Acid Ionomer Research Articles

High Temperature Polymer Electrolyte Membrane Fuel Cells with High Phosphoric Acid Retention

Phosphoric acid loss poses immense hurdles for the durability of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Here we report quaternary ammonium-biphosphate ion-pair HT-PEMFCs that do not lose phosphoric acids under normal and accelerated stress conditions. Our energetics study explains the acid loss behavior of the conventional phosphoric acid-polybenzimidazole (PA-PBI) system by two mechanisms. If PA loss occurs via acid evaporation, the acid loss is constant over time. On the other hand, when water activity in the PA-PBI system is high, exponential decay of PA loss occurs via the water replacement mechanism. Combined 31P NMR and computational studies show that the proposed ion-pair system has six times higher interaction energy, which allows for containing all PAs in the membrane electrode assemblies under a broad range of operating conditions. In addition, polar interactions between the phosphonic acid ionomer and phosphoric acid explain acid retention in the electrodes of the ion-pair HT-PEMFCs.

Advanced Functional Hybrid Proton Exchange Membranes Robust and Conductive at 120 °C

AbstractProton‐exchange membrane fuel cell vehicles offer a low‐carbon alternative to traditional oil fuel vehicles, but their performances still need improvement to be competitive. Raising their operating temperature to 120 °C will enhance their efficiency but is currently unfeasible due to the poor mechanical properties at high temperatures of the state‐of‐the‐art proton‐exchange membranes consisting of perfluorosulfonic acid (PFSA) ionomers. To address this issue, xx designed composite membranes made of two networks: a mat of hybrid fibers to maintain the mechanical properties filled with a matrix of PFSA‐based ionomer to ensure the proton conductivity. The hybrid fibers obtained by electrospinning are composed of intermixed domains of sulfonated silica and a fluorinated polymer. The inter‐fiber porosity is then filled with a PFSA ionomer to obtain dense composite membranes with a controlled fibers‐to‐ionomer ratio. At 80 °C, these obtained composite membranes show comparable performances to a pure PFSA commercial membrane. At 120 °C however, the tensile strength of the PFSA membrane drastically drop down to 0.2 MPa, while it is maintained at 7.0 MPa for the composite membrane. In addition, the composite membrane shows a good conductivity of up to 0.1 S cm−1 at 120 °C/90% RH, which increases with the ionomer content.

Short side chain perfluorosulfonic acid composite membrane with covalently grafted cup stacked carbon nanofibers for polymer electrolyte fuel cells

Efficient proton conduction through polymer electrolyte membrane is critical in the performance of low temperature polymer electrolyte fuel cells (LT-PEFC). Herein, polystyrene sulfonic acid grafted cup stacked carbon nanofiber (PSSA-g-CSNF) incorporated short side chain (SSC)-perfluorosulfonic acid (PFSA) ionomer composite polymer electrolyte is discussed. An optimum level of PSSA-g-CSNF introduced as an additive to SSC-PFSA matrix exhibit higher proton conductivity, improved mechanical, chemical stability, and limited hydrogen crossover towards PEFC application in comparison to recast pristine SSC-PFSA membranes (Aquivion®-720). Maximum current density of 2.5 A cm−2 at 0.5 V and peak power density of 1.4 W cm−2 under H2 and O2 condition is seen for the optimized nanocomposite membrane. Besides, the peak power density of the nanocomposite membrane shows an enhancement higher than 22% and 42% in relation to recast pristine Aquivion and Nafion212 respectively. In addition to this, nanocomposite membrane shows higher chemical stability and retention of 80% initial OCV even after 100 h of accelerated stress test (AST) as compared to 73% for pristine Aquivion. Uniform dispersion of PSSA-g-CSNF in Aquivion facilitates facile proton transport by limiting fuel cross-over via hydrogen bonding interaction with closely spaced hydrophilic groups in the membrane matrix in LT-PEFC.

