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 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 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/cm² 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.