Microenvironment Alters the Oxygen Reduction Activity of Metal/N/C Catalysts at the Triple-Phase Boundary

Abstract

Metal- and nitrogen-doped carbon (M/N/C) catalysts are promising catalysts that may replace platinum in the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells. At the triple-phase boundary, the complex electrochemical interface of M/N/C catalysts involves the water film between highly dispersed active sites and aggregated ionomers, and the electrochemical behavior is different from that at the double-phase boundary, where a rotating disk electrode (RDE) is directly submerged in the aqueous solution. In this work, the ORR kinetics of the same Fe/N/C catalyst at the double-phase boundary at the RDE and the triple-phase boundary at the gas diffusion electrode (GDE) were compared to determine the microenvironmental factors that can tune the ORR activity. A model was created based on the reaction mechanism, microkinetic analysis, and local proton transport in the pores. The model fitting identified the accessible active sites and the water activity as the factors influencing the different ORR kinetics of the RDE and GDE. The surface charge and functional groups were proposed to tune the proton transport in the pores, resulting in a loss of accessible active sites when E > E Q/HQ at the GDE. Low water activity in the GDE was predicted by the model fitting. This low activity triggered a low OH* coverage and facilitated the reaction kinetics, but it limited the proton transport at the GDE. Strategies for improving proton transport are suggested for the future design of catalysts and ionomers. This study quantitatively demonstrates how the microenvironment alters the ORR kinetics of Fe/N/C catalysts.

(B, N)‐Rich B−C−N Nanosheet‐assembled Microwires as Effective Electrocatalysts for Oxygen Reduction Reaction

“…The BCNNAM gives a Tafel slope of 56 mV dec À 1 , which is lower than that of BNNAM (67 mV dec À 1 ), NC (58 mV dec À 1 ), PC (58 mV dec À 1 ) and Pt/C (71 mV dec À 1 ), demonstrating its favorable ORR kinetics. [44] The enhanced ORR activity may be due to the rich active sites provided by B and N and unique porous structure allows for rapid electron/ion transport.…”

Interfacial Micro‐Environment of Electrocatalysis and Its Applications for Organic Electro‐Oxidation Reaction

“…[72][73][74] The significance of IME, of which is buried between the reaction electrode and bulk electrolyte, is thus receiving increasing attention. In fact, substantial work has been contributed to tailoring the IME of electrocatalyst for gas-involving reactions like OER, [30,33,75,76] hydrogen evolution reaction (HER), [77][78][79] oxygen reduction reaction (ORR), [14,[80][81][82][83][84] CRR [85][86][87][88] and N 2 reduction reaction (NRR), and positive impact on the performance are achieved.…”

Co–Pyridinic-N Bond Constructed at the Interface of CoxP and N-Doped Carbon to Effectively Facilitate Oxygen Reduction

“…The increasingly serious energy crisis and environmental contamination caused by excessive expenditures of fossil fuels have accelerated the pace of exploring new energy sources in the world. − Currently, new energy sources’ intermittent, fluctuating, and random nature hinders their development, while energy storage is considered an effective means to solve these problems. − Metal–air batteries are considered to be an ideal energy storage device and conversion system due to their large capacity, the safety of use, low environmental impact, high theoretical energy density, and low expense. − During the charging and discharging process of metal–air batteries, the oxygen reduction reactions (ORR) usually need to be carried out at a high overpotential, and the slow kinetic process severely restricts their energy conversion efficiency. − To accomplish high energy conversion efficiency, it is necessary to explore a robust electrocatalyst to minimize the needed kinetic overpotential. Although noble metal-based compounds such as Pt/C have been widely used in current commercial batteries as a consequence of their excellent catalytic performance, their large-scale application and development are plagued by exorbitant cost and low reserves. − In order to change this status, there is a pressing need to construct a sustainable and efficient electrocatalyst for metal–air batteries.…”