Electrochemical impedance spectroscopy is an important tool for fuel-cell analysis and monitoring. This study focuses on the low-AC frequencies (2–0.1 Hz) to show that the thickness of the catalyst layer significantly influences the overall resistance of the cell. By combining known models, a new equivalent circuit model was generated. The new model is able to simulate the impedance signal in the complete frequency spectrum of 105–10−2 Hz, usually used in experimental work on polymer electrolyte fuel cells (PEMFCs). The model was compared with experimental data and to an older model from the literature for verification. The electrochemical impedance spectra recorded on different MEAs with cathode catalyst layer thicknesses of approx. 5 and 12 µm show the appearance of a third semicircle in the low-frequency region that scales with current density. It has been shown that the ohmic resistance contribution (Rmt) of this third semicircle increases with the catalyst layer’s thickness. Furthermore, the electrolyte resistance is shown to decrease with increasing catalyst-layer thickness. The cause of this phenomenon was identified to be increased water retention by thicker catalyst layers.
A membrane-free direct borohydride fuel cell that is operated with a borohydride containing electrolyte is designed and constructed. Main focus of this study is the cathode catalyst that has to show high tolerance against borohydride. Both catalysts, Ag-Mn 3 O 4 /C as cathode catalyst and commercial Pt/C as anode catalyst, are characterized electrochemically using a rotating disk electrode. The number of exchanged electrons is calculated employing Levich and Koutecky-Levich analysis. Cathodes are fabricated using a cross rolling method with an in-house made gas diffusion layer. A cell with an implemented Luggin capillary is designed in order to follow the electrode potentials. The BH 4 − /O 2 fuel cell shows a power density of 17 mW cm −2 using a 1 M NaBH 4 /1 M NaOH/5 mM thiourea electrolyte.
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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