The water balance in operating hydroxide-exchange-membrane fuel cells (HEMFCs) is a key challenge for improved performance and durability. For every 4 electrons produced in an HEMFC, 4 water molecules are generated in the anode, 2 water molecules are consumed in the cathode. Water is also transported from cathode to anode along with the hydroxide due to electroosmosis. Consequently, there is a concentration gradient of water that transport water from anode to cathode. Ineffective water management could lead to cathode dry-out, where insufficient water in the cathode limits the rate of reaction or causes ionomer degradation through alkaline instability, or to anode flooding, where excess liquid water in the anode limits gas diffusion. To address these issues, it is critical to measure the transport of water across the hydroxide-exchange membrane during cell operation. We built a water-flux station to measure the water in and out of the anode and cathode during cell operation with different inlet relative humidities and back pressures. Here, the effect of the membrane thickness and the effect of stacking hydroxide-exchange membranes on the water transport across the hydroxide exchange membrane is studied. The stacking of hydroxide-exchange membranes creates an interfacial ohmic resistance between membranes, measured from the high frequency resistance, but not an interfacial water transport resistance. This study shows that the water transport across the hydroxide-exchange membrane is sufficient for water consumption in the cathode and is not significantly affected by the membrane thickness or stacking. However, the added ohmic resistance from the increased membrane thickness or stacking membranes significantly affects the performance.

Constructing well-defined heterostructure interfaces in catalysts is an efficient strategy to break the so-called scaling relationships and to accelerate the reactions involving multiple intermediates. Here a cluster–cluster heterostructure catalyst composed of crystalline ruthenium cluster and amorphous chromium oxide cluster is designed to realize high-performance alkaline hydrogen electrocatalysis. The strongly coupled cluster–cluster heterostructure interface induces a unique interfacial interpenetration effect, which simultaneously optimizes the adsorption of intermediates on each cluster. The resulting catalyst exhibits impressive catalytic activities for the hydrogen oxidation reaction (exchange current density of 2.8 A mg −1 Ru ) and the hydrogen evolution reaction (mass activity of 23.0 A mg −1 Ru at the overpotential of 100 mV) in alkaline media. The hydroxide exchange membrane fuel cell delivers a mass activity of 22.4 A mg −1 Ru at 0.65 V and outstanding durability with no voltage loss over 105 h operation at 500 mA cm −2 . The present work demonstrates the superiority of cluster–cluster heterostructure interface towards the development of advanced catalysts.

The anion-exchange membrane (AEM) is the key component to optimize overall water flux in anion-exchange-membrane fuel cells (AEMFCs) for performance and durability. For alkaline hydrogen oxidation reaction (HOR) in the anode, 4 water molecules are produced for every 4 electrons, and for alkaline oxygen reduction reaction (ORR) in the cathode, 2 water molecules are consumed for every 4 electrons. Water molecules also transport across the AEM from the cathode to the anode along with the hydroxide due to electroosmosis. Due to the reactions and electroosmosis, large water concentration gradient develops, driving water transport from the anode back to the cathode which is essential for ORR and ionomer hydration. Insufficient water in the cathode catalyst layer can lead to ionomer dry-out which exacerbates ionomer degradation, so it is imperative to have an AEM with high water permeability and diffusivity. To quantify the water diffusivity for vapor-equilibrated PiperION-A AEMs of 13, 20, 40, and 80 µm thickness, we use an in-house built water flux station under inert conditions with varying degrees of water vapor, and we report the water transport resistance as a function of the average water activity at 80°C, a standard operating temperature for AEMFCs. Insights into understanding the water transport phenomena in AEMs as a function of thickness and water content is crucial for designing AEMFCs to overcome the challenging water balance for improved performance and durability.

