Optimization of porous current collectors for PEM water electrolysers

Transcription

1 Optimization of porous current collectors for PEM water electrolysers S. Grigoriev a, I. Baranov a, P. Millet b, Z. Li c, V. Fateev a a Hydrogen Energy and Plasma Technology Institute of Russian Research Center Kurchatov Institute, 1, Kurchatov sq., Moscow Russia, S.Grigoriev@hepti.kiae.ru b Institut de Chimie Moléculaire et des Matériaux, UMR 8182, Université Paris Sud, bât 420, Orsay cedex France, pierre.millet@lpces.u-psud.fr c GKN Sinter Metals Filters GmbH, 43,Dahlien str., D Radevormwald Germany ABSTRACT: Water electrolysers using proton exchange membranes (PEM) offer high prospective potentialities for the production of pure hydrogen. The basic components of PEM electrolysis cell stacks are membrane, electrocatalytic layers, porous current collectors and bipolar separating plates. The work presented in this paper is devoted to the optimization of the structure of the current collectors, the role of which is to provide efficient electric contact and gas/water transport between electrocatalytic layers and bipolar plates. Using both experiments and modelling, optimum pore sizes, thickness and porosities have been determined for operation at high current densities (above 1 A.cm 2 ). KEYWORDS: water electrolysis, proton exchange membrane, current collector, optimization. 1 ) Introduction Compared to the more conventional alkaline process, PEM (proton exchange membrane) water electrolysis offers a number of advantages for the production of electrolytic-grade hydrogen, such as ecological safety, high gas purity (above 99.99% for hydrogen), the possibility of producing compressed gases (up to 200 bar and more) for direct pressurized storage without additional power inputs, etc. This is why PEM water electrolysers are considered as rather attractive devices to accelerate the transition to the hydrogen economy and to develop a hydrogen infrastructure network (for example, for the development of refilling stations for automotive applications, using electric power stations at night hours and renewable energy sources). The basic components of a PEM electrolysis cell are (fig. 1) : the membrane material onto which are platted two electrocatalytic layers, the porous current collectors (in general sintered titanium powder) and the bipolar plates separating each cell in the stack. In the PEM cell, the current flows from one bipolar plate (4a in figure 1) to the next one (4c in figure 1). At the same time, mass transfer phenomena take place across the porous current collectors (3.a and 3.c in figure 1). In particular, the two gases formed during the electrochemical splitting of water (hydrogen and oxygen), are evacuated to the collecting channels of the bipolar plates. Increasing the porosity of the current collectors may facilitate gas evolution but will introduce significant parasite ohmic losses at contact points between current collector and catalytic layers (front sides) and between current collectors and channels (backsides). An optimization of the geometry of the pore network is therefore required in terms of overall porosity and pore size distribution. 1/6

2 In a previous work [1], the pore size of the current collectors was optimized by considering only current transport. The goal of the present study is to go one step further and to provide optimization results concerning the structure of the current collectors, especially from the viewpoint of mass transport. Such optimization is required for operation at high current densities (1 A.cm 2 ). In the followings, results and discussion pertain only to water electrolysis performed at atmospheric pressure. Different results are expected for operation at higher pressure. 4a O 2 H 2 O 3a 5 1 Н + nh 2 O d 2a 2c 3c 4c H 2 Overall reaction: H 2 O 1/2O 2 + H 2 Fig. 1. Schematic representation (cross-sectional view) of a PEM electrolysis cell. 1 PEM; 2 electrocatalytic layers (a anodic, c cathodic); 3 current collectors (a anodic, c cathodic); 4 bipolar plates (a anodic side, c cathodic side); 5 contacts points between current collectors and electrocatalytic layers; d average pore diameter of current collectors. 2/6

3 2 ) Preparation and characterization of current collectors 2.1. Porosity In order to optimize their porosity, current collectors were prepared by thermal sintering of spherically-shaped titanium powder (fig. 2). Using spherical particles, homogeneously distributed inter-particle pore sizes were obtained. The percolation theory does not fully apply to this case, in comparison with the case of absolutely casual arrangement of pores [2]. Simulation experiments of powder sintering, using plasticine balls disposed within a thin rubber envelope, have shown that particles are packed mainly in hexagonal lattice [3]. This means that if the particles are not put out of shape during the sintering process (i.e. they retain their true spherical shape), the solid phase will amount to 64% of the total volume, and the porosity (void fraction) will amount, in that case, to 36%. If the shape of the particles changes during the sintering process, lower porosity values will be obtained (down to 20-30%). This is not necessarily a real problem as long as percolation of pores prevails in order to allow water and gas transport. For practical applications, a porosity of 30% is recommended as minimum value. Concerning the upper limit, a value of ca. 50% is a threshold above which mechanical strength of the porous plates becomes a limiting factor. Thus, the recommended range of porosity for the porosity of the current collectors is 30-50%. 1 cm (a) (b) Fig. 2. SEM (a) and optical microscope (b) micrographs of a porous current collectors made of sintered titanium spherical-particles Pore size value During PEM water electrolysis, the pores of the titanium current collectors are filled by a biphasic mixture of liquid water and gas bubbles which form at the level of the electrocatalytic layers. These mixtures (water oxygen on the anodic side and water hydrogen on the cathodic side) must flow across the porous current collectors. On cathodic sides, water and hydrogen flow from the catalytic layers to the collecting channels of the bipolar plate at the backside (figure 1). On anodic sides, water and oxygen flow in opposite directions. To 3/6

