Experimentally determined reduction of both ohmic and mass transport overpotential due to femtosecond laser-induced surface structuring of titanium-based porous transport layers at the interface to the catalyst layer.
Information on proton exchange membrane water electrolysis performance is often obtained from full cell measurements. The level of detail of this information is, however, comparably low. This contribution analyzes kinetic parameters for anode and cathode reactions separately as a step towards an extended loss breakdown through a salt bridge reference electrode. The reference electrode setup is shown in detail, and qualitative measurements are discussed. Oxygen evolution reaction and hydrogen evolution reaction Tafel slopes and exchange current densities for both reactions are reported. An outlook on future use cases for the salt bridge reference electrode is given and supported by measurement data.
In this work, we use a method to separate the total oxygen mass transport coefficient into molecular, Knudsen, and ionomer contributions. Therefore, limiting current density measurements are carried out as a function of the diluent gas (He, N2, CO2), temperature (30, 50, 80°C), relative humidity (50, 75, 100%), and oxygen concentration (1, 3, 5, 7%) using state of the art membrane electrode assemblies with three platinum loadings (0.05, 0.1, 0.15 mg/cm2). As expected, the molecular diffusion coefficient is independent of the platinum loading, but increases with temperature to a varying degree depending on the humidity level. On the other hand, the Knudsen diffusion coefficient increases with increasing electrochemical active surface area and temperature, and with decreasing relative humidity. The separation procedure includes a novel feature to isolate the ionomer mass transport resistance. Its interpretation as well as the method’s reliability are critically questioned using operating condition dependencies.
The added constraints of fuel cells and electrolysers can make incorporating reference electrodes into their design a complex and invasive process. One set of solutions to this problem includes placing multiple reference electrodes around the perimeter to be the least invasive as possible, adding a catalyst patch to the membrane to keep potentials from fluctuating, and using additional hydrogen producing electrodes as a reference for potential [1]. The referenced approach has been proven to work in fuel cells; this work seeks to apply the referenced methodology to electrolysers, and expand upon it using electrochemical modelling techniques. By characterizing the net- and half-reactions using polarization curves and Tafel analysis at a series of different temperatures, it is possible to model the half-reaction parameters from polarization curve data collected for the net-reaction. The resulting model will then enable estimates of half-reaction parameters in a conventional two-electrode electrolyser setup. Figure 1 shows the suggested set-up and reference electrode placement for this experiment. The reference electrodes; Re1, Re2, Re4 and Re5 will be platinized platinum wires and will work as Reversible Hydrogen Electrodes in this system. Reference electrodes ReP,0 and ReP,3 will be catalyst patches placed in chambers where the conditions will be held constant. Sensing electrodes will also be added as patches but are not subject to the same conditions as the reference electrodes. The conditions, temperature, pressure and humidity, in the cell will be known, thereby allowing for the characterization and correction of the reference electrode potentials against the Standard Hydrogen Electrode. References: Herrera, O. et al., “New Reference Electrode Approach for Fuel Cell Performance Evaluation” ECS Trans. Vol. 16, p. 1915, (2008) Figure 1
Membrane electrode assemblies (MEA) and specifically the anode catalyst are of particular interest in PEM water electrolysis (WE) research as they are considered key components for the capital expenditure and operating expenditure of a PEMWE system (1). An effective method to understand the operation of the MEA is to apply diagnostic tools during the operation of the electrolysis cell. Previous studies in this regard have been mainly limited to the total cell voltage. However, the individual electrode behavior cannot be investigated in the standard setup. To improve the MEA design, the individual electrode behavior is crucial. This can be captured by using a reference