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.     

Electrodeposited Ni-Co-Sn alloy as a highly efficient electrocatalyst for water splitting

Water splitting to produce hydrogen and oxygen is considered as a feasible solution to solve the current energy crisis. It is highly desirable to develop inexpensive and efficient electrocatalyst for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this paper, nanostructured Ni-Co-Sn alloys were electrodeposited on copper foil and the excellent electrocatalytic performances for both HER and OER in alkaline media were achieved. The optimized Ni-Co-Sn electrode shows a low onset overpotential of −18 mV and a small Tafel slope of 63 mV/dec for the HER, comparable to many state-of-the-art non-precious metal HER catalysts. For the OER, it produces an overpotential of 270 mV (1.50 V vs. RHE) at current density of 10 mA/cm², which is better than that of the commercial Ir/C catalyst. In addition to high electrocatalytic activities, it exhibits good stability for both HER and OER. This is the first report that Ni-Co-Sn is served as a cost-effective and highly efficient bifunctional catalyst for water splitting and it will be of great practical value.     

Recent progress in non-noble metal-based electrocatalysts for urea-assisted electrochemical hydrogen production

Water electrolysis is known as an efficient strategy in the direction of green energy production to remove fossil fuels and generate hydrogen. On the other hand, the slow kinetics of the anodic half-reaction (OER) significantly reduces the efficiency of this system. Therefore, choosing an alternative to OER has become a new and reliable approach. Urea oxidation reaction (UOR) is considered an excellent alternative to OER due to its low required potential (0.37V), the abundance of urea sources (industrial waste and human/animal urine), and harmless by-products (N₂, CO₂). Electrocatalysts based on non-noble metals such as nickel, cobalt, molybdenum, manganese, iron, and copper in electrochemical urea-assisted water splitting due to their high electrocatalytic performance and lower price than noble metals play an essential role in reducing costs and increasing the efficiency of this system. This review investigated the electrochemical water splitting reaction and its anodic and cathodic half-reactions. Then, urea, electro-oxidation of urea, methods of making catalysts, measuring parameters of electrocatalytic properties, solutions to improve performance, and types of non-noble catalysts used in this field were reviewed, and finally, challenges and solutions to improve results in the future were introduced.     

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.     

Interface design and composition regulation of cobalt-based electrocatalysts for oxygen evolution reaction

The efficient, stable and low-cost catalyst for water splitting is a key factor especially for the industrial production of green hydrogen. Oxygen evolution reaction (OER), a critical semi-reaction, severely limits the occurrence of water splitting due to the slow kinetics of four-electron transfer. Cobalt-based materials are the well-stocked conductive catalytic feedstock, which can drive down the energy cost needed for OER via lowering the overpotential. Herein, cobalt-based materials and their derivatives in the field of OER catalysis in alkaline, acidic and neutral media are summarized. The interface design and composition regulation strategy are the key approaches for cobalt-based catalysts design in recent years. The related catalytic mechanisms and structure-function relationship have been deeply discussed and analyzed. Finally, for different media environment, a new prospect is proposed for the design of efficient cobalt OER catalytic materials and their application in industrial large-scale energy output strategies.     

Content-dependent electroactivity enhancement of nickel hexacyanoferrate/multi-walled carbon nanotubes electrocatalyst: Cost-efficient construction and promising application for alkaline water splitting

Exploring cost-efficient electrocatalyst towards oxygen evolution reaction is the key to develop sustainable hydrogen energy from water splitting. In this work, based on the different mass contents of MWCNTs, we construct a series of cost-effective noble metal-free NiHCF/MWCNTs (NFM) catalysts for oxygen evolution reaction (OER) in alkaline medium. Thanks to the conductive and 3D interleaved structure of MWCNTs support, the agglomeration of NiHCF nanoparticles is alleviated. NFM catalysts show comparable OER electroactivity with much lower Tafel slope and higher current density than that of bare NiHCF catalyst. In addition, by tuning the content of MWCNTs, as-prepared NFM-2 catalyst shows extensively enhanced electroactivity with the overpotential of only 360 mV at 10 mA cm⁻² in 1.0 M KOH solution. Meanwhile, under the optimum mass content of 14.29 wt% MWCNTs, NFM-2 catalyst exhibits long-term stability. The excellent electrocatalytic oxygen evolution performances suggest synergistic effect between NiHCF and MWCNTs. As a result, the content-dependent NFM composites are believed as feasible non-precious electrocatalysts for alkaline OER.     

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。     

Construction of hierarchical Prussian Blue Analogue phosphide anchored on Ni2P@MoOx nanosheet spheres for efficient overall water splitting

In response to the energy crisis, molybdenum-based catalyst has been proposed as a high-performance electrocatalytic material due to its low price and excellent HER performance. However, in contrast with its excellent HER performance, its poor OER performance often limits practical application as a high-performance overall water splitting catalyst. In this study, Prussian blue analogue (PBA) is grown in-situ on molybdenum-based nanosheet spheres by a simple and ingenious method and then subjected to phosphorization. The resulting composite catalyst exhibits highly efficient overall water splitting performance, overpotentials at current densities of 10 mA cm⁻² and 100 mA cm⁻² for the HER and OER are −61 mV and 268 mV, respectively. Moreover, an alkaline electrolyzer makes up by the catalyst both as positive and negative can reach a cell voltage 1.494 V at 10 mA cm⁻² for the overall water splitting. This method has provided a new strategy to effective combine PBA and molybdenum-based catalyst.     

