Recent Progress in Electrocatalysts for Acidic Water Oxidation

Hydrogen is a clean and renewable energy carrier for powering future transportation and other applications. Water electrolysis is a promising option for hydrogen production from renewable resources such as wind and solar energy. To date, tremendous efforts have been devoted to the development of electrocatalysts and membranes for water electrolysis technology. In principle, water electrolysis in acidic media has several advantages over that in alkaline media, including favorable reaction kinetics, easy product separation, and low operating pressure. However, acidic water electrolysis poses higher requirements for the catalysts, especially the ones for the oxygen evolution reaction. It is a grand challenge to develop highly active, durable, and cost‐effective catalysts to replace precious metal catalysts for acidic water oxidation. In this article, an overview is presented of the latest developments in design and synthesis of electrocatalysts for acidic water oxidation, emphasizing new strategies for achieving high electrocatalytic activity while maintaining excellent durability at low cost. In particular, the reaction pathways and intermediates are discussed in detail to gain deeper insight into the oxygen evolution reaction mechanism, which is vital to rational design of more efficient electrocatalysts. Further, the remaining scientific challenges and possible strategies to overcome them are outlined, together with perspectives for future‐generation electrocatalysts that exploit nanoscale materials for water electrolysis.     

Active MnOx Electrocatalysts Prepared by Atomic Layer Deposition for Oxygen Evolution and Oxygen Reduction Reactions

The ability to deposit conformal catalytic thin films enables opportunities to achieve complex nanostructured designs for catalysis. Atomic layer deposition (ALD) is capable of creating conformal thin films over complex substrates. Here, ALD‐MnO_x on glassy carbon is investigated as a catalyst for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR), two reactions that are of growing interest due to their many applications in alternative energy technologies. The films are characterized by X‐ray photoelectron spectroscopy, X‐ray diffraction, scanning electron microscopy, ellipsometry, and cyclic voltammetry. The as‐deposited films consist of Mn(II)O, which is shown to be a poor catalyst for the ORR, but highly active for the OER. By controllably annealing the samples, Mn₂O₃ catalysts with good activity for both the ORR and OER are synthesized. Hypotheses are presented to explain the large difference in the activity between the MnO and Mn₂O₃ catalysts for the ORR, but similar activity for the OER, including the effects of surface oxidation under experimental conditions. These catalysts synthesized though ALD compare favorably to the best MnO_x catalysts in the literature, demonstrating a viable way to produce highly active, conformal thin films from earth‐abundant materials for the ORR and the OER.     

A Facile Strategy to Construct Amorphous Spinel‐Based Electrocatalysts with Massive Oxygen Vacancies Using Ionic Liquid Dopant

Oxygen vacancies are demonstrated to be beneficial to various electrocatalytic reactions. However, integrating oxygen vacancies into an amorphous catalyst with a large specific surface area, and investigating its effect on the oxygen evolution reaction remains a great challenge. Herein, oxygen vacancies are introduced into an amorphous N, P, and F tri‐doped CoFe₂O₄ using ionic liquid as a dopant. Simultaneously, ultrafine MoS₂ nanoclusters are anchored onto its surface to increase the specific surface area. The vacancy‐rich MoS₂/NPF‐CoFe₂O₄ exhibits an overpotential of 250 mV and a small Tafel slope of 41 mV dec^?1, which is the best spinel‐based oxygen evolution reaction (OER) electrocatalysts so far. The excellent performance is attributed to massive oxygen vacancies, amorphous structure, large surface area, and synergistic coupling effects among active species. Density‐functional theory calculations reveal that the electronic structure of the catalyst can be modulated in the presence of heteroatoms and MoS₂ nanoclusters, and then the energy barriers of intermediates are decreased as well, which enhances the OER performance. This design not only provides a simple strategy to construct amorphous structures with abundant oxygen vacancies using ionic liquid‐dopants, but also presents an in‐depth insight into the OER mechanism in alkaline solution.     

