance electrocatalysts, including Au, Pt, Pd, Ru, etc. Graphene, a single‐layer 2D carbon nanosheet, possesses many intriguing properties, and has attracted tremendous research attention. Specifically, graphene and graphene derivatives have been utilized as templates for the synthesis of various noble‐metal nanocomposites, showing excellent performance in electrocatalytic‐energy‐conversion applications, such as the hydrogen evolution reaction and CO₂ reduction. Herein, the recent progress in graphene‐based noble‐metal nanocomposites is summarized, focusing on their synthetic methods and electrocatalytic applications. Furthermore, some personal insights on the challenges and possible future work in this research field are proposed.     

Recent Advances on Self‐Supported Arrayed Bifunctional Oxygen Electrocatalysts for Flexible Solid‐State Zn–Air Batteries

Flexible solid‐state Zn–air batteries have been rapidly developed benefiting from the uprising demand for wearable electronic devices, wherein the air electrode integrated with efficient bifunctional oxygen electrocatalysts plays an important role to achieve high performance. Binder‐free self‐supported bifunctional catalysts can provide large active surface area, fast electron transport path, easy ion diffusion, and excellent structural stability and flexibility, thus acting as promising flexible air cathodes. In this review, recent advances on the application of nanoarrayed electrocatalysts as air cathodes in flexible Zn–air batteries are reviewed. Especially, various types of bifunctional oxygen electrocatalysts, including carbonaceous material arrays, transition metal compound arrays, transition metal/carbon arrays, transition metal compound/carbon arrays, and other hybrid arrays, are discussed. The applications of flexible Zn–air batteries with two configurations (i.e., planar stacks and cable fibers) are also introduced. Finally, perspectives on the optimization of arrayed air cathodes for future development to achieve high‐performance flexible Zn–air batteries are shared.     

Chemical Approaches to Carbon‐Based Metal‐Free Catalysts

Highly active and durable catalysts play a key role in clean energy technologies. However, the high cost, low reserves, and poor stability of noble‐metal‐based catalysts have hindered the large‐scale development of renewable energy. Owing to their low cost, earth abundance, high activity, and excellent stability, carbon‐based metal‐free catalysts (CMFCs) are promising alternatives to precious‐metal‐based catalysts. Although many synthetic methods based on solution, surface/interface, solid state, and noncovalent chemistries have been developed for producing numerous CMFCs with diverse structures and functionalities, there is still a lack of effective approaches to precisely control the structures of active sites. Therefore, novel chemical approaches are needed for the development of highly active and durable CMFCs that are capable of replacing precious‐metal catalysts for large‐scale applications. Herein, a comprehensive and critical review on chemical approaches to CMFCs is given by summarizing important advancements, current challenges, and future perspectives in this emerging field. Through such a critical review, our understanding of CMFCs and the associated synthetic processes will be significantly increased.     

Framework‐Porphyrin‐Derived Single‐Atom Bifunctional Oxygen Electrocatalysts and their Applications in Zn–Air Batteries

High‐performance bifunctional oxygen electrocatalysis constitutes the key technique for the widespread application of clean and sustainable energy through electrochemical devices such as rechargeable Zn–air batteries. Single‐atom electrocatalysts with maximum atom efficiency are highly considered as an alternative of the present noble‐metal‐based electrocatalysts. However, the fabrication of transition metal single‐atoms is very challenging, requiring extensive attempts of precursors with novel design principles. Herein, an all‐covalently constructed cobalt‐coordinated framework porphyrin with graphene hybridization is innovatively designed and prepared as the pyrolysis precursor to fabricate single‐atom Co–N_x–C electrocatalysts. Excellent electrochemical performances are realized for both bifunctional oxygen electrocatalysis and rechargeable Zn–air batteries with regard to reduced overpotentials, improved kinetics, and prolonged cycling stability comparable with noble‐metal‐based electrocatalysts. Design principles from multiple scales are proposed and rationalized with detailed mechanism investigation. This work not only provides a novel precursor for the fabrication of high‐performance single‐atom electrocatalysts, but also inspires further attempts to develop advanced materials and emerging applications.     

Engineering Single‐Atom Cobalt Catalysts toward Improved Electrocatalysis

The development of cost‐effective catalysts to replace noble metal is attracting increasing interests in many fields of catalysis and energy, and intensive efforts are focused on the integration of transition‐metal sites in carbon as noble‐metal‐free candidates. Recently, the discovery of single‐atom dispersed catalyst (SAC) provides a new frontier in heterogeneous catalysis. However, the electrocatalytic application of SAC is still subject to several theoretical and experimental limitations. Further advances depend on a better design of SAC through optimizing its interaction with adsorbates during catalysis. Here, distinctive from previous studies, favorable 3d electronic occupation and enhanced metal–adsorbates interactions in single‐atom centers via the construction of nonplanar coordination is achieved, which is confirmed by advanced X‐ray spectroscopic and electrochemical studies. The as‐designed atomically dispersed cobalt sites within nonplanar coordination show significantly improved catalytic activity and selectivity toward the oxygen reduction reaction, approaching the benchmark Pt‐based catalysts. More importantly, the illustration of the active sites in SAC indicates metal‐natured catalytic sites and a media‐dependent catalytic pathway. Achieving structural and electronic engineering on SAC that promotes its catalytic performances provides a paradigm to bridge the gap between single‐atom catalysts design and electrocatalytic applications.