Bioinspired Iron- and Copper-incorporated Carbon Electrocatalysts for Oxygen Reduction Reaction

Abstract

We report an Fe/Cu/N-doped carbon electrocatalyst for the oxygen reduction reaction to water. Pyrolysis of metal precursors supported on oxidized carbon black provides the electrocatalyst with high catalytic activity and selectivity. Detailed physicalchemical studies have revealed that the copresence of iron and copper catalytic sites in carbon in a 1:1 mole ratio, which is the same as that of an enzyme of cytochrome c oxidase, might be the key to synthesize highly active and selective electrocatalysts. Keywords: Oxygen reduction reaction (ORR) |Polymer electrolyte fuel cell | Non-platinum group metal (non-PGM) electrocatalystThe oxygen reduction reaction (ORR) to water is a key reaction in respiration and electricity generation devices of polymer electrolyte fuel cells (PEFCs) and metal-air batteries. In nature, metalloenzymes such as cytochrome c oxidases (CcOs) efficiently catalyze the ORR with almost no overpotential. 1,2Such an enzymatic ORR is efficiently catalyzed at non-preciousmetal-based reaction centers including a dinuclear heme/Cuhistidine complex in CcOs.2 In the energy devices, on the other hand, platinum group metal (PGM) alloys are widely used as the cathodic material for the ORR. 35 The scarcity and high cost of PGM alloys limit the widespread application of PEFCs, and ideally we should develop non-precious-metal-based electrocatalysts for the ORR like the natural enzymes.Intensive efforts have been devoted to developing non-PGM electrocatalysts for the ORR over the past several decades. 69 The most promising approach to highly efficient and durable non-PGM ORR electrocatalysts is pyrolysis. 8Pyrolysis of metal, nitrogen, and carbon sources has provided metal-and nitrogen-doped carbon electrocatalysts, and some of them show high activity and durability comparable with that of PGM alloys. 10,11 Although the pyrolytic approach has been successful, the choice of precursors is unclear, and finding the best precursors is still based on trial-and-error experiments. Thus, guidance on the choice of precursors will open up possibilities to develop non-PGM ORR electrocatalysts for practical applications.Herein we report Fe/Cu/N-incorporated carbon electrocatalysts, inspired by the active site of CcO, and synergistic effects of the dopants on the ORR catalytic activity. We used a dinuclear copper complex with a nitrogen-rich ligand of 3,5-diamino-1,2,4-triazole (Cu-Hdatrz) 12 as the copper source and chloro(protoporphyrinato)iron(III) (hemin) as the iron source (Scheme 1). Cu-Hdatrz is known to show high ORR activity in copper-based ORR electrocatalysts, particularly in alkaline solution, 1214 and pyrolysis of Cu-Hdatrz and graphene oxide provides a Cu-doped carbon electrocatalyst with higher ORR activity than the pristine Cu-Hdatrz. 15 Hemin is also known to catalyze the ORR, and the pyrolysis of hemin gives Fe-doped carbon electrocatalysts for the ORR. 16 Furthermore, hemin and Cu-Hdatrz have similar ligand structures to heme a 3 and the Cuhistidine complex found at the catalytic active site of CcO...

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

“…[3][4][5]17 The improvement of the 4eselectivity is crucial for developing practical non-PGM catalysts. To improve the ORR activity and selectivity of Fe-N-C, heterometal-doped Fe-N-C catalysts of (M,Fe)-N-C (M = Mn, 18 Co, [19][20] Ni 21 or Cu [22][23][24][25][26][27][28][29][30] ) have been synthesized. [31][32] In nature, a highly selective non-PGM ORR catalysts with heterobimetallic active sites is known: a metalloenzyme of cytochrome c oxidase (CcO).…”

“…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. 28 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. [41][42][43] Herein, we report synthesis, characterization and kinetic analysis of Cu-, Fe-and N-doped carbon nanotubes, (Cu,Fe)-N-CNT, for the ORR in acidic media.…”

“…28 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. [41][42][43] Herein, we report synthesis, characterization and kinetic analysis of Cu-, Fe-and N-doped carbon nanotubes, (Cu,Fe)-N-CNT, for the ORR in acidic media. The bimetallic (Cu,Fe)-N-CNT electrocatalyst was synthesized in pyrolysis from a composite of chloro(protoporphyrinato)iron (hemin), 22,44 a trinuclear copper complex of Cutrz ([Cu II 3(trz)3(µ-OH)]Cl2•6H2O, trz = 1,2,4triazole) [45][46] and single-walled carbon nanotubes (SWCNTs), which originate from vertically aligned carbon nanotubes (VACNTs) 7,10 (Scheme 1). Inspired by the CcO binuclear active site of a heme and a copper complex of histidine ligands, hemin and Cutrz were used as metal precursors.…”

Bimetallic M/N/C catalysts prepared from π-expanded metal salen precursors toward an efficient oxygen reduction reaction

“…11,24 Inspired by cytochrome c oxidase, which catalyzes O 2 reduction using a unique bimetallic Fe and Cu active site with ultimate efficiency, a number of examples of construction of bimetallic M/N/C catalysts containing rst row transition metals as active sites have been reported. [25][26][27][28][29][30][31][32] In the enzyme, the active site containing heme iron and a copper ion is known to efficiently promote four-electron reduction of a bound O 2 molecule to water. Thus, M/N/C catalysts prepared from two precursors with two metal ions have been investigated in attempts to improve the ORR activity.…”

Electrocatalytic Oxygen Reduction at Multinuclear Metal Active Sites Inspired by Metalloenzymes

“…In Fe/Cu/N/C electrocatalysts, the co-presence of Cu and Fe in N-doped carbon electrocatalysts is also known to show synergistic effects on the ORR activity. A Fe/Cu/N/C electrocatalyst was prepared from the precursor mixture of chloro(protoporphyrinato)iron(III) (hemin), Cu-Hdatrz and oxidized Vulcan XC-72 (oV) in the short-period heat treatment at 1023 K under Ar for 45 s [ Figure 13(a)] [95]. The Fe/Cu/N/C electrocatalyst shows higher ORR activity in the wide pH range from 1 to 13 than Fe/N/C and Cu/N/C, which are prepared from precursors of hemin/oV and Cu-Hdatrz/ oV, respectively [ Figure 13(b, c)].…”