Acceleration of the reduction of carbon dioxide in the presence of multivalent cations

Abstract

The accelerating effect of various multivalent cations, the halogen anions and the acidity of the solution on the rate of the electrochemical reduction of CO₂ on a Cu (88)–Sn (6)–Pb (6) alloy cathode was studied. In 1.5 mol L⁻¹ HCl containing various cations, at −0.65 V vs. Ag/AgCl the rate increases with the increase of the surface charge of the cation of the supporting electrolyte in the order Na⁺ < Mg²⁺ < Ca²⁺ < Ba²⁺ < Al³⁺ < Zr⁴⁺ < Nd³⁺ < La³⁺. In La³⁺ containing electrolyte the rate was two-times higher than that in the case of Na⁺ at the same potential. The acceleration effect was attributed to the participation of the radical anion (CO₂⁻) in the rate determining step. The effect of the cation was somewhat higher at pH > 4, but less pronounced at quite negative potentials (−1.7 V). This was attributed to the change of the rate-determining step of the reduction at high overpotentials. In strongly acidic solution at −0.65 V the halogen anion increases the rate in the order Cl⁻ < Br⁻ < I⁻, while the increase of the [H₃O⁺] from 0.1 to 2 mol L⁻¹ resulted to an increase in the rate by 53%. The main products of the reduction were CH₃OH, CH₃CHO, HCOOH and CO. The % current efficiency (% CE) of CH₃OH and HCOOH displayed maxima (∼35 and 28%) at −0.6 and −0.65 V respectively. The %CE of CH₃CHO was continuously increased with increasing negative potential from −0.55 to −1.2 V, while that of CO followed the reverse trend. In the presence of Zr⁴⁺ a relatively high %CE of CH₃CHO (17.6%) was obtained. The main conclusion of this work is that the rate of CO₂ reduction can be increased at low overpotentials and the distribution of the products can be controlled simply by varying the composition of the electrolyte.

Introduction

The electrochemical reduction of CO₂ was studied for the first time in 1870 by Royer [1]. Since then, it has been the subject of a large number of publications, especially during the recent twenty years. These works have been reviewed by Sammells and Cook [2], Jitaru et al. [3], Hori [4], Gattrell et al. [5] as well as by Chaplin and Wragg [6].

The conversion of CO₂ to organic chemicals is of crucial importance for the storage of the energy produced by renewable energy sources such as wind, solar, geothermal as well as from nuclear stations. Among various chemicals that can be produced from the reduction of CO₂, methanol has many advantages. The “methanol economy” in which methanol will replace fossil fuels is an alternative way to the so-called “hydrogen economy” due to the limitations and disadvantages of hydrogen [7]. Even though the electrochemical production of methanol from CO₂ is thermodynamically more favorable than the production of hydrogen from water [8], which is the main competitive reaction, it is kinetically difficult and needs suitable electrocatalysts and reaction conditions. Formation of methanol has been observed on various metallic electrodes such as Ru [9], Mo [10], Cu (90.5)–Ni (9.5) [11], Cu in a C₂H₅OH–H₂O–LiCl electrolyte [12] and on Ru–Cu [13] as well as on conductive oxides such as RuO₂ [14] or mixtures of oxides [8]. The % CE usually is high at low cathodic overpotentials and depends strongly on the nature of the cathode and the other electrolysis conditions.

The influence of the electrolyte and its concentration on the electrochemical reduction of CO₂ has been studied in several works [14], [15], [16], [17], [18], [19], [20], [21], [22]. It has been found from steady state electrolytic experiments that the rate of the reduction depends on the Stokes radius of the alkali metal cation in the order Li⁺ < Na⁺ < K⁺ < Cs⁺. Cyclic voltammetry experiments have also proved that the rate of the reduction increases as the size of the anion of the electrolyte increases in the order Cl⁻ < Br⁻ < I⁻ [23].

The present work includes experimental results on the influence of multivalent cations on the rate of the reduction of CO₂. The effect of the halogen anions and the acidity of the electrolyte was also studied.