Unveiling the Structure-Activity Relationship for Cobalt-Based Spinel Oxide with Superior Acidic Oer Performance
Authors: Pu Heting , Liu Haotian , Kwon Soonho , Goddard William A , Huang Yu 
Published: December 2024

Climate change has become an urgent issue due to the high carbon emission from various industrial sectors. As a result, achieving carbon neutrality has become a priority for society. Hydrogen is widely used in various industries and stands out as a promising zero-carbon energy carrier. However, most hydrogen is currently produced from steam reforming of natural gas, which is unsustainable due to its consumption of fossil fuel and the enormous amount of COx and NOx by products released. Water electrolysis presents an attractive alternative for clean hydrogen production when coupled with renewable electricity. In recent years, the proton exchange membrane water electrolyzer (PEMWE) has received significant attention due to its ability to produce hydrogen at high rate, high efficiency, and high purity. The current bottleneck of PEMWE technology comes from its reliance on pricey Ru or Ir-based catalysts with limited durability. Thus, finding a low-cost catalyst with high activity and stability for the acidic oxygen evolution reaction (OER) on the anode is crucial for the next generation PEMWE products.

Co3O4, as an alternative to Ru and Ir-based catalysts, has attracted much attention due to its low cost, good activity, and acid tolerance. A significant amount of effort has been devoted to further improving the performance of Co3O4 and related spinel oxides, in order to reach the superior performance of noble-metal-based catalysts1. However, the maintenance of both high activity and acid corrosion resistivity has remained challenging. These two factors are closely related to the OER reaction mechanism, and direct oxo coupling (DOC) has been proposed as an ideal mechanism that optimizes both activity and stability2. Nevertheless, the DOC mechanism requires tight control of lattice parameters and metal site distances3,4, consequently non-noble-metal-based spinel oxide structures that enable acidic OER via the DOC mechanism have not been reported.

In this work, we present a facile method of hydrothermal synthesis that produces aggregated needle-like spinel NiCo2O4 nanostructures. In 0.5M H2SO4, the NiCo2O4 nanostructure exhibits remarkable activity and record-high durability for acidic OER. In-situ x-ray absorption spectroscopy revealed a higher stability during acidic OER catalysis of the Co and Ni coordination environment in NiCo2O4 compared to its less stable Co3O4 counterpart. This observation indicates the formation of fewer oxygen vacancies on NiCo2O4 under reactive potential, avoiding the critical structural deformation that would impair catalyst durability. Furthermore, we conducted density functional theory calculations to confirm the decreased OER overpotential arising from replacement of Co with Ni in octahedral lattice sites and to learn the preferred reaction pathway for DOC.

In conclusion, this research presents NiCo2O4 as an acidic OER catalyst with superior activity and stability. This investigation into NiCo2O4 provides insights into the design of anode catalysts for PEMWE, advancing toward sustainable hydrogen production with high yield and low cost.

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