Opening of Bottleneck Pores for the Improvement of Nitrogen Doped Carbon Electrocatalysts

A facile synthesis strategy to control the porosity of ionothermal nitrogen doped carbons is demonstrated. Adenine is used as cheap and biomass based precursor and a mixture of NaCl/ZnCl₂ as combined solvent‐porogen. Variation of the ratio between the two salt influences the pore structure over a wide range. The eutectic mixture leads to micro‐ and mesoporous material with high total pore volume (TPV) of 3.0 cm³ g^?1 and very high surface area of 2900 m² g^?1 essentially rendering the product an “all‐surface‐area” nitrogen doped carbon. Increasing NaCl contents cause a continuous increase of the mesopore size and the formation of additional macropores resulting in a very high maximal TPV of 5.2 cm³ g^?1, showing 2540 m² g^?1 specific surface area using 60 mol% NaCl. Interestingly, the electrocatalytic activity of the samples toward oxygen reduction is strongly affected by the detailed pore structure. The different—however, chemically equivalent—catalysts vary up to 70 mV in their half wave potentials (E _1/2).The sample with optimized pore system shows a high selectivity toward the favored four electron process and an outstanding E _1/2 of ≈880 mV versus reversible hydrogen electrode (RHE), which is one of the best values reported for nitrogen doped carbons so far.     

Reduced Mesoporous Co3O4 Nanowires as Efficient Water Oxidation Electrocatalysts and Supercapacitor Electrodes

While electrochemical water splitting is one of the most promising methods to store light/electrical energy in chemical bonds, a key challenge remains in the realization of an efficient oxygen evolution reaction catalyst with large surface area, good electrical conductivity, high catalytic properties, and low fabrication cost. Here, a facile solution reduction method is demonstrated for mesoporous Co₃O₄ nanowires treated with NaBH₄. The high‐surface‐area mesopore feature leads to efficient surface reduction in solution at room temperature, which allows for retention of the nanowire morphology and 1D charge transport behavior, while at the same time substantially increasing the oxygen vacancies on the nanowire surface. Compared to pristine Co₃O₄ nanowires, the reduced Co₃O₄ nanowires exhibit a much larger current of 13.1 mA cm^‐2 at 1.65 V vs reversible hydrogen electrode (RHE) and a much lower onset potential of 1.52 V vs RHE. Electrochemical supercapacitors based on the reduced Co₃O₄ nanowires also show a much improved capacitance of 978 F g^‐1 and reduced charge transfer resistance. Density‐functional theory calculations reveal that the existence of oxygen vacancies leads to the formation of new gap states in which the electrons previously associated with the Co‐O bonds tend to be delocalized, resulting in the much higher electrical conductivity and electrocatalytic activity.     

Few‐Layer Black Phosphorus Nanosheets as Electrocatalysts for Highly Efficient Oxygen Evolution Reaction

Black phosphorus (BP) is a new rediscovered layered material, which has attracted enormous interests in the field of electrocatalysis. Recent investigations reveal that bulk BP is a promising electrocatalyst for oxygen evolution reactions (OER), whereas its bulk crystal structure restricts sufficient active sites for achieving highly efficient OER catalytic performances. Toward this end, few‐layer BP nanosheets prepared by facile liquid exfoliation are applied as electrocatalysts and exhibit preferable electrocatalytic OER activity in association with structural robustness; subsequently, the dependence of current density and applied bias potential on the concentration of OH^? has also been uncovered. Most importantly, we are aware that reduction in the thickness of BP nanosheets would generate extra active sites from the ultrathin planar structure and complimenting to the electrocatalytic activities. It is further anticipated that the current work might provide further implementation about the OER performance of BP nanosheets, thereby, offering extendable availabilities for BP‐based electrocatalysts in constructing high‐performance OER devices.     

Nanocarbon Electrocatalysts for Oxygen Reduction in Alkaline Media for Advanced Energy Conversion and Storage

Alkaline oxygen electrocatalysis, targeting anion exchange membrane fuel cells, Zn‐air batteries, and alkaline‐based Li‐air batteries, has become a subject of intensive investigation because of its advantages compared to its acidic counterparts in reaction kinetics and materials stability. However, significant breakthroughs in the design and synthesis of efficient oxygen reduction catalysts from earth‐abundant elements instead of precious metals in alkaline media remain in high demand. Carbon composite materials have been recognized as the most promising because of their reasonable balance between catalytic activity, durability, and cost. In particular, heteroatom (e.g., N, S, B, or P) doping can tune the electronic and geometric properties of carbon, providing more active sites and enhancing the interaction between carbon structure and active sites. Importantly, involvement of transition metals appears to be necessary for achieving high catalytic activity and improved durability by catalyzing carbonization of nitrogen/carbon precursors to form highly graphitized carbon nanostructures with more favorable nitrogen doping. Recently, a synergetic effect was found between the active species in nanocarbon and the loaded oxides/sulfides, resulting in much improved activity. This report focuses on these carbon composite catalysts. Guidance for rational design and synthesis of advanced alkaline ORR catalysts with improved activity and performance durability is also presented.     

