Highly Porous Materials as Tunable Electrocatalysts for the Hydrogen and Oxygen Evolution Reaction

The facile preparation of highly porous, manganese doped, sponge‐like nickel materials by salt melt synthesis embedded into nitrogen doped carbon for electrocatalytic applications is shown. The incorporation of manganese into the porous structure enhances the nickel catalyst's activity for the hydrogen evolution reaction in alkaline solution. The best catalyst demonstrates low onset overpotential (0.15 V) for the hydrogen evolution reaction along with high current densities at higher potentials. In addition, the possibility to alter the electrocatalytic properties of the materials from the hydrogen to oxygen evolution reaction by simple surface oxidation is shown. The surface area increases up to 1200 m²g^?1 after mild oxidation accompanied by the formation of nickel oxide on the surface. A detailed analysis shows a synergetic effect of the oxide formation and the material's surface area on the catalytic performance in the oxygen evolution reaction. In addition, the synthesis of cobalt doped sponge‐like nickel materials is also delineated, demonstrating the generality of the synthesis. The facile salt melt synthesis of such highly porous metal based materials opens new possibilities for the fabrication of diverse electrode nanostructures for electrochemical applications.     

Bimetallic-Based Electrocatalysts for Oxygen Evolution Reaction

The electrochemical oxygen evolution reaction (OER) is a core electrode reaction for the renewable production of high-purity hydrogen, carbon-based fuel, synthetic ammonia, etc. However, the sluggish kinetics of the OER result in a high overpotential and limit the widespread application of OER-based technologies. Recent studies have shown that bimetallic-based materials with the synergism of different metal components to regulate the adsorption and dissociation energy of intermediates are promising OER electrocatalyst candidates with a lower cost and energy consumption. In the past two decades, tremendous efforts have been devoted to developing OER applications of bimetallic-based materials with a focus on compositions, phase, structure, etc., to highlight the synergism of different metal components. However, there is a lack of critical thinking and organized analysis of OER applications with bimetallic-based materials. This review critically discusses the challenges of developing bimetallic-based OER materials, summarizes the current optimization strategies to enhance both activity and stability, and highlights the state-of-the-art electrocatalysts for OER. The relationship between the componential/structural features of bimetallic-based materials and their electrocatalytic properties is presented to form comprehensive electronic and geometric modifications based on thorough analysis of the reported works and discuss future efforts to realize sustainable bimetallic-based OER applications.     

Facile Synthesis of Medium-Entropy Metal Sulfides as High-Efficiency Electrocatalysts toward Oxygen Evolution Reaction

Developing highly efficient and durable electrocatalysts toward oxygen evolution reaction (OER) is an urgent demand to produce clean hydrogen energy. In this study, a series of medium-entropy metal sulfides (MEMS) of (NiFeCoX)₃S₄ (where X = Mn, Cr, Zn) are synthesized by a facile one-pot solvothermal strategy using molecular precursors. Benefiting from the multiple-metal synergistic effect and the low crystallinity, these MEMS show significantly enhanced electrocatalytic OER activity compared with the binary-metal (NiFe)₃S₄ and ternary-metal (NiFeCo)₃S₄ counterparts. Especially, (NiFeCoMn)₃S₄ delivers a low overpotential of 289 mV at 10 mA cm⁻², a decent Tafel slope of 75.6 mV dec⁻¹ and robust catalytic stability in alkaline medium, outperforming the costly IrO₂ benchmark electrocatalyst and the majority of the reported metal sulfide-based electrocatalysts until now. These MEMS with facile synthesis and excellent electrocatalytic performance bring a great opportunity to design desirable electrocatalysts for practical application.     

