Ultrathin FeOOH Nanolayers with Abundant Oxygen Vacancies on BiVO4 Photoanodes for Efficient Water Oxidation

Photoelectrochemical (PEC) water splitting is a promising method for storing solar energy in the form of hydrogen fuel, but it is greatly hindered by the sluggish kinetics of the oxygen evolution reaction (OER). Herein, a facile solution impregnation method is developed for growing ultrathin (2 nm) highly crystalline β‐FeOOH nanolayers with abundant oxygen vacancies on BiVO₄ photoanodes. These exhibited a remarkable photocurrent density of 4.3 mA cm^?2 at 1.23 V (vs. reversible hydrogen electrode (RHE), AM 1.5 G), which is approximately two times higher than that of amorphous FeOOH fabricated by electrodeposition. Systematic studies reveal that the excellent PEC activity should be attributed to their ultrathin crystalline structure and abundant oxygen vacancies, which could effectively facilitate the hole transport/trapping and provide more active sites for water oxidation.     

A Strongly Coupled Graphene and FeNi Double Hydroxide Hybrid as an Excellent Electrocatalyst for the Oxygen Evolution Reaction

Cost‐effective electrocatalysts for the oxygen evolution reaction (OER) are critical to energy conversion and storage processes. A novel strategy is used to synthesize a non‐noble‐metal‐based electrocatalyst of the OER by finely combining layered FeNi double hydroxide that is catalytically active and electric conducting graphene sheets, taking advantage of the electrostatic attraction between the two positively charged nanosheets. The synergy between the catalytic activity of the double hydroxide and the enhanced electron transport arising from the graphene resulted in superior electrocatalytic properties of the FeNi‐GO hybrids for the OER with overpotentials as low as 0.21 V, which was further reduced to 0.195 V after the reduction treatment. Moreover, the turnover frequency at the overpotential of 0.3 V has reached 1 s^?1, which is much higher than those previously reported for non‐noble‐metal‐based electrocatalysts.     

硫氧键合BiVO4光阳极与FeNi催化剂实现高效水氧化

光电催化分解水可以将充足的太阳能直接转化存储为绿色清洁的氢能,然而光阳极表面缓慢的析氧反应动力学严重限制了太阳能到氢能的转化效率。我们通过一种简单的S-O键合策略实现BiVO4光阳极与FeNi催化剂的界面耦合（S:BiVO4-FeNi）,其光电催化分解水的光电流达到6.43 mA/cm2（1.23 VRHE, AM 1.5G）。进一步研究结果表明：界面S-O键合能够有效实现BiVO4光阳极光生电荷分离并促进空穴向FeNi催化剂表面迁移。同时,S-O键合可以进一步调控FeNi催化剂表面的电荷分布,从而有效提高光电化学分解水析氧活性和稳定性。该工作为设计构建具有高效、稳定的太阳能光电催化分解水体系提供了一种新的研究策略。     

Curing BiVO4 Photoanodes with Ultraviolet Light Enhances Photoelectrocatalysis

Exposure of BiVO₄ photoanodes to ultraviolet (UV) radiation for extended time periods (e.g., 20 h) produces a morphological change and concomitant improvement in photo‐electrocatalytic (PEC) efficiency for driving water splitting directly by sunlight. The ～230 mV cathodic shift in onset potential and doubling of the photocurrent at 1.23 V vs. RHE after UV curing are comparable to the effects engendered by the presence of a secondary catalyst layer. PEC measurements and absorption spectra indicate that the cathodic shift after UV curing corresponds to a suppression of charge recombination and a greater photovoltage generation caused by the shift of the flat‐band potential, and not an improvement in electrocatalytic activity or light absorption. Spectroscopic surface analysis suggests that surface defect sites, which are eliminated by UV curing, for the differences in observed charge recombination.     

Towards Long‐Term Photostability of Nickel Hydroxide/BiVO4 Photoanodes for Oxygen Evolution Catalysts via In Situ Catalyst Tuning

Increasing long‐term photostability of BiVO₄ photoelectrode is an important issue for solar water splitting. The NiOOH oxygen evolution catalyst (OEC) has fast water oxidation kinetics compared to the FeOOH OEC. However, it generally shows a lower photoresponse and poor stability because of the more substantial interface recombination at the NiOOH/BiVO₄ junction. Herein, we utilize a plasma etching approach to reduce both interface/surface recombination at NiOOH/BiVO₄ and NiOOH/electrolyte junctions. Further, adding Fe²⁺ into the borate buffer electrolyte alleviates the active but unstable character of etched‐NiOOH/BiVO₄, leading to an outstanding oxygen evolution over 200 h. The improved charge transfer and photostability can be attributed to the active defects and a mixture of NiOOH/NiO/Ni in OEC induced by plasma etching. Metallic Ni acts as the ion source for the in situ generation of the NiFe OEC over long‐term durability.     

