Role of transition metals in catalyst designs for oxygen evolution reaction: A comprehensive review

With the increasing demand for sustainable energy, human beings put forward higher requirements for the preparation of clean energy. Electrocatalytic water splitting has become a promising way to solve the energy crisis because of the characteristics of high efficiency and cleanliness. The catalyst of oxygen evolution reaction (OER), an important part of electrocatalytic water splitting, plays a critical role in water splitting. Transition metals (TMs) are generally used as active sites to realize high-efficiency electrocatalytic in the water. To better understand the role of TMs in the OER catalysts. This review focused on the relationship between TMs and OER catalyst activity. The mechanism of synthesis strategy in different types of TMs-based catalysts was also summarized. The challenges and prospects of developing economic and efficient TMs-based OER catalysts were discussed.     

Development of NiO/Co3O4 nanohybrids catalyst with oxygen vacancy for oxygen evolution reaction enhancement in alkaline solution

The oxygen evolution reaction (OER) is a significant reaction in water splitting and energy conversion. However, high price and sluggish kinetics catalysts prevent commercial applications. Generally, noble metals (e.g., iridium and ruthenium), which are expensive and unstable, have been used as catalysts for OER because of their high electrocatalytic activity. In this study, we report a high-performance OER catalyst with oxygen vacancies comprising NiO/Co₃O₄ nanohybrids. For OER, the NiO/Co₃O₄ heterostructure show good electrocatalytic performance with a low overpotential of 330 mV. This is higher than those of NiO, Co₃O₄, and benchmark IrO₂ candidates at current density of 10 mA cm^?2. Furthermore, the NiO/Co₃O₄ nanohybrids show long-term electrochemical stability for 10 h. The present research results show that NiO/Co₃O₄ heterostructure is an excellent electrocatalyst for OER.     

Binder-free bifunctional SnFe sulfide/oxyhydroxide heterostructure electrocatalysts for overall water splitting

Developing robust non-noble catalysts towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is vital for large-scale hydrogen production from electrochemical water splitting. Here, we synthesize Sn- and Fe-containing sulfides and oxyhydroxides anchored on nickel foam (SnFeS_xO_y/NF) using a solvothermal method, in which a heterostructure is generated between the sulfides and oxyhydroxides. The SnFeS_xO_y/NF exhibits low overpotentials of 85, 167, 249, and 324 mV at 10, 100, 500 and 1000 mA cm^?2 for the HER, respectively, and a low overpotential of only 281 mV at 100 mA cm^?2 for the OER. When it serves as both anode and cathode to assemble an electrolyzer, the cell voltage is only 1.69 V at 50 mA cm^?2. The sulfides should be the efficient active species for the HER, while the oxyhydroxides are highly active for the OER. The unique sulfide/oxyhydroxide heterostructure facilitates charge transfer and lowers reaction barrier, thus promoting electrocatalytic processes.     

Bundle-shaped cobalt–nickel selenides as advanced electrocatalysts for water oxidation

Searching for high-performance and earth-abundant electrocatalysts for oxygen evolution reaction (OER) is of paramount significance for overall water splitting to produce hydrogen. Herein, an advanced class of CoNi selenides containing rich oxygen vacancies, with a hierarchical bundle-like and holey nanosheets as noble-metal-free catalysts were first synthesized through a facile hydrothermal method. Benefitting from abundant oxygen vacancies, bundle-like nanostructure, as well as strong synergistic effects, such CoNi selenides demonstrate a greatly enhanced surface area to supply more electrocatalytic active sites to contact with electrolyte, accompanied by largely promoted reaction kinetics, which show outstanding electrocatalytic performances for OER. Remarkably, the optimal Co₁N_i0.5Se can display outstanding OER activity with the small Tafel slope of 48 mV dec⁻¹ and low overpotential of 250 mV (at 10 mA cm⁻²), which are much superior to those of Co/Ni-based catalysts. This electrocatalyst can also maintain high activity and structure stability during long-term electrolysis of 35 h, demonstrating a desirable electrocatalyst for OER. This work elucidates the sophisticated construction of well-defined non-noble metal catalysts for the practical applications in water oxidation.     

