Porous Structure Controlling the Selectivity of Oxygen Reduction Reaction on N‐Doped Carbon in Alkaline Solution

Abstract

N-doped carbon materials have been regarded as potential replacements for Pt-based electrocatalysts for oxygen reduction reaction (ORR). However, the exact catalytic mechanisms are still not completely understood and there is a controversy about which type of N determines the selectivity of the reaction. This study reports a simple pyrolysis method to prepare hierarchical porous-structured N-doped graphene using 4-nitroimidazole as both C and N sources without the presence of two-dimensional template. The as-prepared Ndoped graphene exhibits efficient four-electron transfer pathway for ORR with an onset potential of 0.943 V, a half-wave potential of 0.823 V, and a limiting current density of 5.58 mA cm À 2 in 0.1 M KOH solution. In contrast, N-doped platelet-like carbon consisted mainly of micropores prefers the two-electron pathway with a high selectivity toward peroxide generation over 90 % at potential range of 0.20-0.62 V. We demonstrate that the ORR selectivity is dependent on the H 2 O transfer dynamics which are controlled by the porous structure. Sluggish H 2 O transfer rate induces HO 2 À formation, whereas fast H 2 O transfer rate ensures the desired four-electron pathway. This study gives further insight into the ORR selectivity and provides feasible strategies for fabricating N-doped carbon materials as promising catalysts for either four-electron oxygen reduction or H 2 O 2 production in alkaline solution.

Recent progress in electrocatalytic selectivity in heterogeneous electro-Fenton processes

“…52 Also, N-doped platelet-like carbon consisting of micropores prefers the 2e − ORR pathway with high selectivity toward H 2 O 2 generation over 90% in the potential range of 0.20–0.62 V vs. RHE. 53 The well-ordered mesoporous structure of nitrogen-doped carbon with diameters of 3.4–4.0 nm was reported to exhibit high selectivity toward H 2 O 2 due to enhanced mass transport in the catalyst layer. 54 Meanwhile, the honeycomb carbon nanofibers with super hydrophilic O 2 -entrapping surfaces are capable of achieving a high H 2 O 2 selectivity of 97.3%, much higher than that of their solid carbon nanofiber counterparts.…”

Formation of N‐doped Graphene and Carbon Nanoparticles Integrated Composite for Efficient Oxygen Reduction Electrocatalysis

“…[13][14][15][16] What is more, both four-electron (4e À ) and two-electron (2e À ) ORRs can be achieved in N-doped carbon catalysts, and the origin of ORR selectivity is still poorly understood to date. [19][20][21][22] Since thermodynamics favors the 4e À ORR over the 2e À ORR, the selectivity should originate in the kinetics of ORR in N-doped carbon. [21] Our previous work has shown that the ORR selectivity in alkaline electrolyte is dependent on the H 2 O transfer dynamics which are controlled by the porous structure of N-doped carbon catalysts.…”

“…[21] Our previous work has shown that the ORR selectivity in alkaline electrolyte is dependent on the H 2 O transfer dynamics which are controlled by the porous structure of N-doped carbon catalysts. [22] Facile accessibility of H 2 O molecules to the active sites is essential to reduce the adsorbed O 2 to OH À completely. N-dope carbon with abundant porous structure allows the desired 4e À ORR due to the fast H 2 O diffusion, whereas N-doped carbon with only limited micropores exhibits 2e À ORR since there are not enough H 2 O molecules to reach the active sites for a complete 4e À ORR.…”

“…Many studies have highlighted the significant roles of the synergistic effect of N‐doping and the intrinsic carbon defects [13–16] . What is more, both four‐electron (4e − ) and two‐electron (2e − ) ORRs can be achieved in N‐doped carbon catalysts, and the origin of ORR selectivity is still poorly understood to date [19–22] . Since thermodynamics favors the 4e − ORR over the 2e − ORR, the selectivity should originate in the kinetics of ORR in N‐doped carbon [21] .…”

“…Since thermodynamics favors the 4e − ORR over the 2e − ORR, the selectivity should originate in the kinetics of ORR in N‐doped carbon [21] . Our previous work has shown that the ORR selectivity in alkaline electrolyte is dependent on the H 2 O transfer dynamics which are controlled by the porous structure of N‐doped carbon catalysts [22] . Facile accessibility of H 2 O molecules to the active sites is essential to reduce the adsorbed O 2 to OH − completely.…”