Photovoltaic-driven Ni(ii)/Ni(iii) redox mediator for the valorization of PET plastic waste with hydrogen production.

Electrocatalytic valorization of PET plastic waste provides an appealing route by converting intermittent renewable energy into valuable chemicals and high-energy fuels. Normally, anodic PET hydrolysate oxidation and cathodic water reduction reactions occur simultaneously in the same time and space, which increases the challenges for product separation and operational conditions. Although these problems can be addressed by utilizing membranes or diaphragms, the parasitic cell resistance and high overall cost severely restrict their future application. Herein, we introduce a Ni(ii)/Ni(iii) redox mediator to decouple these reactions into two independent processes: an electrochemical process for water reduction to produce hydrogen fuel assisted by the oxidation of the Ni(OH)2 electrode into the NiOOH counterpart, followed subsequently by a spontaneous chemical process for the valorization of PET hydrolysate to produce formic acid with a high faradaic efficiency of ∼96% by the oxidized NiOOH electrode. This decoupling strategy enables the electrochemical valorization of PET plastic waste in a membrane-free system to produce high-value formic acid and high-purity hydrogen production. This study provides an appealing route to facilitate the transformation process of PET plastic waste into high-value products with high efficiency, low cost and high purity.

1D/3D Heterogeneous Assembling Body of Cobalt Nitrides for Highly Efficient Overall Hydrazine Splitting and Supercapacitors.

Herein, the construction of a heterostructured 1D/3D CoN-Co2 N@NF (nickel foam) electrode used for thermodynamically favorable hydrazine oxidation reaction (HzOR), as an alternative to sluggish anodic oxygen evolution reaction (OER) in water splitting for hydrogen production, is reported. The electrode exhibits remarkable catalytic activities, with an onset potential of -0.11 V in HzOR and -71 mV for a current density of 10 mA cm-2 in hydrogen evolution reaction (HER). Consequently, an extraordinary low cell voltage of 53 mV is required to achieve 10 mA cm-2 for overall hydrazine splitting in a two-electrode system, demonstrating significant energy-saving advantages over conventional water splitting. The HzOR proceeds through the 4e- reaction pathway to release N2 while the 1e- pathway to emit NH3 is uncompetitive, as evidenced by differential electrochemical mass spectrometric measurements. The X-ray absorption spectroscopy, in situ Raman spectroscopy, and theoretical calculations identify cobalt nitrides rather than corresponding oxides/(oxy)hydroxides as catalytic species for HzOR and illustrate advantages of heterostructured CoN-Co2 N in optimizing adsorption energies of intermediates/reagents and promoting catalytic activities toward both HzOR and HER. The CoN-Co2 N@NF is also an excellent supercapacitive material, exhibiting an increased specific capacity (938 F g-1 at 1 A g-1 ) with excellent cycling stability (95.8%, 5000 cycles).

Engineering of heterointerface of ultrathin carbon nanosheet-supported CoN/MnO enhances oxygen electrocatalysis for rechargeable Zn-air batteries.

Designing bifunctional electrocatalysts with outstanding reactivity and durability towards the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) has remained a long-term aim for metal-air batteries. Achieving the high level of fusion between two distinct metal components to form bifunctional catalysts with optimized heterointerfaces and well-defined morphology holds noteworthy implications in the enhancement of electrocatalytic activity yet challenging. Herein, the fabrication of numerous heterointerfaces of CoN/MnO is successfully realized within ultrathin carbon nanosheets via a feasible self-templating synthesis strategy. Experimental results and theoretic calculations verify that the interfacial electron transfer from CoN to MnO at the heterointerface engenders an ameliorated charge transfer velocity, finely tuned energy barriers concerning reaction intermediates and ultimately accelerated reaction kinetics. The as-prepared CoN/MnO@NC demonstrates exceptional bifunctional catalytic performance, excelling in both OER and ORR showcasing a low reversible overpotential of 0.69 V. Furthermore, rechargeable liquid and quasi-solid-state flexible Zn-air batteries employing CoN/MnO@NC as the air-cathode deliver remarkable endurance and elevated power density, registering values of 153 and 116 mW cm-2 respectively and exceeding Pt/C + RuO2 counterparts and those reported in literature. Deeply exploring the effect of electron-accumulated heterointerfaces on catalytic activity would contribute wisdom to the development of bifunctional electrocatalysts for rechargeable metal-air batteries.

Self-Assembled Core-Shell Structure MgO@TiO2 as a K2CO3 Support with Superior Performance for Direct Air Capture CO2.