Recent Progress in MOF‐Derived,Heteroatom‐Doped Porous Carbons as Highly Efficient Electrocatalysts for Oxygen Reduction Reaction in Fuel Cells

Currently, developing nonprecious‐metal catalysts to replace Pt‐based electrocatalysts in fuel cells has become a hot topic because the oxygen reduction reaction (ORR) in fuel cells often requires platinum, a precious metal, as a catalyst, which is one of the major hurdles for commercialization of the fuel cells. Recently, the newly emerging metal‐organic frameworks (MOFs) have been widely used as self‐sacrificed precursors/templates to fabricate heteroatom‐doped porous carbons. Here, the recent progress of MOF‐derived, heteroatom‐doped porous carbon catalysts for ORR in fuel cells is systematically reviewed, and the synthesis strategies for using different MOF precursors to prepare heteroatom‐doped porous carbon catalysts, including the direct carbonization of MOFs, MOF and heteroatom source mixture carbonization, and MOF‐based composite carbonization are summarized. The emphasis is placed on the precursor design of MOF‐derived metal‐free catalysts and transition‐metal‐doped carbon catalysts because the MOF precursors often determine the microstructures of the derived porous carbon catalysts. The discussion provides a useful strategy for in situ synthesis of heteroatom‐doped carbon ORR electrocatalysts by rationally designing MOF precursors. Due to the versatility of MOF structures, MOF‐derived porous carbons not only provide chances to develop highly efficient ORR electrocatalysts, but also broaden the family of nanoporous carbons for applications in supercapacitors and batteries.     

Polyformamidine‐Derived Non‐Noble Metal Electrocatalysts for Efficient Oxygen Reduction Reaction

A facile approach for the template‐free synthesis of highly active non‐noble metal based oxygen reduction reaction (ORR) electrocatalysts is presented. Porous Fe?N?C/Fe/Fe₃C composite materials are obtained by pyrolysis of defined precursor mixtures of polyformamidine (PFA) and FeCl₃ as nitrogen‐rich carbon and iron sources, respectively. Selection of pyrolysis temperature (700–1100 °C) and FeCl₃ loading (5–30 wt%) yields materials with differing surface areas, porosity, graphitization degree, nitrogen and iron content, as well as ORR activity. While the ORR activity of Fe‐free materials is limited (i.e., synthesized from pure PFA), a huge increase in activity is observed for catalysts containing Fe, revealing the participation of the metal dopant in the construction of active electrocatalytic sites. Further activity improvement is achieved via acid‐leaching and repeated pyrolysis, a result which is attributed to the creation of new active sites located at the surface of the porous nitrogen‐doped carbon by dissolution of the Fe and Fe₃C nanophases. The best performing catalyst, which was synthesized with a low Fe loading (i.e., 5 wt%) and at a pyrolysis temperature of 900 °C, exhibits high activity, excellent H₂O selectivity, extended stability, in both basic and acidic media as well as a remarkable tolerance toward methanol.     

Iron,Nitrogen Co-Doped Carbon Spheres as Low Cost,Scalable Electrocatalysts for the Oxygen Reduction Reaction

Atomically dispersed transition metal-nitrogen-carbon catalysts are emerging as low-cost electrocatalysts for the oxygen reduction reaction in fuel cells. However, a cost-effective and scalable synthesis strategy for these catalysts is still required, as well as a greater understanding of their mechanisms. Herein, iron, nitrogen co-doped carbon spheres (Fe@NCS) have been prepared via hydrothermal carbonization and high-temperature post carbonization. It is determined that FeN₄ is the main form of iron existing in the obtained Fe@NCS. Two different precursors containing Fe²⁺ and Fe³⁺ are compared. Both chemical and structural differences have been observed in catalysts starting from Fe²⁺ and Fe³⁺ precursors. Fe²⁺@NCS-A (starting with Fe²⁺ precursor) shows better catalytic activity for the oxygen reduction reaction. This catalyst is studied in an anion exchange membrane fuel cell. The high open-circuit voltage demonstrates the potential approach for developing high-performance, low-cost fuel cell catalysts.     

