Radially Aligned Hierarchical Nickel/Nickel-Iron (Oxy)hydroxide Nanotubes for Efficient Electrocatalytic Water Splitting

Densely populated tiny RuO2 crystallites supported by hierarchically porous carbon for full acidic water splitting

The exploitation of highly active bifunctional electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in acidic media has been a subject receiving immense interest. However, the existing catalysts usually suffer from low catalytic efficiency and poor corrosion resistance under acidic conditions. Herein, we report a facile molten salt method to fabricate ruthenium dioxide nanoparticles supported by hierarchically porous carbon (RuO2/PC) as a bifunctional electrocatalyst for full water splitting under strong acidic conditions. The formation of a densely populated nanocrystalline RuO2/carbon heterostructure helps expose catalytic sites, accelerates the mass transfer rate, and further enhances the acid resistance of RuO2 nanoparticles. The as-synthesized RuO2/PC consequently exhibits superior catalytic performance for the OER with an overpotential of 181 mV upon 10 mA cm-2 compared to that of the commercial RuO2 (343 mV) and a comparable performance to Pt/C for the HER (47.5 mV upon 10 mA cm-2) in 0.5 M H2SO4. The RuO2/PC shows promising stability with little degradation over ∼24 h. Impressively, the water electrolyzer based on RuO2/PC shows an overpotential of 326 mV at 10 mA cm-2, much lower than that of the electrolyzer based on the combination of Pt/C and RuO2 (400 mV), indicating its great potential towards practical application.

An electrochemically fabricated cobalt iron oxyhydroxide bifunctional electrode for an anion exchange membrane water electrolyzer

For an anion exchange membrane water electrolyzer (AEMWE), exploring bifunctional electrodes with low cost and high efficiency is a crucial task for future renewable energy systems. Herein, we report a simple method to fabricate cobalt iron oxyhydroxide (Co_zFe_1-zO_xH_y) bifunctional electrodes for AEMWEs. The bifunctional electrodes were prepared via one-pot electrodeposition on Ti paper (TP). By adjusting the electrodeposition conditions, the morphology and composition of Co_zFe_1-zO_xH_y/TP could be controlled. The Co₆₅Fe₃₅O_xH_y/TP electrode demonstrated the highest activity for overall water electrolysis owing to the maximized synergy effect between Co and Fe. The bifunctional activities of Co₆₅Fe₃₅O_xH_y/TP were well retained at -50 and 50 mA cm^-2 for 12 h. Co₆₅Fe₃₅O_xH_y/TP, which shows the highest bifunctional activity, was employed in an AEMWE single cell as the anode and cathode. The AEMWE single cell employing Co₆₅Fe₃₅O_xH_y/TP showed a current density of 0.605 A cm^-2 at a cell voltage of 2.0 V_cell. The calculated energy efficiency of the single cell is 55.7% at 2.0 A cm^-2, which is comparable with those of the state-of-the-art AEMWE single cells with bifunctional electrodes. Furthermore, the cell voltage of the single cell with Co₆₅Fe₃₅O_xH_y/TP showed negligible degradation for 50 h at 0.6 A cm^-2.

Self-standing 2D/2D Co3O4@FeOOH nanosheet arrays as promising catalysts for the oxygen evolution reaction

The rational design of a highly efficient oxygen evolution reaction (OER) is crucial for the practical applications of water electrolysis. Herein, a hybrid Co₃O₄@FeOOH/NF electrode was fabricated by loading FeOOH sheets on the surface of Co₃O₄ nanosheet arrays via a newly developed chemical deposition protocol. The decoration of FeOOH on Co₃O₄ nanosheet arrays not only endows a strong electronic interaction between the two components but also offers sufficient active sites for the OER process. Benefitting from these advantages, Co₃O₄@FeOOH/NF exhibited outstanding OER activity in terms of a low overpotential of 209 mV at 10 mA cm^-2 and a low Tafel slope of 48.9 mV dec^-1. Moreover, nearly steady state operation current and negligible change in the phase and morphology of the catalyst also indicate remarkable stability. This work may provide an important guide for the design of high-performance electrocatalysts for energy conversion applications.

Recent Advances in Layered-Double-Hydroxides Based Noble Metal Nanoparticles Efficient Electrocatalysts

With the energy crisis and environmental pollution becoming more and more serious, it is urgent to develop renewable and clean energy. Hydrogen production from electrolyzed water is of great significance to solve the energy crisis and environmental problems in the future. Recently, layered double hydroxides (LDHs) materials have been widely studied in the electrocatalysis field, due to their unique layered structure, tunable metal species and highly dispersed active sites. Moreover, the LDHs supporting noble metal catalysts obtained through the topotactic transformation of LDHs precursors significantly reduce the energy barrier of electrolyzing water, showing remarkable catalytic activity, good conductivity and excellent durability. In this review, we give an overview of recent advances on LDHs supporting noble metal catalysts, from a brief introduction, to their preparation and modification methods, to an overview of their application in the electrocatalysis field, as well as the challenges and outlooks in this promising field on the basis of current development.