A Versatile Ionomer‐Based Soft Actuator with Multi‐Stimulus Responses, Self‐Sustainable Locomotion, and Photoelectric Conversion

AbstractThe prospects of endowing artificial robotics or devices with increasingly complex and emergent life‐like behaviors have attracted growing interest in the soft functional materials that mimic the versatile motions of living creatures in the iridescent nature. However, despite the flourishing achievements so far, soft actuators capable of sensitive multi‐stimulus responses and self‐sustainable movements, have been extensively pursued to reduce control complexity yet remains a challenging target. Here, through material‐structural synergistic design incorporating stress‐mismatching structure, high pseudo‐negative coefficient of thermal expansion of perfluoro‐sulfonic acid ionomer, comprehensive converting properties of carbon nanotube, and anisotropic large thermal expansion of PE polymer, an ionomer‐based bilayer actuator is proposed, presenting high‐performance actuation of various forms and nice stability, responsive to light (including sunlight without focusing, LED light), low voltage, mild heating, and humidity/solvent change. With a built‐in structural feedback loop, the actuation performances are further explored to realize intelligent systems, including: 1) self‐sustainable locomotion under sunlight irradiation with adjustable photophobic and phototropic direction as well as adaption to different topographies and loading conditions, 2) self‐sustainable oscillation and solar‐electric generating, and 3) bionic floristic reaction according to environmental change. These diversified actuation modes allow promising following‐up designs for bio‐hybrid soft robotics fueled by and harmonized with natural environments.

Molecular dynamics simulation of inhomogeneous density distribution mechanism of Ultra-Thin perfluoro sulfonic acid ionomer films on Triple-phase interface of PEM fuel cells

The proton exchange membrane (PEM) fuel cell is a clean power generation component with high energy conversion rate and high energy density. The triple-phase interfaces of catalytic layer are where the electrochemical reaction occurs in proton exchange membrane (PEM) fuel cell, and its distribution directly affect the reactivity. The triple-phase interface is a nano-thick perfluorosul-fonic acid ionomer (PFSA) film that covers the catalyst particles. When the density of the ultra-thin PFSA film is not uniform, the gas transport resistance increases significantly, which is not conducive to the progress of reaction. However, ultra-thin PFSA film will have inhomogeneous density distribution due to the influence of forming process. Therefore, how to design the process to improve density distribution of ultra-thin films is an urgent problem. Molecular dynamics simulation model is built to study the influence of manufacturing process on inhomogeneous distribution of ultra-thin PFSA film, and the microscopic mechanism is revealed.

Permeation of Ternary Mixture Containing H2S, CO2 and CH4 in Aquivion® Perfluorosulfonic Acid (PFSA) Ionomer Membranes.

Aquivion® E87-12S Perfluorosulfonated acid ionomer material (PFSA) has been studied as a membrane technology for natural gas sweetening from CO2, H2S due to its interesting chemical and mechanical stability and good separation performance for polar compounds in humid environments. In the present work, permeation of the H2S/CO2/CH4 ternary mixture in this short-side PFSA chain was investigated at pressures up to 10 bar, temperatures up to 50 °C, and in a range of relative humidity (RH) from 20% to 90%. The results obtained confirm the strong dependence of Aquivion® on water activity and temperature, and its ability to separate gases based on their water solubility without substantial differences between pure and mixed gas experiments. Indeed, even when tested in ternary mixture, the permeation behavior remains similar to that observed for pure components and binary mixtures. In particular, the permeability of H2S is higher than that of CO2 and methane CH4, reaching values of 500 Barrer at 50 °C and 80% RH, against 450 and 23 Barrer for the other two gases respectively. Additionally, when tested at higher pressures of up to 10 bar under humid conditions, the membrane properties remained largely unchanged, thus confirming the overall stability and durability of Aquivion® E87-12S in acid environments.

Analysis of Platinum Distribution within a Nafion 212 Membrane during Electroless Deposition