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.     

CoP-anchored high N-doped carbon@graphene sheet as bifunctional electrocatalyst for efficient overall water splitting

Electrocatalytic water splitting, as an ideal technology in renewable energy applications, suffers from high electrical energy consumption due to the slow kinetics of HER and OER reactions. Therefore, it is urgent to design efficient bifunctional catalysts to improve the reaction kinetics. Herein, a self-supported electrode, anchoring CoP nanoparticles on N-doped carbon/graphene (NC-G) and chemically growing on Ni foam as a whole electrode (denoted as NC-G-CoP/NF) displays promising electrocatalytic performance in 1.0 M KOH electrolyte, with a low overpotentials of 68 mV at 10 mA cm^?2 for HER and 255 mV at 50 mA cm^?2 for OER. This bifunctional electrocatalyst only needs 1.435 V to generate 10 mA cm^?2 for overall water splitting. The outstanding electrocatalytic performance is ascribed to the following factors: i) inherent nature of transition metal phosphides, ii) abundant and high dispersion N active sites in NC-G, iii) strong interaction between the NC-G and CoP nanoparticles, and iv) rapid electron transfer between the catalytic centers and Nickle foam. This provides a new perspective to design efficient electrocatalysts for electrocatalytic water splitting.     

Bifunctional nanoporous ruthenium-nickel alloy nanowire electrocatalysts towards oxygen/hydrogen evolution reaction

A class of ruthenium-nickel alloy catalysts featured with nanoporous nanowires (NPNWs) were synthesized by a strategy combining rapid solidification with two-step dealloying. RuNi NPNWs exhibit excellent electrocatalytic activity and stability for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in which the RuNi-2500 NPNWs catalyst shows an OER overpotential of 327 mV to deliver a current density of 10 mA cm^?2 and the RuNi-0 NPNWs catalyst requires the overpotential of 69 mV at 10 mA cm^?2 showing the best HER activity in alkaline media. Moreover, the RuNi-1500 NPNWs catalyst was used as the bifunctional electrocatalyst in a two-electrode alkaline electrolyzer for water splitting, which exhibits a low cell voltage of 1.553 V and a long-term stability of 24 h at 10 mA cm^?2, demonstrating that the RuNi NPNWs catalysts can be considered as promising bifunctional alkaline electrocatalysts.     

Take a cue from nature: Promoting electrocatalytic watersplitting with a helping hand of hemoglobin

Electrocatalytic water splitting is a promising approach to generating hydrogen from water. In order to enhance water splitting efficiency, it is essential to promote gas revolution from catalysts surface, reduce the over-potential of oxygen evolution (OER), and inhibit the production of the hydrogen peroxide by-product. To realize them, in this work, we take a cue from nature to promote water splitting activity of hollow porous Fe₃O₄ microspheres (M?Fe₃O₄) with the aids of hemoglobin. Hemoglobin monolayer was self-assembled on the surface of M?Fe₃O₄ catalysts. It transported newly-generated oxygen molecules away from catalysts surface and exhibited chiral-induced spin selectivity (CISS) effect during OER reaction. Owing to the “helping hand” of hemoglobin, the OER onset potential of hollow porous Fe₃O₄ microspheres reduced by 100 mV and the current density was enhanced 2 folds. The results indicate a new strategy for designing earth-abundant catalysts which combine the merits of oxygen transferring and CISS effect for electrocatalytic water splitting.     

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.     

NiFeCo oxide as an efficient and sustainable catalyst for the oxygen evolution reaction

It has become essential to develop efficient, economical, and earth-abundant catalysts for clean, environmentally friendly, and sustainable fuel production. Herein we describe the fabrication of a ternary NiFeCo oxide catalyst with a RF-magnetron sputtering technique and investigated as durable catalyst oxygen evolution reaction (OER). A comparative study of the electrocatalytic activities of pure, bimetallic, and trimetallic catalysts is performed. With the synergetic effects of electronic structural modification and the large electrochemical area of the ternary NiFeCo oxide catalyst, current densities of 1 and 10 mA cm⁻² are attained at small overpotentials of 248 and 280 mV, respectively. The relatively low Tafel slope of the trimetallic electrode (32.25 mV dec⁻¹) indicates favorable catalytic kinetics during OER. This is attributed to the catalytic performance of metal constituents with high oxidation states. The electrode exhibits outstanding durability during rigorous oxygen evolution testing in 1 M KOH for 500 hours. Simple sputtering deposition of multimetallic oxide catalyst films can be applied to fabricate other multimetallic catalysts and electrodes for robust, efficient water spitting, and electrochemical energy storage.     

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.     