FeNi Alloy Nanoparticles Encapsulated in Carbon Shells Supported on N-Doped Graphene-Like Carbon as Efficient and Stable Bifunctional Oxygen Electrocatalysts

The development of cost-effective and durable oxygen electrocatalysts remains highly critical but challenging for energy conversion and storage devices. Herein, a novel FeNi alloy nanoparticle core encapsulated in carbon shells supported on a N-enriched graphene-like carbon matrix (denoted as FeNi@C/NG) was constructed by facile pyrolyzing the mixture of metal salts, glucose, and dicyandiamide. The in situ pyrolysis of dicyandiamide in the presence of glucose plays a significant effect on the fabrication of the porous FeNi@C/NG with a high content of doped N and large specific surface area. The optimized FeNi@C/NG catalyst displays not only a superior catalytic performance for the oxygen reduction reaction (ORR, with an onset potential of 1.0 V and half-wave potential of 0.84 V) and oxygen evolution reaction (OER, the potential at 10 mA cm⁻² is 1.66 V) simultaneously in alkaline, but also outstanding long-term cycling durability. The excellent bifunctional ORR/OER electrocatalytic performance is ascribed to the synergism of the carbon shell and FeNi alloy core together with the high-content of nitrogen doped on the large specific surface area graphene-like carbon.     

A Theoretical Study on the Mechanism of Photocatalytic Oxygen Evolution on BiVO4 in Aqueous Solution

The oxygen evolution reaction (OER) is regarded as one of the key issues in achieving efficient photocatalytic water splitting. Monoclinic scheelite BiVO₄ is a visible‐light‐responsive semiconductor which has proved to be effective for oxygen evolution. Recently, the synthesis of a series of monoclinic BiVO₄ single crystals was reported, and it was found that the (010), (110), and (011) facets are highly exposed and that the photocatalytic O₂ evolution activity depends on the degree of exposure of the (010) facets. To explore the properties of and photocatalytic water oxidation reaction on different facets, DFT calculations were performed to investigate the geometric structure, optical properties, electronic structure, water adsorption, and the whole OER free‐energy profiles on BiVO₄ (010) and (011) facets. The calculated results suggest both favorable and unfavorable factors for OER on the (010) and the (011) facets. Due to the combined effects of the above‐mentioned factors, different facets exhibit quite different photocatalytic activities.     

A Permselective CeOx Coating To Improve the Stability of Oxygen Evolution Electrocatalysts

Highly active NiFeO_x electrocatalysts for the oxygen evolution reaction (OER) suffer gradual deactivation with time owing to the loss of Fe species from the active sites into solution during catalysis. The anodic deposition of a CeO_x layer prevents the loss of such Fe species from the OER catalysts, achieving a highly stable performance. The CeO_x layer does not affect the OER activity of the catalyst underneath but exhibits unique permselectivity, allowing the permeation of OH^? and O₂ through while preventing the diffusion of redox ions through the layer to function as a selective O₂‐evolving electrode. The use of such a permselective protective layer provides a new strategy for improving the durability of electrocatalysts.     

A Combinatorial Approach for Optimization of Oxygen Evolution Catalyst Loading on Mo‐doped BiVO4 Photoanodes

The incident photon to current efficiency (IPCE) of a photoactive surface strongly depends on the loading and thickness of the active materials. We present a combinatorial approach based on an optical scanning droplet cell for simultaneous deposition and systematic characterization of co‐catalysts for the oxygen evolution reaction (OER) on Mo‐doped BiVO₄ (Mo?BiVO₄) photoanodes electrochemically pre‐deposited on transparent conductive FTO substrates. The loading and photoelectrochemical characterization of 10 different OER co‐catalysts deposited by three different deposition techniques on FTO‐supported Mo?BiVO₄ were investigated aiming at determination of the suitable deposition parameters affording the highest enhancement of photoelectrochemical oxygen evolution for the different OER/Mo?BiVO₄ films. A comparison of the photoelectrochemical performance of films of various OER co‐catalyst deposited on FTO‐supported Mo?BiVO₄ by electrodeposition, photo‐assisted electrodeposition and photodeposition revealed the necessity of a material specific optimization with respect to co‐catalyst loading and deposition technique to achieve optimal IPCE for each co‐catalysts.     