In Situ Exfoliated,N‐Doped,and Edge‐Rich Ultrathin Layered Double Hydroxides Nanosheets for Oxygen Evolution Reaction

The number of catalytically reactive sites and their intrinsic electrocatalytic activity strongly affect the performance of electrocatalysts. Recently, there are growing concerns about layered double hydroxides (LDHs) for oxygen evolution reaction (OER). Exfoliating LDHs is an effective method to increase the reactive sites, however, a traditional liquid phase exfoliation method is usually very labor‐intensive and time‐consuming. On the other hand, proper heteroelement doping and edge engineering are helpful to tune the intrinsic activity of reactive sites. In this work, bulk CoFe LDHs are successfully exfoliated into ultrathin CoFe LDHs nanosheets by nitrogen plasma. Meanwhile, nitrogen doping and defects are introduced into exfoliated ultrathin CoFe LDHs nanosheets. The number of reactive sites can be increased efficiently by the formation of ultrathin CoFe LDHs nanosheets, the nitrogen dopant alters the surrounding electronic arrangement of reactive site facilitating the adsorption of OER intermediates, and the electrocatalytic activity of reactive sites can be further tuned efficiently by introducing defects which increase the number of dangling bonds neighboring reactive sites and decrease the coordination number of reactive sites. With these advantages, this electrocatalyst shows excellent OER activity with an ultralow overpotential of 233 mV at 10 mA cm^?2.     

Coupling Adsorbed Evolution and Lattice Oxygen Mechanism in Fe-Co(OH)2/Fe2O3 Heterostructure for Enhanced Electrochemical Water Oxidation

Oxygen evolution reaction (OER) remains a bottleneck for electrocatalytic water-splitting to generate hydrogen. However, the traditional adsorbed evolution mechanism (AEM) possesses sluggish reaction kinetics due to the scaling relationship, while lattice oxygen mechanism (LOM) triggers an unstable structure due to the escaping of lattice oxygen. Herein, a proof-of-concept Fe-Co(OH)₂/Fe₂O₃ heterostructure is put forward, where Fe-Co(OH)₂ following AEM can complete rapidly deprotonation process while Fe₂O₃ following LOM can trigger O─O coupling step. Combining the theoretical and experimental investigation confirmed that the redistributed space-charge of Fe-Co(OH)₂/Fe₂O₃ junction can optimize synergistically adsorbed and lattice oxygen, the coupling mechanism of AEM and LOM can facilitate synchronously the OER activity and stability. As a result, the Fe-Co(OH)₂/Fe₂O₃ heterostructure shows excellent OER performance with low overpotential of only 219 and 249 mV to reach a current density of 10 and 100 mA cm⁻². Specifically, the Fe-Co(OH)₂/Fe₂O₃ electrocatalyst maintains excellent long-term stability for 100 h at a large current density of 100 mA cm⁻². This work paves an avenue to break through the limit of the conventional OER mechanism.     

Metal (Ni,Co)‐Metal Oxides/Graphene Nanocomposites as Multifunctional Electrocatalysts

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) along with hydrogen evolution reaction (HER) have been considered critical processes for electrochemical energy conversion and storage through metal‐air battery, fuel cell, and water electrolyzer technologies. Here, a new class of multifunctional electrocatalysts consisting of dominant metallic Ni or Co with small fraction of their oxides anchored onto nitrogen‐doped reduced graphene oxide (rGO) including Co‐CoO/N‐rGO and Ni‐NiO/N‐rGO are prepared via a pyrolysis of graphene oxide and cobalt or nickel salts. Ni‐NiO/N‐rGO shows the higher electrocatalytic activity for the OER in 0.1 m KOH with a low overpotential of 0.24 V at a current density of 10 mA cm^?2, which is superior to that of the commercial IrO₂. In addition, it exhibits remarkable activity for the HER, demonstrating a low overpotential of 0.16 V at a current density of 20 mA cm^?2 in 1.0 m KOH. Apart from similar HER activity to the Ni‐based catalyst, Co‐CoO/N‐rGO displays the higher activity for the ORR, comparable to Pt/C in zinc‐air batteries. This work provides a new avenue for the development of multifunctional electrocatalysts with optimal catalytic activity by varying transition metals (Ni or Co) for these highly demanded electrochemical energy technologies.     

