Self-supported Nanosheets on Nanorods as High-performance Electrocatalysts Enable Industrial-scale Water Splitting

In nanotechnology, nanorods are a form of nanoobjects with each size ranging from 1–100 nm. They can be synthesized from metallic or semiconductor materials.

Nanorods

Hydrogen (H₂) has the advantages of high energy density and zero carbon emissions, and will play an important role in the carbon neutrality process. Electrolysis of water is a low-cost, environmentally friendly and efficient method to obtain hydrogen. Both the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode require the use of catalysts to reduce overpotential. By using bifunctional electrocatalysts with both HER and OER catalytic functions, the electrolysis system can be simplified and costs can be significantly reduced. At present, bifunctional catalysts have problems such as high electrolysis voltage, limited current density, and poor stability. Therefore, it is crucial to develop efficient, durable, and cheap bifunctional water electrolysis catalysts.

In a recent study, researchers deposited conductive NiMo-based nanorods on the surface of nickel foam, and then electrodeposited NiFe layered double hydroxide (NiFe-LDH) nanosheet arrays on the nanorods to build a three-dimensional (3D) composite structure, and a highly efficient bifunctional alkaline total water splitting electrocatalyst (NiFe-LDH@NiMo-H₂@NF) was obtained.

The catalyst exhibits excellent performance in both HER and OER. When this catalyst is used in water splitting reactions, it only requires a voltage of 1.45/1.61 V to reach current densities of 10/500 cm^-2 respectively. The excellent performance of this material can be attributed to the synergistic effect of the 3D composite structure formed by highly HER active NiMo-based nanorods and highly OER active NiFe-LDH nanosheet arrays. Among them, the NiFe-LDH nanosheet array increases the local pH on the cathode side by inhibiting OH^- diffusion and expands the electrochemically active surface area of the anode side catalyst.

The conductive NiMo-based nanorods on the nickel foam significantly reduce the charge transfer resistance of the two electrodes. This study rationally combines HER and OER active ingredients to provide an effective strategy for the development of advanced bifunctional water electrolysis catalysts.

The researchers first obtained NiMoO₄·xH₂O nanorods grown on NF through a simple hydrothermal reaction, then obtained NiMo-H₂ nanorods through thermal reduction, and finally used electrodeposition to grow NiFe-LDH on the NiMo-H₂ surface. NiFe-LDH@NiMo-H₂@NF was obtained. SEM and TEM photos show that the nanosheets are successfully grown on the surface of the nanorods. The nanorods are mainly composed of Ni and MoO₂, and the nanosheets are NiFe-LDH.

Next, experimental results show that after growing NiFe-LDH, the binding energy of Ni⁰ and Mo⁴⁺ shifts to the direction of high binding energy, while the signal of Fe³⁺ shifts to the direction of low binding energy, which indicates that electrons are transferred from NiMo-H₂ nanorods to NiFe-LDH nanosheets. This electron transfer is beneficial to the water dissociation reaction kinetics and the adsorption and release of intermediates, thereby promoting water electrolysis.

NiFe-LDH@NiMo-H₂@NF can exhibit optimal HER catalytic performance, with only 26 mV overpotential at a current density of 10 mA cm^-2, and at an industrial level of 500 mA cm^-2 At the current density, only an overpotential of 74 mV is required, and the catalyst can be stably catalyzed for more than 400 hours. Unexpectedly, growing NiFe-LDH of HER on NiMo-H₂ greatly promoted the catalytic activity of HER. This is attributed to the NiFe-LDH nanosheet array hindering the diffusion of OH- and increasing the pH near the HER active site, thereby improving the HER catalytic activity.

NiFe-LDH@NiMo-H₂@NF exhibits optimal OER catalytic performance, requiring only 172 mV overpotential at a current density of 10 mA cm^-2, and at an industrial-grade current density of 500 mA cm^-2 , only needs an overpotential of 239 mV, and can stably catalyze for more than 400 hours. The high OER catalytic activity of NiFe-LDH@NiMo-H₂@NF can be attributed to the high density of NiFe-LDH, which enriches the catalytic active sites, increases the electrochemically active surface area, and the highly conductive NiMo-H₂ nanorods reduce the electrochemical chemical impedance.

NiFe-LDH@NiMo-H₂@NF shows the best catalytic performance for total water splitting, requiring a voltage of 1.45/1.61 V to reach a current density of 10/500 cm^-2. It can stably catalyze the total water splitting reaction for more than 1000 h at an industrial-grade current density of 500 mA cm^-2, showing excellent industrial application prospects for water splitting.

Reference

1. Self-supported NiFe-LDH nanosheets on NiMo-based nanorods as high-performance bifunctional electrocatalysts for overall water splitting at industrial-level current densities. Nano Research, 2023