Role of Mass Transport in Electrochemical CO2 Reduction to Methanol Using Immobilized Cobalt Phthalocyanine

Thomas Chan; Calton J Kong; Alex J King; Finn Babbe; Rajiv Ramanujam Prabhakar; Clifford P Kubiak; Joel W Ager

doi:10.1021/acsaem.3c02979

Role of Mass Transport in Electrochemical CO₂ Reduction to Methanol Using Immobilized Cobalt Phthalocyanine

ACS Appl Energy Mater. 2024 Apr 4;7(8):3091-3098. doi: 10.1021/acsaem.3c02979. eCollection 2024 Apr 22.

Authors

Thomas Chan^{1

2

3

4

5}, Calton J Kong^{1

2

6}, Alex J King^{1

2

7}, Finn Babbe^{1

2}, Rajiv Ramanujam Prabhakar^{1

2}, Clifford P Kubiak^{3

4}, Joel W Ager^{1

8

6}

Affiliations

¹ Liquid Sunlight Alliance, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
² Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
³ Liquid Sunlight Alliance, University of California, San Diego, La Jolla, California 92093, United States.
⁴ Department of Chemistry & Biochemistry, University of California, San Diego, La Jolla, California 92093, United States.
⁵ Department of Nanoengineering, University of California, San Diego, La Jolla, California 92093, United States.
⁶ Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California 94720, United States.
⁷ Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, California 94720, United States.
⁸ Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.

Abstract

Electrochemical CO₂ reduction (CO₂R) using heterogenized molecular catalysts usually yields 2-electron reduction products (CO, formate). Recently, it has been reported that certain preparations of immobilized cobalt phthalocyanine (CoPc) produce methanol (MeOH), a 6-electron reduction product. Here, we demonstrate the significant role of intermediate mass transport in CoPc selectivity to methanol. We first developed a simple, physically mixed, polymer (and polyfluoroalkyl, PFAS)-free preparation of CoPc on multiwalled carbon nanotubes (MWCNTs) which can be integrated onto Au electrodes using a poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) adhesion layer. After optimization of catalyst preparation and loading, methanol Faradaic efficiencies and partial current densities of 36% (±3%) and 3.8 (±0.5) mA cm^-2, respectively, are achieved in the CO₂-saturated aqueous electrolyte. The electrolyte flow rate has a large effect. A linear flow velocity of 8.5 cm/min produces the highest MeOH selectivity, with higher flow rates increasing CO selectivity and lower flow rates increasing the hydrogen evolution reaction, suggesting that CO is an unbound intermediate. Using a continuum multiphysics model assuming CO is the intermediate, we show qualitative agreement with the optimal inlet flow rate. Polymer binders were not required to achieve a high Faradaic efficiency for methanol using CoPc and MWCNTs. We also investigated the role of formaldehyde as an intermediate and the role of strain, but definitive conclusions could not be established.