The role of oxygen-vacancy in bifunctional indium oxyhydroxide catalysts for electrochemical coupling of biomass valorization with CO₂ conversion

The study addresses the challenge of high energy consumption and low-value anodic products in conventional CO2 electrochemical reduction reaction (CO2RR) systems that pair CO2RR with the sluggish oxygen evolution reaction (OER). It proposes coupling CO2RR on the cathode with the oxidation of biomass-derived molecules on the anode to improve overall energy efficiency and produce value-added chemicals at both electrodes. Formic acid/formate is a desirable CO2RR product due to its industrial relevance and hydrogen storage potential, while 2,5-furandicarboxylic acid (FDCA) is a high-value product from the oxidation of 5-hydroxymethylfurfural (HMF). The research aims to develop a bifunctional catalyst effective for both CO2RR and HMF oxidation reaction (HMFOR), enabling an asymmetric-pH integrated cell (neutral catholyte and alkaline anolyte). Indium oxides are known to be selective for formate production from CO2, and the work hypothesizes that engineering oxygen vacancies (Ov) in indium oxyhydroxide (InOOH) will distort the lattice and redistribute charge to enhance adsorption/activation of CO2 and HMF, thereby boosting bifunctional performance and selectivity while suppressing competing HER and OER.

- Conventional CO2RR typically uses OER at the anode, which is kinetically sluggish and yields low-value O2, increasing energy costs. Replacing OER with biomass-derived molecule oxidation lowers cell voltages and co-produces valuable chemicals. - Formate/formic acid is an important product for CO2RR due to its roles as an industrial intermediate, hydrogen carrier, and fuel. - HMF, a lignocellulosic biomass-derived molecule, can be electrooxidized to FDCA, a top platform chemical. Alkaline media accelerate FDCA formation. - First-row transition metal oxides are common HMFOR catalysts but partially filled d-orbitals can cause strong adsorption of oxygenated intermediates and hinder desorption, limiting EOR performance. - Main-group p-block metal oxides, with fully occupied d-orbitals and p-bands as host orbitals, can facilitate desorption of oxygenated intermediates and potentially enhance EOR, but may suffer from weaker chemisorption/activation; they remain underexplored for EORs. - Indium oxides have demonstrated high selectivity for CO2-to-formate conversion; oxygen vacancies in indium oxides can enhance CO2 adsorption/activation, suggesting defect engineering as a promising route.

- Catalyst synthesis: InOOH nanosheets grown on carbon black (CB) via solvothermal treatment of In(NO3)3·4H2O and urea in ethanol (90 °C, 12 h). Plasma treatments (120 s, 100 W, 20 Pa) introduced or healed surface oxygen vacancies: Ar plasma to create Ov (InOOH-Ov) and O2 plasma to repair Ov (InOOH-O2). A CB-free InOOH-Ov sample was also prepared for operando Raman. - Structural/chemical characterization: SEM/TEM/HR-TEM, AFM (thickness ~1.68 nm, ~5 atomic layers), SAED (dominant (110) plane), HAADF-STEM (lattice distortions indicating Ov), EELS O K-edge (reduced 532 eV peak in distorted domains evidencing decreased oxygen coordination), XRD (InOOH phase preserved), XPS (In 3d shifts indicating altered In valence; O 1s deconvolution to OL, Ov, OH with Ov fraction highest in InOOH-Ov), EPR (g ≈ 2.0035 signal intensity tracking Ov content), CO2 physisorption isotherms, EIS. - Electrochemistry CO2RR: Gas-tight H-cell with Nafion 117 separator; catholyte 0.1 M KHCO3 saturated with CO2 (30 mL min−1), stirring 400 rpm; working electrode: catalyst ink with 10 wt% Nafion on hydrophobic carbon cloth (loading 2 ± 0.05 mg cm−2, area 1 cm2); Pt counter, Ag/AgCl reference; LSV (0 to −1.1 V vs RHE, 5 mV s−1); potentiostatic electrolysis with 30 min steps; product analysis: online GC (TCD/FID) for gases and 1H NMR for liquid products; ECSA from double-layer capacitance by CV in nonfaradaic region; EIS under CO2RR potentials; iR compensation 90%. - Electrochemistry HMFOR: LSV in 1 M KOH with 50 mM HMF (1.0–1.7 V vs RHE, 5 mV s−1) in undivided cell; potentiostatic electrolysis in H-cell (Nafion 117) with 1 M KOH and 10 or 50 mM HMF; working electrode: catalyst on nickel foam (2 ± 0.05 mg cm−2, area 2 cm2); graphite counter, Hg/HgO reference; iR compensation 90%; product quantification by HPLC (PDA 265 nm, Aminex HPX-87H, 5 mM H2SO4, 0.6 mL min−1, 50 °C) using calibration curves; calculated HMF conversion, FDCA yield, FE of FDCA, and combined electron efficiency. - Operando Raman: Custom H-cell with quartz window; 633 nm laser; for CO2RR in CO2-saturated 0.5 M KHCO3; for HMFOR in 1 M KOH with/without 50 mM HMF; potential-dependent spectra collected to track intermediates and structural changes. - Integrated asymmetric-pH cell: Two-compartment, gas-tight cell with bipolar membrane (BPM) separating catholyte (CO2-saturated 0.1 M KHCO3, pH 6.8) and anolyte (1 M KOH + 10 mM HMF, pH 14); electrodes: InOOH-Ov on carbon paper (cathode, 1×2 cm) and InOOH-Ov on nickel foam (anode, 2×2 cm); LSV of full cell (1.5–2.7 V, 5 mV s−1); monitored individual electrode potentials with Ag/AgCl probes; simultaneous product monitoring by GC/NMR and HPLC. - Computations: Spin-polarized DFT (VASP, PAW, PBE; RPBE checks; HSE06 for DOS validation), plane-wave cutoff 400 eV, Monkhorst-Pack 2×2×1 k-mesh; slabs: InOOH(110) with 5 layers (bottom 3 fixed), Ov created by removing one surface O; implicit solvent; CHE model; free energies computed including ZPE, thermal and entropic contributions; reference corrections for O2 and aqueous species; adsorption configurations, electron localization function (ELF), partial DOS, and free energy diagrams for CO2RR and HMFOR pathways.