Creating Highly Active Atomic Layer Deposited NiO Electrocatalysts for the Oxygen Evolution Reaction

Atomic layer deposition (ALD) provides a promising route for depositing uniform thin coatings of electrocatalysts useful in many technologies, including the splitting of water. For materials such as NiO _x that readily form hydrous oxides, however, the smooth, compact films deposited by ALD may result in higher overpotentials due to low catalyst surface area compared to other deposition methods. Here, the use of ALD–NiO thin films as oxygen evolution reaction (OER) electrocatalysts is explored. Thin films of crystalline ALD­–NiO are deposited and OER activity is tested using cyclic voltammetry (CV). Fe incorporated from the electrolyte can increase the activity of NiO, and it is shown that the turnover frequency (TOF) increases tenfold by going from an Fe‐poor to Fe‐rich KOH electrolyte. Applying a potential exfoliates the NiO, increasing the number of electrochemically accessible Ni sites. Interestingly, by X‐ray photoelectron spectroscopy (XPS) and CV, it is found that an Fe‐rich electrolyte reduces the amount of restructuring and oxidation is found. It is shown that a high surface area, high TOF catalyst may be created by using a two‐step process in which the sample is sequentially conditioned in Fe‐poor then Fe‐rich KOH. This work highlights the importance of pretreatment on catalytic activity for compact NiO films deposited by ALD.     

Graphitic Nanoshell/Mesoporous Carbon Nanohybrids as Highly Efficient and Stable Bifunctional Oxygen Electrocatalysts for Rechargeable Aqueous Na–Air Batteries

Efficient and cost‐effective bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are of vital importance in energy conversion and storage devices. Despite the recent progress in bifunctional oxygen electrocatalysts, their unbalanced and insufficient OER and ORR activities has continued to pose challenges for the practical application of such energy devices. The design of highly integrated, high‐performance, bifunctional oxygen electrocatalysts composed of highly graphitic nanoshells embedded in mesoporous carbon (GNS/MC) is reported. The GNS/MC exhibits very high oxygen electrode activity, which is one of the best performances among nonprecious metal bifunctional oxygen electrocatalysts, and substantially outperforms Ir‐ and Pt‐based catalysts. Moreover, the GNS/MC shows excellent durability for both OER and ORR. In situ X‐ray absorption spectroscopy and square wave voltammetry reveal the roles of residual Ni and Fe entities in enhancing OER and ORR activities. Raman spectra indicate highly graphitic, defect‐rich nature of the GNS/MC, which can contribute to the enhanced OER activity and to high stability for the OER and ORR. In aqueous Na–air battery tests, the GNS/MC air cathode‐based cell exhibits superior performance to Ir/C‐ and Pt/C‐based batteries. Significantly, the GNS/MC‐based cell demonstrates the first example of rechargeable aqueous Na–air battery.     

Co–Ni‐Based Nanotubes/Nanosheets as Efficient Water Splitting Electrocatalysts

One promising approach to hydrogen energy utilization from full water splitting relies on the successful development of earth‐abundant, efficient, and stable electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, homologous Co–Ni‐based nanotube/nanosheet structures with tunable Co/Ni ratios, including hydroxides and nitrides, are grown on conductive substrates by a cation‐exchanging method to grow hydroxides, followed by anion exchanging to obtain corresponding nitrides. These hydroxide OER catalysts and nitride HER catalysts exhibit low overpotentials, small Tafel slopes, and high current densities, which are attributed to their large electrochemically reactive surface, 1D morphologies for charge conduction, and octahedral coordination states of metal ions for efficient catalytic activities. The homologous Co–Ni‐based nanotube hydroxides and nitrides suggest promising electrocatalysts for full water splitting with high efficiency, good stability, convenient fabrication, and low cost.     

