Recent advances in formic acid electro-oxidation: from the fundamental mechanism to electrocatalysts

Abstract

This review summarizes the recent advances of studies on formic acid electro-oxidation, including the reaction mechanism and the electrocatalysts used.

Benchmarking Catalysts for Formic Acid/Formate Electrooxidation

“…Joo et al [ 46 ], instead, claimed the absence of formate intermediate under similar reaction conditions, using in situ attenuated total reflection surface-enhanced infrared adsorption spectroscopy (ATR−SEIRAS), and they proposed a similar dual-pathway mechanism, but involving linearly adsorbed CO and bridge-bounded CO as a reactive intermediate and poisoning species, respectively. Other in situ spectroscopic techniques, like attenuated total reflection–Fourier transform infrared spectroscopy (ATR−FTIR) or electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (EC−SHINERS), as well as density functional theory (DFT) calculations, have also been employed to study the formic acid/formate electrooxidation mechanism, as recently reviewed [ 31 ]. This mechanism is still matter of discussion, especially regarding the nature of the reactive intermediate for the direct oxidation pathway, the CO formation mechanism and even a hypothetical third indirect pathway through a different adsorbed intermediate (triple path).…”

“…The complexity of the FA electroxidation reaction (FAEOR) results in sluggish kinetics and hinders fuel cell power density. Indeed, despite being investigated since the early 1960s of the twentieth century [ 29 , 30 ], the lack of electrocatalysts with suitable catalytic activity and durability is still the main concern limiting the competitiveness of FAFC and hinders its large-scale use [ 6 , 13 , 31 , 32 ]. Similar to other types of low-temperature fuel cells, FAFC are still dependent on PGM catalysts, in particular Pd and Pt.…”

“…These catalysts are generally characterized by a low onset potential (as low as 0.1 V vs. RHE) but are limited in current density/durability, especially due to a severe poisoning effect primarily due to CO species. Several experimental and theoretical studies, primarily with respect to Pt, Pd and Au, have been performed with the aim of elucidating the reaction mechanism as has been recently summarized comprehensively in a review by Fang and Chen [ 31 ]. It is commonly accepted that FAEOR can follow two reaction pathways: (I) A direct pathway involving the formation of CO 2 without the involvement of CO or (II) an indirect pathway, which entails CO formation followed by CO oxidation to CO 2 .…”

“…It is commonly accepted that FAEOR can follow two reaction pathways: (I) A direct pathway involving the formation of CO 2 without the involvement of CO or (II) an indirect pathway, which entails CO formation followed by CO oxidation to CO 2 . The latter should be avoided because CO can poison the catalysts, and can only be oxidated at higher potentials, thus limiting the catalytic activity [ 31 ]. However, there is still debate on the precise nature of the reactive intermediate involved in the reaction.…”

“…However, there is still debate on the precise nature of the reactive intermediate involved in the reaction. Several species have been proposed, including formic acid adsorbed to the catalysts in the formate (deprotonated) or formic acid (protonated), as well as formate in its bidentate or monodentate configurations [ 31 ]. Due to this uncertainty, in this review, we will take into consideration catalysts for both formic acid and formate oxidation—hence referring to both formic acid and formate generally as FA without focusing on the mechanism or the precise intermediates involved.…”