Correlating high temperature thin film ionomer electrode binder properties to hydrogen pump polarization

Ionomer electrode binders are important materials for polymer electrolyte membrane (PEM) fuel cells and electrolyzers and have a profound impact on cell performance. Herein, we report the effect of two different types of high-temperature (HT) ionomers, characterized as thin films (~10 nm), on hydrogen oxidation/evolution reaction (HOR/HER) kinetics and hydrogen permeability using interdigitated electrode (IDE) platforms decorated with nanoscale platinum electrocatalysts. The two different ionomers studied were poly(tetraflurostyrene phosphonic acid- co -pentafluorostyrene) (PTFSPA) and quaternary benzyl pyridinium poly(arylene ether sulfone) imbibed with phosphoric acid (QPPSf-H 3 PO 4 ). The reaction kinetics and limiting current values observed with thin film ionomers on IDEs were commensurate to the values observed in electrochemical hydrogen pumps (ECHPs) that use the ionomers as electrode binders. Using PTFSPA as the binder, an HT-PEM ECHP showed A cm -2 at 55 mV when using 2 mg Pt cm -2 in the membrane electrode assembly.


INTRODUCTION
Ionomer binders strongly influence the performance and stability of numerous electrochemical processes such as fuel cells 1-4 , water 5 and carbon dioxide electrolyzers 6 , and deionization units 7,8 . In low-and high-temperature polymer electrolyte membrane (i.e., LT-PEM and HT-PEM) architectures involving hydrogen, the binders hold the electrocatalyst/electrocatalyst supports, while also delivering protons to and from the electrocatalyst to the PEM separator. Both PEM separators and ionomer electrode binders require high conductivity and stability 4,9 under a wide-range of conditions (e.g., chemical, electrochemical, and thermal), but there are nuanced differences with respect to the properties for PEM separators and electrode binders. PEM separators necessitate low gas permeability for safety and mitigating mixed overpotentials. Ionomer binders require high gas permeability to overcome mass transfer related resistances and enable high current density 10,11 .
In the electrode layers, the ionomer binder often serves as a thin adhesive coating on the electrocatalyst/electrocatalyst support particles 12 . A few research groups have shown that Nafion ® can display substantially different proton conductivity and water uptake properties when confined as a thin film (5 to 100 nm) when compared to its properties as a bulk membrane [13][14][15][16][17][18][19] . It is worth noting that there is a lack of studies investigating how the properties of other thin film ionomers influence electrochemical properties, such as charge-transfer reaction kinetics as well as gas permeability, in addition to other types of ionomer chemistries for hydrogen-based electrochemical systems. These other properties have a more profound impact on electrochemical device performance when compared to ionic conductivity 20 . For example, ionomers can alter redox reaction rates (e.g., by adsorption of the tethered ion to the catalyst) 21,22 and gas reactant mass transfer rates to the electrocatalyst surface 11,20,23,24 .
In this work, we studied the influence of two different types of high-temperature ionomer thin films on hydrogen oxidation/evolution reaction (HOR/HER) kinetics and hydrogen gas permeability on interdigitated electrodes (IDEs). We then correlated the thin film properties to the polarization behavior of electrochemical hydrogen pumps (ECHPs).
Our work was motivated by our previous reports 20,25,26 , and others 3,[27][28][29] , showing that ionpair HT-PEMs operate over a wider temperature range and have greater humidity tolerance when compared to the conventional benchmark based upon phosphoric acid (H 3 PO 4 ) imbibed polybenzimidazole. In HT-PEM fuel cell studies, we observed significant kinetic and mass transfer resistances that hail from the presence of liquid H 3 PO 4 in the electrode layers. To address these resistances, Los Alamos and the University of Stuttgart adopted an alternative ionomer electrode binder based upon tethered phosphonic acid to the polymer backbone (i.e., poly(tetrafluorostyrene phosphonic acid-copentafluorostyrene) (PTFSPA)) 1 . This binder addressed mass transfer resistances in the electrode layers and achieved a peak power density of 1.7 W cm -2 at 240 °C. Drawing inspiration from this group, we investigated how removal of liquid acid from the ionomer influences HOR/HER kinetics and gas permeability. We then related the properties of thin films to the polarization of a single-cell electrochemical hydrogen pump (ECHP).
ECHPs are used for hydrogen separations and compression in industrial settings [30][31][32][33][34][35][36] , in addition to being a diagnostic tool for fuel cells. Combining the ion-pair HT-PEM separator with Pt/C electrodes using the PTFSPA binder, we have demonstrated that an ECHP that can operate at 1 A cm -2 at 120 mV using 1 mg Pt cm -2 in the membrane electrode assembly (MEA) and at 55 mV at 220 °C using 2 mg Pt cm -2 in the MEA. These values represent the best performance in the peer-reviewed literature for a HT-PEM ECHP.
However, it should be noted that LT-PEMs (e.g., perfluorosulfonic acid materials like Nafion ® ), using humidification, can achieve the same current density values at lower voltage (0.04 to 0.09 V) 34,35 at lower platinum loadings (< 1 mg Pt cm -2 for the MEA).
Although LT-PEM ECHPs have better performance, they require substantial gas humidification and suffer more in performance loss with greater concentration of contaminants in the hydrogen mixture (e.g., carbon monoxide (CO)). Raising the temperature above 180 °C makes the ECHP more resilient to contaminants enabling more effective hydrogen separations. Also, the environment for HT-PEM ECHP is not as harsh as HT-PEMFC and LT-PEM ECHP as the cell has no oxygen and water.
Most ECHP studies that operate above 100 °C examine different types of PBI chemistries 30-33, 37, 38 . These studies often use commercially available electrodes (e.g., BASF electrodes) and little attention has been given to how the electrode binder impacts ECHP performance. New materials for fuel cells, ECHPs, and water electrolyzers are characterized in ex-situ experimental setups for assessing their likelihood to improve the electrochemical cell performance. Experimental protocols for ex-situ assessment of bulk membranes (e.g., 4-pt conductivity) and electrocatalyst activity (e.g., rotating disk electrode) are standardized, but there are few precedents and tools to examine the electrochemical properties of thin film ionomers 11,39 outside ionic conductivity and without liquid supporting electrolyte.

