Selectivity of mixed iron-cobalt spinels deposited on a N,S-doped mesoporous carbon support in the oxygen reduction reaction in alkaline media

One of the practical efforts in the development of oxygen reduction reaction (ORR) catalysts applicable to fuel cells and metal-air batteries is focused on reducing the cost of the catalysts production. Herein, we have examined the ORR performance of cheap, non-noble metal based catalysts comprised of nanosized mixed Fe-Co spinels deposited on N,S-doped mesoporous carbon support (N,S-MPC). The effect of the chemical and phase composition of the active phase on the selectivity of catalysts in the ORR process in alkaline media was elucidated by changing the iron content. The synthesized materials were thoroughly characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy (RS). Detailed S/TEM/EDX and Raman analysis of the phase composition of the synthesized ORR catalysts revealed that the dominant mixed iron-cobalt spinel is accompanied by minor fractions of bare cobalt and highly dispersed spurious iron oxides (Fe2O3 and Fe3O4). The contribution of individual phases and their degree of agglomeration on the carbon support directly influence the selectivity of the obtained catalysts. It was found that the mixed iron-cobalt spinel single phase gives rise to significant improvement of the catalyst selectivity towards the desired 4e− reaction pathway, in comparison to the reference bare cobalt spinel, whereas spurious iron oxides play a negative role for the catalyst selectivity

Selectivity of Mixed Iron-Cobalt Spinels Deposited on a N,S-Doped Mesoporous Carbon Support in the Oxygen Reduction Reaction in Alkaline Media

One of the practical efforts in the development of oxygen reduction reaction (ORR) catalysts applicable to fuel cells and metal-air batteries is focused on reducing the cost of the catalysts production. Herein, we have examined the ORR performance of cheap, non-noble metal based catalysts comprised of nanosized mixed Fe-Co spinels deposited on N,S-doped mesoporous carbon support (N,S-MPC). The effect of the chemical and phase composition of the active phase on the selectivity of catalysts in the ORR process in alkaline media was elucidated by changing the iron content. The synthesized materials were thoroughly characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy (RS). Detailed S/TEM/EDX and Raman analysis of the phase composition of the synthesized ORR catalysts revealed that the dominant mixed iron-cobalt spinel is accompanied by minor fractions of bare cobalt and highly dispersed spurious iron oxides (Fe2O3 and Fe3O4). The contribution of individual phases and their degree of agglomeration on the carbon support directly influence the selectivity of the obtained catalysts. It was found that the mixed iron-cobalt spinel single phase gives rise to significant improvement of the catalyst selectivity towards the desired 4e− reaction pathway, in comparison to the reference bare cobalt spinel, whereas spurious iron oxides play a negative role for the catalyst selectivity

The application of the mixed spinels as catalysts for the oxygen reduction reaction.

Celem prowadzonych badań było otrzymanie aktywnych katalizatorów w reakcji ORR na bazie mieszanych spineli o rozmiarach nanometrycznych oraz określenie mechanizmu reakcji redukcji tlenu na zsyntezowanych materiałach w środowisku zasadowym. Przy zastosowaniu metody hydrotermalnej wspomaganej mikrofalami otrzymano następujące nanometryczne spinele: Co3O4, NiCo2O4, MnCo2O4, osadzone na nośnikach węglowych (sadza Vulcan XC-72 oraz węgiel mezoporowaty CMK-1/CMK-1-NO2). Ich struktura została określona za pomocą mikroskopii Ramana oraz dyfraktometrii rentgenowskiej. Analiza XRD pozwoliła określić wielkość ziaren (10-20 nm), a także wykazała, że czas prowadzenia syntezy (5-60 min) nie wpływa znacząco na rozmiar krystalitów. Właściwości katalityczne otrzymanych materiałów sprawdzono przy wykorzystaniu metody wirującej elektrody (RDE). Aktywność katalityczną spineli porównano z komercyjnym katalizatorem platynowym (40% Pt/C). Wyniki pokazały, że zarówno katalizator platynowy jak i tlenki metali przejściowych, umożliwiają bezpośrednią 4-elektronową redukcję tlenu. Na podstawie porównania wartości zarejestrowanych prądów stwierdzono, że lepszą aktywność wykazują tlenki naniesione na sadzy, niż na węglu mezoporowatym. W procesie ORR najbardziej aktywny okazał się spinel MnCo2O4/C.The aim of this research was to obtain active catalysts in the oxygen reduction reaction, ORR, based on the nanometric mixed spinels and to determine the mechanism of this process in the alkaline media. By using the microwave assisted hydrothermal route the following nanoparticles spinels were synthesized: Co3O4, NiCo2O4, MnCo2O4. Spinels were deposited on the carbon supports such as Vulcan XC-72 and mesoporous carbon CMK-1/CMK-1-NO2. The structure of the materials was determined by using Raman microscopy and X-ray diffraction. The XRD measurements were performed in order to determine size of the grain of spinels (10-20 nm). The results show that the duration of the synthesis (5-60 min), did not have any significant effect on the size of crystallites. The catalytic properties of spinels were tested and determined by using rotating disk electrode method (RDE). The catalytic activity of spinels were compared with a commercial platinum catalyst (40% Pt / C). The results confirm that in all the cases direct oxygen reduction by the 4-electrons pathway took place. Furthermore, by comparison of the values of the registered electrical current density it was determined that oxides supported on the Vulcan XC-72 carbon exhibited a better activity than on the mesoporous carbon. It was determined that the spinel MnCo2O4 was be the most active catalysts in the ORR process