Flower-like HEA/MoS2/MoP heterostructure based on interface engineering for efficient overall water splitting

The development of cheap, efficient, and active non-noble metal electrocatalysts for total hydrolysis of water (oxygen evolution reaction (OER) and hydrogen evolution reaction (HER)) is of great significance to promote the application of water splitting. Herein, a heterogeneous structured electrode based on FeAlCrMoV high-entropy alloy (HEA) was synthesized as a cost-effective electrocatalyst for hydrogen and oxygen evolution reactions in alkaline media. In combination of the interfacial synergistic effect and the high-entropy coordination environment, flower-like HEA/MoS₂/MoP exhibited the excellent HER and OER electrocatalytic performance. It showed a low overpotential of 230 mV at the current density of 10 mA cm⁻² for OER and 148 mV for HER in alkaline electrolyte, respectively. Furthermore, HEA/MoS₂/MoP as both anode and cathode also exhibited an overpotential of 1.60 V for overall water splitting. This work provides a new strategy for heterogeneous structure construction and overall water splitting based on high-entropy alloys.     

One-pot preparation of Ni3S2@3-D graphene free-standing electrode by simple Q-CVD method for efficient oxygen evolution reaction

Efficient non-noble metal catalysts for the oxygen evolution reaction (OER) are particularly important in the practical applications of electrocatalytic water splitting (ECWS). Herein, based on a simple quasi chemical vapor deposition (Q-CVD) method, we fabricate a newly Ni₃S₂@3-D graphene free-standing electrode for efficient OER applications. The Ni₃S₂@3-D graphene integrates the advantageous features of 3-D graphene and Ni₃S₂ towards OER, such as more interfacial catalytic sites, pore-rich structure, N-doped structure and good electrical conductivity. Benefiting from the favorable features, the Ni₃S₂@3-D graphene (especially 900 °C sample) exhibits excellent OER performances in alkaline medium, which includes a low on-set potential (1.53 V), low overpotential of 305 mV at a current density of 10 mA cm⁻², and a smaller Tafel slope (50 mV dec⁻¹). This catalyst also shows ultrahigh stability after chronoamperometry response at 10 mA cm⁻² for 48 h with 30% increase in the current density. The present work opens a new approach for the one-pot construction of hybrid materials between metal sulfide and graphene to increase the electrocatalytic activity of non-noble metal OER catalysts.     

Structural engineering of rare-earth-based perovskite electrocatalysts for advanced oxygen evolution reaction

Oxygen evolution reaction (OER) acts an important role in electrochemical water splitting which converts sustainable and renewable energy into hydrogen fuel and the proper electrocatalysts are significant to the high-efficiency electrochemical water splitting. Rare-earth-based perovskites are promising catalysts for OER because of the low cost, the abundance of earth reserves, and coordinated electronic structures. However, their intrinsic OER property is still far from satisfactory from the viewpoint of industrial application, especially the electrocatalytic capability as well as stability under alkaline conditions. Herein, the recent advance in structural engineering of rare-earth-based perovskites for OER is reviewed comprehensively. The electrochemical characteristics enhancement strategy of rare-earth-based perovskites is discussed and organized according to structural engineering including A-site, B-site substitution, composite engineering, and morphology engineering. At last, the challenges and opportunities confronting rare-earth-based perovskites catalysts in superior OER are discussed.     

Bifunctional electrocatalysts for water splitting from a bimetallic (V doped-NixFey) Metal–Organic framework MOF@Graphene oxide composite

