Hydrogen coverage modulation unveils the hydrogenation routes in electrochemical CO(2) reduction

Abstract Understanding the hydrogenation routes in the electrochemical CO (2) reduction reaction (eCO (2) RR) is essential for the selective production of oxygenated and hydrocarbon products. Hydrogenation dictates the selectivity by determining whether hydrogen preferentially attacks oxygen or carbon in one intermediate. Oxygenated products are particularly valuable due to their higher energy density and economic potential, making enhancing their Faradaic efficiency (FE) vital. However, the factors determining hydrogenation selectivity remain unclear, making precise control over product distribution challenging. Herein, we systematically investigate hydrogenation mechanisms from CO to CH 4 , C 2 H 5 OH, and C 2 H 4 using density functional theory (DFT) calculations with an explicit solvation model. Our results reveal that surface hydrogen preferentially attacks carbon atoms via the Langmuir-Hinshelwood (LH) mechanism, while solvent hydrogen attacks oxygen atoms via the Eley-Rideal (ER) mechanism. This insight suggests that enhancing the LH mechanism could promote oxy-generating products when the solvent environment is determined. Microkinetic modeling supports these findings by adjusting the LH mechanism through H 2 partial pressure modulation. Further experiments demonstrate FE change of ethanol, ethylene, and methane under different CO:H 2 /N 2 partial pressures at different currents. Experiment results confirm that increasing the coverage of * H can effectively enhance the FE of oxygenated compounds while also causing rapid saturation of carbon atoms, thereby suppressing C-C coupling and reducing the FE of multi-carbon products. These computational and experimental findings provide a mechanistic foundation for optimizing eCO (2) RR selectivity through hydrogen coverage modulation.