Electrochemical CO₂ reduction reaction (CO₂RR) is a significant area of research for mitigating global warming and producing valuable chemicals. However, the typical pairing of CO₂RR with the oxygen evolution reaction (OER) at the counter electrode results in high energy consumption due to OER's sluggish kinetics. Furthermore, the O₂ byproduct has limited economic value. A promising solution is to replace OER with the oxidation of biomass-derived small molecules, which operates at lower thermodynamic potentials, improving energy efficiency and yielding valuable products at both electrodes. Formic acid (HCOOH), a key intermediate in many industrial processes and a hydrogen storage compound, is a desirable CO₂RR product. Simultaneously, oxidizing 5-hydroxymethylfurfural (HMF), a biomass-derived molecule, to 2,5-furandicarboxylic acid (FDCA), a top-12 platform chemical, is attractive. Electrochemical coupling of CO₂RR (cathode) with HMF oxidation reaction (HMFOR, anode) allows simultaneous production of HCOOH and FDCA. This requires an effective asymmetric electrolysis cell (neutral electrolyte for CO₂RR and basic for FDCA) and catalysts with high activity and selectivity to suppress competing HER and OER reactions. Indium oxides are effective CO₂RR electrocatalysts for formate production, outperforming many transition metal oxides. However, transition metal oxides often struggle with electrochemical oxidation reactions (EOR) due to strong interactions with oxygen-containing molecules, hindering product desorption. Main-group p-block metal oxides, with fully occupied d-orbitals and p-bands, could enhance EOR by facilitating desorption of oxygenated intermediates. This study explores the potential of oxygen-vacancy rich indium oxyhydroxide (InOOH) as a bifunctional catalyst for this coupled system.

Extensive research has focused on CO₂RR, exploring various catalysts and electrolytes to enhance efficiency and selectivity for different reduction products. Significant attention has been given to the development of multicarbon product formation. Coupling CO₂RR with organic oxidation reactions has emerged as a strategy to improve the overall efficiency of electrochemical cells. Previous work has demonstrated the effectiveness of integrating oxidative biomass valorization with hydrogen evolution reactions (HER), decreasing cell voltage. Formic acid has been identified as a high-value product of CO₂RR due to its utility as a chemical intermediate, a hydrogen storage medium, and as a direct fuel source in fuel cells. The oxidation of HMF to FDCA is also a widely studied process, driven by the high market value of FDCA. Numerous studies have explored different catalysts, mostly transition metal oxides, for HMFOR, highlighting the need for efficient and selective catalysts to improve reaction yields. Main group p-block metal oxides have been less explored for EORs despite their potential advantages.

InOOH nanosheets were synthesized using a solvothermal method, employing In(NO₃)₃·4H₂O, urea, and carbon black (CB) as support. Oxygen vacancies (O_V) were introduced via Ar plasma treatment, creating InOOH-O_V. O₂ plasma treatment was used to control the O_V concentration, yielding InOOH-O₂. The samples were characterized using various techniques: Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), High-Resolution TEM (HR-TEM), Atomic Force Microscopy (AFM), Selected Area Electron Diffraction (SAED), Elemental Mapping, High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM), Atomic-Resolution Electron Energy-Loss Spectroscopy (EELS), X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and Electron Paramagnetic Resonance (EPR). Electrochemical CO₂RR tests were conducted in a three-electrode setup using 0.1 M KHCO₃ electrolyte. Linear sweep voltammetry (LSV), chronoamperometry, and electrochemical impedance spectroscopy (EIS) were employed. Product analysis involved online gas chromatography (GC) and ¹H-NMR. Electrochemical HMFOR tests were performed similarly, using 1 M KOH electrolyte containing 50 mM or 10 mM HMF. HPLC was used for product quantification. Density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) to understand the catalytic mechanism and O_V's role. Operando Raman spectroscopy was used to monitor catalyst structure dynamics during CO₂RR and HMFOR. Finally, an integrated two-electrode cell with asymmetric pH values was assembled using a bipolar membrane (BPM) to couple CO₂RR and HMFOR, assessing the overall system performance.

Ar plasma treatment significantly increased the oxygen vacancy concentration in InOOH nanosheets. HAADF-STEM and EELS confirmed lattice distortion due to O_V formation. XPS analysis revealed that InOOH-O_V had a 40.9% O_V proportion, much higher than InOOH and InOOH-O₂. InOOH-O_V exhibited superior electrochemical CO₂RR performance, achieving a formate faradaic efficiency (FE) of 92.6% at -0.85 V vs. RHE and a maximum formate partial current density (jformate) of 56.2 mA cm⁻² at -1.00 V. Tafel analysis showed a Tafel slope of 72 mV dec⁻¹, indicating efficient kinetics. CO₂ adsorption tests confirmed enhanced CO₂ adsorption capacity for InOOH-O_V. For HMFOR, InOOH-O_V showed the lowest onset potential (1.30 V vs. RHE) and Tafel slope (66 mV dec⁻¹), achieving a 91.6% FDCA yield at 1.48 V. HPLC analysis confirmed a high HMF conversion (98.5%) with InOOH-O_V. The long-term stability of InOOH-O_V for both CO₂RR and HMFOR was demonstrated through stability tests. DFT calculations indicated charge redistribution around O_V sites, influencing the adsorption and activation of CO₂ and HMF molecules. Operando Raman spectroscopy confirmed the involvement of key intermediates during both reactions and demonstrated that the oxidation state of In was maintained during CO₂RR. The integrated two-electrode cell, utilizing InOOH-O_V and a BPM, achieved a 99.0% HMF conversion, 87.5% FDCA yield, and >90.0% formate FE at a cell voltage of 2.27 V, demonstrating high system-level efficiency.

The findings demonstrate the crucial role of oxygen vacancies in enhancing the bifunctional catalytic activity of InOOH nanosheets for both CO₂RR and HMFOR. The improved performance is attributed to the charge redistribution induced by O_V, which modifies the adsorption behavior of reactants and intermediates, leading to faster reaction kinetics and higher selectivity. The success in integrating these two processes in a single cell with asymmetric pH conditions highlights the potential of this strategy for efficient and sustainable chemical production. This work expands the understanding of main-group p-block metal oxides as efficient electrocatalysts and provides a promising approach for developing coupled electrochemical systems for simultaneous production of valuable chemicals.

This study successfully synthesized and characterized InOOH nanosheets with tunable oxygen vacancy concentrations. InOOH-O_V showed exceptional bifunctional catalytic activity for CO₂RR and HMFOR. DFT calculations and operando Raman spectroscopy provided mechanistic insights, revealing the role of O_V in enhancing catalytic performance. The successful integration of CO₂RR and HMFOR in a single cell demonstrates a promising approach for sustainable chemical production. Future research could focus on further optimizing the catalyst structure, exploring different main-group p-block metal oxides, and scaling up the integrated system for practical applications.

The study primarily focused on the InOOH system. Further investigation is needed to determine the generality of these findings to other main group p-block metal oxides. The long-term stability tests, while showing promising results, could be extended to even longer durations to obtain more comprehensive data. The current study primarily uses a laboratory-scale setup, and further work is required to assess the scalability and techno-economic feasibility of this approach for industrial applications.