Despite a promising activity of Fe-N-C catalysts at beginning-of-life in protonexchange membrane fuel cells (PEMFCs), their poor durability in operating PEMFCs remains a great challenge for the successful replacement of commercial Pt-based catalysts. One of the key reasons for this poor operando durability is the surface oxidation of carbonaceous supports via Fenton(-like) reactions between the Fe centers and the intermediate product of the oxygen reduction reaction (ORR) in acidic medium, H2O2. In the present study, we have investigated the pH effect on the chemical deactivation of Fe-N-C catalysts by contacting with controlled amount of H2O2. Covering the entire pH range 0-14, we reveal a strong pH dependence of the H2O2-induced deactivation. Especially, acidic H2O2 treatment leads to a severe decrease in ORR activity while almost negligible deactivation is found after the treatment in sufficiently strong alkaline electrolyte. Electron paramagnetic resonance (EPR) study reveals a positive correlation between the magnitude of Fe-N-C activity decrease and the signal intensity of hydroxyl radical spin adduct after H2O2 treatment at a given pH. Reactive oxygen species (ROS) such as the hydroxyl radical is identified as a key deactivating agent of Fe-N-C catalysts operating from acidic to neutral pH environments. This result suggests that controlling the formation and lifetime of ROS at such pH is crucial to secure durable fuel cell operation with Fe-N-C cathodes. Alternatively, fuel cell operation under highly alkaline environment could also be considered to improve the catalytic durability, by virtue of different Fenton(-like) reaction pathway at such pH.

Ammonia has recently received considerable attention as an alternative energy carrier and a carbon-neutral fuel. In this future energy scenario, the ammonia oxidation reaction (AOR) is a pivotal process for onsite hydrogen production and/or electricity generation. However, its implementation is hindered by the nondurable nature of AOR catalysis by platinum. Accordingly, securement of a durable Pt electrocatalysis for the AOR is critical but has been hampered by the well-known chemical deactivation (i.e., poisoning). Additionally, the structural stability, which could also affect durable AOR operation, has scarcely been investigated. Herein, the degradation of Pt catalysts under AOR conditions has been investigated with various operando and in/ex situ spectroscopies. We demonstrate that NH3 (or AOR intermediates/byproducts) modifies the chemical structures of both the Pt surface and dissolved Pt ions, specifically by passivation of the Pt surface with NH3-derived adsorbates and complexation of the dissolved Pt ions, respectively. These modifications lead to a significant acceleration in Pt dissolution but a deceleration in its redeposition, resulting in the augmented structural degradation of Pt catalysts in NH3-containing electrolyte after the Pt has experienced a potential excursion above ca. 1 VRHE. With these understandings, a quasi-stable operation potential window and operational strategy are suggested. The tentative AOR protocol allows prolonged NH3 electrolysis with alleviated Pt dissolution (<0.02 ng cmPt –2 s–1), suggesting that NH3 will be a viable future energy carrier if the rational operational strategy proposed herein is developed further.

Carbon materials have been used as supporting substrates for electrocatalytically active species in many important reactions due to their high electrical conductivity and chemical inertness. To secure durable electrocatalysis, the...

A three-electrode system is typically utilized in many voltammetry studies to understand the behavior of an analyte at the electrode/electrolyte interface. A bulk Pt piece is usually used as a counter electrode in such systems because of its high activity and stability in many electrochemical reactions. However, the dissolution of the Pt counter electrode led to growing concern about inaccurate evaluation of the inherent characteristics of the analyte. In the present study, we have demonstrated that strong interferences emerged from the conventional Pt counter and Ag/AgCl reference electrodes in the photoelectrochemical (PEC) hydrogen evolution reaction (HER) with a model photocathode of p-type silicon (p-Si). Under light illumination, the Pt counter electrode is polarized to as high as 1.6–2.0 VRHE, which leads to a non-negligible Pt dissolution from the oxidized surface, as monitored by operando inductively coupled plasma-mass spectrometry (ICP-MS). Postreaction spectroscopy and microscopy studies confirm the formation of Pt and Ag particles on p-Si photocathode, resulting in erroneous overestimation of the HER activity of p-Si. Various configurations of the three-electrode system, e.g., an H-type cell with a Nafion membrane, have been studied to find a suitable cell structure for prohibiting undesirable contamination of p-Si. Isolation of p-Si from the Pt counter and Ag/AgCl reference electrodes using the Nafion membrane effectively alleviates the contamination of p-Si but, toward the end, the metallic ions can be slowly deposited on p-Si by diffusion through the membrane. Consequently, this work highlights that the careful caution is necessary when the conventional Pt counter and Ag/AgCl reference electrodes are employed; it is recommended to use a graphite counter electrode and separate reference electrode to prevent artifacts originating from the dissolved Pt and Ag species during PEC cathode evaluations.

In the study of electrocatalysis, precise measurement of voltammetric responses for electrode materials is an essential step in evaluating them accurately and understanding electron transfer at the electrode/electrolyte interfaces. Due to growing concerns regarding the validity of Pt counter electrodes in such measurements, largely attributed to their dissolution and redeposition, graphite is increasingly being employed as a versatile counter electrode in conventional three-electrode systems. However, the reliability of graphite in this role has not been fully investigated. Herein, we have demonstrated that using graphite as a counter electrode can significantly hinder the precise evaluations of electrocatalytic materials. For a polycrystalline Pt surface coupled with a graphite counter electrode, the rapid loss of catalytic activity in the hydrogen evolution (HER) and oxygen reduction reactions (ORR) is observed within a few voltammetric evaluation cycles. Online differential electrochemical mass spectroscopy (DEMS) and in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) revealed that the catalytic activity loss is mainly due to the formation of poisonous carbon monoxide (CO) and its subsequent strong adsorption on the catalytic Pt sites. CO coverage on the Pt surface becomes almost saturated within just 50 cycles of HER polarization measurements. The resultant underestimation of activity is effectively avoidable by physically separating the graphite counter electrode from the analyte in an H-type cell. Consequently, our findings highlight that, similar to Pt counter electrodes, graphite counter electrodes must be carefully applied to eliminate any experimental artifacts originating from CO evolved during electrocatalytic reactions.