Cobalt-Free Perovskite-Type Oxygen Reduction Reaction Catalysts for Energy Conversion
Authors: Venkatesan Senthil Velan , Abubaker Orrsam Aadil , Singh Kalpana , Thangadurai Venkataraman 
Published: February 2020

Alkaline fuel cells and metal–air batteries are emerging as an alternative energy conversion devices which can use non-precious and earth-abundant materials as electrocatalysts for oxygen reduction reaction (ORR) [1-3]. Perovskite-type oxides have been widely investigated due to their high activity, long durability and low cost [4]. Mn and Co-based perovskites have shown stable and high ORR performance and their electrochemical properties was explained using the molecular orbital theory. It was shown that the transition-metal-oxide electrocatalysts exhibit maximum catalytic activity when the occupancy of the eg orbital is ~1 which is the primary activity descriptor, whereas spin-state and electronegativity are secondary activity descriptors [5]. However, Co-based oxides have several disadvantages such as high volatility, thermal expansion coefficient, and cost [6-8]. Thus, there is need for alternative Co-free materials for next generation fuel cells. Here, we report Co-free perovskite-type Ba0.5Sr0.5Fe0.8Cu0.2O3-δ as ORR electrode for fuel cells. Effect of Cu-doping in Ba0.5Sr0.5Fe1-xCuxO3-δ (x = 0-0.2) towards the ORR activity in alkaline media has been studied. Partial substitution of Cu in the Fe-site in (Sr,Ba)FeO3 could influence the transition-metal oxygen covalency which replaces and regenerates OH— ions in alkaline ORR process. Thin film rotating-disk electrode (RDE) method with well-defined oxygen transport was previously developed to study the ORR activity of perovskite catalysts [4, 9]. Evaluating the effects of such molecular level engineering on the desired outcome is challenging, since the ORR process consists of multiple adsorption/desorption step involving oxygen containing species. There are fewer literature on understanding the ORR process on transition metal oxides involving the transfer of four electrons and four OH— ions to O2 with subsequent cleavage of double bond O2.

Ba0.5Sr0.5Fe1-xCuxO3-δ (x = 0-0.2) powders were prepared through sol gel synthesis method. Powder X-ray diffraction was employed for structural characterization and the activity towards ORR was studied using RDE method. RDE experiments were performed in oxygen saturated electrolyte at 100-1600 rpm. The voltage scan was performed from 0.4-0.9 V vs. RHE. The relation between current and rotational speed was evaluated through a Koutechy-Levich (K-L) plot to deduce the number of electrons (n) involved in the ORR process, i.e.

n = (0.62∙F∙D0
2/3∙ v-1/6∙Co∙slope)-1

Where, F is Faraday’s constant, F = 96485 C.mol-1, (Do) diffusion coefficient of oxygen in 0.1M KOH = 1.93x10-5 cm2.s-1, (ν) Kinematic viscosity of 0.1 M KOH = 1.09x10-2 cm2.s-1, (Co) saturation concentration of O2 in 0.1 M KOH at 1 atm O2 pressure = 1.26x10-6 mol.cm-3, and slope – slope from K-L plot.

Preliminary results showed that Cu doping in the lattice is affecting the ORR properties of perovskite in alkaline media. The slope of K-L plot gradually increases with increase in Cu content and decreases beyond 0.05 Cu until 0.2 Cu. The number of electron involved in the ORR process is mostly restricted to two.

Acknowledgements

This research was supported by the Canada First Research Excellence Fund (CFREF) at the University of Calgary.

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Figure 1