Promoting Bifunctional Water Splitting by Modification of the Electronic Structure at the Interface of NiFe Layered Double Hydroxide and Ag

Electrochemical water splitting is a promising method for the renewable production of high-purity hydrogen via the hydrogen evolution reaction (HER). Ni-Fe layered double hydroxides (Ni-Fe LDHs) are highly efficient materials for mediating the oxygen evolution reaction (OER), a half-reaction for water splitting at the anode, but LDHs typically display poor HER performance. Here, we report the preparation of self-organized Ag@NiFe layered double hydroxide core-shell electrodes on Ni foam (Ag@NiFe/NF) prepared by galvanic etching for mediating both the HER and OER (bifunctional water-splitting electrocatalysis). This synthetic strategy allowed for the preparation of organized hierarchical architectures which displayed improved the electrochemical performance by tuning the electronic structure of the catalyst and increasing the surface area utilization. X-ray photoelectron spectroscopy (XPS) and theoretical calculations revealed that electron transfer from the Ni-Fe LDH to Ag influenced the adsorption of the reaction intermediates leading to enhanced catalytic activity. The Ag@NiFe/NF electrode displayed overpotentials as low as 180 and 80 mV for oxygen and hydrogen evolution, respectively, at a current density of 10 mA cm^-2, and improvements in the specific activity by ∼5× and ∼1.5× for the oxygen and hydrogen evolution reaction, respectively, compared to benchmark NiFe hydroxide materials. Additionally, an integrated water-splitting electrolyzer electrode can be driven by an AA battery.

Synergistically Integrating Nickel Porous Nanosheets with 5d Transition Metal Oxides Enabling Efficient Electrocatalytic Overall Water Splitting

An integration hydrogen adsorption benign component such as a metal with an oxygen-containing reactant adsorption benign component such as metal oxide allows for efficient overall water splitting in alkaline solutions and yet remains a considerable challenge. Herein, 5d transition metal oxide WO₂ and WO₃ (denoted as WO_x) nanoparticles are purposely integrated with a porous Ni nanosheet array grown on nickel foam (NF) to design a strongly coupled Ni/WO_x/NF porous nanosheet array electrocatalyst. Through the anion exchange of Ni(OH)₂ nanosheets with tungstate, followed by hydrogenation treatment, abundant Ni/WO_x interfaces with strong coupling interaction are generated. Benefiting from the strong synergies between Ni and WO_x and the unique nanostructure, Ni/WO_x/NF only requires the overpotentials of 42 mV for hydrogen evolution reaction (HER) and 395.7 mV for oxygen evolution reaction (OER) to achieve the current densities of 10 and 100 mA cm^-2, respectively. Furthermore, the Ni/WO_x/NF can achieve a current density of 10 mA cm^-2 at a low cell voltage of 1.54 V in a two-electrode system. This work opens a novel avenue for the design of high-performance but low-cost electrocatalysts for overall water splitting.

Self-supported cobalt–nickel bimetallic telluride as an advanced catalyst for the oxygen evolution reaction

The Ni-doped Co@CoTe2 electrode exhibits an outstanding OER activity and excellent long-term stability and outperforms most of the well-studied Co-based dichalcogenides.

Laser-Assisted Synthesis and Oxygen Generation of Nickel Nanoparticles

Nowadays, more than ever, environmental awareness is being taken into account when it comes to the design of novel materials. Herein, the pathway to the creation of a colloid of spherical, almost purely metallic nickel nanoparticles (NPs) through pulsed laser ablation in ethanol is presented. A complex description of the colloid is provided through UV-vis spectroscopy and dynamic light scattering analysis, ensuring insight into laser-induced nanoparticle homogenization and size-control of the NPs. The transmission electron spectroscopy revealed spherical nanoparticles with a narrow size distribution, whereas the energy-dispersive X-ray spectroscopy accompanied by the X-ray photoelectron spectroscopy revealed their metallic nature. Furthermore, an example of the application of the colloidal nanoparticles is presented, where a quick, five-min ultrasound modification results in over an order of magnitude higher current densities in the titania-based electrode for the oxygen evolution reaction.