Ionic polymer metal composite (IPMC) is an electrically-responsive material made of perfluorinated sulfonic acid (PFSA) ionomers electroless plated with noble metal, such as platinum. Recently, interest in thin IPMC has been rising because of its potential applications in micro-electromechanical systems. However, little effort has been made so far on studying metal electroless deposition within thin PFSA membranes (thinner than 100 um). In this paper we study the platinum distribution within a thin Nafion membrane (Nafion 212, 50 µm-thick) using standard manufacturing recipe. We compare the obtained distribution with the distribution reported in literature based on thick Nafion membrane (Nafion 117, 180 µm-thick). We show that in Nafion 212 the maximum Pt concentration is situated in the bulk of the membrane, while in Nafion 117 the maximum peak is situated at the liquid-membrane boundary, suggesting a different rate determining deposition mechanism. Electroless deposition of platinum in Nafion 212 Ionic polymer metal composite (IPMC) is rising interest in the microelectromechanical system (MEMS) since smaller and thinner IPMC substrates would expand the applications. IPMCs are mostly made of a perfluorinated sulfonic acid ionomers (PFSA) membrane, such as commercially-available Nafion, coated with platinum. The prevalent way of manufacturing IPMC relies on the chemical reduction of platinum on the surface of the PFSA membrane, i.e. electroless deposition (1). Most of the reports on IPMC manufacturing rely on Nafion 117 (thickness of 0.180 mm, equivalent weight 1100 g/mol) while Nafion 212 has been poorly studied (thickness 0.050 mm, equivalent weight 1100 g/mol). This contribution aims at filling this gap.Materials and methodsA PFSA membrane (Nafion 212 membrane) has been coated with platinum using the standard recipe already describe elsewhere (1). An immersion-reduction step was performed in an immersion bath composed of tetraammineplatinum chloride (Pt(NH3)4Cl2, 0.2%), and reduction baths made of sodium borohydride (NaBH4, 5%). After the first immersion (30 min and 2 hours, respectively), the samples were immerged in the reduction bath until the end of the precipitation. Samples were then frozen in liquid nitrogen and cut, and analyzed with electrons dispersive x-ray spectroscopy (EDS) and processing of scanning electron microscopy (SEM) images (Fig 1a).Figure 1. a) EDS detection of platinum (yellow) overlapped with SEM picture of the cross-section of IPMC. Platinum distribution and Rayleigh fit for single reduction step (NaBH4) of platinum in a Nafion 212 membrane, after 30 minutes (b) and 2 hours (c) of immersion.Results and discussionThe distribution of platinum within Nafion 117 has already been studied by Millet et al. (2). The Pt distributions thereby reported strongly differs from the profiles we obtained (Fig 2). In the aforementioned profiles, the maximum Pt concentration is located at the liquid-substrate boundary of the Nafion membrane (2), whereas the Pt distribution in Nafion 212 reaches the maximum inside the Nafion membrane, between 0.5 µm and 1 µm below the surface (Fig 1b 1c).As reported by Millet et al., two limiting types of kinetics may be encountered in a dynamic exchange process, such that the deposition rate-determining mechanism is either diffusion in the liquid-film boundary (F-mechanism), or diffusion in the membrane (M-mechanism).The position of the peaks in our experimental data suggests that the mechanism determining the Pt precipitation rate inside Nafion 212 is diffusion inside the membrane while using Nafion 117 with the same reducing agent concentration leads to a precipitation rate determined by diffusion at the liquid/film boundary. It has been proven experimentally that ionic conductivity and water uptake of Nafion 212 is higher than for Nafion 117 (3). Thus we can hypothesize that the different behavior of thinner Nafion could explain the different empirical Pt distribution obtained using same reducing agent concentration. We could obtain a good fit of the Pt distribution (R2=0.919 and R2=0.945) within Nafion 212 with the Rayleigh distribution for immersion time of 30 minutes and 2 hours respectively (Eq 1.):f(x,σ 2) = (x/σ 2) exp(-x/2σ 2) [1]The scale parameter σ was found to be 4.19 and 4.58 for the sample immerged for 30 minutes and 2 hours, respectively. Further investigation of the precipitation mechanism inside Nafion 212 membrane and estimations of the phenomenological equations of mass transport correlated to Rayleigh distribution is reserved for future work.References K. Kim, M. Shahinpoor, Smart Mater. Struct., 12, 1, p. 65 (2003).P. Millet, R. Durand, E. Dartyge, G. Tourillon, and A. Fontaine J. Electrochem. Soc., 140, 5, pp. 1373-1380 (1993).A. Kusoglu, A.Z. Weber Chem. Rev. 117, 3 pp. 987-1104 (2017). Figure 1

