Real-time investigation of reactive oxygen species and radicals evolved from operating Fe-N-C electrocatalysts during the ORR: potential dependence, impact on degradation, and structural comparisons

Abstract

Improving the stability of platinum-group-metal-free (PGM-free) catalysts is a critical roadblock to the development of economically feasible energy storage and conversion technologies. Fe-N-C catalysts, the most promising class of PGM-free catalysts, suffer from rapid degradation. The generation of reactive oxygen species (ROS) during the oxygen reduction reaction (ORR) has been proposed as a central cause of this loss of activity. However, there is insufficient understanding of the generation and dynamics of ROS under catalytic conditions due to the difficulty of detecting and quantifying short-lived ROS such as the hydroxyl radical, OH˙. To accomplish this, we use operando scanning electrochemical microscopy (SECM) to probe the production of radicals by a commercial pyrolyzed Fe-N-C catalyst in real-time using a redox-active spin trap methodology. SECM showed the monotonic production of OH˙ which followed the ORR activity. Our results were thoroughly backed using electron spin resonance confirmation to show that the hydroxyl radical is the dominant radical species produced. Furthermore, OH˙ and H₂O₂ production followed distinct trends. ROS studied as a function of catalyst degradation also showed a decreased production, suggesting its relation to the catalytic activity of the sample. The structural origins of ROS production were also probed using model systems such as iron phthalocyanine (FePc) and Fe₃O₄ nanoparticles, both of which showed significant generation of OH˙ during the ORR. These results provide a comprehensive insight into the critical, yet under-studied, aspects of the production and effects of ROS on electrocatalytic systems and open the door for further mechanistic and kinetic investigation using SECM.

Scheme 1. SECM substrate generation–tip collection (SG/TC) for OH˙ detection using DMPO and for H₂O₂ detection. (A) Schematic depiction of substrate-generated OH˙ detection using the DMPO spin trap with a 25 μm diameter gold ultramicroelectrode as the tip biased to oxidize the DMPO–OH adduct. (B) A depiction of the SECM H₂O₂ detection experiment using a platinum UME biased to perform H₂O₂ oxidation.

Fig. 1. SECM results for radical and H₂O₂ detection on pyrolyzed Fe–N–C catalysts. (A) Substrate LSVs of the catalyst on a rotating GC electrode and a stationary FTO substrate in oxygen-saturated 0.5 M H₂SO₄, as well as a catalyst-free FTO blank. (B) The radical SG/TC response at the gold tip as the substrate potential is swept at 10 mV s⁻¹ with 25 mM DMPO in solution. (C) The H₂O₂ SG/TC response on a Pt tip as the substrate potential is swept at 10 mV s⁻¹. (D) Redox competition mode SECM with a Pt tip located above the Fe–N–C in 0.5 mM H₂O₂ solution in 0.5 M H₂SO₄ purged with argon, with the substrate biased to the ORR and under open circuit conditions.

Fig. 2. ESR spectra with DMPO and DEPMPO. (A) ESR spectra of 50 mM DMPO after two minutes of electrolysis at the pyrolyzed Fe–N–C catalyst substrate at increasingly reducing potentials. (B) ESR spectra of 50 mM DEPMPO after two minutes of electrolysis at the pyrolyzed Fe–N–C catalyst substrate at increasingly reducing potentials. (C) Fitting of the 0 V DMPO ESR experiment to obtain the hyperfine coupling constants and confirm the radical species present. Simulated spectra for different possible adducts are provided in the ESI.

Fig. 3. Accelerated stress test (AST) experiments. (A) OH˙ SG/TC detection using 50 mM DMPO as the catalyst is degraded using AST. The arrows indicate a decrease in the current going from cycle 1 to 9. The inset shows the tip current at 0 V as a function of cycle number. The solid line shows an exponential decay fit and the dashed line shows a logistic fit. (B) The H₂O₂ SG/TC response as the catalyst is degraded. The arrows indicate a decrease in the current going from cycle 1 to 9. The inset shows the tip current at 0.55 V (peak current) as a function of cycle number. The solid line shows an exponential decay fit and the dashed line shows a logistic fit. (C) The normalized D/G peak intensity ratio from the Raman spectra of pyrolyzed Fe–N–C catalysts degraded in different solutions (with or without DMPO). Representative Raman spectra are shown in Fig. S10.

Fig. 4. SECM measurements of OH˙ and H₂O₂ generation on active site model systems. (A) The structure of iron(ii) phthalocyanine, phthalocyanine, and schematic of the 20 nm Fe₃O₄ nanoparticle. (B) H₂O₂ SG/TC response of each model catalyst system on a GC electrode in oxygen-saturated 0.5 M H₂SO₄. (C) OH˙ SG/TC detection using 50 mM DMPO of each model catalyst system on a GC electrode in oxygen saturated 0.5 M H₂SO₄.