Pyrolyzed M–N_x catalysts for oxygen reduction reaction: progress and prospects

Abstract

Fuel cell, a sustainable technology that assures a cleaner earth, once experienced disillusion due to the issue of economic viability associated with the massive usage of Pt-based catalysts. To address...

Advanced Atomically Dispersed Metal–Nitrogen–Carbon Catalysts Toward Cathodic Oxygen Reduction in PEM Fuel Cells

“…The theoretical calculation further disclosed that introduction of adjacent pore defect dramatically decreased the adsorption energy of intermediate species (O 2 * and OOH*), enhancing thermodynamic limiting potential, which ultimately contributes to the higher intrinsic activity than the configuration with no defect. Despite a high ORR activity with acceptable stability for Fe–N–C‐based SACs in acidic medium, there are still some issues that must be solved: [ 36,86,87 ] (i) Fe–N x active site is subjected to the demetallation during ORR; and (ii) PEM degradation and carbon oxidation due to the attack by free radicals from the Fenton reactions (particularly, Fe with a high Fenton reaction activity), which notably decreases activity and stability. Hence, designing other non‐Fe based atomically dispersed M–N–C catalysts with negligible Fenton reaction activity, promising activity and high stability under acidic condition is significant for promoting the applications of M–N–C catalysts in MEA of practical fuel cell, and relevant recent progresses are discussed below.…”

“…), which gives rise to PEM decomposition through the promoted Fenton reaction. Thus, before incorporating into the MEA, removing some unstable species via acid‐washing has been verified as an efficient way of the mitigation [ 36,170 ] ; (ii) the FeN x sites are subjected to the demetallation at fuel cell actual operation, causing a rapid degradation of performance. Recently, Dodelet et al.…”

“…in atomically dispersed M–N–C catalysts are mainly responsible for the observed high activity. [ 36–38 ] Although the reported ORR performance of M–N–C catalysts is comparable to state‐of‐the‐art Pt/C catalysts in acidic solution under rotating disk electrode (RDE) measurements, when used for membrane electrode assembly (MEA) in practical, they still suffer from insufficient fuel cell performance and poor stability. [ 36,39–42 ] The following three factors are considered when looking to improve the performance of M–N–C catalysts: (i) the intrinsic activity; (ii) the density and availability of M–N x active sites; and (iii) the balanced porous structure (micro‐, meso‐, and macroporous) and accessible reaction interface.…”

“…[ 36–38 ] Although the reported ORR performance of M–N–C catalysts is comparable to state‐of‐the‐art Pt/C catalysts in acidic solution under rotating disk electrode (RDE) measurements, when used for membrane electrode assembly (MEA) in practical, they still suffer from insufficient fuel cell performance and poor stability. [ 36,39–42 ] The following three factors are considered when looking to improve the performance of M–N–C catalysts: (i) the intrinsic activity; (ii) the density and availability of M–N x active sites; and (iii) the balanced porous structure (micro‐, meso‐, and macroporous) and accessible reaction interface. By virtue of designing the appropriate precursors (containing nitrogen, carbon, transition metal, and other elements), and advanced synthesis technologies, combining with a versatile regulating strategy at an atomic or molecular scale, great advancements have been made in the research of atomically dispersed M–N–C catalysts (including single atom catalysts (SACs) and double atom catalysts (DACs)) in recent years, meanwhile, the corresponding membrane electrode assembly (MEA) performance in PEMFCs based on atomically dispersed M–N–C catalysts has also been significantly improved.…”

Active site engineering of single-atom carbonaceous electrocatalysts for the oxygen reduction reaction

“…[18][19][20] in the recently published excellent review articles, examining both the design of M-N-C materials and their ORR activities. 22,38,42,[75][76][77][78][79] Until now, the M-N-C electrocatalysts have achieved outstanding ORR activity in alkaline solutions, exceeding that of benchmark Pt/C catalyst. 80 The distinguished ORR activity of such M-N-C materials empowered the asassembled metal-air batteries with superior performance.…”