Traditional carbon capture and storage technologies for large point sources can at best slow the rate of increase in atmospheric CO2 concentrations. In contrast, direct capture of CO2 from ambient air, or "direct air capture" (DAC), offers the potential to become a truly carbon-negative technology. Composite solid adsorbents fabricated by impregnating a porous matrix with K2CO3 are promising adsorbents for the adsorption capture of CO2 from ambient air. Nevertheless, the adsorbent can be rapidly deactivated during continuous adsorption/desorption cycles. In this study, MgO-supported, TiO2-stabilized MgO@TiO2 core-shell structures were prepared as supports using a novel self-assembled (SA) method and then impregnated with 50 wt % K2CO3 (K2CO3/MgO@TiO2, denoted as SA-KM@T). The adsorbent exhibits a high CO2 capture capacity of ∼126.6 mg CO2/g sorbent in direct air adsorption and maintained a performance of 20 adsorption/desorption cycles at 300 °C mid-temperature, which was much better than that of K2CO3/MgO. Analysis proved that the core-shell structure of the support effectively inhibited the reaction between the active component (K2CO3) and the main support (MgO) by the addition of TiO2, resulting in higher reactivity, thermal stability, and antiagglomeration properties. This work provides an alternative strategy for DAC applications using adsorbents.

Self-supporting, hierarchically hollow structured NiFe-PBA electrocatalyst for efficient alkaline seawater oxidation.

Seawater electrolysis, taking advantage of the huge seawater resource, holds great promise for sustainable hydrogen generation. Compared to conventional water electrolysis, seawater electrolysis is more challenging because of the more complex and corrosive electrolyte and competitive side reactions, which necessitates the development of highly efficient and stable electrocatalysts. In this study, a self-supporting, highly porous NiFe-PBA (Prussian-blue-analogue) electrocatalyst with a hierarchically hollow nanostructure is introduced, which exhibits impressive catalytic performance towards the oxygen evolution in alkaline seawater electrolytes. In NiFe-PBA, the synergistic interaction between Ni and Fe improves intrinsic conductivity for efficient electron transfer, enhances chemical stability in seawater, and boosts overall electrocatalytic activity. The direct use of self-supporting NiFe-PBA as an electrocatalyst avoids the energy-intensive and tedious pyrolysis procedure during the preparation process while making use of the tailored morphological, structural, and compositional benefits of PBA-based materials. By combining the NiFe-PBA catalyst with the NiMoN cathode, the constructed two-electrode electrolyzer achieved a high current density of 500 mA cm-2 at a low cell voltage of 1.782 V for overall electrolysis of alkaline seawater, demonstrating excellent durability for 100 hours. Our findings have important implications for the hydrogen economy and sustainable development through the development of robust and efficient PBA-based electrocatalysts for seawater electrolysis.

Selective CO2 Photoreduction to Acetate at Asymmetric Ternary Bridging Sites.

Photoreduction of CO2 is a promising strategy to synthesize value-added fuels or chemicals and realize carbon neutralization. Noncopper catalysts are seldom reported to generate C2 products, and the selectivity over these catalysts is low. Here, we design rich-interface, heterostructured In2O3/InP (r-In2O3/InP) for highly competitive photocatalytic CO2-to-CH3COOH conversion with a productivity of 96.7 µmol g-1 and selectivity > 96% along with water oxidation to O2 in pure water (no sacrificial agent) under visible light irradiation. The hard X-ray absorption near-edge structure (XANES) shows that the formation of r-In2O3/InP with the isogenesis cation adjusts the coordination environment via interface engineering and forms O-In-P polarized sites at the interface. In situ FT-IR and Raman spectra identify the key intermediates of OCCO* for acetate production with high selectivity. Density functional theory (DFT) calculations reveal that r-In2O3/InP with rich O-In-P polarized sites promotes C-C coupling to form C2 products because of the imbalanced adsorption energies of two carbon atoms. This work reports an interesting indium-based photocatalyst for selective CO2 photoreduction to acetate under strict solution and irradiation conditions and provides significant insights into fabricating interfacial polarization sites to promote the process.

Fabrication of complex, 3D, branched hollow carbonaceous structures and their applications for supercapacitors.

