Electrochemical reduction of Formic acid using an earth abundant catalyst

Abstract

Copper is one of the most useful electrocatalysts for electrochemically converting CO2 to hydrocarbons and alcohols. Unfortunately, copper suffers from a lack of selectivity and efficiency. Earth-abundant electrocatalysts such as metal porphyrins have been shown to be highly stable and highly selective for products such as carbon monoxide and formic acid. Formic acid is formed with high efficiency on a wide range of materials and can be further reduced to other useful products such as hydrocarbons and alcohols. The aim of this project is to conduct electrochemical formic acid reduction to hydrocarbons or alcohols using copper (II) tetraphenyl porphyrin. Electrochemical Formic acid reduction was conducted using a Proton Exchange Membrane (PEM) cell electrolyser. Water electrolysis was conducted at the anode using a 70:30 IrO2: TaC electrocatalyst. A control experiment was first conducted with a freebase tetraphenyl porphyrin cathode. Thereafter all experiments were repeated with Cu (II) tetraphenyl porphyrin. Products were characterized using liquid injection gas chromatography. Formic acid reduction with freebase tetraphenyl porphyrin did not yield any products at both -1.8 V and -2.1 V. Copper (II) tetraphenyl porphyrin yielded isopropanol and the most conductive copper (II) tetraphenyl porphyrin electrode produced isopropanol with a faradic efficiency of 4.5 % at -2.1 V with current density of -1.71 mA/cm2. The least conductive Cu (II) tetraphenyl porphyrin electrode exhibited a current density of -0.055 mA/cm2 at -2.1 V but produced isopropanol with a faradaic efficiency of 30.4 %. This shows that a high current density does not necessarily equate to an enhanced faradaic efficiency of formic acid reduction to isopropanol. No isopropanol was detected from formic acid reduction using freebase tetraphenyl porphyrin. This indicates that the mechanism of formic acid reduction to isopropanol requires the presence of the copper central metal as the active site.