Coordination Shell Dependent Activity of CuCo Diatomic Catalysts for Oxygen Reduction,Oxygen Evolution,and Hydrogen Evolution Reaction

Challenges in rational designing dual-atom catalysts (DACs) give a strong motivation to construct coordination-activity correlations. Here, thorough coordination-activity correlations of DACs based on how the changes in coordination shells (CSs) of dual-atom Cu,Co centers influence their electrocatalytic activity in oxygen reduction reaction(ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) is constructed. First, Cu,Co DACs with different CSs modifications are fabricated by using a controlled “precursors-preselection” approach. Three DACs with unique coordination environments are characterized as secondary S atoms that directly bond to Cu,Co-N₆ in lower CSs, indirectly bond in neighboring CSs, and are doped in higher CSs, respectively. Then, experimentally and theoretically, a coordination correlation resembling a planet-satellite system, where satellite coordinated atoms (heteroatom N, S) surround Cu-Co dual-atom entity in various orbitals CSs. By evaluating electrocatalytic activity indicators, differences are identified in electronic structure and electrocatalytic performance of Cu and Co centers in ORR, OER, and HER. Interestingly, initial CSs modifications for DACs may not always be advantageous for electrocatalysis. This work offers valuable insight for designing DACs for diverse applications.     

Re-Looking into the Active Moieties of Metal X-ides (X- = Phosph-, Sulf-, Nitr-, and Carb-) Toward Oxygen Evolution Reaction

Non-precious metal-based catalysts for oxygen evolution reaction (OER) have been extensively studied, among which the transition metal X-ides (including phosph-ides, sulf-ides, nitr-ides, and carb-ides) materials are emerging as promising candidates to replace the benchmark Ir/Ru-based materials in alkaline media. However, it is controversial whether the metal Xides host the real active sites since these metal Xides are thermodynamically unstable under a harsh OER environment—it has been reported that the initial metal Xides can be electrochemically oxidized and transformed into corresponding oxides and (oxy)hydroxides. Therefore, the metal Xides are argued as “pre-catalysts”; the electrochemically formed oxides and (oxy)hydroxides are believed as the real active moieties for OER. Herein, the recent advances in understanding the transformation behavior of metal Xides during OER are re-looked; importantly, hypotheses are provided to understand why the electrochemically formed oxides and (oxy)hydroxides catalysts derived from metal Xides are superior for OER to the as-prepared metal oxides and (oxy)hydroxides catalysts.     

Nanoporous Surface High-Entropy Alloys as Highly Efficient Multisite Electrocatalysts for Nonacidic Hydrogen Evolution Reaction

Electrocatalytic hydrogen evolution in alkaline and neutral media offers the possibility of adopting platinum-free electrocatalysts for large-scale electrochemical production of pure hydrogen fuel, but most state-of-the-art electrocatalytic materials based on nonprecious transition metals operate at high overpotentials. Here, a monolithic nanoporous multielemental CuAlNiMoFe electrode with electroactive high-entropy CuNiMoFe surface is reported to hold great promise as cost-effective electrocatalyst for hydrogen evolution reaction (HER) in alkaline and neutral media. By virtue of a surface high-entropy alloy composed of dissimilar Cu, Ni, Mo, and Fe metals offering bifunctional electrocatalytic sites with enhanced kinetics for water dissociation and adsorption/desorption of reactive hydrogen intermediates, and hierarchical nanoporous Cu scaffold facilitating electron transfer/mass transport, the nanoporous CuAlNiMoFe electrode exhibits superior nonacidic HER electrocatalysis. It only takes overpotentials as low as ≈240 and ≈183 mV to reach current densities of ≈1840 and ≈100 mA cm⁻² in 1 m  KOH and pH 7 buffer electrolytes, respectively; ≈46- and ≈14-fold higher than those of ternary CuAlNi electrode with bimetallic Cu–Ni surface alloy. The outstanding electrocatalytic properties make nonprecious multielemental alloys attractive candidates as high-performance nonacidic HER electrocatalytic electrodes in water electrolysis.     

