Large-Scale Nanofiber Manufacturing| Tailoring the electrolyte microenvironmentof indium catalysts for enhanced formic acidelectrosynthesis

Professor Yi Xiaodong, Assistant Professor Chen Zhou & Associate Professor Zhang Zhihao (Xiamen University): Optimizing Electrolyte Microenvironment for Enhanced Electrosynthesis of Formic Acid via CO₂RR

The electrochemical CO₂ reduction reaction (CO₂RR) to produce high-value liquid formic acid (HCOOH) is considered a promising solution to energy and sustainability challenges. Indium-based catalysts have attracted widespread attention for their high efficiency in electrocatalytic CO₂RR to formate, primarily due to their slow hydrogen evolution reaction (HER) kinetics. However, current indium-based CO₂RR catalysts still suffer from insufficient activity, selectivity, and stability, limiting practical applications. Moreover, few indium-based CO₂RR catalysts maintain long-term stability under continuous operation, especially in harsh acidic/alkaline media and at high current densities, posing significant challenges for developing CO₂ electrolyzers and compatible electrolytes.

Recently, Yi Xiaodong, Chen Zhou, Zhang Zhihao et al. from Xiamen University published research in Journal of Energy Chemistry titled "Tailoring the Electrolyte Microenvironment of Indium Catalysts for Enhanced Formic Acid Electrosynthesis." The team synthesized oxygen-vacancy-rich In₂O₃ nanofibers via electrospinning. Results showed that 500-In₂O₃ exhibited exceptional performance in CO₂RR-to-formate conversion, achieving 92.1% Faradaic efficiency (FE) for HCOOH at −600 mA cm⁻². It also demonstrated remarkable stability in neutral electrolytes, operating continuously for over 100 hours at −300 mA cm⁻², outperforming unmodified indium catalysts.

Additionally, the researchers designed a dual-electrode system capable of simultaneously producing formate at both cathode and anode. At −100 mA cm⁻², this system achieved a 12.5% reduction in energy consumption and a 39.9% improvement in electrical energy conversion efficiency.

Figure 1: (a) Schematic of electrocatalyst synthesis. (b,c) SEM images of 500-In₂O₃. (d) TEM image. (e) HRTEM image. (f) SAED pattern. (g) Elemental mapping.

Fibrous indium oxide (In₂O₃) was synthesized via electrospinning technology, with oxygen vacancies in In₂O₃ regulated by varying calcination temperatures. The resulting materials were named 400-In₂O₃, 500-In₂O₃, and 600-In₂O₃ based on their respective temperatures (Figure 1a). Scanning electron microscopy (SEM) images showed that all three samples maintained consistent nanofiber morphology with diameters around 300 nm (Figures 1b, c). Transmission electron microscopy (TEM, Figure 1d) further confirmed the nanofiber structure of 500-In₂O₃. High-resolution TEM (HRTEM) images (Figure 1e) clearly revealed lattice spacings of 0.18 nm, 0.42 nm, and 0.30 nm, corresponding to the (440), (112), and (222) crystal planes of In₂O₃, respectively. Selected area electron diffraction (SAED) patterns (Figure 1f) further confirmed the existence of these crystal planes. High-angle annular dark-field scanning TEM (HAADF-STEM) images of 500-In₂O₃ demonstrated uniform distribution of indium (In) and oxygen (O) elements throughout the sample (Figure 1g).

Figure 2: (a) XRD patterns of 400-In₂O₃, 500-In₂O₃, and 600-In₂O₃. (b) In 3d XPS spectra. (c) O 1s XPS spectra.

X-ray diffraction (XRD) patterns of In₂O₃ treated at different annealing temperatures are shown in Figure 2a. All samples displayed characteristic diffraction peaks of In₂O₃ (PDF #06-0416), indicating that varying annealing temperatures did not alter the crystal structure of In₂O₃. The In 3d XPS spectrum showed two peaks at 451.9 eV and 443.4 eV, corresponding to In 3d₃/₂ and In 3d₅/₂, respectively, further confirming successful synthesis of In₂O₃. The O 1s XPS spectrum (Figure 2c) exhibited two peaks near 529.6 eV and 531.6 eV, attributed to lattice oxygen (OL) and vacancy oxygen (OD), with 500-In₂O₃ showing the highest OD concentration (26.55%).

Figure 3: Electrochemical CO₂ reduction performance of In₂O₃ electrocatalysts. (a-c) Faradaic efficiencies (FE) of products from 400-In₂O₃, 500-In₂O₃, and 600-In₂O₃ in 1 M KCl electrolyte. (d) Maximum formate FE (FEₘₐₓ) and corresponding current densities of indium-based CO₂RR catalysts. (e) Long-term stability test of 500-In₂O₃ at -300 mA cm⁻² in 1 M KCl electrolyte.

Figure 4: (a) Bader charge distribution of *OCHO adsorption on Cl-In (101) surface. (b) CO₂-to-HCOOH conversion processes on In, Cl-In, Br-In, and I-In. (c) HER on In, Cl-In, Br-In, and I-In, with corresponding geometric adsorption models shown. (d) Mechanism diagram of CO₂RR on halogen-adsorbed indium surfaces.

Density functional theory (DFT) calculations were performed on indium (101) surfaces with and without different halogen adsorptions to investigate their effects on CO₂RR selectivity. Figure 4(a) shows optimized adsorption configurations of CO₂, COOH, CO, OCHO, HCOOH, and H on these surfaces, along with Bader charge distribution of *OCHO adsorption on In (101). Experimental and DFT results demonstrated that different halogen atoms played distinct roles in CO₂RR. Bromine (Br) and iodine (I) on indium surfaces facilitated water dissociation and promoted the hydrogen evolution reaction (HER) to some extent. In contrast, chlorine (Cl) adsorption modulated water dissociation to balance CO₂ activation and reduced the formation energy of *HCOOH. Consequently, Cl-modified indium catalysts exhibited superior electrocatalytic performance for CO₂RR to formate (Figure 4d). These insights are significant for selective CO₂RR using natural seawater as electrolyte under mild conditions.

Given the sluggish kinetics of the anodic oxygen evolution reaction (OER), replacing OER with selective electrocatalytic oxidation of organic compounds is an effective strategy to improve overall energy efficiency of CO₂RR electrolyzers. A hybrid CO₂RR electrolysis system (denoted as GOR//CO₂RR) combining anodic glycerol oxidation reaction (GOR) with cathodic CO₂RR was further optimized. This system not only enhanced energy efficiency but also enabled simultaneous production of high-value-added formate, significantly improving its practical application potential in electrochemical CO₂ conversion.

Figure 5: (a) Schematic of flow cell. (b) Linear sweep voltammetry curves of electrolyzer with different anodic reactions. (c) Faradaic efficiency of formate from GOR catalyzed by NiV-LDH in 1 M KOH/0.1 M glycerol solution. (d) Faradaic efficiency of CO₂RR catalyzed by 500-In₂O₃ in 1 M KCl solution. (e) Stability test of GOR||CO₂RR system at -300 mA cm⁻².