The Ionomer Molecular Structure Effect in the PEFC Ink & Applications

The interaction between the perfluorosulfonic acid (PFSA) ionomer and platinum group metal (PGM) catalyst in polymer electrolyte fuel cell (PEFC) inks is a critical issue for designing high-performance PEFC electrodes. During the ink fabrication, the complex interparticle interaction of the ink components determines the agglomerate morphology and size distribution. Among the ink components, the ionomer, which has amphiphilic in nature due to its hydrophobic backbone and the hydrophilic ionic group, mostly effective parameter for interparticle interaction. Therefore, understanding the interaction and molecular structure of ionomer is required for a high-performance catalyst layer. The backbone length of the ionomer which controls the spacing between ionic pendant groups is the main parameter of the ionomer character. In this study, the three different ionic spacing ionomers, made in the Solvay industry: Aquivion D72-25BS, Aquivion D83-24BS, and D98-25BS, were used for investigating the adsorption behavior of the catalyst ink and PEFC catalyst layer morphology. During the fabrication of PEFC ink and catalyst layer, all samples have same sulfonic acid molality to avoid the ionic clustering effect in the side chain. Ex-situ measurements were carried out to quantify the interparticle interaction and agglomerate size distribution in the PEFC ink. The dynamic lights scattering (DLS) carry out to determine the aggregation size distribution. The rheological behavior was conducted to understand the interparticle interactions and the agglomeration behavior of the inks by steady-shear and dynamic-oscillatory-shear measurements. It is noted that increasing the ionomer backbone length, which favors interaction via hydrophobic interaction on a carbon support, suppresses the agglomeration of ink components and decreases ink viscosity. The in situ electrochemical tests were carried out to figure out the relationship between ink formulation and catalyst layer morphology. It was found that the long ionomer backbone length helps to suppress the O2 transport resistance and ORR kinetic loss by uniform distribution from ionomer backbone-carbon supports interaction.

(Invited) Biomolecular Engineering for Electrochemical Applications in Fuel Cells/Electrolyzers and Beyond

Control of ionomer thin film structures on metal surfaces is pivotal for efficient performance in electrochemical devices such as fuel cells and electrolyzers. Unfortunately, the assembly of these thin film structures is difficult to control using conventional methods and ideal arrangements remain unknown. Engineered polypeptides have emerged as powerful biomolecular tools in electrode assembly because binding sites and polypeptide structures can be easily modulated by changing the amino acid sequence. However, no studies have been conducted showing polypeptides can be engineered to interact with ionomers, attach them to metal surfaces, and control their arrangement. Our lab has recently developed this technology, using an elastin-like polypeptide to bind to metals, and bind to acidic and basic ionomer via ionic interactions. We use a quartz crystal microbalance with dissipation to provide detailed information about the loading, thickness, and binding behavior of the polypeptide and ionomer layers. We also use atomic force microscopy and grazing-incidence small-angle X-ray scattering to understand the impact of the biomolecules on ionomer phase separation. Finally, we have analyzed the performance (ionic conductivity) of the assembled films using interdigitated electrodes and electrochemical impedance spectroscopy. Through these techniques, we show that 1) our biomolecular system is highly flexible, easily adapting to different materials, 2) the polypeptide sequence can dictate ionomer phase separated structures, and 3) this system can be used to improve the performance of ionomer thin films and gain structure-function understanding. Generally, our results demonstrate that engineered polypeptides are promising tools for ionomer control in electrode engineering for fuel cells/electrolyzers and beyond. Furthermore, this work demonstrates that the creative combination of applied electrochemistry and biomolecular engineering supported and encouraged by leaders in each field can lead to unique research pathways that benefit heath, energy and the environment.

Evaluation of High Oxygen Permeability Ionomer (HOPI) Oxygen Permeability for Proton Exchange Membrane Fuel Cells (PEMFCs)

The performance of proton exchange membrane fuel cells (PEMFCs) is sensitive to Pt loading in the cathode catalyst layer, with lower Pt loadings significantly increasing the oxygen transport resistance of the catalyst-ionomer interface when using perfluorosulfonic acid (PFSA) ionomers. The oxygen permeability is suspected to be locally decreased at Pt interfaces due to ionomer densification caused by the Pt-ionomer interactions. However, due to the high cost of Pt, it is important to be able to reduce Pt loadings without increasing oxygen transport resistance. With this goal in mind, new precommercial high oxygen permeability ionomers (HOPIs) with blocky backbone units are being studied. These HOPIs show significantly lower local oxygen transport resistance in fuel cells as well as greater efficiency and durability. However, there was a question of whether HOPI’s higher in-situ oxygen permeability is due solely to reduced densification or if it is also due to increased bulk permeability of the films. Here we evaluated this question using a unique oxygen permeability measurement apparatus that measures the oxygen permeability of thin-films supported on mesoporous substrates. This apparatus allows films to be characterized without the polarized Pt surfaces used in other oxygen permeability diagnostics, providing insights into the bulk permeability of cast thin-films with minimized substrate effects. This presentation will present our findings of the oxygen permeability of pristine thin-film HOPIs characterized against that of the baseline PFSA ionomer. Results show that the HOPI is significantly more oxygen permeable than the baseline. This finding is supported by our molecular dynamics simulations of these ionomer films. This increased bulk film oxygen permeability indicates that a significant portion of the decreased oxygen transport resistance in the cathodes is associated with the greater bulk permeability of these films and not just local Pt interface effects.This material is based upon work supported by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office, Award Number DE-EE0008822.