electrode (RE). In this work, a salt bridge is used for the first time as RE concept for polarization curves and electrochemical impedance spectroscopy (EIS). The implementation is shown in figure 1.a. Via a porous transport layer impregnated with Nafion, the reference electrode can access the ionic potential of the catalyst layer under investigation. This isolates the kinetic overvoltage from other cell voltage contributions such as ohmic membrane losses, see figure 1.b. The experiments in this study include polarization curves with conventional materials and operating conditions. In addition, EIS is performed to test the applicability of the measurement method in RE operation. Exemplary results are shown in Figure 1.c. In addition, a previous analysis of RE from fuel and electrolytic cells is presented and a classification by electrode type and positioning is made. The key results are subsequently summarized. First, a distinction is made according to the principle behavior of the electrode or its possible configurations. The principal electrode behavior includes the dynamic hydrogen electrode (DHE), the quasireference electrodes, external REs and other configurations. Concrete concepts follow from this. For example, the DHE can be implemented with two platinum wires. By applying a current in the microampere range, the hydrogen evolution reaction takes place in the presence of water. The platinum wire in hydrogen atmosphere becomes the RE (2). Quasireference electrodes are very commonly used with ionic liquids. They are often metal wires (3). A platinum wire is used in a recent work for recording individual electrochemical impedance spectra of anode and cathode (4). Any type of RE can be used as an external RE, such as the silver-silver chloride electrode. Another possibility is to create a free-standing catalyst strip by laser ablation of the MEA. This strip can be used as a reversible electrode to measure the voltage difference with the active catalyst layer participating in the reaction (5). In addition to the behavior of the electrode, a subdivision is made into the position of the RE. In order to separate the potential of one of the two electrodes from the total potential, the measurement can be made as close as possible to the electrode under investigation. This can be implemented by a direct measurement in the catalyst layer (6) or a special geometry of the active area (7). Alternatively, the potential can be measured at the membrane and a correction of the ohmic membrane losses can be used to infer the electrode potential to be investigated. Here, too, two implementations are possible. On the one hand, the RE can be positioned between two membrane halves (4). On the other hand, the membrane can be contacted outside the active area (8). One requirement chosen for the presented concept is the applicability of the RE independent of the geometry of the MEA. Furthermore, the RE should be insensitive to misalignment of the catalyst layers. The exact alignment of the anodic and cathodic catalyst layers is challenging and even small deviations lead to a shift of the potential in the membrane (8). For the two reasons mentioned above, the concept of salt bridge with external RE is used in the direct approach at the electrode. References The national hydrogen strategy (Juni 2020). M. V. Lauritzen, P. He, A. P. Young, S. Knights, V. Colbow and P. Beattie, Journal of New Materials for Electrochemical Systems, 10(3), 143–145 (2007). G. Inzelt, A. Lewenstam, F. Scholz and F. G. K. Baucke, Editors, Handbook of reference electrodes, Springer, Berlin (2013). A. Hartig-Weiß, M. Bernt, A. Siebel and H. A. Gasteiger, J. Electrochem. Soc., 168(11), 114511 (2021). D. Gerteisen, J Appl Electrochem, 37(12), 1447–1454 (2007). E. Brightman, J. Dodwell, N. van Dijk and G. Hinds, Electrochemistry Communications, 52, 1–4 (2015). A. A. Kulikovsky and P. Berg, J. Electrochem. Soc., 162(8), F843-F848 (2015). G. Li and P. G. Pickup, Electrochimica Acta, 49(24), 4119–4126 (2004). Figure 1