RESULTS AND DISCUSSIONS
To understand how thin film ionomers impact other electrochemical properties beyond ionic conductivity, our group developed an interdigitated electrode (IDE) platform that features a thin film (< 30 nm) of nanoscale platinum group metal (PGM) electrocatalyst afforded from self-assembled block copolymer templates 40 (Figure 2a). The electron micrograph of platinum nanowires on IDEs is shown in Figure S1, while images of the chamber for HOR/HER experiments are provided in Figure S2. The presence of this periodic nanostructure PGM electrocatalyst across the IDE is useful for assessing HOR/HER kinetics in the presence of a thin film ionomer (previously Nafion ® at room temperature) 40 .
Here, we extended this platform to assess the electrochemical properties (HOR/HER kinetics, ionic conductivity, and hydrogen permeability) for two different types of thin film high-temperature (HT-) ionomers. The chemical structures of the HT-ionomers (PTFSPA and quaternary benzyl pyridinium poly(arylene ether sulfone) imbibed with H 3 PO 4 (QPPSf H 3 PO 4 )) are shown in Figure 1a. PTFSPA was synthesized following the procedure of Atanasov et al. 41 QPPSf H 3 PO 4 was prepared as described in our previous work 26     Next, the linear regime of the iR-corrected polarization curves was analyzed in order to assess how the different thin film ionomers affect HOR/HER kinetics. The iRcorrected potential values in the linear regime, which also corresponds to low current density values, is under mixed-control (i.e., reaction kinetics and mass transfer resistances dictate the current response). Here, we assume that the mass transfer resistance is not severe but not negligible because of the presence of the thin film ionomer coating. Figure   2d shows the current density at 150 mV (i.e., i @η=150 mV ) for two different high-temperature thin film ionomers as a function of temperature. At 160 °C, the i @η=150 mV value for the  Figure 3a, and the non-iR corrected polarization curves are provided in Figure S8. There are two salient features in Figure 3a: i.) the MEA with a PTFSPA electrode binder outperformed the MEA with a QPPSf H 3 PO 4 electrode binder and ii.) as the cell temperature increased, the polarization decreased more    Table 1) and best performance compared to the current HT-PEM ECHPs 6,30 .
To optimize the HT-PEM ECHP, future work with these materials will examine overpotential differences between the HOR and HER, similar to the asymmetric MEA PTFSPA materials as electrode binders in HT-PEM ECHPs results in excellent performance of 1 A cm -2 at 55 mV (see Table 1).