Selectively Etching Vanadium Oxide to Modulate Surface Vacancies of Unary Metal–Based Electrocatalysts for High‐Performance Water Oxidation

Electrocatalytic water splitting for hydrogen generation is hindered by the sluggish kinetics of water oxidation, and highly efficient electrocatalysts for the oxygen evolution reaction (OER) are urgently required. Numerous bi‐ and multimetal‐based, low‐cost, high‐performance OER electrocatalysts have been developed. However, unary metal–based high‐performance electrocatalysts are seldom reported. In the present study, Co₂(OH)₃Cl/vanadium oxide (VO_y) composites are synthesized, from which VO_y is completely etched out by a simple cyclic voltammetry treatment, which simultaneously transforms Co₂(OH)₃Cl in situ to ultrafine CoOOH. The selective removal of VO_y modulates the nature of the surface in the obtained CoOOH by creating surface oxygen vacancies (V_o), along with disordered grain boundaries. The best‐performing CoOOH with optimum V_o is found to be associated with a low overpotential of 282 mV at 10 mA cm^?2 catalytic current density on a simple glassy carbon electrode for OER. This facile protocol of selectively etching VO_y to modulate the nature of the surface is successfully applied to synthesize another Fe‐based electrocatalyst with high OER performance, thus establishing its utility for unary metal–based electrocatalyst synthesis.     

Nitrogen and Sulfur Codoped Graphite Foam as a Self‐Supported Metal‐Free Electrocatalytic Electrode for Water Oxidation

The oxidation of water to produce oxygen gas is related to a variety of energy storage systems. Thus, the development of efficient, cheap, durable, and scalable electrocatalysts for oxygen evolution reaction (OER) is of great importance. Here, a high‐performance OER catalyst, nitrogen and sulfur codoped graphite foam (NSGF) is reported. This NSGF is prepared from commercial graphite foil and directly applied as an electrocatalytic electrode without using a current collector and a polymeric binder. It exhibits an extremely low overpotential of 0.380 V to reach a current density of 10 mA cm^?2 and shows fast kinetics with a small Tafel slope of 96 mV dec^?1 in 0.1 m KOH. This electrocatalytic performance is superior or comparable to those of previously reported metal‐free OER catalysts.     

From Water Oxidation to Reduction: Homologous Ni–Co Based Nanowires as Complementary Water Splitting Electrocatalysts

A homologous Ni–Co based nanowire system, consisting of both nickel cobalt oxide and nickel cobalt sulfide nanowires, is developed for efficient, complementary water splitting. The spinel‐type nickel cobalt oxide (NiCo₂O₄) nanowires are hydrothermally synthesized and can serve as an excellent oxygen evolution reaction catalyst. Subsequent sulfurization of the NiCo₂O₄ nanowires leads to the formation of pyrite‐type nickel cobalt sulfide (Ni_0.33Co_0.67S₂) nanowires. Due to the 1D nanowire morphology and enhanced charge transport capability, the Ni_0.33Co_0.67S₂ nanowires function as an efficient, stable, and robust nonnoble metal electrocatalyst for hydrogen evolution reaction (HER), substantially exceeding CoS₂ or NiS₂ nanostructures synthesized under similar methods. The Ni_0.33Co_0.67S₂ nanowires exhibit low onset potential of ?65, ?39, and ?50 mV versus reversible hydrogen electrode, Tafel slopes of 44, 68, and 118 mV dec^?1 at acidic, neutral, and basic conditions, respectively, and excellent stability, comparable to the best reported non‐noble metal‐based HER catalysts. Furthermore, the homologous Ni_0.33Co_0.67S₂ nanowires and NiCo₂O₄ nanowires are assembled into an all‐nanowire based water splitting electrolyzer with a current density of 5 mA cm^?2 at a voltage as 1.65 V, thus suggesting a unique homologous, earth abundant material system for water splitting.     

