Boron and nitrogen doping modulating the coordination environment of copper in biochar for reformative electrocatalytic CO2 reduction

Abstract

In the past decades, the widespread application of heavy metal phytoremediation technology in soil has led to an increase in the yield of hyperaccumulating plants year by year. Therefore, there was a pressing demand for exploring an efficient reprocessing method to utilize the plant tissue loaded with heavy metals. Elsholtzia Harchowensis is the hyperaccumulating plant that can enrich copper, and it has a common distribution in copper areas and has been applied in the field of soil remediation of copper tails. Carbon dioxide (CO2), as a major greenhouse gas, its emission reduction and resource utilization are of great significance for China to achieve the "carbon peaking and carbon neutrality strategy". Electrochemical methods have received widespread attention due to their advantages such as mild conditions and controllable products. Copper-based materials, which can convert CO2 into utilizable fuels or chemicals, are currently hot catalysts in this field. This study prepared two types of copper containing biochar, Cu/C-BN and Cu/C, using Elsholtzia Harchowensis as raw material. The rich types of elements in plant cells make the catalyst have special spatial structure and element composition, and both catalysts exhibited good pore structures. Meanwhile, the introduction of two non-metallic elements, B and N, suppressed the hydrogen evolution reaction (HER) in the system, and the Faraday efficiency (FE) of methane (CH4) was significantly improved. In a 1.0 M KHCO3, the Cu/C-BN material electrode had an instantaneous current density of 13.8 mA/cm2 at -0.82 V vs. RHE. The electrode potential was -0.32 V vs. RHE under the condition of electrolytic 8 h, the current reached a stable value of 0.33 mA/cm2, resulting in a FE of 31.3 % for CH4 and 26.9 % for carbon monoxide (CO). On contrary, the FE of CH4 and CO was only 1.5 % and 19.8 % for Cu/C material at -0.32 V vs. RHE, respectively. The doping of both boron and nitrogen significantly improved the electrochemical activity and CH4 selectivity towards CO2RR.