Toward Efficient Tandem Electroreduction of CO2 to Methanol using Anodised Titanium

Abstract

The electroreduction of CO2 (CO2RR) using renewable electricity is an appealing route to synthesize methanol (CH3OH), a valuable C1 feedstock and fuel. Unfortunately, there are still no workhorse electrocatalysts with suitable activity and selectivity for this reaction. Currently, formic acid (HCOOH), CO and methane are the most common C1 products. Since multi-electron electrocatalytic reactions can be severely affected by adsorption-energy scaling relations, a tandem process likely offers higher efficiency. We, therefore, strategize to reduce CO2 to HCOOH, and then reduce HCOOH to CH3OH. While the former step can be accomplished with ease using post-transition metals, the latter is extremely difficult due to the electrochemical inertness of HCOOH. Herein, we develop anodised titanium catalysts containing Ti3+ sites and oxygen vacancies (termed as TOVs), which can reduce HCOOH to CH3OH with a remarkable Faradaic efficiency of 12.6 % and a partial current density of –2 mA/cm2 at –1.0 V vs the reversible hydrogen electrode (RHE). Using electron paramagnetic resonance spectroscopy and cyclic voltammetry, we show that the population of TOVs on the catalyst is positively correlated with CH3OH production. DFT calculations identify TOVs at defects as the active sites, in a vacancy-filling pathway mediated by *H2COOH. We further provide holistic screening guidelines based on the *HCOOH and *H2COOH binding energies, alongside TOV formation energies. These can open the path for the high-throughput, automated design of catalysts for CH3OH synthesis from tandem CO2 electrolysis.