Hierarchically Structured 3D Integrated Electrodes by Galvanic Replacement Reaction for Highly Efficient Water Splitting

A NiFe‐based integrated electrode is fabricated by the spontaneous galvanic replacement reaction on an iron foam. Driven by the different electrochemical potentials between Ni and Fe, the dissolution of surface Fe occurs with electroless plating of Ni on iron foam with no need to access instrumentation and input energy. A facile cyclic voltammetry treatment is subsequently applied to convert the metallic NiFe to NiFeO_x . A series of analytical methods indicates formation of a NiFeO_x film of nanosheets on the iron foam surface. This hierarchically structured three dimensional electrode displays high activity and durability against water oxidation. In 1 m KOH, a current density of 1000 mA cm^?2 is achieved at an overpotential of only 300 mV. This method is readily extended to fabricate CoFe or NiCoFe‐based integrated electrodes for water oxidation. Phosphorization of the bimetallic oxide (NiFeO_x ) generates the bimetallic phosphide (NiFe‐P), which can act as an excellent electrocatalyst for hydrogen production in 1 m KOH. An alkaline electrolyzer is constructed using NiFeO_x and NiFe‐P coated iron foams as anode and cathode, which can realize overall water splitting with a current density of 100 mA cm^?2 at an overpotential of 630 mV.     

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.     

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.     

Ultrasensitive Iron‐Triggered Nanosized Fe–CoOOH Integrated with Graphene for Highly Efficient Oxygen Evolution

Effectively active oxygen evolution reaction (OER) electrocatalysts are highly desired for water splitting. Herein, the design and fabrication of nanometer‐sized Fe‐modulated CoOOH nanoparticles by a novel conversion tailoring strategy is reported for the first time and these nanoparticles are assembled on graphene matrix to construct 2D nanohybrids (Fe? CoOOH/G) with ultrasmall particles and finely modulated local electronic structure of Co cations. The Fe components are capable of tailoring and converting the micrometer‐sized sheets into nanometer‐sized particles, indicative of ultrasensitive Fe‐triggered behavior. The as‐made Fe? CoOOH/G features highly exposed edge active sites, well‐defined porous structure, and finely modulated electron structure, together with effectively interconnected conducting networks endowed by graphene. Density functional theory calculations have revealed that the Fe dopants in the Fe? CoOOH nanoparticles have an enhanced adsorption capability toward the oxygenated intermediates involved in OER process, thus facilitating the whole catalytic reactions. Benefiting from these integrated characteristics, the as‐made Fe? CoOOH/G nanohybrids as an oxygen evolution electrocatalyst can deliver a low overpotential of 330 mV at 10 mA cm^?2 and excellent electrochemical durability in alkaline medium. This strategy provides an effective, durable, and nonprecious‐metal electrocatalyst for water splitting.     

Amorphous Cobalt Boride (Co2B) as a Highly Efficient Nonprecious Catalyst for Electrochemical Water Splitting: Oxygen and Hydrogen Evolution

It is demonstrated that amorphous cobalt boride (Co₂B) prepared by the chemical reduction of CoCl₂ using NaBH₄ is an exceptionally efficient electrocatalyst for the oxygen evolution reaction (OER) in alkaline electrolytes and is simultaneously active for catalyzing the hydrogen evolution reaction (HER). The catalyst achieves a current density of 10 mA cm^?2 at 1.61 V on an inert support and at 1.59 V when impregnated with nitrogen‐doped graphene. Stable performance is maintained at 10 mA cm^?2 for at least 60 h. The optimized catalyst, Co₂B annealed at 500 °C (Co₂B‐500) evolves oxygen more efficiently than RuO₂ and IrO₂, and its performance matches the best cobalt‐based catalysts reported to date. Co₂B is irreversibly oxidized at OER conditions to form a CoOOH surface layer. The active form of the catalyst is therefore represented as CoOOH/Co₂B. EXAFS observations indicate that boron induces lattice strain in the crystal structure of the metal, which potentially diminishes the thermodynamic and kinetic barrier of the hydroxylation reaction, formation of the OOH* intermediate, a key limiting step in the OER.     

