Toward Efficient Tandem Electroreduction of CO2 to Methanol using Anodized Titanium

The electroreduction of CO2 (CO2RR) using renewable electricity is an appealing route to synthesize methanol (CH3OH), a valuable C-1 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 C-1 products. Since multielectron electrocatalytic reactions can be severely affected by adsorption-energy scaling relations, a tandem process likely offers a higher efficiency. Therefore, we strategized 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 anodized 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/cm(2) at -1.0 V versus 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 the production of CH3OH. Density functional theory (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.