A unique "integrated hard-templating strategy" is described for facile synthesis of a carbonaceous material with a novel three-dimensional (3D) branched hollow architecture. A set of steps, including template formation, surface coating and template removal, all occur in a spontaneous and orderly manner in the one-pot hydrothermal process. Investigations on structural evolution during the process reveal that pre-synthesized zeolitic imidazolate framework-8 (ZIF-8) nanoparticles are first dissociated and then self-assembled into 3D branched superstructures of ZnO as templates. Initial self-assembly is followed by coating of the glucose-derived carbonaceous materials and etching of interior ZnO by organic acids released in situ by hydrolysis of glucose. The 3D-branched hollow architecture is shown to greatly enhance supercapacitor performance. The research described here provides guidance into the development of strategies for complex hollow carbonaceous architectures for a variety of potential applications.

Integrating Bimetal Alloy into N-Doped Carbon Nanotubes@Nanowires Superstructure for Zn-Air Batteries.

Exploring bifunctional oxygen electrocatalysts with low cost and high performance is critical to the development of rechargeable zinc-air batteries, but it still remains a huge challenge. In this work, a "coordination construction-pyrolysis/self-catalyzed growth" approach was employed to fabricate branches@trunks-like, N-doped carbon nanotubes@nanowires superstructure with uniformly incorporated CoFe alloy nanoparticles (CoFe@CNTs-NWs). The rational design of such hierarchical architecture could effectively enlarge the exposure of active sites, modulate their electronic structure, and assist the electron transfer and mass diffusion, thus benefiting both ORR and OER. The resultant CoFe@CNTs-NWs displayed prominent bifunctional electrocatalytic activity and stability with a minimized oxygen overpotential of 0.71 V. When used as a cathode for zinc-air batteries, it provided a high peak power density of 131 mW cm-2 and remarkable charge-discharge stability for at least 400 cycles (130 h). This study presents a successful demonstration for optimizing the electrocatalytic performance by elaborate nanostructure and carbon matrix hybridization with simultaneous modulation of electronic structure, thus providing a new avenue to the rational design of transition metal-based oxygen electrocatalysts.

Superaerophobic copper-based nanowires array for efficient nitrogen reduction.

Electrocatalytic N2 reduction reaction (NRR) provides a promising route for NH3 production under ambient conditions to replace traditional Haber-Bosch process. For this purpose, efficient NRR electrocatalysts with high NH3 yield rate and high Faradaic efficiency (FE) are required. Cu-based materials have been recognized catalytic active for some multi-electron-involved reduction reactions and usually exhibit inferior catalytic activities for hydrogen evolution reaction. We report here the preparation and characterization of a series of Cu-based nanowires array (NA) catalysts in situ grown on Cu foam (CF) substrate, including Cu(OH)2 NA/CF, Cu3N NA/CF, Cu3P NA/CF, CuO NA/CF and Cu NA/CF, which are directly used as self-supported catalytic electrodes for NRR. The electrochemical results show that CuO NA/CF achieves a highest NH3 yield rate of 1.84 × 10-9 mol s-1 cm-2, whereas Cu NA/CF possesses a highest FE of 18.2% for NH3 production at -0.1 V versus reversible hydrogen electrode in 0.1 M Na2SO4. Such catalytic performances are superior to most of recently reported metal-based NRR electrocatalysts. The contact angle measurements and the simulated calculations are carried out to reveal the important role of the superaerophobic NA surface structure for efficient NRR electrocatalysis.

Engineering Two-Phase Bifunctional Oxygen Electrocatalysts with Tunable and Synergetic Components for Flexible Zn-Air Batteries.

Metal-air batteries, like Zn-air batteries (ZABs) are usually suffered from low energy conversion efficiency and poor cyclability caused by the sluggish OER and ORR at the air cathode. Herein, a novel bimetallic Co/CoFe nanomaterial supported on nanoflower-like N-doped graphitic carbon (NC) was prepared through a strategy of coordination construction-cation exchange-pyrolysis and used as a highly efficient bifunctional oxygen electrocatalyst. Experimental characterizations and density functional theory calculations reveal the formation of Co/CoFe heterostructure and synergistic effect between metal layer and NC support, leading to improved electric conductivity, accelerated reaction kinetics, and optimized adsorption energy for intermediates of ORR and OER. The Co/CoFe@NC exhibits high bifunctional activities with a remarkably small potential gap of 0.70 V between the half-wave potential (E1/2) of ORR and the potential at 10 mA cm‒2 (Ej=10) of OER. The aqueous ZAB constructed using this air electrode exhibits a slight voltage loss of only 60 mV after 550-cycle test (360 h, 15 days). A sodium polyacrylate (PANa)-based hydrogel electrolyte was synthesized with strong water-retention capability and high ionic conductivity. The quasi-solid-state ZAB by integrating the Co/CoFe@NC air electrode and PANa hydrogel electrolyte demonstrates excellent mechanical stability and cyclability under different bending states.

