CO₂-Assisted Regeneration of a Polymer Electrolyte Water Electrolyzer Contaminated with Metal Ion Impurities

Abstract

Polymer electrolyte water electrolysis (PEWE) is perceived as a key technology for conversion of renewable electricity in large-scale energy storage applications and deep decarbonization of the traditionally carbon-intensive mobility and chemical industry sectors. Since the price of hydrogen produced by PEWE is dominated by the price of electricity, it is crucial to ensure low degradation rates during the lifetime of a PEWE stack. This study aims to shed light onto the effects of cationic impurities that are responsible for the water impurity related failures of a majority of commercial stacks by employing operando neutron imaging coupled with electrochemical impedance spectroscopy measurements. A detailed overpotential analysis is conducted to elucidate how different voltage losses are triggered depending on the relative position of the cationic contaminant in the catalyst coated membrane. Based on this, a novel, CO 2 -assisted, method for the extraction of cationic impurities from the ionomer of the electrolyzer to recover performance during operation of the cell is presented.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 195.176.113.2 Downloaded on 2019-06-17 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 195.176.113.2 Downloaded on 2019-06-17 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 195.176.113.2 Downloaded on 2019-06-17 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 195.176.113.2 Downloaded on 2019-06-17 to IP

Dynamic Neutron Imaging and Modeling of Cationic Impurities in Polymer Electrolyte Water Electrolyzer

“…Recovery of the voltage is immediately observed as the contaminant cations are moving to positions near the cathode catalyst layer where the impact of their presence is less severe. 36 When the electrochemical conditions reached steady state (∼30 min from switch to the fresh feed loop) all transient experiments were carried out, which took approximately 8 h. The last step of the experiment included operando regeneration of the electrolyzer by introducing dilute H 2 SO 4 acid (0.5 mmol l −1 ) to the cathode cell compartment. Initially, a drop of the performance can be seen due to a temperature effect after the acid (which was at room temperature) was injected.…”

“…IV curve simulation and HFR hysteresis.-In our previous publication, which focused mostly on the impact of contaminants on electrochemical performance, 36 it has been shown that for contaminated membranes a very distinct hysteresis in the HFR appears when measuring Current-Voltage (IV) curves with ascending and descending current densities in cycles. Using the model that has been validated with experimental data, we inserted new transport parameters that match Fe 3+ .…”

“…The amount of contaminants was left the same as the contamination procedure followed an identical protocol and we expect that the cation uptake should not vary significantly as the rate of contamination with Fe 3+ and Gd 3+ was already shown to be similar to each other. 36 The diffusion coefficient however was adjusted according to literature 47 and set to D i = 2•10 −7 cm 2 s −1 . The potential profile from the IV curve measurement was reproduced in the simulation and plotted.…”

“…Gadolinium proved to be a representative model cation in terms of electrochemical behavior according to our previous study. 36 Thanks to its very high neutron cross-section of Gd atoms (over 300 times more than that of a water molecule) good contrast can be obtained while maintaining reasonably low concentration of contaminants in the CCM. An in-house developed mobile test bench and custom designed electrochemical cell have been used to perform experiments.…”

“…39 In our previous study, we have shown the distribution of Gd-ions in the CCM cross-section using operando neutron imaging, from which regeneration strategies have been derived. 36 It has been shown that in situ generated carbonic acid made by flowing CO 2 gas through the gas water separator can be used to remove cationic impurities from the CCM and thereby successfully recover the performance of the cell. Such strategies prolong the overall lifetime of the stack and help to avoid a costly disassembly procedure to exchange CCMs when the cationic contamination is too high to operate the electrolyzer economically.…”