An ongoing challenge still lies in the exploration of proficient electrocatalysts from earth-abundant non-precious metals instead of noble metal-based catalysts for clean hydrogen energy through large-Scale electrochemical water splitting. However, developing a non-precious transition metals based, stable electrocatalyst for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) is important challenge for modern energy conversion technology. In this report Vanadium doped bimetallic nickel-iron nanoarray, fabricated by carbon supported architecture through carbonization process for electrochemical water splitting. Three types of catalysts were prepared in different molar ratio of Ni/Fe. The electrocatalytic performance demonstrated that the catalyst with equal mole ratio (0.06:0.06) of Ni/Fe possess high catalytic activity for both OER and HER in alkaline and acidic medium. Besides, our findings revealed that the doping of vanadium could play a strong synergetic effect with Ni/Fe, which provide a small overpotential of 90 mV and 210 mV at 10 mA cm^?2 for HER and OER respectively compared to the other two catalyst counterparts. Also, the catalyst with 1:1 (Ni/Fe) molar ratio showed a high current density of 208 mA cm^?2 for HER at 0.5 M H₂SO₄ and 579 mA cm^?2 for OER at 1 M KOH solution, the both current densities are much higher than the other two catalysts (different Ni/Fe ratio). In addition, the presented catalysts showed extremely good durability, reflecting in more than 20 h of consistent Chronoamprometry study at fixed overpotential η = 250 mV without any visible voltage elevation. Similarly, the (Ni/Fe) equal ratio catalyst showed better corrosion potential 0.209 V vs Ag/AgCl and lower current density 0.594 × 10^?12 A cm^?2 in high alkaline medium. The V-doping, MOF/GO surface defects are significantly increased the corrosion potential of the V-Ni_xFe_y-MOF/GO electrocatalyst. Besides, the water electrolyzed products were analysed by gas chromatography to get clear insights on the formed H₂ and O₂ products.     

Engineering FeNi-based electrocatalysts conjoined with Mo2C grown on carbon spheres toward efficient water oxidation via structure and electronic modulation

The development of active non-noble electrocatalysts for oxygen evolution reaction (OER) is urgently desired to accomplish high-performance electrocatalytic water splitting. Here, we report a unique structure electrocatalyst composed of FeNi and Mo₂C heterojunction as spherical shell supported on carbon sphere as core for efficient OER. With the aid of Mo₂C incorporation, FeNi–Mo₂C@C shows increased specific surface area, more electrocatalytic active sites, improved surface water adsorption, and reduced energy barrier. Benefiting from the synergy of shell-core and heterojunction sturcture, the optimized FeNi–Mo₂C@C exhibits superior activity for OER with an overpotential of 283 mV at 10 mA cm⁻² as well as Tafel slope of 29.2 mV dec⁻¹, which is comparable to that of the benchmark ruthenium oxide. The feasible bond between structural design and electronic alteration enhances the charge transfer efficiency, conductivity, and catalytic kinetics, thus intrinsically boost the electrocatalytic performance. This study hence supports a viable strategy to develop highly-efficient non-precious OER electrocatalysts through structural and electronic modification.     

Spinel NiCo2O4 3-D nanoflowers supported on graphene nanosheets as efficient electrocatalyst for oxygen evolution reaction

Efficient oxygen evolution reaction (OER) electrocatalysts with non-noble metals are very critical for the large-scale exploitation of electrocatalytic hydrogen production systems. To improve the catalytic activity of OER electrocatalysts, several design strategies, such as construction of nanostructures, porous structures and composite materials have been proposed. Herein, spinel NiCo₂O₄ 3-D nanoflowers supported on graphene nanosheets (GNs) are prepared by a simple solvothermal synthesis method as non-noble metal electrocatalysts for OER. The present NiCo₂O₄/GNs composite integrates multiple advantages of nanostructures, porous structures and composite materials, including high surface area, abundant catalytic sites and high stability. Benefiting from the favorable features, the NiCo₂O₄/GNs composite exhibits a better OER performance than NiCo₂O₄ and RuO₂ in alkaline medium, which has a low onset potential (1.50 V), a small Tafel slope (137 mV dec⁻¹). The present work opens a new window for the construction of the carbon-supported 3-D nanostructure of transition metal catalysts with optimizable electrocatalytic performances for electrocatalytic hydrogen production.     