Construction of self-supporting, hierarchically structured caterpillar-like NiCo2S4 arrays as an efficient trifunctional electrocatalyst for water and urea electrolysis.

In this study, we have developed intriguing self-supporting caterpillar-like spinel NiCo2S4 arrays with a hierarchical structure of nanowires on a nanosheet skeleton, which can be used as a self-supporting trifunctional electrocatalyst for the oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). The caterpillar-like NiCo precursor arrays are first in situ grown on carbon cloth (NiCo2O4/CC) by a facile hydrothermal reaction, which is followed by an anion exchange process (or sulfuration treatment) with Na2S to form self-supporting spinel NiCo2S4 arrays (NiCo2S4/CC) with a roughened nanostructure. Taking advantage of the bimetallic synergistic effect, the unique hierarchical nanostructure, and the self-supporting nature, the resultant NiCo2S4/CC electrode exhibits high activities toward the OER, HER and UOR, which are highly superior to the monometallic counterparts of NiS nanosheets and Co9S8 nanowires on a carbon cloth substrate. The comparison of the three electrodes also indicates that the hierarchically structured bimetallic electrode combines the morphological and structural characteristics of monometallic Ni-based nanosheets and Co-based nanowires. When assembling a two-electrode electrolytic cell with NiCo2S4/CC as both the anode and cathode, an applied cell voltage of only 1.66 V is required to deliver a current density of 10 mA cm-2 in water electrolysis. By using the same two-electrode setup, the applied voltage for urea electrolysis is further reduced to 1.45 V that produces hydrogen at the cathode with the same current density. This study paves the way for exploring the feasibility of future less energy-intensive and large-scale hydrogen production.

Self-Supported Vanadium Carbide by an Electropolymerization-Assisted Method for Efficient Hydrogen Production.

Exploring efficient electrodes toward the hydrogen evolution reaction (HER) remains a great challenge for large-scale hydrogen production. Owing to its high earth abundance, low electrical resistivity, and small density, vanadium carbide (VC) is a promising HER electrode candidate but has been rarely explored. In this work, VC nanoparticles encased in nitrogen-doped carbon matrix on carbon cloth (VC@NC/CC) were prepared as a binder-free HER cathode through electropolymerization followed by carbothermal reduction under argon. In the first step of pyrrole electropolymerization, the VO4 3- anions, serving as both vanadium source and supporting electrolyte, were homogeneously incorporated in the positively charged polypyrrole (PPy) framework through coulombic interaction. The electropolymerization was effective for preparation of binder-free metal carbide materials with various polymer monomers as carbon source, which was favorable for the high performance of metal carbide electrodes. During the pyrolysis process, the polymeric hybrids were converted to VC nanoparticles and entrapped in the PPy-derived N-doped carbon matrix. The optimized VC@NC/CC electrode exhibited high catalytic activity and durability in both acidic and alkaline media. The use of VC for efficient HER is remarkable, and such a convenient and versatile electropolymerization-assisted method is appealing for the fabrication of industrially scalable large-area VC electrodes for efficient hydrogen production.

The preparation of Ni/Mo-based ternary electrocatalysts by the self-propagating initiated nitridation reaction and their application for efficient hydrogen production.

In the hydrogen evolution reaction (HER), multi-component electrocatalysts with a synergistic effect may possess enhanced catalytic activity and broadened applicability in both acidic and alkaline media. Herein, we developed a novel strategy via the self-propagating initiated nitridation reaction for the synthesis of Mo2C, MoNi4, and Ni2Mo3N nanocrystals as active components assembled in a multiscale porous honeycomb-like carbon (Ni/MoCat@HCC). This strategy can be realized by simply calcining (NH4)6Mo7O24 and Ni(NO3)2 precursor hybrids under a H2/Ar atmosphere at a fairly low temperature of 600 °C. It relies on the in situ thermal decomposition of (NH4)6Mo7O24 and the subsequent nitridation reaction with released NH3, thus avoiding the continuous purging of NH3 in the conventional method. The rich reaction intermediates during the calcination of bimetallic precursors also offer other catalytically active components that are controllable by varying the calcining procedure. Benefiting from the multiscale porous structure, ultrafine size of catalyst particles, and strong synergistic effect of several catalytically active components, the as-prepared Ni/MoCat@HCC exhibits extraordinary HER electrocatalytic activity with low onset overpotentials, small Tafel slopes, and excellent cycling stability in both acidic and alkaline media, outperforming most current noble-metal-free electrocatalysts. This study paves a novel way for synthesizing multi-component electrocatalysts with enhanced catalysis performance.

