Electrocatalysis of Oxygen Reduction Reaction in a Polymer Electrolyte Fuel Cell with a Covalent Framework of Iron Phthalocyanine Aerogel

Abstract

Carbon aerogels have been studied in the context of fuel cell electrodes mainly as catalyst support materials due to their high surface area, porosity, and electrical conductivity. Recently, aerogels composed solely of inorganic molecular complexes have shown to be promising materials for the electrocatalysis of oxygen reduction reaction (ORR). These aerogels consist of atomically dispersed catalytic sites. Herein, we report on the synthesis and characterization of an aerogel-based catalyst: iron phthalocyanine aerogel. It was synthesized by coupling of ethynylterminated phthalocyanine monomers and then heat-treated at 800 °C to increase its electrical conductivity and catalytic activity. The aerogels reported here were tested as catalysts for ORR in acidic conditions for the first time and found to have a ultra-high number of atomically dispersed catalytic sites (7.11 × 10 20 sites g −1 ) and very good catalytic activity (E onset = 0.9 V vs RHE and TOF = 9.2 × 10 −3 e − s −1 site −1 at 0.8 V vs RHE). The iron phthalocyanine aerogel was also studied in a proton exchange membrane fuel cell, reaching a peak power density of 292 mW cm −2 and an open circuit voltage of 0.83 V.

Design of advanced aerogel structures for oxygen reduction reaction electrocatalysis

“…Ligands such as bipyridine, porphyrin, and phthalocyanine have been used in different studies conducted by the Elbaz group (Figure 4A,B). 60,67,68 This strategy is based on adjusting the ligands with the suitable functional groups to enable their polymerization. For example, terminal alkyne bonds were added to bipyridine and phthalocyanine to enable their polymerization by Glaser coupling, 67,68 and terminal amines were used to synthesize porphyrin aerogels by polycondensation with dialdehydes 30 .…”

“…These aerogels were heat‐treated to increase their electronic conductivity, but the overall active sites (M–N x ), and their distribution was retained. In these papers, the introduction of ligands as monomers enables high metal loading, reaching very high active site densities of up to 9.7 × 10 20 sites g −1 60,67,68 …”

“…One example is of an iron phthalocyanine aerogel that was studied in 0.5 M H 2 SO 4 and showed promising activity, with onset potential of 0.9 V versus RHE 68 . Another example is from the work of the Berthon‐Fabry group performed in moderate acidic conditions (0.05 M H 2 SO 4 ).…”

“…60,67,68 This strategy is based on adjusting the ligands with the suitable functional groups to enable their polymerization. For example, terminal alkyne bonds were added to bipyridine and phthalocyanine to enable their polymerization by Glaser coupling, 67,68 and terminal amines were used to synthesize porphyrin aerogels by polycondensation with dialdehydes. 30 In these procedures, the metal was inserted at different stages, depending on the polymerization process.…”

Modulating the Fe–N4 Active Site Content by Nitrogen Source in Fe–N–C Aerogel Catalysts for Proton Exchange Membrane Fuel Cell

“…However, no highly active carbon aerogel-derived Fe–N–C catalysts in acidic media have been reported hitherto due to the lack of Fe–N 4 active sites. − Therefore, it is still a challenge to develop a practical method to prepare the Fe–N–C aerogel catalyst with both high content of Fe–N 4 active site and excellent mass transport properties. Elbaz et al combined the advantages of the three-dimensional covalent organic framework and carbon aerogel in heat-treated iron porphyrin aerogels, which achieve both high surface area and density of active sites. − …”