4 optimize flow of the biphasic mixture across the current collector, it is therefore necessary to determine the best geometry of the pore network. According to the capillary theory, the radius r of a bubble which forms at the solid body - gas - liquid boundary is a function of the pore radius (channel) R and the wetting angle α (Fig. 3) : r = R cos α. Fig. 3. Schematic representation (longitudinal-sectional view) of a gas bubble in a capillary tube. According to the Laplace equation, the difference of pressure between the gas and liquid phases in the pore is : 2 σ P = (1) r where P = Р m Р s ; Р m and Р s are the pressures of saturated water vapor above a meniscus of radius r and a plane surface, respectively; σ is the surface tension coefficient. When the water-wetting in the pore is good (the wetting angle is close to 180 ), the radius of the bubbles is close to the radius of the pores. Introducing the surface tension of water in Eq. (1), it can be seen that acceptable additional pressure (less than 1 bar) corresponds to a minimum pore diameter value of ca. 15 microns. This is the minimum pore size value which can be recommended for the porous titanium collectors contacting the electrocatalytic layer. In a previous work [1], an optimal pore size value of 7-10 microns was obtained from minimization of electric losses considerations. Thus, taking into account both mass and electron transfer processes, as a first approach, a pore mean diameter of microns should be recommended for current collectors. Exact calculations of pore s diameter for different current densities (up to 2 A.cm 2 ) and pressures (up to 50 bar) will be reported in a subsequent work. 3 ) Electrochemical performances of PEM cells To support the estimations obtained above, current-voltage relationships and current efficiencies of a PEM cell were measured during water electrolysis. Experiments were performed using a laboratory electrolysis cell (7 cm 2 working area). Nafion 115 membrane (thickness 127 microns) was used as solid electrolyte. The membrane electrode assemblies 4/6

5 (MEAs) were prepared using iridium black (2.0 mg.cm 2 and 5 wt. % of ion-exchange polymer in alcoholic solution) as anodic catalyst, and platinum (0.8 mg.cm 2 deposited on black carbon and 10 wt. % of the same polymer solution) as cathodic catalyst. Catalytic mixtures were applied onto the membrane surface using a spray method. The MEAs and the current collectors were then installed in the electrolysis cell. Current collectors with different porosities (between 25 and 40%) and different mean pore size values (between 8 and 100 microns) were tested. It can be seen from figure 4 that the structure of the current collectors plays a significant role. An optimum value of d = 21 microns was obtained. For lower and higher values, less efficient polarization curves were obtained. It is not yet clear whether the higher cell voltages are due to either anodic or cathodic higher overvoltages. A deeper analysis of the problem requires separate measurements of current overvoltage relationships on both anodic and cathodic sides. In particular, increased pressure at the catalytic layer current collector interfaces (resulting from an inappropriate pore size of the current collector) may decrease the kinetics of either the hydrogen evolution or the oxygen evolution reactions. 2 1,9 1,8 U, V 1,7 1,6 1,5 d=100 mcm d=8 mcm d=20 mcm * d=21 mcm 1,4 0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2 2,2 i, A/cm 2 Fig. 4. Current-voltage curves measured during water electrolysis using a PEM cell made of current collectors with different mean pore size values (d). Polarization curves were recorded at t = 90 C, after 10 hours of continuous operation. (* - both surfaces of this current collector are rolled during fabrication, and real pore size on the boundary with electrocatalytic layer is less than 20 µm). Results obtained so far indicate that the optimum pore size value is ca. 20 microns. Also, experimental results show that the porosity of the current collectors in the range 25-40% has no significant influence on current-voltage curves. 5/6

6 4 ) Conclusions and perspectives The structure of the porous titanium plates used as current collectors in PEM water electrolysis cells requires an optimization in terms of porosity and mean pore size value. Results presented in this communication show that at atmospheric pressure, for current densities ranging from 0 to 1 A.cm 2, an optimum pore size value of ca. 20 microns is required. By recording the polarization curves at 90 C, it has been shown that an inappropriate pore size of the current collectors can increase the cell voltage up to 100 mv at 2 A.cm 2. The role of the operating pressure is currently under investigation. Acknowledgements This work has been supported by the Commission of the European Communities (6th Framework Programme, STREP program GenHyPEM n ) and by the Council for Grants at the President of the Russian Federation (grant n MK ). References [1] Grigoriev S.A., Kalinnikov A.A., Millet P., Etievant C., Porembsky V.I., Fateev V.N. Mathematical modeling and experimental study of PEM electrolysis // Proceedings of the 7th European Symposium on Electrochemical Engineering (Toulouse, France, 3-5 October 2005), pp [2] Baranov I.E., Grigoriev S.A., Ylitalo D., Fateev V.N., Nikolaev I.I. Transfer processes in PEM fuel cell: Influence of electrode structure // International Journal of Hydrogen Energy, Volume 31, Issue 2, pp , [3] Arsentiev P.P., Koledov L.A. Metallurgical melts and their properties // Moscow, Metallurgy, p. 375, /6