Rational construction of self-supported triangle-like MOF-derived hollow (Ni,Co)Se2 arrays for electrocatalysis and supercapacitors.

In this work, we have adopted a facile three-step method for constructing an intriguing bifunctional electrode of self-supported hollow (Ni,Co)Se2 arrays with a metal-organic framework (MOF) precursor. The triangle-like cobalt-based MOF arrays are first grown on a carbon cloth at room temperature, which is followed by an ion exchange/etching process with Ni(NO3)2 to form a critical hollow nanostructure with an incorporated hetero-metal element. The intermediate is then transformed into the final product through solvothermal selenization treatment. Taking advantages of the structural and compositional merits as well as the self-supporting nature, the resultant (Ni,Co)Se2 electrode exhibits excellent electrochemical activity and stability. When tested as an electrocatalyst for the oxygen evolution reaction (OER), the (Ni,Co)Se2 array electrode displayed a low onset overpotential of 226 mV and a small overpotential of 256 mV to afford a current density of 10 mA cm-2. The (Ni,Co)Se2 electrode is also utilized in a supercapacitor, which delivers a high specific capacitance of 2.85 F cm-2 at 2 mA cm-2 and exhibits excellent cycling stability with a capacitance retention of 80.8% after 2000 charge-discharge cycles at 20 mA cm-2. These results demonstrate the significance of the rational design of electrode materials and disclose the potential of our MOF-derived hollow (Ni,Co)Se2 array electrode for a variety of practical applications in energy conversion and storage.

In situ autologous growth of self-supporting NiFe-based nanosheets on nickel foam as an efficient electrocatalyst for the oxygen evolution reaction.

A highly efficient and low-cost oxygen evolution reaction electrocatalyst is essential for water splitting. Herein, a simple and cost-effective autologous growth method is developed to prepare NiFe-based integrated electrodes for water oxidation. In this method, a Ni(OH)2 nanosheet film is first developed on nickel foam by oxidative deposition in a chemical bath solution. The as-prepared nanosheet electrode is then immersed into a solution containing Fe(iii) cations to form an Fe-doped Ni(OH)2 electrode by utilization of the different solubility of metal cations. Benefiting from its unique and integrated nanostructure, this hierarchically structured electrode displays extremely high catalytic activity toward water oxidation. In 1 M KOH, the electrode can deliver a current density of 1000 mA cm-2 at an overpotential of only 330 mV. This work provides a facile way to produce an efficient, durable, and Earth-abundant OER electrocatalyst with no energy input, which is attractive for large-scale water splitting.

Conjugated hybrid films based on a new polyoxotitanate monomer.

Electropolymerisation of the novel polyoxotitanate (POT) hexamer [Ti(µ3-O)(OiPr)(TA)]6 (TA = thiophene-3-acetate) with 3,4-ethylenedioxythiophene (EDOT) gives films of hybrid conjugated copolymer, Poly-(EDOT-POT)s, the morphologies of which are, uniquely, influenced by the electropolymerisation potential. Nanoporous Poly-(EDOT-POT)-1 is a fast-ion electrode material and has improved electrochromic properties and significantly higher capacitance than that of the parent poly(3,4-ethylenedioxythiophene) (PEDOT).

Walnut-like Transition Metal Carbides with Three-Dimensional Networks by a Versatile Electropolymerization-Assisted Method for Efficient Hydrogen Evolution.

Mo2C@NPC (N,P-doped carbon) electrocatalysts are developed on carbon cloth (CC) as binder-free cathodes for efficient hydrogen evolution through a facile route of electropolymerization followed by pyrolysis. Electropolymerization of pyrrole to form polypyrrole occurs with the homogeneous incorporation of PMo12, driven by Coulombic force between the positively charged polymer backbone and PMo12 anions. This electrochemical synthesis is easily scaled up, requiring neither complex instrumentation nor an intentionally added electrolyte (PMo12 also acts as an electrolyte). After pyrolysis, the resultant Mo2C@NPC/CC electrode exhibits a unique interconnected walnut-like porous structure, which ensures strong adhesion between the active material and the substrate and favors electrolyte penetration into the electrocatalyst. This method is effective with other monomers such as aniline and is readily extended to fabricate other metal carbide electrodes such as WC@NPC/CC. These carbide electrodes exhibit high catalytic performance for hydrogen production, for example, WC@NPC/CC can deliver an unprecedented current density of 600 mA cm-2 at an overpotential of only 200 mV either in an acidic or an alkaline solution. Considering the simplicity, scalability, and versatility of the synthetic method, the unique electrode structure, and the excellent catalysis performance, this study opens up new avenues for the design of various novel binder-free metal carbide cathodes based on electropolymerization.