The cost and durability of proton exchange membrane fuel cells (PEMFCs) still limit their adoption as sustainable power systems [1]. Operation at high current densities, which decreases the inventory of costly materials [2], is still considered an option [3] in addition to concurrent reductions in Pt catalyst loadings. However, these approaches intensify mass transfer losses especially at the cathode because the oxidant stream is more dilute (21 % O2 in air) than the fuel (100 % H2) and the heavier O2 molecule moves more slowly. The improvement of characterization methods able to measure and separate oxygen mass transfer coefficients is desirable for more accurate identification and localization of resistances in membrane/electrode assemblies (MEAs). Average limiting currents iave are first measured with a dilute oxidant stream for different flow rates. Subsequently, data are fitted to a mathematical model [4] to obtain the overall mass transfer coefficient k: iave =ie (1–exp(–nFkpr /RTief)) (1) with ie the inlet reactant flow rate equivalent current density, n the number of electrons exchanged in the electrochemical reaction, F the Faraday constant, pr the dry inlet reactant stream pressure, R the ideal gas constant, T the temperature, and f the inert gas to reactant fraction in the dry inlet reactant stream. The oxidant diluent is also varied (He, N2, CO2) to separate the molecular diffusion mass transfer coefficient km by extrapolating to the origin the linear correlation between the overall mass transfer resistance and the diluent molecular mass M [4]: k –1=km (M)–1+k e+K (c)–1=km (M)–1+ke (c)–1+kK –1 (2) The ionomer permeability mass transfer coefficient contribution ke was separated from 1/k e+K by taking advantage of the constant Knudsen diffusion resistance (equation 2 above and equation 7 in [5]). For that last step, the linear correlation between the lumped ionomer permeability and Knudsen diffusion mass transfer resistance 1/k e+K and the oxygen concentration c (1 to 7 %) was extrapolated to the origin, which yielded the Knudsen diffusion mass transfer coefficient kK . This approach to isolate the Knudsen contribution is novel. Other techniques are used, including temperature variations [5]. Experiments were conducted with a 50 cm2 active area cell and General Motors MEAs with a commercially relevant cathode catalyst loading of 0.05 to 0.15 mg Pt cm–2 (Pt catalyst supported on a high surface area carbon). All MEA tests were duplicated and completed for several temperatures (30 to 80 °C) and relative humidities (50 to 100 %). Four diagnostic methods were employed to supplement the analysis of mass transfer data. The use of O2, 21 % O2 in He, and air enabled the derivation of overpotentials from polarization curves [6]. The high frequency cell resistance was measured with a milliohmmeter. The catalyst area was extracted from the hydrogen adsorption region of cyclic voltammograms [7]. The overall cell water balance was also calculated to assess the presence of liquid water [8]. The smallest mass transfer resistance was assigned to the ionomer permeability, which supported the need for repetitive measurements and statistics for an accurate quantification. The slope of the linear correlation between the ionomer permeability and Knudsen diffusion mass transfer resistance and the oxygen concentration was dependent on the water balance. A positive slope occurred under sub-saturated streams whereas a negative slope was correlated with the wettest operating conditions, which will require a method modification to extract the Knudsen diffusion resistance. A plot of the ionomer and molecular diffusion mass transfer resistance versus the total mass transfer overpotential showed two regimes that are differentiated by the absence/presence of liquid water (see figure). Catalyst layers, only several microns thick, have a larger mass transfer resistance and overpotential under flooding conditions. [1] Y. Wang, D. F. R. Diaz, K. S. Chen, Z. Wang, X. C. Adroher, Mater. Today, 32 (2020) 178. [2] A. Kongkanand, M. F. Mathias, J. Phys. Chem. Lett., 7 (2016) 1127. [3] N. Ramaswamy, W. Gu, J. M. Ziegelbauer, S. Kumaraguru, J. Electrochem. Soc., 167 (2020) article 064515. [4] T. Reshetenko, J. St-Pierre, J. Electrochem. Soc., 161 (2014) F1089. [5] N. Nonoyama, S. Okazaki, A. Z. Weber, Y. Ikogi, T. Yoshida, J. Electrochem. Soc., 158 (2011) B416. [6] J. St-Pierre, M. Angelo, K. Bethune, J. Huizingh, T. Reshetenko, M. Virji, Y. Zhai, Modern Fuel Cell Testing Laboratory, in Springer Handbook of Electrochemical Energy, Part D, Chapter 19, Edited by C. Breitkopf, K. Swider-Lyons, Springer, 2017, p. 611. [7] R. N. Carter, S. S. Kocha, F. T. Wagner, M. Fay, H. A. Gasteiger, ECS Trans., 11(1) (2007) 403. [8] J. St-Pierre, N. Jia, M. van der Geest, A. Atbi, H. R. Haas, United States patent 7,132,179, November 7, 2006. Figure 1
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.