Experimental Reasons for the Variable Efficiencies of Organic Electrocatalysts Used for Converting Carbon Dioxide to Methanol

Abstract

Pyridinium (the protonated form of pyridine) and related compounds have been proposed to be promising homogenous electrocatalysts in the electrochemical reduction of CO 2 to methanol due to the low overpotential required to achieve faradaic yields of about 20 %, although the percentage yields vary dramatically between different research groups. In this study, experimental conditions were varied during the electrolysis of CO 2 at a platinum electrode in the presence of pyridinium to determine the reasons for the discrepancies in the yields of methanol reported between different research groups. Two other vitamin-based and environmentally friendly nitrogen-containing heterocyclic compounds (nicotinamide and nicotinic acid) which have structural similarities to the promising but toxic pyridine homogeneous electrocatalyst, were also investigated as alternative electrocatalysts for the reduction of CO 2 in aqueous acidic media. Cyclic voltammetry studies suggest that nicotinamide and nicotinic acid (forms of vitamin B 3 ) follow a similar reaction mechanism as pyridine in the reduction of CO 2 . Relatively low faradaic yields of methanol were obtained during controlled potential electrolysis experiments for all the electrocatalysts (0.4-1.9 %) which can be attributed to the low solubility of CO 2 along with the competing hydrogen evolution reaction. It was found that adventitious sources of methanol were responsible for greatly inflating the apparent yields of methanol unless scrupulous care was taken in controlling the experimental conditions. The problem with background methanol is particularly difficult to control due to the electrochemical reactions typically generating products in the parts per million range, considerably lower than normal synthetic reactions.

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“…Interestingly, this decline in peak intensity after 3 h of reaction implied a second electrochemical reaction involved reduction of this initial product. It should be noted that when products were left in the reactor cell and subjected to low potentials, more highly reduced products may result; 44 earlier studies showed situations where products were seemingly lost from the reaction solution, and this was exacerbated by performing the reactions at low concentrations 45 . Moreover, different potentials resulted in various products (Fig.…”

“…Based on the products identified by NMR, we can propose a reduction mechanism that results in formic acid as the main product. This mechanism is based on multiple recent publications describing the use of a similar copper-based catalyst and formation of similar products 20,44,45 . A plausible mechanism for the HKUST-1/C composite is shown in Fig.…”

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“…35 Moreover, different potentials result in various products (Figure S11). [35] The faradaic e ciency of formic acid and hydrogen gas produced with different potentials is displayed in Figure S11. The formation of formic acid at lower potential is higher and reduces with an increase in potential because of the hydrogen evolution reaction (Figure S11).…”

“…It should be noted that when products are left in the reactor cell subjected to low potentials, further reduced products may result, 34 and studies have identi ed situations where products may be seemingly lost from the reaction solutions, exacerbated by the reactions being carried out at low concentrations. 35 Moreover, different potentials result in various products (Figure S11). [35] The faradaic e ciency of formic acid and hydrogen gas produced with different potentials is displayed in Figure S11.…”