Multiscale porous molybdenum phosphide of honeycomb structure for highly efficient hydrogen evolution.

The hydrogen evolution reaction (HER) based on electrochemical water splitting is considered a promising strategy to produce clean and sustainable hydrogen energy. Searching for non-noble metal based electrocatalysts with high efficiency and durability toward the HER is vitally necessary. In this work, we report a novel method for synthesizing molybdenum phosphide (MoP) supported on multiscale porous honeycomb carbon (MoP@HCC) and the application of this catalyst material in acidic media for water electrolysis. Due to the unique structure of the catalyst material, the as-prepared MoP@HCC shows remarkable electrocatalytic activity and stability in 0.5 M H2SO4 aqueous solution. The hybrid catalyst could deliver a current density of 10 mA cm-2 at a low overpotential of 129 mV, with an onset overpotential of 69 mV and a Tafel slope of 48 mV dec-1, outperforming most of the current noble-metal-free electrocatalysts. This study demonstrates an effective way for multiscale control of the MoP structure via overall consideration of the mass transport, and the accessibility, quantity and capability of active sites toward the HER.

A Novel Bimetallic Nickel-Molybdenum Carbide Nanowire Array for Efficient Hydrogen Evolution.

The design and fabrication of noble-metal-free hydrogenevolution electrocatalysts with high activity is significant to future renewable energy systems. In this work, self-supported NiMo carbide nanowires on carbon cloth (Ni3 Mo3 C@NPC NWs/CC; NPC=N,P-doped carbon) were developed through an electropolymerization-assisted procedure. During the synthesis process, NiMoO4 nanowires were first grown on CC through a hydrothermal reaction that was free of any polymer binder such as Nafion. By use of electropolymerization, the as-prepared NiMoO4 NWs/CC sample was then coated by a layer of polypyrole (PPy) that served as the carbon source for subsequent conversion into Ni3 Mo3 C@NPC NWs/CC by carbothermal reduction. The experimental results indicated that judicious choices of the amount of coated PPy and the pyrolysis temperature were essential for obtaining the pure-phase, nanowire array structure of Ni3 Mo3 C@NPC NWs/CC. Benefitting from the pure phase of the bimetallic carbide, the unique architecture of the nanowire array, and its self-supported nature, the optimized Ni3 Mo3 C@NPC NWs/CC electrode exhibited excellent performance in the hydrogen evolution reaction (HER) in both acidic and alkaline media. Low overpotentials of 161 and 215 mV were required to afford a high current density of 100 mA cm-2 toward the HER in acidic and alkaline media, respectively, and the catalytic activity was maintained for at least 48 h, which puts Ni3 Mo3 C@NPC NWs/CC among the best HER electrocatalysts based on metallic carbides yet reported.

Ni nanotube array-based electrodes by electrochemical alloying and de-alloying for efficient water splitting.

The design of cost-efficient earth-abundant catalysts with superior performance for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is extremely important for future renewable energy production. Herein, we report a facile strategy for constructing Ni nanotube arrays (NTAs) on a Ni foam (NF) substrate through cathodic deposition of NiCu alloy followed by anodic stripping of metallic Cu. Based on Ni NTAs, the as-prepared NiSe2 NTA electrode by NiSe2 electrodeposition and the NiFeOx NTA electrode by dipping in Fe3+ solution exhibit excellent HER and OER performance in alkaline conditions. In these systems, Ni NTAs act as a binder-free multifunctional inner layer to support the electrocatalysts, offer a large specific surface area and serve as a fast electron transport pathway. Moreover, an alkaline electrolyzer has been constructed using NiFeOx NTAs as the anode and NiSe2 NTAs as the cathode, which only demands a cell voltage of 1.78 V to deliver a water-splitting current density of 500 mA cm-2, and demonstrates remarkable stability during long-term electrolysis. This work provides an attractive method for the design and fabrication of nanotube array-based catalyst electrodes for highly efficient water-splitting.