Operando X-Ray Absorption Spectroscopic Study on Influence of Specific Adsorption of Sulfo Group in Perfluorosulfonic Acid Ionomer Towards ORR Activity of Pt/C Catalyst

Polymer electrolyte fuel cells (PEFCs) are clean energy devices with polymer electrolytes and are expected to play an important role in the development of a global renewable energy system and a pure hydrogen energy society in the future. PEFCs have already been applied for small-scale energy systems such as fuel cell vehicles (FCVs). Pt-based catalysts are basically used due to the relatively inefficient reaction kinetics and catalyst degradation under strong acidic conditions during the oxygen reduction reaction (ORR). However, there are still many difficulties in realizing acceptable cost-effective FCVs. It is known that the ORR activity of Pt-based catalysts is reduced by specific adsorption of anionic species. Perfluorosulfonic acid ionomers such as Nafion® are commonly included in the catalyst layers of PEFCs to provide protonic pathways for promoting the ORR1. However, direct observation of anion adsorption on practical Pt nanoparticle/C catalysts for perfluorosulfonic acid ionomers using X-ray absorption spectroscopy (XAS) still has less report. In this study, to achieve specific adsorption over a higher potential range that is at least equal to the ORR active potential (around 0.90 V vs. RHE), operando XAS measurements were performed; the electronic status and local structural change of Pt atoms induced by the specific adsorption of anions at any potential were observed2.Pt/C (29.1 wt.%; TEC10V30E) catalyst supported was purchased from TKK. Nafion® solution was employed to prepare Pt/C catalyst ink with I/C ranging from 0.0 to 1.0. And the amount of Pt/C was adjusted to form a 20 µgcarbon cm-2 catalyst layer after dropping 10 μL on a glassy carbon RDE. The electrochemical cell was constructed using a model electrode fabricated in the working electrode, a Pt mesh for the counter electrode, a reversible hydrogen electrode for the reference electrode, and 0.1 M HClO4 aq for the electrolyte. operando XAS measurement of Pt L-edge was carried out in SPring-8 (Japan).The specific activity decreased as the I/C ratio increased from 0.0 to 0.20. We utilized many methods to evaluate the adsorption species separately, including the measurements of ECSA and oxygen coverage, CO stripping voltammetry, operando X-ray absorption fine structure, and analysis of 5d orbital vacancy. The ECSA and oxygen coverage did not change with increasing ionomer content, indicating that the Pt/C catalyst activity was affected by other adsorption species. A comparison of the CO displacement charge and 5d orbital vacancies of the Pt/C catalysts with I/C = 0.0 and 1.0 suggests that the ionomer-specific adsorption increases when the I/C ratio of the Pt/C catalyst is 1.0; active sites on the surface of the Pt/C catalyst are occupied, resulting in lower catalyst activity. Acknowledgment This work was supported by the project (JPNP20003) and a NEDO FC-Platform project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Probing the Electrochemical Behavior of High-Temperature Ionomer Blends

In the past two years, tremendous improvements in high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs)(1) and HT-PEM hydrogen pumps(2) using membrane electrode assemblies containing ion-pair HT-PEMs(3, 4) and electrode phosphonic acid ionomer binders. A challenge in using phosphonic acid ionomers at high temperatures and under anhydrous conditions is their propensity to form anhydrides that compromise ionic conductivity(5). Poly(tetraflurostyrene phosphonic acid-co-pentafluorostyrene) (PTFSPA) is a good electrode ionomer binder material as it is less susceptible to anhydride formation when compared to poly(vinyl phosphonic acid) because of the electron-withdrawing fluorine in the styrene ring that increases the acidity of the phosphonic acid moiety. However, PTFSPA can still form anhydrides. Recently, Kim and co-workers blended PTFSPA with Nafion, a perfluorosulfonic acid (PFSA) material, as a strategy to mitigate anhydride formation(1). The sulfonic acid in PFSA was shown to protonate the phosphonic acid group in PTFSPA to foster proton conductivity. However, much is still not known about how the PFSA and PTFSA polymer blends affect the electrochemical properties of porous electrodes used in HT-PEM electrochemical systems. It is posited that PFSA may promote redox reaction kinetics because it is superacid material and kinetic rates are enhanced at extreme pH values. PFSA is also known to have higher gas permeability rates when compared to other polymer materials (6). This talk will present our research examining the electrochemical properties, and other thin-film properties, of PTFSPA-PFSA blends. PFSA materials will consist of Nafion and other PFSA analogous that have shorter side chains and lower equivalent weight values (e.g., Aquivion). Preliminary data has shown that Aquivion, which has a shorter side chain than Nafion, is more effective for promoting thin film ionic conductivity on interdigitated electrode (IDE) substrates under anhydrous (i.e., 0% RH) and 100% RH in PTFSPA-PFSA blends.