- Oxygen-vacancy engineering: Ar plasma created higher Ov content in InOOH-Ov (O 1s XPS Ov fraction ~40.9%) vs InOOH (~higher Ov than OL) and InOOH-O2 (Ov ~29.0%); HAADF-STEM showed lattice disorder; EPR g=2.0035 signal intensity tracked Ov content (InOOH-O2 < InOOH < InOOH-Ov). - CO2RR performance (0.1 M KHCO3): InOOH-Ov achieved maximum FE for formate of 92.6% at −0.85 V vs RHE; InOOH and InOOH-O2 reached 80.5% (−0.90 V) and 71.5% (−0.95 V), respectively. Formate partial current density jformate for InOOH-Ov: 16.0 mA cm−2 at −0.85 V, 43.7 mA cm−2 at −0.95 V, 56.2 mA cm−2 at −1.00 V. Positive correlation between Ov proportion and formate FE. Tafel slopes: 72 mV dec−1 (InOOH-Ov) < 101 mV dec−1 (InOOH) < 140 mV dec−1 (InOOH-O2), indicating faster kinetics with higher Ov. CO2 adsorption capacity: InOOH-Ov > InOOH > InOOH-O2. EIS showed lowest charge transfer resistance for InOOH-Ov. 30 h stability at −0.85 V with sustained performance. - HMFOR performance (1 M KOH, 10–50 mM HMF): In presence of 50 mM HMF, HMFOR onset potentials: 1.30 V (InOOH-Ov), 1.37 V (InOOH), 1.41 V (InOOH-O2). Potential for 10 mA cm−2: 1.34 V (InOOH-Ov), 1.42 V (InOOH), 1.49 V (InOOH-O2). Tafel slopes: 66 mV dec−1 (InOOH-Ov), 95 mV dec−1 (InOOH), 118 mV dec−1 (InOOH-O2). Electrolysis at 1.48 V: HMF conversion reached ~98.5% (InOOH-Ov), FDCA yield 91.6%, FE for FDCA 90.7% at 117 C charge; inferior performance for InOOH (conversion ~89.4%, yield 75.9%) and InOOH-O2 (conversion ~84.3%, yield 45.0%). Six sequential batches on InOOH-Ov maintained FDCA yield and FE >90% with structural stability; InOOH and InOOH-O2 degraded (In(OH)3 formation). - Operando Raman: During CO2RR, In–O bands at ~354 and 459 cm−1 remained, indicating InOOH-Ov resists further reduction; *HCOO intermediate band at ~1350 cm−1 appeared and intensified to −0.8 V, consistent with formate formation. During HMFOR, In–OH bands at 307 and 390 cm−1 in KOH; with HMF, a 313 cm−1 band appeared at OCP (blue-shift due to HMF adsorption at Ov), delaying OH− adsorption band emergence until higher potentials, evidencing preferential HMF adsorption at Ov and explaining HMFOR selectivity window (1.30–1.50 V). - DFT insights: Ov induces charge redistribution with increased In p-DOS at Fermi level and electron aggregation near Ov, strengthening adsorption/activation. For CO2RR, PDS on pristine InOOH is CO2 protonation (ΔG 1.38 eV); on InOOH-Ov, PDS shifts to HCOO* hydrogenation/desorption with lower ΔG (1.26 eV), further reduced to 0.93 eV with co-adsorbed CO2, explaining enhanced activity/selectivity for HCOO−. Competing HER/CO less favorable. For HMFOR, Ov lowers barriers for hydroxyl-to-aldehyde oxidation steps: HMFCA→FFCA ΔG becomes −0.41 eV (vs 0.54 eV on InOOH); HMF→DFF barrier 0.19 eV (vs 0.63 eV), consistent with higher FFCA formation and FDCA yields. - Integrated asymmetric-pH cell (BPM-separated): At cell voltage 2.27 V, electrode biases align with optimal potentials (anode ~1.48 V, cathode ~−0.95 V). With charge accumulation to 185 C, HMF conversion ~99.0%, FDCA yield 87.5%, and cathodic FE for formate remained >90%. Combined electron efficiency reached 172.1%, nearly doubling the efficiency compared to separate processes, demonstrating energetic advantages of coupling HMFOR with CO2RR using a single bifunctional catalyst.

