Development of a reference electrode for a PEMFC single cell allowing an evaluation of plate potentials Johan ANDRE1, Nicolas GUILLET2, Jean-Pierre PETIT3, Laurent ANTONI4, * 1
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CEA – LITEN 17 rue des Martyrs 38054 Grenoble cedex 9 –
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CEA - LITEN 17 rue des Martyrs 38054 Grenoble cedex 9 - 04 38 78 66 89 –
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Abstract Increasing lifetime and performance is critical for PEMFC using stainless steel plates. A good compromise between passivity and electrical contact resistance of the plate material is required. Measuring the potential of each plate during fuel cell operation is of paramount importance to lead to relevant ex situ tests in order to investigate new materials. From a review on methods used for potential measurement, the present work focused on the realization and use of a Dynamic Hydrogen Electrode (DHE) device as a reference electrode in a PEMFC single cell, its evaluation in terms of accuracy and drift. With classic reference electrodes introduced into the flow field, measurements were shown to be irrelevant because of the impossibility to ensure good and stable ionic conductivity between the reference electrode and the plate when operating the cell. Several examples of DHE found in the literature were reviewed and used to realize a DHE which showed correct accuracy and stability of its potential under fully humidified conditions. The experimental device was shown to be reliable and easily adaptable for different single cells. It was used to investigate transient phenomena while cycling a cell, but needs some improvement when the cell is operated with unsaturated gases. Keywords Reference electrode; DHE; PEMFC; corrosion; bipolar plate; fuel cell
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1. Introduction Among the new technologies for energy, fuel cells and in particular PEMFC (Proton Exchange Membrane Fuel Cell) represent an attractive solution, convenient for many applications [1]. However, some technical issues still have to be solved: increasing lifetime while maintaining performance is of major interest in the challenge for PEMFC development. Bipolar plate is a key component of PEMFC, with electrical, chemical, thermal, and mechanical functions [2-5]. Some efforts are focused on finding a compromise between passivity and electrical contact resistance of passive films formed on stainless steel bipolar plates [6]. Cathodic and anodic bipolar plate voltages are usually considered to reach respectively about 0.74 to 0.84 V/SHE (for standard use) and 0.04 to 0.1 V/SHE [7-9]. Nevertheless, based on previous studies of Andreaus et al. [10,11] about humidification aspects and analysis of performance losses at high current densities, Kuhn et al. [12] claimed that anode could contribute to some extent to the cell voltage variation. To perform electrochemical studies which reproduce as well as possible fuel cell operating conditions, it is critical to determine precisely these values. Little information about reference electrodes suited for PEMFC was published elsewhere [13-20]. The kinds of reference electrode presented in this paper were selected in order to get a device with a reduced or null perturbation to the cell, and an improved accuracy. An original version is the DHE (Dynamic Hydrogen Electrode) [18,21] which requires two platinum wires. At the surface of one of them, hydrogen is produced by electrolysis. If stable, the current enables good hydrogen coverage of the cathodic wire and a stable electrode potential. First studies about DHE date back to 1964 (Giner et al. [22]). As main advantages, this kind of electrode was shown to be convenient to use and non intrusive, because of the absence of salt bridge and foreign ions. A simpler version of hydrogen electrode named RHE (Reversible Hydrogen Electrode) was shown to be useful [13], but hydrogen coverage is not steady on its surface, and adsorption of contaminants is possible. Unlike pseudo reference electrodes [12,17] presented elsewhere, the potential of a DHE can be referred to a classical 2
reference electrode so that not only overvoltage values can be determined, but also absolute values of potentials i.e. referable to any reference electrode. Some recommendations [17-19] were given by researchers for the use of both kinds of electrodes such as providing a good alignment between cathode and anode and/or using several reference electrodes [21], positioning the electrode in an area of stable potential (at least further than 3x membrane thickness), and in an “edge-type” configuration rather than in a “sandwich-type”. In order to get an easy-to-use reference device, Siroma et al. [20] even proposed a DHE device hot pressed within the MEA (membrane electrode assembly), but such a system, with the reference electrode very close to the MEA, implies the use of a high electrolysis current and an increased difference from the equilibrium potential. Operation of commercial electrodes was tried in order to get directly the potential of bipolar plates versus a classical reference electrode. Then, with the help of previous works, a DHE easily adaptable for different single cells was developed. 2. Experimental 2.1 Single cell configuration and use Single cell tests were performed with AISI 316L monopolar plates (thickness: 8mm, width x length: 90 mm x 80 mm with machined single serpentine channel. Homemade 25cm² 3-layers membrane electrodes assemblies (MEA) were employed using a Nafion® 112 membrane. Pt loading was about 0.3 mg/cm² for each electrode (anode / cathode). Gas diffusion layers were prepared with 10% Teflon Freudenberg felt and homemade micro porous layer. Unless stated different, all experiments were conducted feeding the cell by pure and fully humidified hydrogen and oxygen or air gases. In order to get information about potential evolution during transient states, a current cycle was defined with some rapid variations (increase and decrease) of the current load with five minute steps, as shown on Figure 1. 3
2.2 Commercial reference electrodes A first idea was to introduce a reference electrode into a hydrogen flow channel. Due to the limited space available, microelectrodes were used: WPI Flexref™ with flexible PTFE body and WPI RC-6™ silver chloride half cell with glass body. The first model presents a limited leakage rate (5.7*10-8 mL/h) and is quite easy to handle, whereas the second one shows an excessive brittleness but no leakage. The electrode is introduced through a tight connector shown in Figure 2, close to an outlet of the cell. Because the fuel cell is fed with fully humidified gases, and due to the short distance (