Parametric Study of the Influence of Support Type, Presence of Platinum on Support, and Ionomer Content on the Microstructure of Polymer Electrolyte Fuel Cell Catalyst Layers

Ultra-small angle X-ray scattering (USAXS) was employed to investigate the effects of carbon support type, the presence of platinum on carbon, and ionomer loading on the microstructure of polymer electrolyte fuel cell (PEFC) catalyst layers (CLs). Particle size distributions (PSDs), obtained from fitting the measured scattering data were used to interpret the size of carbon aggregates (40–300 nm) and agglomerates (>400 nm) from two-component carbon/ionomer and three-component platinum/carbon/ionomer CLs. Two types of carbon supports were investigated: high surface area carbon (HSC) and Vulcan XC-72. CLs with a range of perfluorosulfonic acid (PFSA) ionomer to carbon (I/C) ratios (0.2–1.2) and also with perfluoroimide acid (PFIA) ionomer were studied to evaluate the effect of ionomer on CL microstructure. The carbon type, the presence of platinum, and ionomer loading were all found to significantly impact carbon agglomeration. The extent of Pt/C agglomeration in the CL was found to increase with increasing ionomer and platinum concentration and to decrease with increasing carbon surface area. Platinum electrochemically-active surface area (ECSA) and local oxygen transport resistance (RnF) were correlated to the CL microstructure to yield relationships affecting electrode performance.

Self-Healability of Poly(Ethylene-co-Methacrylic Acid): Effect of Ionic Content and Neutralization.

Self-healing polymers such as poly(ethylene-co-methacrylic acid) ionomers (PEMAA) can heal themselves immediately after a projectile puncture which in turn lowers environmental pollution from replacement. In this study, the thermal-mechanical properties and self-healing response of a library of 15 PEMAA copolymers were studied to understand the effects of the ionic content (Li, Na, Zn, Mg) and neutralization percentage (13 to 78%) on the results. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and tensile testing were used to study the thermo-mechanical properties of PEMAA copolymers while the self-healing response was studied using the projectile test. Puncture sites were observed using scanning electron microscopy (SEM) and the healing efficiency was quantitatively measured using the water leakage test. Five different self-healing responses were observed and correlated to ionic content and neutralization. At high neutralization, divalent neutralizing ions (Zn and Mg) that have stronger ionic interactions exhibited brittle responses during projectile testing. PEMAA samples neutralized with Mg and Li at low concentrations had a higher healing efficiency than PEMAA samples neutralized with Zn and Na at low neutralization. The PEMAA copolymers with higher tensile stress and two distinct peaks in the graph of loss factor versus temperature that indicate the presence of sufficient ionic aggregate clusters had improved healing efficiency. By increasing the neutralization percentage from 20% to 70%, the tensile strength and modulus of the samples increased and their self-healability generally increased. Among the investigated samples, the copolymer with ~50% neutralization by Li salt showed the highest healing efficiency (100%). Overall, the strength and elastic response required for successful self-healing responses in PEMAA copolymers are shown to be governed by the choice of ion and the amount of neutralization.

Effect of ionomer dispersions on the performance of catalyst layers in proton exchange membrane fuel cells

In this study, various ionomer dispersions having different equivalent weights, length of side chains, and dispersing solvents have been extensively analyzed to investigate the effect of the microstructure of catalyst layers on the performance of membrane-electrode assembly in proton exchange membrane fuel cells. Five different commercially available perfluorinated sulfonic acid ionomer dispersions have been used to prepare laboratory-made propylene glycol-based ionomer dispersions. The thickness of self-assembled ultra-thin films on silicon wafer for all types of ionomer dispersions has been measured and correlated mainly with the type of dispersing solvents (i.e., water, 2-propanol, and propylene glycol), the length of ionomer side chain, and the wettability. As a result, the short side chain and the propylene glycol-based ionomer dispersions form thicker films due to the smaller average size of their ionomer aggregates. The thick-ionomer wrapped catalyst layers especially prepared by the propylene glycol-based ionomer dispersions result in no influence on activation loss but approximately 42% less Ohmic loss and 43% higher limiting current density in proton exchange membrane fuel cells. In addition, it is confirmed that the slightly higher performance is exhibited as the thickness of ultra-thin ionomer film increases due to the effect of the length of side chains. The catalyst layers with the ionomer film over 100 nm show the higher performance than those below 100 nm. The MEA using Nafion D2020 having hydrophobic surface shows the lower increasing ratio (2.26%) in HFR than the one using Aquivion D98-25BS having hydrophilic surface due to the difference of the water-retaining ability.