Spatially confined growth of ultrathin NiFe layered double hydroxide nanosheets within carbon nanofibers network for highly efficient water oxidation

The rational design of catalysts with low cost, high efficient and robust stability toward oxygen evolution reaction (OER) is greatly desired but remains a formidable challenge. In this work, a one-pot, spatially confined strategy was reported to fabricate ultrathin NiFe layered double hydroxide (NiFe-LDH) nanosheets interconnected by ultrafine, strong carbon nanofibers (CNFs) network. The as-fabricated NiFe-LDH/CNFs catalyst exhibits enhanced OER catalytic activity in terms of low overpotential of 230 mV to obtain an OER current density of 10 mA cm^?2 and very small Tafel slope of 34 mV dec^?1, outperforming pure NiFe-LDH nanosheets assembly, commercial RuO₂, and most non-noble metal catalysts ever reported. It also delivers an excellent structural and electrocatalytic stability upon the long-term OER operation at a large current of 30 mA cm^?2 for 40 h. Furthermore, the cell assembled by using NiFe-LDH/CNFs and commercial Pt/C as anode (+) and cathode (?) ((+)NiFe-LDH/CNFs||Pt/C(?)) only requires a potential of 1.50 V to deliver the water splitting current of 10 mA cm^?2, 130 mV lower than that of (+)RuO₂||Pt/C(?) couple, demonstrating great potential for applications in cost-efficient water splitting devices.     

Microwave-assisted growth of spherical core-shell NiFe LDH@CuxO nanostructures for electrocatalytic water oxidation reaction

The high energy demand for electrochemical water splitting arises from sluggish oxygen evolution reaction (OER) kinetics. In this regard, Layered double hydroxide (LDH) has been introduced as an outstanding catalyst for the OER due to its exceptional physiochemical and 2D infrastructure properties. Herein, we report the design and synthesiss of core-shell nanostructured electrocatalyst by rationally decorating vertically oriented NiFe LDH ultrathin nanosheets on Cu_xO support (NiFe LDH@Cu_xO) via microwave-assisted hydrothermal reaction. For OER, the NiFe LDH@Cu_xO core-shell nanostructured catalyst demonstrated promising electrocatalytic performance, requiring only 1.43 V onset potential and 270 mV overpotential at 10 mA cm^?2. The NiFe LDH@Cu_xO also outperformed pristine NiFe LDH and iridium oxide (IrO₂) in terms of electrocatalytic activity, durability, and Faradaic efficiency. The fabricated NiFe-LDH@Cu_xO electrocatalyst with outer shell NiFe-LDH ultrathin nanosheets provides numerous exposed active sites, benefits electrolyte diffusion and oxygen gas releasing and also reduces the interfacial charge transfer resistance to enhance OER activity. Furthermore, exclusive core-shell 3D infra-structure effectively prevents NiFe-LDH nanosheets agglomeration and restacking, enhancing electrochemical stability.     

Fe-doped NiCo2S4 catalyst derived from ZIF – 67 towards efficient hydrogen evolution reaction

The development of economical, efficient and stable non-noble metal catalysts plays a key role in electrocatalytic hydrogen evolution. NiCo₂S₄ has been proved to be an efficient non-noble catalyst, to further improve its electrocatalytic performance is a meaningful work. In this paper, the effects of Fe doping on electrochemical performance of NiCo₂S₄ is investigated. The Fe-doped NiCo₂S₄ catalyst is prepared by a facile solvothermal method with metal-organic-framework (MOF, ZIF-67) as template, and it exhibits an improved hydrogen evolution reaction (HER) performance with an overpotential of 181 mV at 10 mA cm^?2, a Tafel slope of 125 mV dec^?1 compared with that of NiCo₂S₄ (252 mV overpotential and 149 mV dec^?1 Tafel slope). The combination of improved conductivity, mesopores architecture retained with the ZIF-67 template, which result the reduced internal resistance, enhanced charge transportation as well as large electrochemical double-layer capacitance. This work provides an effective and synergistic strategy for fabricating NiCo₂S₄-based catalysts toward electrochemical water splitting.     

Directly application of bimetallic 2D-MOF for advanced electrocatalytic oxygen evolution

The development of non-noble oxygen evolution reaction (OER) catalysts with low energy consumption and cost is imperative to produce hydrogen energy from water splitting and enlarge its application. Two-dimensional (2D) metal organic frameworks (MOF) and their derivatives are widely regarded as promising electrocatalysts (EC) for OER due to its unique structural characteristics. Here, by optimizing the molar ratio of Ni to Co, bimetal 2-methylimidazole based 2D-MOF is synthesized and its derivatives are also obtained by phosphorization or oxidation. OER measurements prove that the original MOF structure with 5% Ni/Co molar ratio shows excellent activity with 310 mV overpotential (?) at the current density of 10 mA cm^?2 in 1 M KOH solution for OER, as well as a fairly good electrochemical stability rather than their oxidation or phosphorus derivatives. This work introduces a facile method to prepare bimetal imidazole-based 2D-MOF directly applied in energy conversion field without transformation, making a contribution for opening a new window of widely application of 2D-MOF and improving the hydrogen production efficiency from water splitting.     