Anion-exchange membrane fuel cells (AEMFCs) are promising alternative hydrogen conversion devices. However, the sluggish kinetics of the hydrogen oxidation reaction in alkaline media hinders further development of AEMFCs. As a synthesis method commonly used to prepare disordered PtRu alloys, the impregnation process is ingeniously designed herein to synthesize sub-3 nm Pt@Ru core-shell nanoparticles by sequentially reducing Pt and Ru at different annealing temperatures. This method avoids complex procedures and synthesis conditions for organic synthesis systems, and the atomic structure evolution of the synthesized core-shell nanoparticles can be tracked. The synthesized Pt@Ru electrocatalyst shows an ultrasmall average size of ∼2.5 nm and thereby a large electrochemical surface area (ECSA) of 166.66 m2 gPt+Ru-1. Exchange current densities (j0) normalized to the mass (Pt + Ru) and ECSA of this electrocatalyst are 8.0 and 5.8 times as high as those of commercial Pt/C, respectively. To the best of our knowledge, the achieved mass-normalized j0 measured by rotating disk electrodes is the highest reported so far. The membrane electrode assembly test of the Pt@Ru electrocatalyst shows a peak power density of 1.78 W cm-2 (0.152 mgPt+Ru cmanode-2), which is higher than that of commercial PtRu/C (1.62 W cm-2, 0.211 mgPt+Ru cmanode-2). The improvement of the intrinsic activity can be attributed to the electron transfer from the Ru shell to the Pt core, and the ultrafine particles further enhance the mass activity. This work reveals the feasibility of using simple impregnation to synthesize fine core-shell nanocatalysts and the importance of investigating the atomic structure of PtRu nanoparticles and other disordered alloys.

In this contribution, a series of characterizations, such as water uptake, acid uptake, membrane density, and conductivity measurements, have been utilized to study the effect of equivalent weights (EW) on ion partitioning and transport behavior of perfluorosulfonic acid membranes produced by 3M Corp. It is observed that water sorption and acid uptake both increase with a decrease in EW. This phenomenon results from a higher sulfonic acid site density in the membrane with a lower EW. Just as important, the Donnan potential opposing co-ion uptake on the membrane surface decreases with decreasing equivalent weight. The membrane with a lower EW is found to have a higher membrane density at the same water content, but the porosity does not show a large discrepancy among the three membranes we studied. Even though the higher acid concentration in membranes results in lower activity coefficients of ions, its higher water content still facilitates faster proton mobility and higher ion conductivity. This analysis helps draw a picture of the relationship between membrane morphology and dynamics of PFSA membranes.

An in-depth understanding of pretreatment effects on a perfluorosulfonic acid membrane is essential for its application to vanadium redox flow batteries (VRFBs). The physicochemical properties and transport properties of perfluorosulfonic acid membranes (PFSA) can be modified and controlled by conditioning treatments. This work investigates the effects of boiling pretreatment on the water uptake, acid uptake, membrane density, and conductivity of 3M PFSA membranes when they are in contact with H2SO4 or VOSO4/H2SO4 electrolytes. Increased water and acid uptake and much higher membrane conductivity are outcomes of the boiling pretreatment. This is mainly attributed to the pretreatment-induced morphology change, which includes a higher normalized swelling volume and a larger number of continuous water channels. The reduction of the Donnan potential for boiled membrane contributes to a higher uptake and faster proton transport but also inevitably leads to a much higher vanadium permeability. The trade-off between membrane conductance and vanadium transport across the membrane is crucial and primarily driven by the propensity to absorb various solution components, as well as their intrinsic mobility. These findings shed light on addressing relationships among physicochemical properties, transport behavior, membrane morphology, and the pretreatment effects, all of which are crucial to the performance of PFSA membranes when applied in vanadium redox flow batteries.

The high pH environment in hydroxide exchange membrane fuel cells (HEMFCs) has the potential to reach lower costs than the current proton exchange membrane fuel cells (PEMFCs), the incumbent technology. A significant difference between HEMFCs and PEMFCs is the location of water production within the cell. In PEMFCs, the water is produced on the cathode, limiting oxygen transport. In HEMFCs, the water is produced on the anode where the fuel is pure hydrogen. This allows the cathode to be optimized for oxygen transport without the presence of excess liquid water. Limiting current analysis, a technique previously used in PEMFCs, is adopted in HEMFCs to evaluate the oxygen mass transport resistances for different sections of the cathode. Through elimination of the microporous layer (MPL), gas diffusion layer (GDL), and traditional flow field and using porous nickel foam for gas distribution, the transport resistance at an operating condition of 150 kPa(g) and with the cell temperature at 80 °C was decreased from 112 s m−1 to 48 s m−1, effectively halved. The optimal configuration for performance was found with Ni foam and a GDL, eliminating the MPL and traditional flow field, which vastly improved oxygen transport while maintaining adequate electrical contact with the cathode catalyst layer.