Unveiling the Promotion of Surface-Adsorbed Chalcogenate on the Electrocatalytic Oxygen Evolution Reaction

Transition metal chalcogenides (TMCs) are efficient oxygen evolution reaction (OER) pre-electrocatalysts, and will in situ transform into metal (oxy)hydroxides under OER condition. However, the role of chalcogen is not fully elucidated after oxidation and severe leaching. Here we present the vital promotion of surface-adsorbed chalcogenates on the OER activity. Taking NiSe₂ as an example, in situ Raman spectroscopy revealed the oxidation of Se-Se to selenites (SeO₃²⁻) then to selenates (SeO₄²⁻). Combining the severe Se leaching and the strong signal of selenates, it is assumed that the selenates are rich on the surface and play significant roles. As expected, adding selenites to the electrolyte of Ni(OH)₂ dramatically enhance its OER activity. And sulfates also exhibit the similar effect, suggesting the promotion of surface-adsorbed chalcogenates on OER is universal. Our findings offer unique insight into the transformation mechanism of materials during electrolysis.     

Plasma‐Engraved Co3O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction

Co₃O₄, which is of mixed valences Co²⁺ and Co³⁺, has been extensively investigated as an efficient electrocatalyst for the oxygen evolution reaction (OER). The proper control of Co²⁺/Co³⁺ ratio in Co₃O₄ could lead to modifications on its electronic and thus catalytic properties. Herein, we designed an efficient Co₃O₄‐based OER electrocatalyst by a plasma‐engraving strategy, which not only produced higher surface area, but also generated oxygen vacancies on Co₃O₄ surface with more Co²⁺ formed. The increased surface area ensures the Co₃O₄ has more sites for OER, and generated oxygen vacancies on Co₃O₄ surface improve the electronic conductivity and create more active defects for OER. Compared to pristine Co₃O₄, the engraved Co₃O₄ exhibits a much higher current density and a lower onset potential. The specific activity of the plasma‐engraved Co₃O₄ nanosheets (0.055 mA cm^?2_BET at 1.6 V) is 10 times higher than that of pristine Co₃O₄, which is contributed by the surface oxygen vacancies.     

镍钴复合氧化物的制备和析氧活性

用浸碱热分解法制备了具有较高析氧活性和稳定性的复合氧化物ＮｉＣｏ＿２Ｏ＿４。考察了涂液成分、热分解温度和涂覆次数对ＮｉＣｏ＿２Ｏ＿４析氧活性的影响；测定了氧在ＮｉＣｏ＿２Ｏ＿４从电极上的稳态电化学行为。     

界面增强的CeO2/FeNi MOF高效析氧催化剂

在众多非贵金属基材料中，金属有机骨架（MOFs）因其高比表面积和丰富的金属活性中心而成为最有前景的氧气析出反应（OER）催化剂之一.但MOFs的本征催化活性、导电性和稳定性较差，从而影响其在OER电催化中的应用.本工作通过电沉积法在泡沫镍支撑的FeNi MOF纳米片表面引入5 nm的CeO₂纳米团簇来提高MOFs的催化活性.CeO₂纳米团簇与FeNi MOF纳米片之间的固-固界面相互作用以及CeO₂纳米团簇的掺杂有效调控了MOF表面金属位点的电子结构，提高了金属位点的本征电催化活性；同时，CeO₂团簇良好的导电性促进了FeNi MOF表面的电荷迁移，从而使CeO₂/FeNi MOF的OER活性优于FeNi MOF.在1 mol·L^-1 KOH溶液中CeO₂/FeNi MOF达到50 mA·cm^-2和100 mA·cm^-2的电流密度所需要的过电位分别只有220 mV和233 mV，同时表现出快速的反应动力学和优异的稳定性.     

Enriched Surface Oxygen Vacancies of Photoanodes by Photoetching with Enhanced Charge Separation

A facile photoetching approach is described that alleviates the negative effects from bulk defects by confining the oxygen vacancy (O_vac) at the surface of BiVO₄ photoanode, by 10‐minute photoetching. This strategy could induce enriched O_vac at the surface of BiVO₄, which avoids the formation of excessive bulk defects. A mechanism is proposed to explain the enhanced charge separation at the BiVO₄ /electrolyte interface, which is supported by density functional theory (DFT) calculations. The optimized BiVO₄ with enriched surface O_vac presents the highest photocurrent among undoped BiVO₄ photoanodes. Upon loading FeOOH/NiOOH cocatalysts, photoetched BiVO₄ photoanode reaches a considerable water oxidation photocurrent of 3.0 mA cm^?2 at 0.6 V vs. reversible hydrogen electrode. An unbiased solar‐to‐hydrogen conversion efficiency of 3.5 % is realized by this BiVO₄ photoanode and a Si photocathode under 1 sun illumination.     