Bi‐Functional Iron‐Only Electrodes for Efficient Water Splitting with Enhanced Stability through In Situ Electrochemical Regeneration

Scalable and robust electrocatalysts are required for the implementation of water splitting technologies as a globally applicable means of producing affordable renewable hydrogen. It is demonstrated that iron‐only electrode materials prove to be active for catalyzing both proton reduction and water oxidation in alkaline electrolyte solution with superior activity to that of previously established bi‐functional catalysts containing less abundant elements. The reported bi‐functionality of the iron electrodes is reversible upon switching of the applied bias through electrochemical interconversion of catalytic species at the electrode surface. Cycling of the applied bias results in in‐situ electrochemical regeneration of the catalytic surfaces and thereby extends the catalyst stability and lifetime of the water electrolyzer. Full water splitting at a current density of I = 10 mA cm^?2 is achieved at a bias of ≈2 V, which is stable over at least 3 d (72 one hour switching cycles). Thus, potential‐switching is established as a possible strategy of stabilizing electrode materials against degradation in symmetrical water splitting systems.     

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.     

Multifunctional Single‐Crystallized Carbonate Hydroxides as Highly Efficient Electrocatalyst for Full Water splitting

The controllable synthesis of single‐crystallized iron‐cobalt carbonate hydroxide nanosheets array on 3D conductive Ni foam (FCCH/NF) as a monolithic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) bifunctional electrocatalyst for full water splitting is described. The results demonstrate that the incorporation of Fe can effectively tune the morphology, composition, electronic structure, and electrochemical active surface area of the electrocatalysts, thus greatly enhancing the intrinsic electrocatalytic activity. The optimal electrocatalyst (F_0.25C₁CH/NF) can deliver 10 and 1000 mA cm^?2 at very small overpotentials of 77 and 256 mV for HER and 228 and 308 mV for OER in 1.0 m KOH without significant interference from gas evolution. The F_0.25C₁CH‐based two‐electrode alkaline water electrolyzer only requires cell voltages of 1.45 and 1.52 V to achieve current densities of 10 and 500 mA cm^?2. The results demonstrate that such fascinating electrocatalytic activity can be ascribed to the increase in the catalytic active surface area, facilitated electron and mass transport properties, and the synergistic interactions because of the incorporation of Fe.     

A Large‐Scale Graphene–Bimetal Film Electrode with an Ultrahigh Mass Catalytic Activity for Durable Water Splitting

The practical industralization of water splitting needs high‐efficient and cost‐effective catalytic electrodes. A versatile and scalable solution‐processing method to prepare such a catalytic electrode with high flexibility and conductivity is introduced. This preparation method is applicable for a wide variety of metal species and takes graphene sheets as metal carriers and film‐forming agents, resulting in 100% utilization of raw materials. The obtained graphene–bimetal film has excellent comprehensive performance with high areal activity and superior turnover frequency at a low mass loading of 0.05 mg cm^?2, as well as a record‐high mass activity for oxygen or hydrogen evolution. The assembled two‐electrode configuration can be used in a practical full water splitting system, requiring a cell voltage of 1.58 or 1.50 V at 30 or 70 °C to afford a current density of 10 mA cm^?2; it also exhibits a long‐term durability over 200 h, superior to most of the reported systems for the same purpose. This work provides a new platform for large‐scale and high‐yield production of electrocatalysts and also uncovers the design principles of catalytic electrodes with high mass activity toward industralization.     

Robust Sub‐Monolayers of Co3O4 Nano‐Islands: A Highly Transparent Morphology for Efficient Water Oxidation Catalysis

The scalable synthesis of highly transparent and robust sub‐monolayers of Co₃O₄ nano‐islands, which efficiently catalyze water oxidation, is reported. Rapid aerosol deposition of Co₃O₄ nanoparticles and thermally induced self‐organization lead to an ultra‐fine nano‐island morphology with more than 94% light transmission at a wavelength of 500 nm. These transparent sub‐monolayers demonstrate a remarkable mass‐weighted water oxidation activity of 2070–2350 A g_Co3O4^?1 and per‐metal turnover frequency of 0.38–0.62 s^?1 at an overpotential of 400 mV in 1 m NaOH aqueous solution. This mixed valent cobalt oxide structure exhibits excellent long‐term electrochemical and mechanical stability preserving the initial catalytic activity over more than 12 h of constant current electrolysis and 1000 consecutive voltammetric cycles. The potential of the Co₃O₄ nano‐islands for photoelectrochemical water splitting has been demonstrated by incorporation of co‐catalysts in GaN nanowire photoanodes. The Co₃O₄‐GaN photoanodes reveal significantly reduced onset overpotentials, improved photoresponse and photostability compared to the bare GaN ones. These findings provide a highly performing catalyst structure and a scalable synthesis method for the engineering of efficient photoanodes for integrated solar water‐splitting cells.     

