Correlations between Synthesis and Performance of Fe-Based PGM-Free Catalysts in Acidic and Alkaline Media: Evolution of Surface Chemistry and Morphology

Abstract

The effects of the synthesis steps of a platinum group metal-free (PGM-free) catalyst on the surface chemistry, morphology, and electrochemical activity in acidic and alkaline media toward the oxygen reduction reaction (ORR) were studied. Each step exhibits a positive impact on catalyst activity. In acid media, etching of the silica template is the major contributor to the enhancement of the half-wave potential, as the ORR active sites formed during the first pyrolysis become more accessible. Further processing steps result even in higher accessibility and utilization of the 4e − transfer sites. In alkaline media, the second pyrolysis is a critical step that favors the complete reduction of oxygen to water, as the peroxide production is significantly diminished. The large heterogeneity in the porosity at each synthesis step indicates that this parameter needs to be further studied to attain better control of the morphology of the PGM-free catalyst, as it is an important factor that contributes to the active site utilization. The acid etching and second pyrolysis increase the meso-and macroporosity. Understanding the effects of each of the synthesis steps on the chemical composition, morphology and ORR activity of the PGM-free catalyst provides the necessary feedback for the design of synthetic schemes that increase the catalysts' performance.

Relevant Properties of Carbon Support Materials in Successful Fe-N-C Synthesis for the Oxygen Reduction Reaction: Study of Carbon Blacks and Biomass-Based Carbons

“…In order to produce such catalysts there are two different pathways, a template-based and a carbon support-based synthesis. Silica [ 8 ] or quite complex and expensive metal-organic-frameworks (MOFs) [ 9 ] can be used as a template, whereas for the support-based synthesis commercial carbon blacks [ 7 , 10 ] are utilized. The latter approach has the advantage of a simple and cheap synthesis without the requirement of a template removing step and the targeted use of carbon material with known morphology, porosity, surface area, and functionalities.…”

Recent Advances in Waste Plastic Transformation into Valuable Platinum‐Group Metal‐Free Electrocatalysts for Oxygen Reduction Reaction

“…The initial preparation conditions for the electrocatalysts such as mixing of different waste plastics percentage, homogenization of the sample, grounding the precursors to a fine powder should be the subject of extended studies. Moreover, process parameters such as pyrolysis temperature, atmosphere, heating and cooling rates must be studied and optimized to obtain a controlled and defined morphology and surface chemistry [126,127] . For doping and surface functionalization, the role of precursors choice could be an important research topic [128] .…”

Iron-Based Electrocatalysts for Energy Conversion: Effect of Ball Milling on Oxygen Reduction Activity

“…The samples were further subjected to a pyrolysis treatment to overcome a possible heterogeneity in porosity and modification in surface chemistry induced by ball-milling, which may have contributed to decrease the active site utilization [59]. The pyrolysis step after ball-milling led to an increase in ECSA only for the sample prepared with no ball-milling (146 m 2 g −1 for FeNC_P vs. 19 m 2 g −1 for FeNC) and the sample obtained with 1 h of ball-milling time (69 m 2 g −1 for FeNC_BM1_P vs. 54 m 2 g −1 for FeNC_BM1).…”

“…Atanassov and coworkers demonstrated that a ball-mill treatment followed by pyrolysis induced a decrease in relative amount of both mesopores and macropores homogenizing morphology of Fe-N-C catalysts. In an alkaline environment, these materials directly catalysed oxygen reduction to water, with a reduction potential of 0.93 V vs. RHE [59].…”