Tungsten Carbide As a Catalyst for CO2 Electrochemical Reduction

Abstract

Currently, conversion of CO2 and through its reduction reaction is considered as a promising method for significantly reducing CO2 emission while permitting the use of fossil fuels to meet the ever-increasing energy demands worldwide [1]. CO2 is an extremely stable molecule, because it has high thermodynamic stability and kinetic inertia. Thus, its conversion is an energy-intensive process. As a consequence, the large scale conversion of CO2 requires highly efficient catalysts capable of converting CO2 to target chemicals, such as carbon monoxide, methane, ethane, ethylene, methanol, etc. This is particularly important in the electrochemical reduction of CO2 since a highly selective and energy-efficient catalyst is a prerequisite, to ensure the high selectivity for the products and greatly reduce the energy consumption, an important topic worth of extensive exploration and in-depth studies. For the catalysts containing noble metals such as Ag, Au and Pt, the high costs of metals are the main hindrance for their large-scale industrial applications [2]. Recent progress has been made in the improvement of the catalytic activities of noble metals, the required noble metal content and the associated costs to achieve a certain level of catalytic efficiency may be reduced significantly with those new catalysts [3]. In the meantime, transition metal carbides (TMCs) have received considerable attention as the alternative electrocatalysts and supporting materials because TMCs display remarkable catalytic activities owing to their similar electronic and catalytic properties to Pt-group metals (by inducing carbon into the metal lattice). Therefore, TMCs have been identified as the most promising candidates to replace Pt or reduce its content in catalysis reactions [4]. In particular, carbides of group 4-6 TMs have been investigated extensively for their catalytic properties towards various reactions including oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) [5]. However, carbides as catalysts in electrochemical CO2 reduction have not been investigated so far. Herein, we developed a tungsten carbide (WC) catalyst to efficiently reduce CO2 to CO in a KHCO3 solution. The XRD pattern of the prepared WC powders shows that no impurity phases were detected. Furthermore, the electrocatalytic activity of WC towards the CO2 reduction was studied in a full electrochemical cell. A catalyst suspension was prepared by mixing WC powders with Nafion solution, 2-propanol, and de-ionized water, and then dropped onto the glassy carbon electrode (GCE) after ultrasonic for dispersion. The cyclic voltammetry curves were recorded in an argon saturated solution and CO2 saturated solution between -0.5 V (vs. SCE) and -1.5 V (vs. SCE). It shows that the onset potential for the hydrogen evolution reaction (HER) was -1.11 V (vs. SCE), a high overpotential for HER. Normally, for HER at pH 7, the onset potential is -0.41 V. When the gas supply was changed to CO2, the current density increased significantly, this increment in the current density indicates that the CO2 reduction reaction occurred and the WC powders had high catalytic activity for CO2 reduction. Therefore, this group of carbides can be the promising materials in the electrochemical CO2 reduction at room temperature. Figure 1