Carbon Supported Nanocrystalline Manganese Oxide: Surpassing Platinum as Oxygen Reduction Catalyst in Direct Borohydride Fuel Cells

Abstract

Manganese oxide (MnO 2 ) nanodispersed on high surface area carbon was tested as cathode catalyst for direct borohydride fuel cells with an anion conducting membrane. In order to investigate the effects of borohydride crossover, ex-situ experiments toward oxygen reduction reaction were conducted employing rotating disk electrodes in presence of borohydride and thiourea. Although platinum showed superior catalytic properties at ideal conditions, manganese oxide outperformed platinum significantly in presence of borohydride. Direct borohydride fuel cell tests were conducted using Pt/C-based and MnO 2 /C-based. After one hour of operation at 0.40 V the platinum based membrane electrode assembly lost approx. 30% of its initial peak power density while the non-precious metal MEA showed constant performance. The peak power density of the single test cell with a Pt/C based anode, an anion exchange membrane and a MnO 2 /C based cathode was 38 mW cm Direct liquid fuel cells (DLFC) offer certain advantages compared to hydrogen based fuel cell systems. Liquid fuels are generally easier to store and safer in handling than pressurized gases. Furthermore, the adaptation of the existing fossil based fuel involves considerably lower investment costs. Among various alcohols, most importantly methanol, borohydride proves to be an interesting and promising fuel candidate for DLFCs.1-3 As a result the number of publications concerning borohydride as hydrogen storage compound and direct borohydride fuel cells (DBFCs) is strongly increasing in the last decade. 4 Borohydride which is usually applied in the form of sodium borohydride NaBH 4 is a white crystalline solid which exhibits a theoretical energy density of 9.3 Wh g −1 . 5The electrooxidation of BH 4 − (shown in Reaction 1) releases eight electrons at a very low theoretical potential. 6 In combination with an oxygen electrode the theoretical cell voltage is 1.64 V (see Reaction 3).Due to fuel crossover with any kind of alkaline electrolytes most publications dealing with DBFCs report the usage of cation exchange membranes.7 In this arrangement sodium acts as charge carrier and is conducted from the anode to the cathode side. The main issue of using cation exchange membranes is the formation of NaOH at the cathode and the undesirable concentration ratio [OH − ]/[BH 4 − ] with a minimum of eight in the fuel.To overcome this major drawback there are in principle two approaches. First, the establishment of hydroxide permeable membranes those are impermeable for borohydride or second, the development of cathode catalysts that show high catalytic selectivity toward the oxygen reduction reaction (ORR) and high tolerance toward borohydride.State-of-the-art catalysts for DBFCs are platinum and gold at the anode and platinum at the cathode. 7,8 Platinum is still the cathode * Electrochemical Society Member. z E-mail: christoph.grimmer@tugraz.at catalyst of choice due to its high ORR activity and sufficient longterm stability.9 However, the usage of platinum at the cathode side harbors si...

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