Surface Engineering of Cobalt Spinel Oxide by Cation Incorporation for High-Performance Electrocatalysis

Spinel oxides, characterized by the AB2O4 crystal structure, have gained attention for their ability to fine-tune the physicochemical properties such as electronic structure and chemical composition through meticulous adjustment of the A and B site cations. The ability to be fine-tuned enables precise modification of surface characteristics, which are crucial for the performance of heterogeneous catalysts. A particularly effective method for enhancing the catalytic performance of cobalt spinel oxides (CSOs, Co3O4) involves introducing different metal cations, which has shown promise for improving various electrocatalytic reactions.

One of the key focuses in electrocatalysis field is the development of catalysts for the oxygen evolution reaction (OER) under acidic conditions. One major challenge in this area is overcoming the sluggish reaction kinetics without relying on noble metals such as Ir or Ru. CSOs have emerged as promising candidates for non-noble metal catalysts in acidic OER, offering a more affordable alternative. However, bare CSOs exhibit poor durability in acidic OER environments. Strategies such as heteroatoms introduction or oxygen vacancies induction in the CSO lattice have been explored to address their long-term stability issues.

A particularly promising approach is the incorporation of metal cations into the CSO structure. However, current research has primarily been confined to first-row transition metals (e.g., Fe, Mn, Ni, Cu, Zn), making it difficult to identify the key factors that influence the stability of CSO catalysts. The difficulty to identify the key factors leads to challenges in developing more durable and high-performance catalysts. Expanding the range of metal cations incorporated into CSOs could unlock new opportunities for advancing catalyst design and improving performance in acidic OER applications.

Despite this potential, identifying suitable cations and successfully incorporating them into CSO is challenging due to limited understanding of the factors controlling metal integration. In particular, incorporating third-row early transition metals like Hf, Ta, and W is difficult due to their tendency to react with water, forming unwanted heterogeneous oxides. This reaction complicates the atomic-level incorporation of these metals, preventing the development of atomically dispersed catalysts based on these elements in CSOs.

In this study, we offer insights into metal cation incorporation in CSOs. We developed a simple and versatile synthetic method that enables doping CSOs with various metals, ranging from early transition metals to metalloids. The use of a metal-organic framework (MOF) was key in successfully incorporating metals such as Hf, Ta, W, Ti, Pd, Ga, and Ge with the atomically dispersed form. We also found that each metal species tends to stabilize at distinct sites within the CSO structure. For example, Ta, W, and Ge prefer to stabilize at octahedral sites on the surface of CSO, which increases the density of surface Co2+. This creates a protective and active layer, leading to high OER performance in acidic environments. Notably, Ta-doped CSO achieved an overpotential of 378 mV at 10 mA cm-2 and maintained stability for over 140 hours. In situ X-ray absorption spectroscopy (XAS), inductively coupled plasma (ICP), post OER X-ray photoemission spectroscopy (XPS), and transmission electron microscopy (TEM) analyses showed that this protective shell helps prevent over-oxidation and the dissolution of cobalt during the reaction. On the one hand, the electronic configuration of the t2g and eg level, as indicated by O K-edge analysis, is modulated by Ta, which may contribute to the enhanced catalytic activity. DFT calculations also suggested that the incorporation of Ta atoms enhances the activity of peripheral Co sites through the adsorbate evolution mechanism (AEM).