High-Pressure PEM Water Electrolysis: in-Situ Measurement of Hydrogen Crossover

To ensure safe and efficient operation of polymer electrolyte membrane (PEM) water electrolyzers the determination of hydrogen crossover through the membrane-electrode-assemblies (MEA) is an important issue. Hydrogen crossover leads to a contamination of the oxygen product stream with the risk of the formation of explosive atmospheres within the anode channels. Additionally, the crossover flux represents a loss of product hydrogen, leading to an increase of the specific energy demand for the electrolysis. Especially under asymmetric (also named unbalanced or differential) pressure conditions (e.g. discussed by [1]), these losses can easily exceed the benefits connected with the electrochemical co-compression of the produced hydrogen [2]. The present contribution proposes an experimental method for the in-situ determination of hydrogen crossover in polymer electrolyte membrane water electrolysis cells [3]. The measurement concept is based on the electrochemical compensation of the hydrogen crossover flux, which translates the mass flux determination into an electric current measurement. The proposed method is based on a very simple set-up and measurement procedure, as well as high accuracy. It allows for measurement with a fully assembled electrolysis cell at standard water electrolysis conditions by use of standard equipment, also installed in industrial electrolyzer plants. The technique is especially suitable for high-pressure PEM electrolyzers operated under asymmetric pressure conditions. In Fig. 1(a) schematic of the employed set-up is shown. The anode side of the electrolyzer under investigation is operated throughout the whole experiment under the same conditions as during normal operation. The cathode side is equipped with a pressure transmitter and is disconnected from the rest of the plant, e.g. by means of a cutoff valve. The measurement principle is as follows: a small current is applied to the set-up described above. On the anode side, water is consumed and oxygen evolves. In the sealed cathode compartment hydrogen is evolved, which leads to a pressure increase over time. The cathode pressure reaches a steady-state value when the hydrogen loss, which is driven by the increasing pressure difference, levels out the hydrogen evolution (Eq. 1). d p cathode / d t = 0 <-> 0 = -G H2,crossover + i/2/F (1) Fig. 1(b),(c) illustrates one application of the described basic measurement principle. If the experiment is carried out repeatedly at different currents, it allows for a quick and simple characterization of MEA materials under electrolysis, since a relation between the cathode pressure and the hydrogen crossover flux can be obtained. The applicability of the suggested method for a broad pressure range is briefly illustrated with a laboratory scale electrolyzer plant and by comparison of the measured data with available literature values (Fig. 2) by use of the membrane permeability coefficient K p,H2 (Eq. 2): G H2,crossover = K p,H2 Δp H2 /t m (2) [1.] F. Marangio, M. Santarelli, M. Cali, International Journal of Hydrogen Energy 34 (2009) 1143–1158. [2.] B. Bensmann, R. Hanke-Rauschenbach, I. K. Pena Arias, K. Sundmacher, Electrochimica Acta 110 (2013), pp. 570-580 [3.] B. Bensmann, R. Hanke-Rauschenbach, K. Sundmacher, International Journal of Hydrogen Energy, In press. DOI 10.1016/j.ijhydene.2013.10.085