Electroless Plating of Highly Efficient Bifunctional Boride‐Based Electrodes toward Practical Overall Water Splitting

Exploring highly‐efficient and low‐cost electrodes for both hydrogen and oxygen evolution reaction (HER and OER) is of primary importance to economical water splitting. Herein, a series of novel and robust bifunctional boride‐based electrodes are successfully fabricated using a versatile Et₂NHBH₃‐involved electroless plating (EP) approach via deposition of nonprecious boride‐based catalysts on various substrates. Owing to the unique binder‐free porous nodule structure induced by the hydrogen release EP reaction, most of the nonprecious boride‐based electrodes are highly efficient for overall water splitting. As a distinctive example, the Co‐B/Ni electrode can afford 10 mA cm^?2 at overpotentials of only 70 mV for HER and 140 mV for OER, and can also survive at large current density of 1000 mA cm^?2 for over 20 h without performance degradation in 1.0 m KOH. Several boride‐based two‐electrode electrolyzers can achieve 10 mA cm^?2 at low voltages of around 1.4 V. Moreover, the facile EP approach is economically viable for flexible and large size electrode production.     

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.     

Novel Route to Fe‐Based Cathode as an Efficient Bifunctional Catalysts for Rechargeable Zn–Air Battery

Efficient bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts are of great importance for rechargeable metal–air batteries. Herein, FeN_x/C catalysts are synthesized by pyrolysis of thiourea and agarose containing α‐Fe₂O₃ nanoplate as Fe precursor, where α‐Fe₂O₃ nanoplate can prevent the aggregation of carbon sheets to effectively improve the specific surface area during the carbonization process. The FeN_x/C‐700‐20 catalyst displays excellent catalytic performance for both ORR and OER activity in alkaline conditions with more positive onset potential (1.1 V vs the reversible hydrogen electrode) and half‐wave potential, higher stability, and stronger methanol tolerance in alkaline solution, which are all superior to that of the commercial Pt/C catalyst. In this study, the detailed analyses demonstrate that the coexistence of Fe‐based species and high content of Fe‐N_x both play an important role for the catalytic activity. Furthermore, FeN_x/C‐700‐20 as cathode catalyst in Zn–air battery possesses higher charge–discharge stability and power density compared with that of commercial Pt/C catalyst, displaying great potential in practical implementation of for the rechargeable energy devices.     

In Situ Grown Bimetallic MOF‐Based Composite as Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting with Ultrastability at High Current Densities

A newly designed water‐stable NH₂‐MIL‐88B(Fe₂Ni)‐metal–organic framework (MOF), in situ grown on the surface of a highly conducting 3D macroporous nickel foam (NF), termed NFN‐MOF/NF, is demonstrated to be a highly efficient bifunctional electrocatalyst for overall water splitting with ultrastability at high current densities. The NFN‐MOF/NF achieves ultralow overpotentials of 240 and 87 mV at current density of 10 mA cm^?2 for the oxygen evolution reaction and hydrogen evolution reaction, respectively, in 1 m KOH. For the overall water splitting, it requires only an ultralow cell voltage of 1.56 V to reach the current density of 10 mA cm^?2, outperforming the pairing of Pt/C on NF as the cathode and IrO₂ on NF as the anode at the same catalyst loading. The stability of the NFN‐MOF/NF catalyst is also outstanding, exhibiting only a minor chronopotentiometric decay of 7.8% at 500 mA cm^?2 after 30 h. The success of the present NFN‐MOF/NF catalyst is attributed to the abundant active centers, the bimetallic clusters {Fe₂Ni(µ₃‐O)(COO)₆(H₂O)₃}, in the MOF, the positive coupling effect between Ni and Fe metal ions in the MOF, and synergistic effect between the MOF and NF.     

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.     

Efficient Solar‐Driven Water Oxidation over Perovskite‐Type BaNbO2N Photoanodes Absorbing Visible Light up to 740 nm

Photoelectrochemical water splitting using semiconductors absorbing a wide range of visible light is a potentially attractive means of harvesting large portions of the solar spectrum. However, this is also very challenging because narrowing the semiconductor band gap lowers the driving force for photoreactions. Herein, a highly active perovskite BaNbO₂N exhibiting photoexcitation up to 740 nm for water oxidation is reported. The synthesis route, consisting of moderate nitridation and subsequent annealing in inert Ar flow, enhances the crystallinity of the BaNbO₂N surface without inducing the reduction of the Nb species. As a result, a particulate BaNbO₂N photoanode exhibits a photocurrent of 5.2 mA cm^?2 at 1.23 V_RHE under simulated solar irradiation, which is the highest yet reported for an oxynitride responsive at wavelengths above 600 nm. Suppressing the reduction of B‐site cations during the synthesis of perovskite AB(O,N)₃, which otherwise results in surface defects or impurities, is critical for achieving high water oxidation activity.