Coverage enhancement accelerates acidic CO

Coverage enhancement accelerates acidic CO₂ electrolysis at ampere-level current with high energy and carbon efficiencies

Acidic CO₂ electroreduction (CO₂R) using renewable electricity holds promise for high-efficiency generation of storable liquid chemicals with up to 100% CO₂ utilization. However, the strong parasitic hydrogen evolution reaction (HER) limits its selectivity and energy efficiency (EE), especially at ampere-level current densities. Here Miao Zhong's group at the College of Engineering and Applied Sciences at Nanjing University presents that enhancing CO₂R intermediate coverage on catalysts promotes CO₂R and concurrently suppresses HER. They identified and engineered robust Cu₆Sn₅ catalysts with strong *OCHO affinity and weak *H binding, achieving 91% Faradaic efficiency (FE) for formic acid (FA) production at 1.2  cm⁻² and pH 1. Notably, the single-pass carbon efficiency reaches a new benchmark of 77.4% at 0.5  cm⁻² over 300 hours. In situ electrochemical Fourier-transform infrared spectroscopy revealed Cu₆Sn₅ enhances *OCHO coverage ~2.8× compared to Sn at pH 1. Using a cation-free, solid-state-electrolyte-based membrane-electrode-assembly (SSE-MEA), 0.36 M pure FA at 88% FE was produced over 130 hours with a marked full-cell EE of 37%.

Figure 1 Characterization of Cu₆Sn₅ catalysts on a gas diffusion electrode.

Fig.1a shows a schematic of the CO₂R process on Cu₆Sn₅/PTFE electrodes. The top-view and cross-sectional scanning electron microscope (SEM) images in Fig. 1b and 1c show the 300–600 nm Cu₆Sn₅ nano-to-micro particles densely packed on the PTFE substrate. This Cu₆Sn₅-on-PTFE structure was synthesized over a large scale via thermal evaporation (Fig. 1d), exhibiting hydrophobicity, as indicated by a water contact angle of 128°. This hydrophobicity enables CO₂ diffusion through the gaps between the particles on the PTFE sides to the catalyst surface (Fig. 1a). Energy dispersive X-ray spectroscopy in transmission electron microscopy (STEM-EDX) analysis revealed a uniform distribution of Sn and Cu over the majority of the Cu₆Sn₅ particles (Fig. 1e).

Figure 2 Electrochemical performance of CO₂ reduction with Cu₆Sn₅ catalyst in strong acid (pH 1) electrolytes.

In a 3 M KCl and 0.05 M H₂SO₄ electrolyte at pH 1, Cu₆Sn₅ exhibited the highest selectivity, reaching up to 96% at −1.4 V_RHE for CO₂R to formic acid in Fig. 2b. Remarkably, Cu₆Sn₅ showed a high FE of above 90% for FA production across a wide range of current densities from 0.4 to 1.2 A cm⁻² (Fig. 2b). We examined the SPCE for Cu₆Sn₅ under a constant current density of 0.5 A cm⁻² in a flow cell equipped with a 1.7 × 1.7 cm² serpentine channel reaction area at various CO₂ flow rates and electrolyte pH levels. Fig.2d presents a maximum SPCE of 77.4%, achieved at a CO₂ flow rate of 3 standard cubic centimeters per minute (sccm) under pH 1 conditions. According to DFT calculations, the selectivity for FA across Cu (111), Cu_0.86Sn_0.14 (111), Cu₆Sn₅ (−113), and Sn (100) surfaces exhibits a volcano plot (Fig. 2a), with Cu₆Sn₅ (−113) displaying the highest energy difference between the two pivotal intermediates, favoring the adsorption of *OCHO over CO. Notably, adhering the established calibration relationship, in situ electrochemical attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy shows the plateau *OCHO peak intensity on Cu₆Sn₅ measured at potentials above −1.04 V_RHE in the CO₂R electrolyte is ~2.8 times higher than that of Sn, with all measurements conducted under identical conditions (Fig. 2c). These results point toward an enhanced surface coverage of *OCHO on Cu₆Sn₅, which facilitates selective FA production during acidic CO₂R. We compared this work with previous reports on the CO₂R-to-FA production under acidic conditions in terms of elevated current density, FE, SPCE, and stability in Fig. 2e.

Figure 3 Electrochemical CO₂R performance with Cu₆Sn₅ catalyst using a cation-free, solid-state-electrolyte-based membrane electrode assembly (SSE-MEA) electrolyzer.

To produce a pure FA solution, we conducted CO₂R using a cation-free, SSE-MEA electrolyzer (Fig. 3). Using the Cu₆Sn₅ catalyst, the FA FE reached ~96% at 100 mA cm⁻². The full-cell voltage was −3.7 V and the full-cell energy efficiency was over 37%. We quantified the amount of FA produced using IC, NMR, and pH measurements. The results confirmed the production of 2.6 liters of 0.36 M pure FA solution with a production rate of 20 mL h⁻¹ over a continuous 130-h CO₂R process.

The above results were published as a research article online (Nat. Commun. 2024, 15, 1711).Miao Zhong from the College of Engineering and Applied Sciences, Nanjing University, and Fanglin Che from the University of Massachusetts Lowell are the co-corresponding authors.Xiaohan Yu (College of Engineering and Applied Sciences, Nanjing University), Yuting Xu (University of Massachusetts Lowell), and Le Li (College of Engineering and Applied Sciences, Nanjing University) are the co-first authors. The authors thank the National Natural Science Foundation of China, the National Key R&D Program of China, the Program for Innovative Talents and Entrepreneurs in Jiangsu, etc for the funding support.

DOI: 10.1038/s41467-024-45988-4