A DFT study of CO₂ electroreduction catalyzed by hexagonal boron-nitride nanosheets with vacancy defects

In addition to providing a sustainable route to green alternative energy and chemical supplies from a cheap and abundant carbon source, recycling CO₂ offers an excellent way to reduce net anthropogenic global CO₂ emissions. This can be achieved via catalysis on 2D materials. These materials are atomically thin and have unique electrical and catalytic properties compared to bigger nanoparticles and conventional bulk catalysts, opening a new arena in catalysis. This paper examines the efficacy of hexagonal boron nitride (h-BN) lattices with vacancy defects for CO₂ electroreduction (CO₂RR). We conducted in-depth investigations on different CO₂RR electrocatalytic reaction pathways on various h-BN vacancy sites using a computational hydrogen model (CHE). It was shown that CO binds to h-BN vacancies sufficiently to ensure additional electron transfer processes, leading to higher-order reduction products. For mono-atomic defects V_N (removed nitrogen), the electrochemical path of (H⁺ + e^-) pair transfers that would lead to the formation of methanol is most favorable with a limiting potential of 1.21 V. In contrast, the reaction pathways via V_B (removed boron) imposes much higher thermodynamic barriers for the formation of all relevant species. With a divacancy V_BN, the hydrogen evolution reaction (HER) would be the most probable process due to the low rate-determining barrier of 0.69 eV. On the tetravacancy defects V_B3N the pathways toward the formation of both CH₄ and CH₃OH impose a limiting potential of 0.85 V. At the same time, the HER is suppressed by requiring much higher energy (2.15 eV). Modeling the edges of h-BN reveals that N-terminated zigzag conformation would impose the same limiting potential for the formation of methanol and methane (1.73 V), simultaneously suppressing the HER (3.47 V). At variance, the armchair conformation favors the HER, with a rate-determining barrier of 1.70 eV. Hence, according to our calculations, V_B3N and V_N are the most appropriate vacancy defects for catalyzing CO₂ electroreduction reactions.