Electrocatalytic Oxygen Reduction at Multinuclear Metal Active Sites Inspired by Metalloenzymes

Abstract

Polymer electrolyte fuel cells (PEFCs) are a clean, sustainable device to convert chemical energy to electricity and can provide power for automobiles, trains, and ships. In PEFCs, the oxygen reduction reaction (ORR) occurs at the cathode and is catalyzed at electrocatalysts. The activity of ORR electrocatalysts is known to limit the overall performance of PEFCs because the ORR is more sluggish than the hydrogen oxidation reaction at the anode. In the state-of-the-art PEFC, platinum group metal (PGM)-based ORR electrocatalysts are used. Since PGMs are rare and expensive, highly active and durable non-PGM ORR electrocatalysts are required for widespread applications of PEFCs. In nature, metalloenzymes such as cytochrome c oxidase and multicopper oxidases efficiently catalyze the ORR and utilize multinuclear iron and/or copper complexes as active sites. The structure of these active sites and enzyme reaction mechanisms would give us design concepts of artificial non-PGM electrocatalysts for the ORR, possibly leading us to develop next-generation non-PGM electrocatalysts. Herein, recent research progress on understanding enzymatic ORR reaction mechanisms and developing non-PGM ORR electrocatalysts is reviewed from the viewpoint of bio-inspired approaches.

Impact of Heterometallic Cooperativity of Iron and Copper Active Sites on Electrocatalytic Oxygen Reduction Kinetics

“…Metal-nitrogen-carbon (M-N-C) electrocatalysts containing earth abundant metals are promising alternatives to PGM-based ORR electrocatalysts 2 and especially Fe-N-C electrocatalysts have been intensively studied because they show relatively high electrocatalytic activity even in acidic media. [3][4][5][6][7][8][9][10][11][12][13][14][15][16] The production of H2O2 as the byproduct for the ORR is a major concern in many non-PGM electrocatalysts including Fe-N-C. The 4ereduction of O2 produces H2O whereas partial 2ereduction of O2 gives H2O2.…”

“…Electrochemical studies of these synthetic models suggest that the Cu site acts as the initial O2 binding site to form bridging heme-peroxo-Cu intermediates and suppresses the release of reactive oxygen species. 16,[37][38][39][40] Cu-doped Fe-N-C, (Cu,Fe)-N-C, electrocatalysts have also been synthesized for the ORR and synergistic effects of the copresence of iron and copper active sites on the ORR activity have been found. [22][23][24][25][26][27][28][29][30] Furthermore, the suppression of the H2O2 production was also observed for (Cu,Fe)-N-C electrocatalysts, compared with the corresponding Cu-N-C and Fe-N-C catalysts.…”

“…56% of the cost in a fuel cell stack comes from expensive PGMs, developing highly active and durable ORR electrocatalysts based on earth-abundant elements is necessary for large-scale production of PEFCs. Metal–nitrogen–carbon (M–N–C) electrocatalysts containing earth-abundant metals are promising alternatives to PGM-based ORR electrocatalysts and especially Fe–N–C electrocatalysts have been intensively studied because they show relatively high electrocatalytic activity even in acidic media. − …”

“…The C c O utilizes a binuclear complex containing a heme and a copper ion as the active center. , The selectivity toward the 4e – reduction for the ORR reaches >99% for C c O. , Inspired by the active site structure of C c O, molecular-based C c O mimics have been synthesized to understand the mechanistic insights into the enzymatic ORR. Electrochemical studies of these synthetic models suggest that the Cu site acts as the initial O 2 binding site to form bridging heme–peroxo–Cu intermediates and suppresses the release of reactive oxygen species. ,− Cu-doped Fe–N–C, (Cu,Fe)–N–C, electrocatalysts have also been synthesized for the ORR and synergistic effects of the copresence of iron and copper active sites on the ORR activity have been found. − Furthermore, the suppression of the H 2 O 2 production was also observed for (Cu,Fe)–N–C electrocatalysts, compared with the corresponding Cu–N–C and Fe–N–C catalysts . However, heterobimetallic effects of Cu and Fe active sites on the ORR kinetics remain unclear although intensive kinetic studies have been done for single metallic M–N–C catalysts such as Fe–N–C. − …”

Effects of Interfacial Interactions on Electrocatalytic Activity of Cytochrome c Oxidase in Biomimetic Lipid Membranes on Gold Electrodes

“…This work demonstrates that PFE coupled with SEIRA spectroscopy is a powerful analytical technique to understand impacts of interfacial interactions of membrane proteins with biomolecules including water-soluble proteins and phospholipids on the protein functionality in ptBLMs. This technique would enable us to gain mechanistic insights into protein dynamics, functionality, and synchronization at biomembrane interfaces involving protein–protein and protein–lipid interactions and encourage us to develop electrocatalysts , and electrochemical devises including biofuel cells and biosensors. , …”