Spatially-Resolved Current Distribution Measurements in Polymer Electrolyte Water Electrolyzers

Abstract

Operational cost, primarily power consumption, is one of the main cost obstacles for hydrogen produced by polymer electrolyte water electrolyzers (PEWE). Roughly 70% of the hydrogen production cost stems from electricity [2]. Thus, overall process efficiency has a direct influence on the hydrogen price. Dominating sources of electrochemical loss in PEWE cells are the kinetic, electric, and mass transport overpotentials. Ex-situ and modeling work have yielded insights into the physical processes occurring within an operating PEWE, but there is still need for in-situ measurements for verification [3]. Work carried out by Zhang et al. [4, 5] has successfully shown the bubble formation of water electrolysis inside a PEWE by means of an in-situ, high-speed micro-scale visualization system (HMVS). However the HMVS does not explicitly capture the local electrochemical behavior of the cell. Therefore, complex CFD models have been implemented to shed light on the inner workings of PEWE; such models need to be validated with experimental data. This work aims to provide a reliable source of experimental data by developing a technique that allows for in-situ spatial and temporal current distribution measurements, which can be used to validate existing modeling efforts.Current distribution measurements are described extensively in the works of Bender et al. [6, 7] and Mench et al. [1, 8]; a major innovation was the inclusion of a printed-circuit board (PCB) with an array of shunt resistors attached to each segment as shown in Figure 1a (4.5 mm x 4.5 mm segments in this work). The measured voltage drop across each shunt resistor is used to calculate the current in each segment. Passive data acquisition on the distributed current board is done simultaneously to electrochemical control of the electrolyzer, without interrupting its operation. This apparatus is capable of using flow fields of both 5cm2 (16 segments) and 25cm2 active area (100 segments). The distributed current is then interpolated to obtain smoothed contours as shown in Figure 1b. Preliminary current distribution experiments were carried out at 80⁰C with a water flow rate of 0.5 to 15 mL/min/cm2. Experiments were also carried out with two types of porous transport layers from Zhang and co-workers [4, 5]: thin titanium foil LGDLs with 80 nm pore size and Ir-coated titanium felt. Two types of flow field where also used – parallel and triple serpentine. Current distributions were found to respond to transport layer porosity and thickness, flow field design, flow rate, and current density. A non-uniform current distribution is suspected to be ultimately dominated by mass transport characteristics and these experiments support that. Additionally, conditions that yielded near-identical polarization performance were found to have disparate current distributions. This work demonstrates the benefit of a spatially-resolved current distribution measurement technique and the insights it offers which would otherwise be impossible to probe. This work is part of DOE project #DE-EE0008426 "Developing novel electrodes with ultralow catalyst loading for high-efficiency hydrogen production in proton exchange membrane electrolyzer cells." References Clement, J.T., D.S. Aaron, and M.M. Mench, In Situ Localized Current Distribution Measurements in All-Vanadium Redox Flow Batteries. Journal of The Electrochemical Society, 2015. 163(1): p. A5220-A5228.Grigoriev, S., V. Porembsky, and V. Fateev, Pure hydrogen production by PEM electrolysis for hydrogen energy. International Journal of Hydrogen Energy, 2006. 31(2): p. 171-175.Schuler, T., T.J. Schmidt, and F.N. Büchi, Polymer Electrolyte Water Electrolysis: Correlating Performance and Porous Transport Layer Structure: Part II. Electrochemical Performance Analysis. Journal of The Electrochemical Society, 2019. 166(10): p. F555-F565.Mo, J., et al., Discovery of true electrochemical reactions for ultrahigh catalyst mass activity in water splitting. Science Advances, 2016. 2(11): p. e1600690.Li, Y., et al., Wettability effects of thin titanium liquid/gas diffusion layers in proton exchange membrane electrolyzer cells. Electrochimica Acta, 2019. 298: p. 704-708.Bender, G., M.S. Wilson, and T.A. Zawodzinski, Further refinements in the segmented cell approach to diagnosing performance in polymer electrolyte fuel cells. Journal of Power Sources, 2003. 123(2): p. 163-171.Reshetenko, T.V., et al., A segmented cell approach for studying the effects of serpentine flow field parameters on PEMFC current distribution. Electrochimica Acta, 2013. 88: p. 571-579.Ertugrul, T.Y., et al., In-situ current distribution and mass transport analysis via strip cell architecture for a vanadium redox flow battery. Journal of Power Sources, 2019. 437: p. 226920. Figure 1