A Bifunctional Electrocatalyst for Oxygen Evolution and Oxygen Reduction Reactions in Water

Oxygen reduction and water oxidation are two key processes in fuel cell applications. The oxidation of water to dioxygen is a 4 H⁺/4 e^? process, while oxygen can be fully reduced to water by a 4 e^?/4 H⁺ process or partially reduced by fewer electrons to reactive oxygen species such as H₂O₂ and O₂^?. We demonstrate that a novel manganese corrole complex behaves as a bifunctional catalyst for both the electrocatalytic generation of dioxygen as well as the reduction of dioxygen in aqueous media. Furthermore, our combined kinetic, spectroscopic, and electrochemical study of manganese corroles adsorbed on different electrode materials (down to a submolecular level) reveals mechanistic details of the oxygen evolution and reduction processes.     

Bifunctional Catalysts for Reversible Oxygen Evolution Reaction and Oxygen Reduction Reaction

Metal-air batteries (MABs) and reversible fuel cells (RFCs) rely on the bifunctional oxygen catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Finding efficient bifunctional oxygen catalysts is the ultimate goal and it has attracted a great deal of attention. The dilemma is that a good ORR catalyst is not necessarily efficient for OER, and vice versa. Thus, the development of a new type of bifunctional oxygen catalysts should ensure that the catalysts exhibit high activity for both OER and ORR. Composites with multicomponents for active centers supported on highly conductive matrices could be able to meet the challenges and offering new opportunities. In this Review, the evolution of bifunctional catalysts is summarized and discussed aiming to deliver high-performance bifunctional catalysts with low overpotentials.     

Self-Assembled Two-Dimensional NiO/CeO2 Heterostructure Rich in Oxygen Vacancies as Efficient Bifunctional Electrocatalyst for Alkaline Hydrogen Evolution and Oxygen Evolution

The development of high-efficiency bifunctional electrocatalysts toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline surroundings is essential and challenging for the large-scale generation of clean hydrogen. Herein, a novel self-assembled two-dimensional (2 D) NiO/CeO₂ heterostructure (HS) consisting of NiO and CeO₂ nanocrystals is prepared through a facile two-step approach, and utilized as an enhanced bifunctional electrocatalyst for the HER and OER under alkaline conditions. It is concluded that this 2 D NiO/CeO₂ HS, rich in oxygen vacancies, demonstrates attractive electrocatalytic properties for both the HER and OER in 1 m KOH, including low onset overpotential (η₁), η₁₀ and Tafel slope, excellent durability, as well as large active surface area. Therefore, the self-assembled 2 D NiO/CeO₂ HS is believed to be an efficient bifunctional electrocatalyst toward the HER and OER.     

A Bifunctional Perovskite Catalyst for Oxygen Reduction and Evolution

La_0.3(Ba_0.5Sr_0.5)_0.7Co_0.8Fe_0.2O_3?δ is a promising bifunctional perovskite catalyst for the oxygen reduction reaction and the oxygen evolution reaction. This catalyst has circa 10 nm‐scale rhombohedral LaCoO₃ cobaltite particles distributed on the surface. The dynamic microstructure phenomena are attributed to the charge imbalance from the replacement of A‐site cations with La³⁺ and local stress on Co‐site sub‐lattice with the cubic perovskite structure.     

用于电催化析氧反应电极的制备策略

随着能源枯竭和环境污染等问题的日益严重,开发清洁可再生能源及相关新型技术迫在眉睫。近年来,水分解、金属-空气电池等电化学能源储存和转换技术得到人们的广泛关注。电催化析氧反应(Oxygen evolution reaction,OER)是其中的一个关键反应,大量高性能的OER电催化剂不断见诸报道。除了材料本征催化活性的影响,不同的电极制备方式同样会对催化剂性能的发挥起到重要作用,越来越多的研究者致力于探索高效OER电极的设计与制备方法。本综述从方法论的角度,详细介绍了目前高效OER电极的制备策略,讨论了各类制备方式的优势和不足,总结了相关工作的最新研究进展,概述了新型电极的制备方法。最后,对电极制备策略的发展方向进行了总结与展望。