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.     

Interfacial Scaffolding Preparation of Hierarchical PBA‐Based Derivative Electrocatalysts for Efficient Water Splitting

The development of highly efficient and durable electrocatalysts is crucial for overall water splitting. Herein, the in situ scaffolding formation of 3D Prussian blue analogues (PBAs) on a variety of 2D or 1D metal hydroxides/oxides to fabricate hierarchical nanostructures is first demonstrated. Typically, cobalt hydroxide or oxide nanoarrays are used as the precursor and structural oriented template for the subsequent growth of 3D PBA nanocubes. The mechanism study reveals that the interfacial scaffolding process can be reversibly controlled via the in situ ion exchange process with adjusting coordination ions. Thus, the facile, versatile strategy can extend to successfully fabricate a variety of hierarchical PBA‐based nanostructures including on cobalt fluoride hydroxide, copper hydroxide, monometal or bimetal nickel–cobalt hydroxides, cobalt oxide, and manganese oxide nanosheets with structural tailor‐ability and chemical diversity. More interestingly, the metal nitride derivatives obtained via controlled calcination process exhibit good electrocatalytic activity for water splitting with low overpotentials, and remarkable durability for 1200 h, thanks to the superior intrinsic activity of bimetal nature and the scrupulous hierarchical structure. This versatile strategy provides a paradigm for rational design of PBA‐based functional nanomaterials, which is highly promising in energy conversion, storage, and electrocatalytic fields.     

Graphitic‐Shell Encapsulation of Metal Electrocatalysts for Oxygen Evolution,Oxygen Reduction,and Hydrogen Evolution in Alkaline Solution

Developing highly efficient, cost effective, and environmentally friendly electrocatalysts for the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) is of interest for sustainable and clean energy technologies, including metal–air batteries and fuel cells. In this work, the screening of electrocatalytic activities of a series of single metallic iron, cobalt, and nickel nanoparticles and their binary and ternary alloys encapsulated in a graphitic carbon shell toward the OER, ORR, and HER in alkaline media is reported. Synthesis of these compounds proceeds by a two‐step sol–gel and carbothermal reduction procedure. Various ex situ characterizations show that with harsh electrochemical activation, the graphitic shell undergoes an electrochemical exfoliation. The modified electronic properties of the remaining graphene layers prevent their exfoliation, protect the bulk of the metallic cores, and participate in the electrocatalysis. The amount of near‐surface, higher‐oxidation‐state metals in the as‐prepared samples increases with electrochemical cycling, indicating that some metallic nanoparticles are not adequately encased within the graphite shell. Such surface oxide species provide secondary active sites for the electrocatalytic activities. The Ni–Fe binary system gives the most promising results for the OER, and the Co–Fe binary system shows the most promise for the ORR and HER.     

Unraveling Geometrical Site Confinement in Highly Efficient Iron‐Doped Electrocatalysts toward Oxygen Evolution Reaction

Introduction of iron in various catalytic systems has served a crucial function to significantly enhance the catalytic activity toward oxygen evolution reaction (OER), but the relationship between material properties and catalysis is still elusive. In this study, by regulating the distinctive geometric sites in spinel, Fe occupies the octahedral sites (Fe³⁺_(Oh)) and confines Co to the tetrahedral site (Co²⁺_(Td)), resulting in a strikingly high activity (η_{j = 10 mA cm}^?2 = 229 mV and η_{j = 100 mA cm}^?2 = 281 mV). Further enrichment of Fe ions would occupy the tetrahedral sites to decline the amount of Co²⁺_(Td) and deteriorate the OER activity. It is also found that similar tafel slope and peak frequency in Bode plot of electrochemical impedance spectroscopy indicate that Co²⁺_(Td) ions are primarily in charge of water oxidation catalytic center. By means of electrochemical techniques and in situ X‐ray absorption spectroscopy, it is proposed that Fe³⁺_(Oh) ions mainly confine cobalt ions to the tetrahedral site to restrain the multipath transfer of cobalt ions during the dynamic structural transformation between spinel and oxyhydroxide, continuously activating the catalytic behavior of Co²⁺_(Td) ions. This material‐related insight provides an indication for the design of highly efficient OER electrocatalysts.