Influence of the metal center of metalloprotoporphyrins on the electrocatalytic CO2 reduction to formic acid

- Faradaic efficiency for formic acid depends on the metal center, pH and potential.
- In, Sn and Rh metal centers are the most active for formic acid formation.
- Cr, Mn, Co, Fe metal centers are inactive for formic acid formation.
- Faradaic efficiency of ca. 70% is obtained with In-protoporphyrin.
- Formic acid activity is differentiated between metal centers by their HER activity.

Electrocatalytic conversion of carbon dioxide has gained much interest for the synthesis of value-added chemicals and solar fuels. Important issues such as high overpotentials and competition of hydrogen evolution still need to be overcome for deeper insight into the reaction mechanism in order to steer the selectivity towards specific products. Herein we report on several metalloprotoporphyrins immobilized on a pyrolytic graphite electrode for the selective reduction of carbon dioxide to formic acid. No formic acid is detected on Cr-, Mn-, Co- and Fe-protoporphyrins in perchloric acid of pH 3, while Ni-, Pd-, Cu- and Ga-protoporphyrins show only a little formic acid. Rh, In and Sn metal centers produce significant amounts of formic acid. However, the faradaic efficiency varies from 1% to 70% depending on the metal center, the pH of the electrolyte and the applied potential. The differentiation of the faradaic efficiency for formic acid on these metalloprotoporphyrins is strongly related to the activity of the porphyrin for the hydrogen evolution reaction. CO2 reduction on Rh-protoporphyrin is shown to be coupled strongly to the hydrogen evolution reaction, whilst on Sn- and In-protoporphyrin such strong coupling between the two reactions is absent. The activity for the hydrogen evolution increases in the order In < Sn < Rh metal centers, leading to faradaic efficiency for formic acid increasing in the order Rh < Sn < In metal centers. In-protoporphyrin is the most stable and shows a high faradaic efficiency of ca. 70%, at a pH of 9.6 and a potential of −1.9 V vs RHE. Experiments in bicarbonate electrolyte were performed in an attempt to qualitatively study the role of bicarbonate in formic acid formation.

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