Developments and Perspectives in 3d Transition‐Metal‐Based Electrocatalysts for Neutral and Near‐Neutral Water Electrolysis

Technology for producing highly pure hydrogen (99.999%) by water electrolysis is a field of importance in terms of the planets' current energy scenario. A much needed transition from a carbon economy to a hydrogen economy further adds importance to the field of hydrogen generation from water for a sustainable future. To avoid energy losses in the production process, the use of highly acidic (Proton Exchange Membrane (PEM) water electrolyzer) and alkaline (alkaline water electrolyzer) electrolytes is conventional practice in this field. Unfortunately, there are several other issues associated with the use of acidic and alkaline electrolytes such as the requirement of specific ion exchanging membranes with good stability, acid or alkali stable catalysts and corrosive environment withstanding cell stacks, etc. To overcome these issues, researchers have shown interest in the field of electrochemical water splitting in neutral and near‐neutral conditions. In this review, the chronological development of 3d transition‐metal‐based electrocatalysts for neutral and near‐neutral water splitting is extensively discussed with emphases on screening methodologies, mechanisms, structure‐activity correlations, and detailed catalyst specific evolution. In addition, catalysts reported so far, are also benchmarked based on their performance separately for different electrolytes used.     

Electronically Double‐Layered Metal Boride Hollow Nanoprism as an Excellent and Robust Water Oxidation Electrocatalysts

Metal–metalloid compounds have been paid much attention as new high‐performance water oxidation catalysts due to their exceptional durability for water oxidation in alkaline media originating from the multi‐dimensional covalent bonding of the metalloid with the surrounding metal atoms. However, compared to the excellent stability, a relatively low catalytic activity of metal‐metalloids often limits their practical application as high‐performance water oxidation catalysts. Here, for the first time, disclosed is a novel self‐templating strategy combined with atomic layer deposition (ALD) to design the electrochemically active and stable quaternary metal boride (vanadium‐doped cobalt nickel boride, VCNB), hollow nanoprism by inducing electronic double layers on the surface. The incorporation of V in a double‐layered structure can substantially increase the number of surface active sites with unsaturated electronic structure. Furthermore, the induced electronic double layers of V can effectively protect the dissolution of the surface active sites. In addition, density functional theory (DFT) calculations reveal that the impressive water oxidation properties of VCNB originate from the synergetic physicochemical effects of the different metal elements, Co and B as active sites, Ni as a surface electronic structure modifier, and V as a charge carrier transporter and supplier.     

Efficient Bifunctional Polyalcohol Oxidation and Oxygen Reduction Electrocatalysts Enabled by Ultrathin PtPdM (M = Ni,Fe, Co) Nanosheets

Despite intense research in past decades, the development of high‐performance bifunctional catalysts for direct ethylene glycol or glycerol oxidation reaction (EGOR or GOR) and oxygen reduction reaction (ORR) remains a grand challenge in realizing fuel‐cell technologies for portable electronic devices and fuel‐cell vehicle applications. Here, a general method is reported for controllable synthesis of a class of ultrathin multimetallic PtPdM (M = Ni, Fe, Co) nanosheets (NSs) with a thickness of only 1.4 nm by coreduction of metal precursors in the presence of CO and oleylamine. With the optimized composition and components, ultrathin Pt₃₂Pd₄₈Ni₂₀ NSs exhibit the highest electrocatalytic activity for EGOR, GOR, and ORR among all different ultrathin PtPdM NSs, ultrathin PtPd NSs, and the commercial catalysts. The mass activities of ultrathin Pt₃₂Pd₄₈Ni₂₀ NSs for EGOR, GOR, and ORR are 7.7, 5.4, and 7.7 times higher respectively than a commercial catalyst, and they are the most efficient nanocatalysts ever reported for EGOR/GOR. The ultrathin PtPdNi NSs are also very stable for EGOR/GOR/ORR. It is further demonstrated that these ultrathin multimetallic NSs can be readily generalized to other sensor‐related electrocatalysis system such as high‐sensitivity electrochemical detection of H₂O₂.     