Steering the construction of amorphous/crystalline hierarchical structure for accelerating the oxygen evolution reaction rate

As electrochemical water splitting is an extremely promising direction for solving future energy problems and the oxygen evolution reaction (OER) is the key half-reaction in water splitting, it is necessary to develop a new type of highly active and stable non-noble metal catalyst. Herein, we construct a series of hierarchical structure by modifying ZIF-67 on clustered-MnO₂ nanotubes which are used as precursor. The fruit@branch-shaped compounds, marked as X@MnO₂ (X = CoS_x, Co₃O₄ and CoP). Compared with pure MnO₂ nanotubes, the obtained derivatives imaginably result in the unique hierarchical structure and introduce synergistic effect between MnO₂ and Co-based species that result in higher performance. In Particular, owing to the abundant active sites, a facil mass transfer pathway and improved electrical conductivity in amorphous/crystallize hierarchical structure, CoS_x@MnO₂ possesses the highest electrocatalytic activity with a lower overpotential of 329 mV at the current density of 10 mA cm^?2, which is superior to most MnO₂-based catalyst previously reported. Most importantly, to expand this strategy is an effective way to synthesize more high-activity catalysts.     

Influence of cobalt dopant in NiFe2-xCoxO4 (0≤x≤2) on electrochemical catalytic properties

Environmental pollution has increased owing to the excessive consumption of fossil fuels. Thus, the combustion of which does not emit pollutants, has attracted attention for as an alternative fuel in various energy application systems. Using hydrogen energy has considered as an environmentally friendly solution to significantly reduce the dependency on fossil fuels and it can be produced by electrochemical water splitting. However, the sluggish kinetic reaction of the oxygen evolution reaction (OER) during electrochemical water splitting reduces the efficiency of hydrogen energy. Therefore, an excellent electrochemical catalyst with superior electrochemical activity in OER is required to increase the energy conversion efficiency. In this study, a NiFe_2-xCo_xO₄ (0 ≤ x ≤ 2) electrochemical catalyst was prepared by hydrothermal synthesis by controlling the amount of Co dopant. The influence of Co doping on the electrochemical catalytic activity of the prepared sample was evaluated by comparing the oxygen vacancies, bandgap, macroporosity and active surface area. Previous research have reported that abundant oxygen vacancies can improve the electrochemical catalytic activity. However, NiCo₂O₄ with the least number of oxygen vacancies exhibited excellent bifunctional catalytic activity for OER/oxygen reduction reaction (ORR) owing to the development of macropore, large active surface areas, and lower bandgap. Therefore, the bandgap and porosity such as macroporosity, large surface area, are an important factor that exert a greater influence than oxygen vacancies in electrochemical catalysts.     

双金属Ni-Co基析氧反应电催化剂研究进展

  目的  能源消耗的持续增长和化石燃料燃烧带来的环保和能源安全问题已经引起世界各国的广泛关注。因此,发展清洁能源生产技术已成为世界范围内的主要研究重点。氢能具有无污染、比能密度高、资源丰富等特点,是最具潜力的传统化石燃料替代品之一。电催化分解水被认为是最有希望的制氢方法,但阳极上的析氧反应动力学缓慢,能量转换效率低,是大规模制氢的主要瓶颈。与稀有和昂贵的贵金属催化剂相比,镍-钴（Ni-Co）基电催化剂由于具有可调的电子结构、高导电性和低成本优势,有望在碱性溶液中实现卓越的OER活性和耐久性。  方法  文章总结并讨论了在OER中Ni-Co基电催化剂的最新研究发展。重点讨论了Ni-Co基电催化剂的设计和合成,以及在OER过程中提高其电催化性能的研究策略。  结果  为了代替钌、铱等贵金属催化剂,研究者们对Ni-Co基非贵金属催化剂进行了大量研究。包括氧化物、氢氧化物、合金、氮化物、硫化物、磷化物等在内的多种Ni-Co基催化剂通过化学结构的调控,从阳极角度提高了电催化制氢的活性。但这些催化剂又分别面临不同的缺陷,有待进一步研究克服。  结论  开发具有高OER活性的非贵金属催化剂是降低电解水制氢成本,促进氢能产业发展的重要途径。虽然仍有一些技术问题尚未解决限制了Ni-Co基催化剂替代贵金属催化剂,但作为重要的贵金属催化剂替代品,Ni-Co基催化剂的研究为新型催化剂的开发提供了重要选择。     