The work demonstrates that engineering oxygen vacancies in InOOH nanosheets creates active sites that induce lattice distortion and charge redistribution, enhancing adsorption and activation of both CO2 and HMF. This dual effect enables high selectivity and activity for CO2-to-formate conversion while simultaneously promoting key oxidation steps in HMFOR, particularly hydroxyl-to-aldehyde transformations, thereby boosting FDCA yield. Operando Raman confirms that Ov sites stabilize the InOOH oxidation state under CO2RR and preferentially bind HMF over hydroxide in alkaline media within a specific potential window, helping suppress competing OER and establishing a selective HMFOR regime. The positive correlation between Ov content and kinetic/adsorptive metrics (Tafel slopes, CO2 uptake, EIS) supports Ov as the primary descriptor governing bifunctional performance. Integrating CO2RR and HMFOR in a BPM-enabled asymmetric-pH electrolyzer aligns each half-reaction with its optimal environment and potential, achieving high product selectivities on both electrodes with improved system-level efficiency (combined electron efficiency 172.1%). The findings substantiate main-group p-block metal oxides as viable bifunctional electrocatalysts and illustrate how defect engineering can enable practical co-electrolysis strategies for simultaneous CO2 conversion and biomass valorization.

Oxygen-vacancy-rich indium oxyhydroxide nanosheets (InOOH-Ov) were developed as a bifunctional electrocatalyst that efficiently couples CO2 reduction to formate (maximum FE 92.6%, jformate up to 56.2 mA cm−2) with HMF oxidation to FDCA (yield 91.6%, FE 90.7%). Atomic-scale imaging, spectroscopy, and DFT reveal that Ov sites distort the lattice and redistribute charge, facilitating adsorption/activation of CO2 and HMF, shifting the CO2RR potential-determining step while lowering barriers for key HMFOR steps. Operando Raman shows Ov-stabilized catalyst states, preferential HMF adsorption over OH− in alkaline media, and the presence of reactive intermediates. A BPM-based asymmetric-pH integrated cell using InOOH-Ov on both electrodes achieved ~99% HMF conversion, 87.5% FDCA yield, and >90% FE to formate at 2.27 V, with a combined electron efficiency of 172.1%. These results highlight defect-engineered main-group p-block oxides as promising bi/multifunctional catalysts and provide a blueprint for integrating CO2 conversion with biomass valorization. Future studies could explore scale-up, long-term durability under industrially relevant current densities, broader biomass substrates, optimization of BPMs and cell architectures, and tuning vacancy concentration and distribution for further performance gains.