Hydroxide exchange membrane fuel cells (HEMFCs) are a potentially lower-cost hydrogen fuel cell technology; however, ambient levels of CO 2 in air significantly reduce HEMFCs’ performance. In this work, we demonstrate an electrochemically-driven CO 2 separator (EDCS) which can be used to remove ambient levels of CO 2 from air upstream of the HEMFC stack in fuel cell vehicles, protecting it from CO 2 -related performance losses. The EDCS operating window was explored for current density, anode flow, and cathode flow with respect to its impact on CO 2 separation performance. Additionally, gas-phase mass transport was improved by selecting flow fields and gas diffusion layers conducive to the EDCS operating regime. The use of a carbon-ionomer interlayer at the cathode was explored and improved CO 2 removal performance from 77.7% to 98.2% at 20 mA cm −2 . An analytical, 1-D model is used to explain the experimental observations and design improvements. A single-cell, 25 cm 2 EDCS using the aforementioned improved design demonstrated greater than 98% CO 2 removal at a cathode flow rate of 1300 sccm for 100 h with 2.7% hydrogen stack consumption.

Development of Hydroxide Exchange Membrane Fuel Cells (HEMFC) is motivated by the promise of significantly reduced overall module costs compared to Proton Exchange Membrane Fuel Cells (PEMFC). By transitioning from a low-pH to a high-pH environment, less expensive materials become stable 1 . Specifically, platinum-group metal loading can be greatly reduced or potentially eliminated from the catalyst formulations 2 , and the bipolar plates can be manufactured more cost effectively. However, HEMFCs are currently in the early stages of development, and improvements in performance and durability are needed. A key challenge in the design of PEM and HEM fuel cells is maximizing O 2 transport from the inlet air stream to the triple phase boundary in the cathode catalyst layer. O 2 transport resistance causes voltage losses at high current densities, limiting maximum power density. Limiting current analysis has been used extensively to experimentally determine O 2 transport resistance in PEMFCs, and we demonstrate that this technique is valid for HEMFCs and produces similar results. Our analysis quantifies the individual O 2 transport resistance contributions from several factors: molecular diffusion through the gas diffusion layer (GDL), Knudsen diffusion through the microporous layer (MPL) and catalyst layer, and diffusion through ionomer in the catalyst layer. Importantly, PEMFC and HEMFC have a different water balance. In PEMFC, an MPL is added to relieve the cathode GDL of flooding but also contributes to O 2 transport losses. Conversely, in HEMFCs, the cathode dries-out and the anode floods; therefore, the MPL is an unnecessary component on the cathode GDL 3 . We report that the elimination of the MPL significantly decreases the O 2 transport resistance and improves the performance under air, especially at high current density. References Setzler, B. P., Zhuang, Z., Wittkopf, J. A. & Yan, Y. Activity targets for nanostructured platinum-group-metal-free catalysts in hydroxide exchange membrane fuel cells. Nat. Nanotechnol. 11 , 1020–1025 (2016). Wang, J. et al. Poly(aryl piperidinium) membranes and ionomers for hydroxide exchange membrane fuel cells. Nat. Energy 4 , 392–398 (2019). Kaspar, R. B., Wittkopf, J. A., Woodroof, M. D., Armstrong, M. J. & Yan, Y. Reverse-Current Decay in Hydroxide Exchange Membrane Fuel Cells. J. Electrochem. Soc. 163 , F377–F383 (2016). Figure 1

Alkaline pretreatment is perceived as an essential step for high-performance hydroxide exchange membrane fuel cells (HEMFCs), but its exact function is not fully understood. Here we show that alkaline pretreatment is only necessary when carboxylates are generated from platinum- or palladium-catalyzed oxidation of primary alcohol solvents during membrane electrode assembly (MEA) fabrication. When alkaline pretreatment is needed, bicarbonates are a better choice than the most commonly used hydroxide bases. We further demonstrate that MEAs with Pt/Pd-free catalysts, which can be used in HEMFCs, exhibit a better performance without the alkaline pretreatment: a voltage of 0.64 V at 1.0 A cm−2 and a peak power density of 0.69 W cm−2 in H2/O2. The optimization or elimination of the alkaline pretreatment will simplify the fabrication process for fuel cells and thus reduces their manufacturing costs.