Multicomponent Intermetallic Nanoparticles on Hierarchical Metal Network as Versatile Electrocatalysts for Highly Efficient Water Splitting

Developing high-efficiency and cost-effective alloy catalysts toward hydrogen-evolution reaction (HER) is crucial for large-scale hydrogen production via electrochemical water splitting, but conventional single-principal-element alloy design usually causes insufficient activity and durability of state-of-the-art multimetallic catalysts based on non-precious transition metals. Herein, we report multicomponent intermetallic Mo(NiFeCo)₄ nanoparticles seamlessly integrated on hierarchical nickel network (Mo(NiFeCo)₄/Ni) as robust hydrogen-evolution electrocatalysts with remarkably improved activity and durability by making use of iron and cobalt atoms partially substituting nickel sites to form high-entropy NiFeCo sublattice in intermetallic MoNi₄ matrix, which serve as bifunctional electroactive sites for both water dissociation and adsorption/combination of hydrogen intermediate and improves thermodynamic stability. By virtue of bicontinuous nanoporous nickel skeleton facilitating electron/ion transportation, self-supported nanoporous Mo(NiFeCo)₄/Ni electrode exhibits exceptional HER electrocatalysis, with low Tafel slope (≈35 mV dec⁻¹), high current density (≈2300 mA cm⁻²) at low overpotential (200 mV) and long-term durability in 1 m KOH. When coupled to its electrooxidized and nitrified derivative for oxygen-evolution reaction, their alkaline water electrolyzers operate with a superior overall water-splitting output, outperforming the one constructed with commercially available noble-metal-based catalysts. These electrochemical properties make it an attractive candidate as electrocatalyst in alkaline water electrolysis for large-scale hydrogen generation.     

Rational Design of Transition Metal Phosphide-Based Electrocatalysts for Hydrogen Evolution

Developing efficient and inexpensive electrocatalysts for the hydrogen evolution reaction (HER) is critical to the commercial viability of electrochemical clean energy technologies. Transition metal phosphides (TMPs), with the merits of abundant reserves, unique structure, tunable composition, and high electronic conductivity, are recognized as attractive HER catalytic materials. Nevertheless, the HER electrocatalytic activity of TMPs is still limited by various thorough issues and inherent performance bottlenecks. In this review, these issues are carefully sorted, and the corresponding reasonable explanations and solutions are elucidated on the basis of the HER catalytic activity origins of TMPs. Subsequently, highly targeted multiscale strategies to improve the HER performance of TMPs are comprehensively presented. Additionally, critical scientific issues for constructing high-efficiency TMP-based electrocatalysts are proposed. Finally, the HER reaction process, catalytic mechanism research, TMP-based catalyst construction, and their application expansion are mentioned as challenges and future directions for this research field. Expectedly, this review offers professional and targeted guidelines for the rational design and practical application of TMP-based HER catalysts.     

Hexagonal Defect-Rich MnOx/RuO2 with Abundant Heterointerface to Modulate Electronic Structure for Acidic Oxygen Evolution Reaction

Developing green hydrogen energy to power future societies has driven the progress of proton-exchange membrane water electrolyzers (PEMWE). However, due to the complex anode oxygen evolution reaction (OER) electron transfer process and the strong acidic environment, the most effective catalysts are still Ir-based nanomaterials. Therefore, exploiting low cost acidic OER catalysts to meet the needs of PEMWE remains a challenging and rewarding task. Herein, hexagonal-shaped and defect-rich MnO_x/RuO₂ heterojunction nanosheets (H/d-MnO_x/RuO₂) is designed. The oxygen vacancies and heterogeneous structure enable the H/d-MnO_x/RuO₂ catalyst to reach 10 mA cm⁻² with only overpotential 178 mV in 0.5 m H₂SO₄. Density functional theory shows that the oxygen vacancies and heterogeneous interface facilitates the reduction of the adsorption energy of *OOH and the reduction of the energy level of Ru-Oads, thus suppressing the involvement of lattice oxygen and enhancing the durability. This study provides an effective way to design efficient catalysts for hydrogen production in PEMWE.     

