Electrocatalytic reduction of carbon dioxide to carbon monoxide and methane at an immobilized cobalt protoporphyrin

Abstract

The electrochemical conversion of carbon dioxide and water into useful products is a major challenge in facilitating a closed carbon cycle. Here we report a cobalt protoporphyrin immobilized on a pyrolytic graphite electrode that reduces carbon dioxide in an aqueous acidic solution at relatively low overpotential (0.5 V), with an efficiency and selectivity comparable to the best porphyrin-based electrocatalyst in the literature. While carbon monoxide is the main reduction product, we also observe methane as by-product. The results of our detailed pH-dependent studies are explained consistently by a mechanism in which carbon dioxide is activated by the cobalt protoporphyrin through the stabilization of a radical intermediate, which acts as Brønsted base. The basic character of this intermediate explains how the carbon dioxide reduction circumvents a concerted proton-electron transfer mechanism, in contrast to hydrogen evolution. Our results and their mechanistic interpretations suggest strategies for designing improved catalysts.

Figure 1. Voltammetry and volatile product identification by online electrochemical mass spectrometry.

This figure shows the electrochemical reduction of CO₂ on Co protoporphyrin immobilized on a PG electrode and the various volatile products detected by OLEMS. (a) CV in 0.1 M HClO₄; (b) CV in 10 mM HClO₄+90 mM NaClO₄; (c) CV in 1 mM HClO₄+99 mM NaClO₄; (d) m/z=2 (H₂) signal in 0.1 M HClO₄; (e) m/z=2 (H₂) signal in 10 mM HClO₄+90 mM NaClO₄; (f) m/z=2 (H₂) signal in 1 mM HClO₄+99 mM NaClO₄; (g) m/z=15 (CH₄) signal in 0.1 M HClO₄; (h) m/z=15 (CH₄) signal in 10 mM HClO₄+90 mM NaClO₄; (i) m/z=15 (CH₄) signal in 1 mM HClO₄+99 mM NaClO₄; (j) m/z=28 (CO) signal in 0.1 M HClO₄; (k) m/z=28 (CO) signal in 10 mM HClO₄+90 mM NaClO₄; (l) m/z=28 (CO) signal in 1 mM HClO₄+99 mM NaClO₄. Scan rate was in all cases 1 mV s⁻¹. Blue lines are negative-going (forward) scans; magenta lines are positive-going (return) scans. Supplementary Fig. 4 shows the same data with the unnormalized MS signals, as well as the signals obtained in the first and second CV scan.

Figure 2. pH dependence of hydrogen evolution reaction on the CoPP-PG electrode.

Hydrogen evolution reaction at pH=1 (black curve), pH=2 (red curve) and pH=3 (blue curve) on Co protoporphyrin-modified PG electrode in the absence of CO₂. Inserted: highlight of the voltammetry at pH=3. Scan rate was in all cases is 100 mV s⁻¹. All electrolyte solutions were 0.1 M perchlorate, with different ratios of H⁺ and Na⁺.

Figure 3. FE of carbon dioxide reduction to CO and methane.

FEs to CO and CH₄ were determined for yellow bars: pH=1, P_CO2=1 atm; blue bars: pH=1, P_CO2=10 atm; magenta bars: pH=3, P_CO2=1 atm and black bars pH=3, P_CO2=10 atm. FE of (a) CH₄ and (b) CO in 0.1 M perchlorate solution saturated with CO₂. At each potential, the electrolysis was conducted for 1 h at P_CO2=1 atm, while it is 90 min at P_CO2=10 atm due to the longer time to reach the steady state. Error bars were determined from 3–8 data points based on samples taken every 6 min during the steady state of a single electrolysis run.

Figure 4. Identification of volatile products by OLEMS during electrochemical reduction of CO and HCHO.

CV of CO reduction in (a) 100 mM HClO₄ and (b) 1 mM HClO₄+99 mM NaClO₄ saturated with CO with associated mass fragments of volatile products detected with OLEMS. (c) CV of HCHO (5 mM) reduction in 100 mM HClO₄ with associated mass fragments measured with OLEMS. (d–f) The corresponding OLEMS signals for m/z=2 (H₂); (g–i) The corresponding OLEMS signals for m/z=15 (CH₄). Scan rate: 1 mV s⁻¹. Blue lines are negative-going (forward) scans; magenta lines are positive-going (return) scans. Supplementary Fig. 14 shows the same data with the unnormalized MS signals, as well as the signals obtained in the first and second CV scan.

Figure 5. Proposed mechanistic scheme for the electrochemical reduction of CO₂ on Co protoporphyrin.

H⁺ and H₂O are the hydrogen source for the hydrogen evolution reaction at pH=1 and 3, respectively. CO₂^·− is the initial intermediate for the CO₂ reduction to CO. CO can be further reduced to methane with HCHO as an intermediate. The catalytically inactive ‘resting' state of the Co is assumed to be 2+. The reduction of Co²⁺ to Co⁺ is supposed to trigger both the H₂ evolution and CO₂ reduction pathways.