CoNiFe Layered Double Hydroxide/RuO2.1 Nanosheet Superlattice as Carbon-Free Electrocatalysts for Water Splitting and Li-O2 Batteries

Efficient electrocatalysts are highly demanded for oxygen evolution reaction (OER) in water splitting and metal-air batteries. Here, superlattice structured materials composed of CoNiFe layered double hydroxide (LDH)/ruthenium oxide nanosheets are synthesized as carbon-free electrocatalysts for OER. The positively charged CoNiFe LDH and negatively charged RuO_2.1 are alternately stacked at the molecular level into superlattice-like hybrids by electrostatic interaction upon mixing their dispersions under suitable conditions. Such heterostructured composites are found to act as effective catalysts toward OER of water splitting with a small overpotential of 281 mV and Tafel plot of 48.9 mV/decade. Such composites also serve as efficient carbon-free cathode catalysts for aprotic Li-O₂ batteries with remarkable higher specific capacities and lower overvoltages than RuO₂ nanoparticles. The superior performance may be attributed to the peculiar superlattice structure, resulting in strong interfacial electronic coupling, better electrical conductivity, and the suppression of side reactions caused by traditional carbon-based materials. Furthermore, potential difference between RuO_2.1 and CoNiFe LDH nanosheets is observed directly by scanning Kelvin probe microscopy, indicating that electrostatic fields might be induced in the superlattice structures to benefit the transport of electrons and charged ions as well as the catalytic process.

Optimizing Platinum Location on Nickel Hydroxide Nanosheets to Accelerate the Hydrogen Evolution Reaction

Rational electrode design is crucial to promote the performance of the hydrogen evolution reaction (HER) via further enhancing the activity, stability, and utilization of platinum (Pt) in an alkaline electrolyte. Herein, a binder-free low-Pt-content HER electrode, Pt (∼20 μg cm^-2) decorated on nickel hydroxide grown on nickel foam (Pt-Ni(OH)₂-2h-NF20), is fabricated at near room temperature in a test tube. To lower the ohmic resistance, for the first time, the Pt nanoparticles were location-selectively anchored on the bottom of height-controlled vertical Ni(OH)₂ nanosheets via utilizing the mass transfer resistance of the dense Ni(OH)₂ film for chloroplatinate. Furthermore, the excellent mass transfer, high specific surface area of Pt, synergistic effect between Pt with Ni(OH)₂, and stable structure together prompt the resulting electrode with a special structure to exhibit superior HER electrocatalytic activity and stability in 1 M KOH. Typically, this electrode reaches a current density of 35.9 mA cm^-2 at an overpotential of 100 mV, which is over 8 times higher than that of commercial Pt/C, and the overpotential only increases by 20 mV at 100 mA cm^-2 over 150,000 s of stability test. Benefiting from the simple fabrication process, the electrode with an area of 840 cm² was successfully prepared with a steady overpotential of 370 mV at 1000 mA cm^-2 and increased potential of 23 mV over 50 h of stability test.

Construction of hierarchical nickel/cobalt iron-hydroxide and nickel/cobalt selenide nanotubes for efficient electrocatalytic water splitting

Hierarchical Ni/CoFeOH and Ni/CoSe₂ nanotube arrays were fabricated via a three-step strategy, and exhibited enhanced electrochemical performance for OER and HER, respectively.

Nanocomposites Based on Ruthenium Nanoparticles Supported on Cobalt and Nitrogen-Codoped Graphene Nanosheets as Bifunctional Catalysts for Electrochemical Water Splitting

Rational design and engineering of high-efficiency electrocatalysts toward overall water splitting is crucial for the development of hydrogen energy technology. Herein, a facile procedure is described for the preparation of effective bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), where ruthenium nanoparticles are supported on graphene nanosheets that are codoped with atomic cobalt and nitrogen by controlled pyrolysis of melamine-functionalized graphene oxide and metal ion precursors. The obtained nanocomposites (CoNG/Ru) exhibit a remarkable electrocatalytic activity toward both HER and OER in alkaline media, with a respective overpotential of only -15 and +350 mV to reach the current density of 10 mA cm^-2, which is much better than the monometallic counterparts and relevant catalysts in the literature. With CoNG/Ru as bifunctional catalysts for overall water splitting in a two-electrode system, a low potential of 1.58 V is needed to reach the current density of 10 mA cm^-2, which is even better than that with commercial Pt/C and RuO₂ catalysts. This is ascribed to the synergistic interactions between the metal species by metal-metal charge transfer. These results highlight the significance of exploiting the electronic interactions between metal species in carbon-based nanocomposites to develop bifunctional catalysts for electrochemical energy technologies.

