Facile Synthesis of CeO₂ Decorated Co₄N Nanoparticles as Highly Active and Durable Electrocatalyst for Oxygen Evolution Reaction

Growing concerns about depletion of fossil fuels and global warming demand the exploration of eco-friendly and efficient energy sources. Recently, hydrogen gas has been considered as a promising candidate for future energy source owing to its environmental friendliness and high energy density. Electrochemical water splitting, involving cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER), is an ideal technology for hydrogen production without any other impurities such as carbon dioxide. Unfortunately, the overall efficiency of water electrolysis is seriously limited by the sluggish kinetics and large overpotential of OER due to the energetically unfavorable four-electron reaction pathway. Therefore, the development of efficient and durable OER electrocatalyst is essential to decrease the reaction energy barrier for OER and enhance the performance of the overall water electrolysis.<br/>Among the noble metal-free materials such as transition metal oxides, sulfides, phosphides, nitrides <i>etc</i>., the transition metal nitrides (TMNs) showed promising electrocatalytic activity toward the OER. The TMNs have anti-corrosion stability in alkaline solution and high electrical conductivity due to their metallic feature. Furthermore, the nitrogen atoms in TMNs induce expansion of metal lattice and modify transition metal d-band structure, thereby shifting their d-band center into the optimal energy state for OER. Despite the noticeable progress in TMNs-based electrocatalysts, their unsatisfactory OER performance need to be further improved because of inferior redox reaction kinetics of TMNs.<br/>To boost electrocatalytic OER activity of TMNs, construction of heterostructure with other species which has rich redox properties can be adopted as efficient strategy. The formation of well-defined heterostructure not only offer abundant electrochemical active sites but optimize the electronic properties of catalysts owing to strong interfacial coupling effect between the two components, thus leading to enhanced oxygen evolution performance. Recently, cerium oxide (CeO₂) has been employed as an activating agent for promoting electrocatalytic oxygen evolution, which is attributed to its rich redox property owing to versatile oxidation state between Ce³⁺ and Ce⁴⁺. Furthermore, the CeO₂ has abundant empty d orbitals and oxygen vacancy defects, leading to strong electron interaction and introduction of efficient active sites for OER in alkaline solution.<br/>Herein, inspired by above mentioned considerations, we proposed a facile method to prepare heterostructured Co₄N/CeO₂ nanoparticles using bimetallic CoCe-BTC (benzene-1,3,5 tricarboxylate) MOF (metal organic framework). As a self-template and precursor of Co₄N/CeO₂, bimetallic BTC MOF has substantial merits such as chemical and structural tunability, low cost, high surface area and porosity. The CoCe-BTC was converted into heterostructured Co₄N/CeO₂ via simple annealing and selective nitration process. Benefiting from electrical conductive metallic feature of Co₄N and multivalent nature of CeO₂, the synthesized Co₄N/CeO₂ electrocatalyst exhibited exceptional OER performance with a low overpotential of 297 mV to acquire a current density of 10 mA cm⁻², small Tafel slope of 88 mV dec⁻¹, and excellent long-term durability over 100 h in alkaline electrolyte. This research will pave the way for development of heterostructured noble metal-free electrocatalyst for various energy storage and conversion system.