Proton exchange membrane electrolysis of methanol for simultaneously synthesizing formaldehyde and hydrogen

Abstract

Electrolysis for upgrading molecules has received attention as a green process that can contribute to a sustainable society. In this study, methanol (MeOH) was converted into formaldehyde (FA) and H₂via electrolysis using a membrane electrode assembly (MEA) device, wherein Nafion was used as a proton exchange membrane (PEM). Comparing various anode catalysts for PEM electrolysis, Pd/C is found to produce FA at a high faradaic efficiency (CH₃OH → HCHO + 2H⁺ + 2e⁻). Pd loading and electrolysis voltage affect the FA formation rate. In the PEM electrolysis, the formed protons move toward the cathode through the Nafion membrane, and H₂ is evolved on the cathode (2H⁺ + 2e⁻ → H₂), which indicates that this electrolysis system simultaneously yields FA and H₂. A by-product, dimethoxymethane (DMM), is formed through the non-electrochemical acetalization of FA and MeOH, where Nafion serves as an acid catalyst. The formation of DMM is inhibited by stirring the electrolysis solution and adding a small amount of water. The FA formation rate (682 mmol g_cat⁻¹ h⁻¹) in this study exceeds those reported in previous studies wherein FA is synthesized from the dehydrogenation of MeOH via heterogeneous catalytic and photocatalytic systems.