Effect of Ball-Milling on the Oxygen Reduction Reaction Activity of Iron and Nitrogen Co-doped Carbide-Derived Carbon Catalysts in Acid Media

Abstract

Iron- and nitrogen-doped carbon-based catalysts are one of the most promising alternatives to platinum-group metal-based ones currently used in the fuel cell industry. Here, we study the effect of ball-milling conditions and compositions of catalyst precursors comprising a silicon carbide-derived carbon (CDC) on the properties of the final catalysts, most importantly their activity toward the oxygen reduction reaction (ORR). Ball-milling rates from 100 to 800 rpm were investigated with 400 rpm proving to be the optimum value. The effect of 1,10-phenanthroline-to-iron ratio in the precursor mixture was also studied, with a 12/1 molar ratio leading to the highest activity. Finally, ZnCl2 addition to the precursor mixture was explored as a pore former during pyrolysis. A ZnCl2-to-CDC mass ratio of 1 resulted in the highest ORR activity. 57Fe Mössbauer spectroscopy reveals that the iron is atomically dispersed as Fe–N x moieties. The most active catalyst showed a kinetic current density of 4.6 mA cm–2 at 0.8 V vs RHE in rotating disk electrode tests and a current density of 18.6 mA cm–2 at 0.8 V in a single-cell proton exchange membrane fuel cell.

Establishing reactivity descriptors for platinum group metal (PGM)-free Fe–N–C catalysts for PEM fuel cells

“…57 Fe Mössbauer spectroscopy. 57 Fe Mössbauer spectra were measured with a Rh matrix 57 Co source. The measurements were performed keeping both the source and the absorber at room temperature, unless otherwise mentioned.…”

“…Theoretical-computational research has offered a much clearer, albeit not fully resolved, picture of the chemical structure of favorable, catalytically active Fe-N x single metal sites. 16,69 Advanced experimental analytical techniques such as 57 Fe Mössbauer spectroscopy and high resolution STEM-EELS microscopy have now qualitatively proven the existence of such sites in active Fe-N-C materials. [13][14][15]26,60,[70][71][72][73] A serious hurdle in the rational improvement of the catalytic activity of Fe-N-C catalysts, however, has been the lack of suitable methods that accurately enumerate the electrochemically accessible Fe-N x sites (SD).…”

“…A range of spectroscopic methods based on X-rays and g-rays have been applied in order to probe and quantify bulk and/or surface Fe-based sites, namely X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and 57 Fe Mössbauer spectroscopy. 7,13,15,31,63,70,71 However, there exist inherent shortcomings for each of these analysis methods.…”

“…7,13,15,31,63,70,71 However, there exist inherent shortcomings for each of these analysis methods. XAS and 57 Fe Mössbauer spectroscopy are inherently bulk methods, so they identify both electrochemically accessible and inaccessible Fe-based sites. X-ray photoelectron spectroscopy (XPS) are element specific but not surface sensitive for carbon-based materials with high surface area, due to the escape path of several nm of photoelectrons through the carbon matrix.…”

“…More specifically, starting from the rotating ring-disk electrode (RRDE) based ORR mass activity (MA), we derive quantitative values for (i) SD for each benchmark catalyst, on a massbasis and/or surface-area basis, and (ii) TOF values. In parallel, the Fe-N x coordination environment and elemental composition in the bulk of the sample were determined by 57 Fe Mössbauer spectroscopy and XPS, respectively. The pore structure and specific surface area were evaluated with nitrogen sorption isotherms.…”