Earth‐Abundant Iron Diboride (FeB2) Nanoparticles as Highly Active Bifunctional Electrocatalysts for Overall Water Splitting

Developing efficient, durable, and earth‐abundant electrocatalysts for both hydrogen and oxygen evolution reactions is important for realizing large‐scale water splitting. The authors report that FeB₂ nanoparticles, prepared by a facile chemical reduction of Fe²⁺ using LiBH₄ in an organic solvent, are a superb bifunctional electrocatalyst for overall water splitting. The FeB₂ electrode delivers a current density of 10 mA cm^?2 at overpotentials of 61 mV for hydrogen evolution reaction (HER) and 296 mV for oxygen evolution reaction (OER) in alkaline electrolyte with Tafel slopes of 87.5 and 52.4 mV dec^?1, respectively. The electrode can sustain the HER at an overpotential of 100 mV for 24 h and OER for 1000 cyclic voltammetry cycles with negligible degradation. Density function theory calculations demonstrate that the boron‐rich surface possesses appropriate binding energy for chemisorption and desorption of hydrogen‐containing intermediates, thus favoring the HER process. The excellent OER activity of FeB₂ is ascribed to the formation of a FeOOH/FeB₂ heterojunction during water oxidation. An alkaline electrolyzer is constructed using two identical FeB₂‐NF electrodes as both anode and cathode, which can achieve a current density of 10 mA cm^?2 at 1.57 V for overall water splitting with a faradaic efficiency of nearly 100%, rivalling the integrated state‐of‐the‐art Pt/C and RuO₂/C.     

Self-Supported Fe-Based Nanostructured Electrocatalysts for Water Splitting and Selective Oxidation Reactions: Past,Present, and Future

Electrochemical water splitting plays a vital role in facilitating the transition towards a sustainable energy future by enabling renewable hydrogen (H₂) production, energy storage, and emission-free transportation. Developing earth-abundant electrocatalysts with outstanding overall water-splitting performance, excellent catalytic activity, and robust long-term stability is highly important in the practical application of water electrolysis. Self-supported electrocatalysts have emerged as the most appealing candidate for practical H₂ production due to their increased active site loading, rapid mass and charge transfer, and strong interaction with the underneath conducting support. Additionally, these electrocatalysts also provide enhanced reaction kinetics and stability. Here, a comprehensive review of recent progress in developing self-supported Fe-based electrocatalysts for water splitting and selective oxidation reactions is presented with examples of oxyhydroxides, layered double hydroxides, oxides, chalcogenides, phosphides, nitrides, and other Fe-containing electrocatalysts. A comprehensive historical development in the synthesis of self-supported Fe-based electrocatalysts is provided, with an emphasis on the various deposition methods and the choice of self-supported conducting substrates considering large-scale commercial applications. An overview of mechanistic understanding and approaches for enhanced H₂ production are also presented. Finally, the challenges and opportunities associated with developing Fe-based electrocatalysts for practical applications in water splitting and alternative oxidation reactions are discussed.     

Eutectic‐Derived Mesoporous Ni‐Fe‐O Nanowire Network Catalyzing Oxygen Evolution and Overall Water Splitting

The development of efficient and abundant water oxidation catalysts is essential for the large‐scale storage of renewable energy in the form of hydrogen fuel via electrolytic water splitting, but still remains challenging. Based upon eutectic reaction and dealloying inheritance effect, herein, novel Ni‐Fe‐O‐based composite with a unique mesoporous nanowire network structure is designed and synthesized. The composite exhibits exceptionally low overpotential (10 mA cm^?2 at an overpotential of 244 mV), low Tafel slope (39 mV dec^?1), and superior long‐term stability (remains 10 mA cm^?2 for over 60 h without degradation) toward oxygen evolution reaction (OER) in 1 m KOH. Moreover, an alkaline water electrolyzer is constructed with the Ni‐Fe‐O composite as catalyst for both anode and cathode. This electrolyzer displays superior electrolysis performance (affording 10 mA cm^?2 at 1.64 V) and long‐term durability. The remarkable features of the catalyst lie in its unique mesoporous nanowire network architecture and the synergistic effect of the metal core and the active metal oxide, giving rise to the strikingly enhanced active surface area, accelerated electron/ion transport, and further promoted reaction kinetics of OER.