(Invited, Digital Presentation) Nanostructured Thin Film (NSTF) Iridium Catalyst Powder for Proton Exchange Membrane Water Electrolyzers

Proton exchange membrane water electrolyzers (PEMWEs) are electrochemical devices which generate hydrogen (H2) gas from water and electrical energy feedstocks. PEMWEs produce H2 renewably and carbon-free when the electricity is from renewable sources, and are a pathway to enable deep decarbonization across multiple industrial and energy sectors[1]. However, commercial deployment of PEMWEs is currently limited to megawatt-scale due to relatively higher H2 production costs and capital costs than hydrocarbon reforming [2]. The higher costs are due in part to the use of significant quantities of expensive materials (Pt and Ir electrocatalysts and perfluorinated ionomers), insufficient operating performance and durability, and high manufacturing costs. Additionally, commercial PEMWEs additionally use high Ir loadings [3] for the oxygen evolution reaction (OER), and the limited abundance of Ir [4] may limit PEMWE annual deployment of those technologies to gigawatt (GW) scale.3M Nanostructured Thin Film (NSTF) PEMWE OER powder catalysts and electrodes are a unique approach to address the cost and Ir utilization barriers noted above. NSTF catalysts [5] are comprised of nm-scale catalyst metal thin films on a high aspect ratio inert support (Fig. A). NSTF Ir OER catalysts enable high efficiency and high durability due to high OER mass activity and intrinsic resistance to dissolution, imparted by the unique agglomerated thin film catalyst structure. NSTF OER electrodes [6] consist of a dispersed matrix of NSTF catalyst powder particles within a perfluorosulfonic acid (PFSA) ionomer binder (Fig. B), which have high catalyst utilization due to the high electronic conductivity of the primary catalyst particles.One of the key challenges associated with development of OER catalysts and electrodes is the lack of qualified accelerated stress tests (ASTs) to enable rapid assessments of durability under conditions relevant for end-use. The challenge is in part magnified by the long lifetime requirements of 80,000 hours and low required decay rates of single microvolts per hour, which traditionally has required long testing times and multiple replicates to obtain needed statistical significance. Additionally, evaluations of durability have often occurred under steady state testing with fixed current densities, which do not reflect anticipated use profiles when integrated with renewables such as wind and solar with significant power production variability over time. Lastly, operation at increased stack power densities is considered a key strategy to reduce stack capital costs and Ir requirements on a gram per kW basis.In this paper, we will report recent work on our durability assessment of NSTF OER powder catalysts and electrodes under aggressive testing protocols with low catalyst loadings relevant for PEM electrolyzers at large scale. Assessments included steady state durability tests, an accelerated stress test, and a protocol intended to simulate integration with a wind variable renewable energy (VRE) load profile. An example of results from the wind VRE protocol are summarized in Figs. C and D. The wind VRE protocol generated by Alia et al. [7] was modified from voltage control to current control and the maximum current density was scaled to 4.5A/cm2. The protocol was applied to a 3M laboratory CCM comprising a 0.20 mg/cm2 of 78wt% Ir/NSTF powder catalyst OER electrode, 0.09 mg/cm2 of 78wt% Pt/NSTF powder hydrogen evolution reaction (HER) electrode, and a 100 micron thick PEM (800EW 3M PFSA). After 500 hours of the wind VRE protocol, the cell performance was essentially unchanged (1mV voltage decrease at 2A/cm2). S. Dept. of Energy “H2@Scale”, https://www.energy.gov/eere/fuelcells/h2scale.S. Dept. of Energy H2USA Model, Current Forecourt Hydrogen Production v. 3.101.Ayers et al., Catalysis Today 262 121-132 (2016).Babic et al., Electrochem. Soc. 164 F387 (2017).Debe et al., ECS Trans. 45(2) 47-68 (2012).Steinbach et al., 2019 U.S. DOE Annual Merit Review, Project ta026.Alia et al., Electrochem. Soc. 166 F1164 (2019). Figure 1