Engineering anion defect in LaFeO2.85Cl0.15 perovskite for boosting oxygen evolution reaction

Development of cost-effective, highly-active and durable catalysts for oxygen evolution reaction (OER) is critical to sustainable energy conversion and storage devices. Perovskite oxides are exploited as a research frontier on OER, however, their activity and stability are still far from desirable. Herein, we highlight a paradigm shift in the design of highly efficient perovskite-type catalysts for OER by engineering anion defect. The Cl-doped LaFeO_2.85Cl_0.15 perovskite demonstrates an enhanced OER activity, which is attributed to the abundant surface oxygen vacancies and low adsorption energy of H₂O molecule after Cl doping. Moreover, the LaFeO_2.85Cl_0.15 catalyst also delivers an improved electrocatalytic durability, in contrast to the pristine LaFeO₃ catalyst.     

Multi-active site CoNi-MOFs as non-noble bifunctional electrocatalysts for highly efficient overall water splitting

The high-efficiency non-precious metal catalysts for oxygen evolution (OER) and hydrogen evolution (HER) are of great significance to the development of renewable energy technologies. Herein, a multiple active sites CoNi-MOFs-DBD electrocatalyst modified by low temperature plasma (DBD) was successfully synthesized by converting metal hydroxyfluoride on nickel foam into a well-arranged MOFs array using vapor deposition. The as-prepared CoNi-MOFs-DBD electrode showed better HER and OER catalytic activity, super hydrophilicity, and excellent stability. In an alkaline medium, the overpotential of HER is 203 mV at 10 mA cm^?2 and that of OER is 168 mV at 40 mA cm^?2. When CoNi-MOFs-DBD was used as a bifunctional electrocatalyst for overall water splitting in a two-electrode system, a current density of 10 mA cm^?2 can be achieved at a low voltage of 1.42 V, which shows great potential in electrocatalytic water splitting.     

Design of Ni/NiO–TiO2/rGO nanocomposites on carbon cloth conductors via PECVD for electrocatalytic water splitting

The facile synthesis and design of noble metal-free efficient catalysts to accelerate the sluggish kinetics of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is still a big challenge for electrolytic water splitting. In that context, the preparation of efficient catalysts with superior catalytic activity from cheap raw materials on a large scale is crucial. Briefly, Ni/NiO/TiO₂/rGO is designed using the environmental-friendly and easily up-scalable PECVD technique. This trinary composite presents significance in regulating the crystalline structure, composition and electronic properties towards superior HER and OER activity in acidic solution as bifunctional electrocatalysts for efficient water splitting. Together with the promising long-term stability and durability, Ni/NiO/TiO₂/rGO displays excellent electrocatalytic activity towards HER with η₁₀ of 130 mv vs RHE and a Tafel slope of 40 mV/dec.     

Interface engineering and heterometal doping Co–Mo/FeS for oxygen evolution reaction

The sluggish kinetics of the oxygen evolution reaction (OER) limits the development of water electrolysis technology and the long-term efficiency of hydrogen energy production. In addition, it is important to evaluate the reconstruction performance of OER catalysts for actual water electrolysis. We created a self-supported electrode with FeS film coated Fe foam as a substrate, ordered resoluble molybdate (MoO₄²⁻) anions in interlayers, and Co-doped as a catalytically active phase for the OER. The catalyst is capable of electrochemical self-reconstruction (ECSR). With the dissolution of molybdate and sulfur ions, the catalyst surface cobalt iron oxide (CoFe₂O₄) forms an active amorphous FeCoOOH, which is favorable for alkaline OER. We realized the introduction of new active sites in the catalyst reconstruction process. Finally, the composite CoFeO_x catalyst increased the specific surface area, promoted bubble transport, and enhanced electron mass transfer. The synergistic coupling effect of the catalyst makes it have excellent OER activity and stability. Remarkably, Co–Mo/FeS nanosheets afforded an electrocatalytic OER with a current density of 100 mA cm⁻² at a low overpotential of 321 mV. These discoveries open up new opportunities for the application of doping and template-directed surface reconfiguration, which holds promise as an effective electrocatalyst for the OER.     

Chiral CuO@Ni with continuous macroporous framework and its high catalytic activity for electrochemical water oxidation

Electrochemical water splitting is considered as a promising approach to storing renewable electricity in the form of hydrogen fuel. In this work, we report the design and electrocatalytic properties of chiral CuO@Ni with three dimensional (3D) continuous macroporous framework. With the chiral CuO@Ni as anode, the OER overpotential required to achieve the current density of 10 mA/cm² was as low as 110 mV in 0.1 M KOH electrolyte, and the hydrogen production rate of water splitting reaction could reach 1070 nL/s. Moreover, the OER overpotential could be regulated easily by controlling the deposition time of chiral CuO layer on Ni. The high catalytic activity of chiral CuO@Ni for water splitting is closely associated with the Chiral-induced spin selectivity (CISS) effect, the large reactive area provided by its 3D macroporous structure, and the effective cooperation between chiral CuO and Ni foam that facilitated the transportation of spin aligned electrons from chiral CuO layer to Ni. The results shown in this work indicate a simple and promising strategy to improve the electrocatalytic activity of other chiral earth-abundant catalysts for water splitting.