Self-template synthesis of defect-rich NiO nanotubes as efficient electrocatalysts for methanol oxidation reaction

Developing robust and inexpensive non-noble metal based anode electrocatalysts is highly desirable for alkaline direct methanol fuel cells (ADMFCs). Herein, we successfully develop a facile self-template synthetic strategy for gram-grade porous NiO nanotubes (NTs) by pyrolyzing a nanorod-like Ni-dimethylglyoxime complex. The pyrolysis temperature highly correlates with the morphology and crystallinity of NiO NTs. The optimal NiO NTs exhibit a large electrochemically active surface area, a fast catalytic kinetics, and a small charge transfer resistance, which induce an outstanding electrocatalytic activity for the methanol oxidation reaction (MOR). Compared with conventional NiO nanoparticles, NiO NTs achieve a 11.5-fold increase in mass activity at 1.5 V for the MOR due to nanotubal morphology and abundant non-vacancy defects on the NiO NT surface. Moreover, NiO NTs have a higher electrocatalytic activity for the intermediates of the MOR (such as formaldehyde and formate) than conventional NiO nanoparticles, which also contribute to MOR activity enhancement. Given the facile synthesis and enhanced electrocatalytic performance, NiO NTs may be promising anode electrocatalysts for ADMFCs.

Fe2+ -Doped Layered Double (Ni, Fe) Hydroxides as Efficient Electrocatalysts for Water Splitting and Self-Powered Electrochemical Systems

Developing nonprecious electrocatalysts with superior activity and durability for electrochemical water splitting is of great interest but challenging due to the large overpotential required above the thermodynamic standard potential of water splitting (1.23 V). Here, in situ growth of Fe²⁺ -doped layered double (Ni, Fe) hydroxide (NiFe(II,III)-LDH) on nickel foam with well-defined hexagonal morphology and high crystallinity by a redox reaction between Fe³⁺ and nickel foam under hydrothermal conditions is reported. Benefiting from tuning the local atomic structure by self-doping Fe²⁺ , the NiFe(II,III)-LDH catalyst with higher amounts of Fe²⁺ exhibits high activity toward oxygen evolution reaction (OER) as well as hydrogen evolution reaction (HER) activity. Moreover, the optimized NiFe(II,III)-LDH catalyst for OER (O-NiFe(II,III)-LDH) and catalyst for HER (H-NiFe(II,III)-LDH) show overpotentials of 140 and 113 mV, respectively, at a current density of 10 mA cm^-2 in 1 m KOH aqueous electrolyte. Using the catalysts for overall water splitting in two-electrode configuration, a low overpotential of just 1.54 V is required at a benchmark current density of 10 mA cm^-2 . Furthermore, it is demonstrated that electrolysis of the water device can be drived by a self-powered system through integrating a triboelectric nanogenerator and battery, showing a promising way to realize self-powered electrochemical systems.

Water Oxidation Catalysts: The Quest for New Oxide-Based Materials

The expected shortage of fossil fuels as well as the accompanying climate change are among the major challenges of the 21st century. A global shift to a sustainable energy landscape is, therefore, of utmost importance. Over the past few years, solar technologies have entered the energy market and have paved the way to replace fossil-based energy sources, in the long term. In particular, electrochemical solar-to-hydrogen technologies have attracted a lot of interest—not only in academia, but also in industry. Solar water splitting (artificial photosynthesis) is one of the most active areas in contemporary materials and catalysis research. The development of low-cost, efficient, and stable water oxidation catalysts (WOCs) remains crucial for artificial photosynthesis applications, because WOCs still represent a major economical and efficient bottleneck. In the following, we summarize recent advances in water oxidation catalysts development, with selected examples from 2016 onwards. This condensed survey demonstrates that the ongoing quest for new materials and informed catalyst design is a dynamic and rapidly developing research area.

3D Coral-Like Ni3S2 on Ni Foam as a Bifunctional Electrocatalyst for Overall Water Splitting

High-performance noble metal-free electrocatalysts are extremely desired for overall water splitting, but there are huge challenges. Herein, we report a novel one-step electrochemical anodic oxidation and cathodic deposition strategy to in situ fabricate three-dimensional coral-like Ni₃S₂ on Ni foam (NF) for electrocatalytic overall water splitting. In a typical two-electrode cell, NF acts as both cathodic and anodic electrodes with thiourea aqueous solution as the electrolyte. The nickel ions from the anodic oxidation of NF are directly used as nickel sources to form 3D coral-like Ni₃S₂/NF by the cathodic deposition method simultaneously. The optimal 3D Ni₃S₂/NF-4 electrode shows high electrocatalytic activity for hydrogen evolution reaction and oxygen evolution reaction with low overpotentials of 89 and 242 mV, respectively, to afford 10 mA cm^-2. When the as-obtained Ni₃S₂/NF-4 is used as a bifunctional electrocatalyst in an electrolyzer, a low applied voltage of 1.577 V is needed to reach 10 mA cm^-2, with extremely long durability. This work focuses on the rational design of unique structures as efficient non-noble metal-based electrocatalysts, which hold great potential for practical applications in electrocatalytic water splitting.