Molybdenum Phosphide/Carbon Nanotube Hybrids as pH‐Universal Electrocatalysts for Hydrogen Evolution Reaction

Molybdenum phosphide (MoP) has received increasing attention due to its high catalytic activity in hydrogen evolution reaction (HER). However, it remains difficult to construct well‐defined MoP nanostructures with large density of active sites and high intrinsic activity. Here, a facile and general method is reported to synthesize an MoP/carbon nanotube (CNT) hybrid featuring small‐sized and well‐crystallized MoP nanoparticles uniformly coated on the sidewalls of multiwalled CNT. The MoP/CNT hybrid exhibits impressive HER activities in pH‐universal electrolytes, and requires the overpotentials as low as 83, 102, and 86 mV to achieve a cathodic current density of 10 mA cm^?2 in acidic 0.5 m H₂SO₄, neutral 1 m phosphate buffer solution, and alkaline 1 m KOH electrolytes, respectively. It is found that the crystallinity of MoP has significant influence on HER activity. This study provides a new design strategy to construct MoP nanostructures for optimizing its catalytic performance.     

Catalyst-Support Interactions in Zr2ON2-Supported IrOx Electrocatalysts to Break the Trade-Off Relationship Between the Activity and Stability in the Acidic Oxygen Evolution Reaction

The development of highly active and durable Ir-based electrocatalysts for the acidic oxygen evolution reaction (OER) is challenging because of the corrosive anodic conditions. Herein, IrO_x/Zr₂ON₂ electrocatalyst is demonstrated, employing Zr₂ON₂ as a support material, to overcome the trade-off between the activity and stability in the OER. Zr₂ON₂ is selected due to its excellent electrical conductivity and chemical stability, and the fact that it induces strong interactions with IrO_x catalysts. As a result, IrO_x/Zr₂ON₂ electrocatalysts exhibit outstanding OER performances, reaching an overpotential of 255 mV at 10 mA cm⁻² and a mass activity of 849 mA mg_Ir⁻¹ at 1.55 V (vs the reversible hydrogen electrode). The activity of IrO_x/Zr₂ON₂ is maintained at 10 mA cm⁻² for 5 h, while in contrast, IrO_x/ZrN and an unsupported IrO_x catalyst undergo drastic degradation. Combined experimental X-ray analyses and theoretical interpretations reveal that the reduced oxidation state of Ir and the extended Ir O bond distance in IrO_x/Zr₂ON₂ effectively increase the activity and stability of IrO_x by altering reaction pathway from a conventional adsorbate evolution mechanism to a lattice oxygen-participating mechanism. These results demonstrate that it is possible to effectively reduce the Ir content in OER catalysts through interface engineering without sacrificing the catalytic performance.     

Cobalt Assisted Synthesis of IrCu Hollow Octahedral Nanocages as Highly Active Electrocatalysts toward Oxygen Evolution Reaction

Development of oxygen evolution reaction (OER) catalysts with reduced precious metal content while enhancing catalytic performance has been of pivotal importance in cost‐effective design of acid polymer electrolyte membrane water electrolyzers. Hollow multimetallic nanostructures with well‐defined facets are ideally suited for saving the usage of expensive precious metals as well as boosting catalytic performances; however, Ir‐based hollow nanocatalysts have rarely been reported. Here, a very simple synthetic scheme is reported for the preparation of hollow octahedral nanocages of Co‐doped IrCu alloy with readily tunable morphology and size. The Co‐doped IrCu octahedral nanocages show excellent electrocatalytic activity and long‐term durability for OER in acidic media. Notably, their OER activity represents one of the best performances among Ir‐based acidic OER catalysts.     

Superaerophobic Electrode with Metal@Metal‐Oxide Powder Catalyst for Oxygen Evolution Reaction

A stable and highly active oxygen evolution reaction (OER) electrode is the key for fast and robust O₂ production, which is one of the essential points for various kinds of energy conversion systems, such as water splitting, lithium‐O₂ battery and artificial photosynthesis. Here, superaerophobic electrodes with metal@metal‐oxide powder catalysts are shown, which demonstrate high and stable OER activity. The active‐site‐density of metal@metal‐oxide catalysts is increased over one order of magnitude than those of pure metal oxides due to the large enhancement of electrical conductivity, revealing the substantial enhancement of electrochemical OER kinetics. Furthermore, the superaerophobic property of electrodes is favorable for fast O₂ desorption, which improves electrochemical active surface area (EASA) during OER. The superaerophobic electrode with metal@metal‐oxide powder catalysts provides the new insight for increase of active‐site‐density and EASA simultaneously, which are the key factors to determine the activity of OER electrode.