Manganese-doped nickel-iron bimetallic hydroxide catalyst for efficient electrocatalytic oxygen evolution reaction

Hydrogen energy has many advantages such as wide source, high calorific value, clean and renewable energy, which is considered ideal secondary energy. Under the background of “carbon peak” and “carbon neutral”, the development of hydrogen energy has become the strategic deployment of all countries in the world. Renewable energy is converted into electric energy, and hydrogen production from water electrolysis is further realized through electric energy, which is currently considered as one of the safe and green way of hydrogen production. However, in the actual process of hydrogen production by electrolysis of water, there are problems such as high reaction overpotential and low energy conversion efficiency, which seriously restrict the cost of hydrogen production. During water electrolysis, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) occur at negative and positive electrodes respectively. Compared with the HER process of the two-electron reaction, the four-electron OER process requires a higher overpotential. Therefore, the four-electron OER process becomes the decisive step of the reaction. In order to achieve high efficiency and low energy consumption of hydrogen production process, it is urgent to use cheap, efficient and stable OER catalyst. At present, the search for efficient and low-cost OER catalyst is still the “holy grail” of water splitting. Among many non-noble metal catalysts, NiFe-layered double hydroxides (NiFe-LDHs) are considered as an ideal OER electrocatalyst in alkaline conditions due to their low raw material cost and adjustable structure. However, for NiFe-LDHs laminates, it is generally believed that the edge metal sites have higher catalytic activity than the internal metal sites, which leads to the insufficient utilization of the metal sites inside the laminates and reduces the catalytic activity of the OER reaction. In order to solve the above problems, based on the microstructural regulation of NiFe-LDHs, some methods have been used to improve its catalytic activity and stability in the OER process. Heterogeneous element doping is considered to be an effective method to regulate the electronic structure and electrochemical activity of catalysts. Metal ion doping (Cr, Cu, V, etc.) can optimize the electronic structure of nickel metal active site, reduce reaction overpotential, and improve catalytic efficiency. However, the high biotoxicity of traditional transition metals (Cr, V, Cu, etc.) limits their industrial application. Mn (Mn²⁺Mn³⁺Mn⁴⁺) with rich variation characteristics has a potential role in regulating lamellar charge characteristics. Herein, in order to fully improve the utilization of NiFe-LDHs laminates, this study introduces manganese ions with variable valence characteristics into NiFe-LDHs laminates (Mn-NiFe-LDHs), and utilizes the variable valence characteristics of manganese ions to fully enhance the carrier mobility and promote electron transfer in the laminate. At the same time, due to the electronegativity characteristics of manganese ions, part of the electrons will be transferred from the vicinity of the nickel site to the vicinity of the manganese site, causing the nickel site to exhibit electron-deficient characteristics, which enhance the overall capture of the electron-rich oxygen-containing functional groups of the laminate, thereby effectively enhancing the OER catalytic activity. According to the results of the catalytic reaction, the Mn-NiFe-LDHs electrode exhibits an overpotential of only 332 mV at a current density of 10 mA/cm², which is lower than the initial NiFe-LDHs of 384 mV. The reasonable doping of manganese ions can effectively adjust the Ni²⁺ site activity and enhance its electrocatalytic activity. And to further clarify the effect of doping on the activity of LDHs laminates to provide experimental facts support.