Pt-Ni Decorated Metal Oxide Electrodes for Borohydride Fuel Cells
Authors: Martins Marta , Sljukic Biljana , Sequeira Cesar A.C. , Soylu Gülin S.P. , Bayrakceken Ayse , Sener Tansel , Santos Diogo M.F. 
Published: February 2020

Electrochemical energy conversion devices including fuel cells, batteries, and supercapacitors are expected to replace fossil-fuel based technologies in different applications. Direct borohydride fuel cells (DBFCs) are a type of alkaline fuel cells that use sodium borohydride (NaBH4) as a fuel and oxygen (O2) as an oxidant. DBFCs have several advantages over other types of fuel cells, from easier handling and cell management, to better cell performance, i.e. higher power densities [1].

Oxygen reduction reaction (ORR) is one of the most studied reactions in electrochemistry and, specifically, for electrochemical energy conversion, due to its application in fuel cells and metal–air batteries [2]. Sluggish kinetics of this reaction is one of the crucial restraining factors in the energy conversion efficiency of these devices. Namely, ORR proceeds at high overpotential and includes several steps [3,4]. Though ORR in alkaline media has faster kinetics compared to that in acid media, platinum (Pt) is still the most frequently employed electrocatalyst even in alkaline fuel cells.

Borohydride (BH4
-) oxidation reaction (BOR) is normally accompanied by BH4
- hydrolysis so that the faradic efficiency of this reaction is reduced. So far, gold (Au) has been demonstrated as the only electrode material active for BOR and inactive for BH4
-hydrolysis. Still, Pt is among the most commonly used electrocatalysts for BOR due to the faster reaction kinetics at Pt compared to Au.

Having in mind scarcity of Pt and, consequently, its high price, wide search for electrocatalysts with similar activity, but lower price is ongoing. One way to reduce amount of Pt is combining it with other active materials, such as transition metals [5]. Further reductions in the amount of Pt used in the electrocatalysts can be obtained by supporting smaller amounts of metal onto high-surface area supports.

Herein, PtNi nanoparticles are decorated on different metal oxide supports, namely MnO2, MnO2+NiO and MnO2+TiO2. Then, the prepared samples are tested as electrocatalysts for both ORR and BOR in alkaline media. Dependence of electrocatalysts behavior on reaction conditions, namely fuel and oxidant concentrations, and electrolyte temperature was studied using voltammetric techniques. Reaction parameters, such as number of exchanged electrons, and reaction mechanism were evaluated using rotating disk and rotating ring disk electrode measurements. Finally, the stability of the prepared electrocatalysts was assessed using chronoamperometry.

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