(326e) Understanding of Active Sites and Reaction Mechanisms for Oxygen Evolution and Biomass Upgrading Reaction on Oxide-Based Electrocatalysts

The transition from fossil fuel-based energy to renewable energy requires an environmentally friendly, highly efficient, and sustainable energy source to store electricity such as solar, wind, biomass as well as hydrogen energy. Hydrogen is a clean, abundant, and highly dense chemical fuel, and can be produced by the electrochemical water splitting approach. However, the water splitting efficiency is largely limited by poor kinetics of the four-electron based oxygen evolution reaction (OER) at the anode. Extensive research has been conducted to explore highly efficient electrocatalysts to overcome the barrier for OER. To date, noble metal-based oxides such as IrO₂ and RuO₂ exhibit superior activity and stability for OER but, the scarcity and high cost severely limit their large-scale applications.¹ Therefore, the demand for the development of 3d transition metal oxide is a great challenge to promote the application for water splitting reaction. I will be talking about the design strategy of high entropy oxides for the OER applications and electrochemical biomass conversion reaction on oxyhydroxide surface.

High entropy oxides (HEOs) emerge as a new kind of oxide-based electrocatalysts consisting of five or more metals bonded with neighboring oxygens. Different structures of oxides such as rock salt, perovskite, spinel-type, and (oxy)hydroxide have been evaluated for various electrochemical applications^2,3 and demonstrated excellent physical/chemical stability and activity. Due to the difficulty of the synthesis process and the lack of an efficient computational model to precisely define active sites and stability, a thorough investigation of the HEO catalytic activity and stability for OER is currently missing. Here, we investigate OER electrochemical activity and stability on 3d transition metals (Co, Fe, Ni, Cr, and Mn) based spinel-type HEOs using both theoretical and experimental methods (Fig. a). In computation, we develop feasible HEO impurity model to study both stability and OER activity. Our DFT calculations predict that randomly mixed metal sites are thermodynamically stable and show wide distribution of absorbates binding energy due to the development of equatorial strain within the HEO structures under mixing. Later, we use rapid sol-flame method to synthesize spinel-type HEOs consisting of Co, Fe, Ni, Cr, and Mn. Our experimental measurements exhibit good OER activity with the overpotential of 309 mV to reach 10 mA cm^-2 and the smallest Tafel slope (30 mV dec^-1), as well as excellent long-term stability for 168 hours under alkaline conditions, showing good agreement with theoretical activity prediction.

On the other hand, electrochemical biomass conversion reaction to value-added chemicals is considered one of the sustainable alternatives to fossil fuels energy over traditional thermal heterogeneous catalysis technique.⁴ The electrode potential can be used to activate the biomass conversion thermodynamics and kinetics well ahead of OER at ambient reaction conditions, indicating the prospects of alternative anodic reactions in fuel cell electrolyzers. Transition metal oxides and oxyhydroxides show great potential as electrocatalyst for biomass upgrading, however, lack of systematic investigation about active sites selectivity, controlled electrochemical activity and moreover, robust reaction mechanism limits its industrial applications. Here, we use Fe-doped NiOOH catalysts to study benzyl alcohol oxidation (BAO) selectivity, activity, and its mechanistic pathways via experimental measurements and computational predictions.⁵ Our experimental study shows BAO onset potential is largely dependent on Ni(II/III) redox potential for various electrochemical conditions. In theory, we find that the adsorption of benzyl alcohol is site-selective, where edge sites are thermodynamically more active than the basal plane. According to the reaction mechanism, once the surface transforms to Ni³⁺, BAO steps are thermodynamically downhill (similar to a cascade reaction), termed as redox mechanism. Our DFT study also introduces the possibility of a secondary vacancy-driven pathway, generated per mole of BA conversion via the redox mechanism (Fig. b).

The above systematic experimental and theoretical approaches have revealed valuable insights into electrochemical reactions (such as OER and BAO) activity, stability, and mechanisms on oxide-based heterogeneous catalysts.

References

1. Lee, Y. et al. Synthesis and Activities of Rutile IrO₂ and RuO₂ Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. Phys. Chem. Lett. 3, 399–404 (2012).

2. Duan, C. et al. Nanosized high entropy spinel oxide (FeCoNiCrMn)3O4 as a highly active and ultra-stable electrocatalyst for the oxygen evolution reaction. Energy Fuels 6, (2022).

3. Zhang, Y. et al. Stabilizing Oxygen Vacancy in Entropy-Engineered CoFe₂O₄-Type Catalysts for Co-prosperity of Efficiency and Stability in an Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces 12, (2020).

4. Albert, J. et al. Spectroscopic and Electrochemical Characterization of Heteropoly Acids for Their Optimized Application in Selective Biomass Oxidation to Formic Acid. Green Chem. 2014, 16 (1), 226–237.

5. Lingze, W. et al. Insights into active sites and mechanisms of benzyl alcohol oxidation on nickel iron oxyhydroxide electrodes. ACS Catal. 2023, 13, 4272−4282