Hydrogen fuel cell vehicles are a promising, emerging alternative to the internal combustion engine but the commercialized proton exchange membrane fuel cells (PEMFCs) are comparatively expensive. Hydroxide exchange membrane fuel cells (HEMFCs) are a potentially lower cost hydrogen fuel cell technology under development; unfortunately, ambient levels of CO2 in air reduce HEMFC performance and impede HEMFCs from becoming competitive with the incumbent fuel cell technology.1 Current levels of atmospheric CO2 have been shown to decrease fuel cell performance up to 200 mV.2 The hydroxide produced during the electrochemical reaction in a HEMFC reacts readily with CO2 in air at the cathode due to its acid-base reaction and transports CO2 across the hydroxide exchange membrane. At the anode, the bicarbonates build up lowering the local pH until CO2 evolution is favorable, concurrently the pH gradient saps cell voltage. However, this transport results in the effective separation of CO2 from air. By optimizing a HEMFC for CO2 capture, rather than power production, an electrochemically-driven CO2 separator (EDCS) was developed using a poly(aryl piperidinium) membrane.3 The continuous and compact EDCS has the potential to enable HEMFCs to operate efficiently with ambient air in automotive applications. This work will demonstrate the ability of the EDCS to effectively and continuously remove CO2 from air at ambient levels using minimal hydrogen flow to power the separation. This work will explore the effect of operating conditions such as flow rates, current density, and relative humidity on CO2 separation performance with the innovative EDCS. A carbon-ionomer interlayer between the catalyst layer and membrane was added to improve CO2 capture by creating an accessible volume for hydroxide and CO2 gas to react. Additionally, various design features of the cell were investigated to improve the mass transport such as flow fields and gas diffusion layers. The CO2 removal performance of a single-cell 25 cm2 EDCS is shown in Figure 1. Currently, the optimal design configuration of the EDCS has achieved 98% CO2 removal for 100 hours (Figure 2) with the potential to improve CO2 removal as well as processed gas throughput by further development of the module’s design. References: B.P. Setzler et al., Nature Nanotechnology 11, 12, 1020-1025 (2016). N. Ziv et al., Chemsuschem, 11, 7, 1136-1150 (2018). J. Wang et al., Nature Energy, 4, 5, 392-398 (2019). Figure 1

As an emerging alternative to proton exchange membrane fuel cells (PEMFCs), hydroxide exchange membrane fuel cells (HEMFCs) are more cost-effective due to PGM-free catalysts options, more affordable bipolar plate production, and other potential savings.1 However, unlike PEMFCs, the HEMFC technology is in its early phases and there is a continuous effort to improve the performance and durability through the advancement of materials and optimization of operating conditions. The polarization curve is one of the most common methods of testing a fuel cell. A typical polarization curve for a fuel cell consists of open circuit potential (crossover losses), low-current behavior (kinetics losses), moderate-current behavior (ohmic losses), and high-current behavior (transport losses). Crossover and ohmic losses are determined by electrolytes, kinetics losses depend on catalysts and transport losses involve gas diffusion and water management. In addition, a triple phase boundary comprised of an electrolyte, an electrode and a gaseous reactant is crucial for the electrochemical reactions. Thus, the optimization of all these aspects is critical for HEMFC performance improvement. Here, we focus our work on transport phenomena in a fuel cell. The materials we used are hydroxide exchange membranes (HEMs) and ionomers (HEIs) based on poly(aryl piperidinium) (PAP).2 The specific chemical structure (shown in Figure 1) endows PAP HEMs with excellent chemical stability, high conductivity and mechanical robustness, making PAP HEMs/HEIs one of the best choices for HEMFC electrolyte. Furthermore, the use of PAP HEMs/HEIs enables operating HEMFCs at 95 °C, which accelerates reaction rates and decreases heat rejection. Optimization of fuel cell operating conditions is vital for water management. Water production by hydrogen oxidation reaction (HOR) tends to cause flooding in the anode, while water consumption from oxygen reduction reaction causes cathode to dry-out. In addition, the electro-osmotic drag pulls water from the cathode to the anode (in the reverse direction of PEMFCs), thus worsening the water management problem further. In this work, observable characteristics in the polarization curve for a flooded HEMFC were found, and a model was developed to describe this issue. Then we demonstrate that balanced water management can be obtained by adjusting the anode relative humidity and back pressure. Limiting current technology is widely employed for PEMFCs to characterize the transport resistance in order to reduce mass-transport losses; however, no one has done this systematically to HEMFCs, although there is a difference between limiting current behavior of PEMFCs and HEMFCs. We have found this technique useful to screen catalysts, optimize catalyst loading and adjust ionomer loading to improve the efficiency of gas diffusion, which is very helpful when switching from oxygen to air on cathode. Our optimized HEMFC can achieve a peak power density of 2.02 W cm-2 in H2/O2 and 1.33 W cm-2 in H2/air with platinum group metal (PGM) based catalysts (shown in Figure 2). The current density at 0.663 V for cell voltage is about 1.70 A cm-2, which is suitable for hydrogen fuel cell vehicles. References B. P. Setzler, Z. Zhuang, J. A. Wittkopf, and Y. Yan, Nat Nanotechnol, 11 (12), 1020-1025 (2016). J. Wang, Y. Zhao, B. P. Setzler, S. Rojas-Carbonell, C. Ben Yehuda, A. Amel, M. Page, L. Wang, K. Hu, L. Shi, S. Gottesfeld, B. Xu, and Y. Yan, Nature Energy, (2019) doi:10.1038/s41560-019-0372-8. Figure 1

