Mass Transport Effects on Electroreduction of Carbon Dioxide into Formic Acid

Abstract

The electrochemical reduction of carbon dioxide ( 2 ) into formic acid () on a cathode is governed by mass transport.This work investigates the effects of mass transport of 2 to a plated electrode on 2 reduction.First, a potential of -1.9 V vs SCE is applied to the cathode for two hours at different rotating cathode speeds.It is found that at a rotating speed of 240 rpm, the obtained current density is 70.91 Am - 2 which increases to 108.67 Am -2 at 540 rpm and to 123.47 Am -2 at 960 rpm.The samples are analyzed using HPLC and is found to contain . decreases when the rotating speed of cathode is increased.To account for 2 evolution, cyclic voltammetry is carried out.The catholyte undergoes pre-electrolysis with a Pt cathode and purged with 2 .The Pt cathode is then replaced with a plated carbon cathode and cyclic voltammetry with a maximum negative potential of -2.5 V vs SCE is carried out on the system to account for 2 evolution.Then the catholyte is saturated with 2 and cyclic voltammetry is repeated on the system. 2 reduction is accounted by subtracting the effects of 2 evolution from the 2 cyclic voltammgram.This is done at varying rotating cathode speed at 240 rpm, 540 rpm, and 960 rpm.When the cathode is rotated at 240 rpm, 2 reduction initaties at circa -0.7 V vs SHE and the difference in area under the 2 and 2 curve is calculated to be 7.926 × 10 -4 AV.When the cathode rotating speed is increased to 540 rpm, the difference in area under the 2 and 2 curve is calculated to be 4.85 ×10 -4 AV.When the cathode rotating speed is further increased to 960 rpm, a slight drop is suffered with the difference in area under the 2 and 2 curve calculated to be 4.54 × 10 -4 AV.It can be concluded that increasing the mass transport of 2 does not necessarily improve reduction performance possibly due to migration of to the anode where it is oxidized back into 2 and -to the cathode to form 2 , further reduction of into other products, the formation of a - 2 complex, and competing reduction of impurities at the cathode.