Nickel sulfide-based electrocatalysts for overall water splitting

Hydrogen is a green energy with sustainability and high energy density. Electrochemical water splitting (EWS) is a promising green strategy for hydrogen production. Noble metal electrocatalysts exhibit excellent electrocatalytic activity in EWS. However, the applications of noble metals in EWS are limited because of their scarcity and high price. Therefore, the research on non-noble metal electrocatalysts has attracted much attention. Among them, nickel sulfide electrocatalysts, with a unique 3D structure, pretty conductivity, and adjustable electronic structure, show significant electrocatalytic activity in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this review, the mechanism of the electrocatalytic reaction, electrochemical parameters, and preparation methods of nickel sulfide are introduced first. Then, the five methods including atomic doping (including cations, anions and diatoms), morphological control, hybridization, integration with nanocarbon, and high-index facets exposure to regulate the electronic structure and active sites of nickel sulfide were illustrated, so as to improve the electrocatalytic activity of nickel sulfide. The electrocatalytic properties of these nickel sulfides were reviewed. However, there are some problems in the research of electrocatalysis, such as how to further improve the conductivity of the electrocatalyst, and the calculation method of current density is not unified. Therefore, our future development direction is to prepare a stable nickel sulfide electrocatalyst, study relevant strategies to simultaneously increase active sites and improve conductivity, and effectively make nickel sulfide into an EWS catalyst with higher performance.     

Rapid synthesis of porous Ni/Co/Fe-LDHs nanosheets for effective electrochemical oxygen evolution reaction and zinc-air batteries

Transition metal compounds, especially layered double hydroxide materials (LDHs), show excellent catalytic activity in oxygen evolution reaction (OER). The ethanol oxidation reaction (EOR) is an innovative alternative anodic reaction to OER for improving the efficiency of water splitting to produce hydrogen. In order to improve the reactivity and explore the similarities and differences of active sites in the two reactions, three kinds of porous LDHs (NiFe, NiCo, CoFe LDHs) were synthesized and a series of tests were carried out. Among them, the best performing OER catalyst is NiFe-LDHs with a low overpotential of 1.44 V vs. RHE at 10 mA cm^?2 and a Tafel slope of 23.85 mV dec^?1. As for the EOR reaction, NiCo-LDHs is the best, with an overpotential of only 1.38 V vs. RHE at 10 mA cm^?2 and a Tafel slope of 71.58 mV dec-1. In addition, compared with OER, the LHDs material exhibited better stability in the EOR. This work provides a new direction for studying the electrocatalytic activity of LDHs materials in OER and EOR.     

ZIF-67-derived nickel–cobalt phosphide nanocubes/N-doped carbon/nickel form composite for efficient overall water splitting

Hydrogen production from electrochemical water splitting is a promising strategy to generating green energy, which requires development of efficient and stable bifunctional catalysts for hydrogen and oxygen evolution reaction (HER/OER). Herein, dual transition metal phosphides/N-doped carbon/Nickel foam composite (CoNiP/NC-NF) is prepared via direct phosphidation of ZIF-67, in which ZIF-67 can control the size and N-doping content of CoNiP/NC, boosting the bifunctional activities for the OER and HER. Then, the overall water splitting is performed by using CoNiP/NC-NF as the cathode and anode, showing a low cell voltage of 1.60 V to reach current density of 10 mA cm⁻². Experimental studies indicate that ZIF-67 influences the electrocatalytic performance, and theoretical studies identify the active component of CoNiP/NC-NF for HER and OER, respectively.