Vanadium redox flow batteries (VRFBs) offer a scalable, long lasting, highly efficient means of energy storage and are a good option for large scale energy storage systems that can work on hand with renewable energy sources such as wind and solar farms. The proton exchange membrane that separates the negative and positive electrodes is a fundamental part of the system, since it allows the diffusion of H+ ions between electrodes to make possible the oxidation/reduction reactions in the half cells.1 In VRFBs the membrane electrolyte is desired to have high proton conductivity, low vanadium permeability to avoid battery discharge caused by vanadium crossover and good durability. The most common membrane used in VRFBs has been Nafion due to its success in fuel cells. Previous work in our laboratory suggests that other membranes offer improved performance.2,3 However, in a VRFB the working environment differs greatly from that of a fuel cell, for example, the membrane can adsorb electrolyte species other than protons and the high electrolyte concentration will tend to dehydrate the membrane. These conditions can substantially affect the membrane’s conductivity as well as other transport properties. In the flow battery, the trade-off between membrane conductivity and species transport across the membrane is key and is largely driven by the tendency of the membrane to imbibe different solution components as well as their intrinsic mobility. Our group has developed a core set of measurements for membrane characterization that includes conductivity, porosity, acid, water and vanadium uptake, and vanadium permeability.2,4 In this work we studied a series of 3M ionomer membranes, conditioned in solutions of different concentrations of sulfuric acid and vanadium. Membranes were tested as received and also after boiling them in water to expand the polymer channels by hydrating the ionic clusters.5 The effect of channel expansion in the polymer electrolyte was studied as function of the membranes equivalent weight. In addition, a set of membranes was subjected to heat treatments near the polymer glass transition temperature, to determine the temperature effect on the membranes properties. Boiling the membranes dramatically increases their conductivity, porosity, water, acid and vanadium uptake, which is ideal for VRFBs, but also increases the vanadium permeability which is directly related to the undesired crossover. Figure 1, shows an example of how the membrane porosity, which is determined from density measurements obtained by gas pycnometry, is affected in a 3M825EW membrane. The boiled membrane shows higher porosity compared to that of the as received membrane. Higher porosity is an indication of the channels expansion in the membrane after boiling. Acknowledgements We would like to thank Dr. Greg Haugen and Dr. Tyler Matthews at 3M company for providing the membranes and funding for this study. References M. Sukkar, T., Skyllas-Kazacos, J. Memb. Sci., 222, 249–264 (2003). Z. Tang et al., J. Electrochem. Soc., 161, A1860–A1868 (2014). R. A. Elgammal, Z. Tang, C.-N. Sun, J. Lawton, and T. A. Zawodzinski, Electrochim. Acta, 237, 1–11 (2017) J. S. Lawton et al., J. Electrochem. Soc., 163, A5229–A5235 (2016). T. E. S. and S. G. Thomas A. Zawodzinski Jr., Charles Derouin, Susan Radzinski, Ruth J. Sherman, Van T. Smith, J. Electrochem. Soc., 140, 1041–1047 (1993). Figure 1