(Invited) Engineering Polypeptides for Controlled Assembly of Ionomer Thin Films: Expanding Beyond Typical Assembly Tools

Control of ionomer thin film structures on metal surfaces is pivotal for efficient performance in electrochemical devices such as fuel cells and electrolyzers. Unfortunately, the assembly of these thin film structures is difficult to control using conventional methods and ideal arrangements remain unknown. Engineered polypeptides have emerged as powerful biomolecular tools in electrode assembly because binding sites and polypeptide structures can be easily modulated by changing the amino acid sequence. However, no studies have been conducted showing polypeptides can be engineered to interact with ionomers, attach them to metal surfaces, and control their arrangement. Our lab has recently developed this technology, using an elastin-like polypeptide to bind to metals, and bind to acidic and basic ionomer via ionic interactions. We use a quartz crystal microbalance with dissipation to provide detailed information about the loading, thickness, and binding behavior of the polypeptide and ionomer layers. We also use atomic force microscopy and grazing-incidence small-angle X-ray scattering to understand the impact of the biomolecules on ionomer phase separation. Finally, we have analyzed the performance (ionic conductivity) of the assembled films using interdigitated electrodes and electrochemical impedance spectroscopy. Through these techniques, we show that 1) our biomolecular system is highly flexible, easily adapting to different materials, 2) the polypeptide sequence can dictate ionomer phase separated structures, and 3) this system can be used to improve the performance of ionomer thin films and gain structure-function understanding. Finally, alternative ionomer assembly techniques are discussed for future exploration. Generally, our results demonstrate that engineered polypeptides are promising tools for ionomer control in electrode engineering.

Effects of Reinforcement Type on the Structure and Properties of Perfluorosulphonic Acid Membranes for Polymer Electrolyte Membrane Fuel Cells

Fuel cell membrane durability remains one of the key challenges limiting the wide scale adoption of fuel cell technology. Membrane degradation in polymer electrolyte membrane (PEM) fuel cells limits the operational lifetime of the fuel cell and prevents the industrial targets from being met. A common approach to increasing the lifetime of membranes includes the addition of a reinforcement layer to increase the mechanical durability of the membrane while not adversely affecting the performance [1,2]. Although reinforced membranes are widely used in industry, there is a literature gap considering membrane structure and properties in relation to durability. This work contributes to characterizing the effects of reinforcement type on the membrane properties. In this study a selection of perfluorosulphonic acid (PFSA) ionomer membranes with expanded polytetrafluoroethylene (ePTFE) reinforcements were tested, these include two novel DMR100 membranes with different reinforcements and Nafion XL compared to a conventional, non-reinforced Nafion NRE-211 for reference. All of these membranes contain common PFSA ionomer and differ primarily in the type of reinforcement layer and membrane thickness. The study addresses comparison between the membrane chemical composition, water uptake, crystallinity, and mechanical strength. Methods of ex situ characterization include solid state nuclear magnetic resonance (SS_NMR), small angle X-ray scattering (SAXS), wide angle X-ray scattering (WAXS), Fourier transform infrared spectroscopy (FTIR), and dynamic mechanical analysis (DMA). Membrane chemical structure and properties are measured by SS-NMR and FTIR, whereas the water uptake, domain spacing, and crystallinity are assessed by SAXS/WAXS study of hydrated and dry membranes [3-5]. Tensile mechanical properties are measured under room temperature (23°C, 50% RH) and fuel cell conditions (80°C, 90% RH) by DMA [6]. Overall, this paper contributes both qualitative and quantitative understanding of the key structural properties of membranes with different reinforcement resulting in novel knowledge about membranes that can be leveraged for improved fuel cell durability.

(Invited) Catalyst Layers for Fluorine-Free Hydrocarbon PEMFCs

Electrochemical energy conversion devices such as fuel cells are crucial for the development of renewable, sustainable alternative energy vectors. The implementation of fuel cell technology is of particular interest in portable, transportation, and stationary energy conversion sectors. There are challenges that still impede fuel cell commercialization at scale: cost, performance, and durability of the membrane-electrode-assemblies (MEA)s that comprise fuel cell stacks. MEAs typically consist of catalyst layers (CL) coated on either side of a proton-exchange membrane (PEM), and gas diffusion layers (GDL) which control water management and reactant gas mass transport. The CL provides a path for proton, electron, and gas transport to (and from) the Pt catalyst surface. Commonly, a perfluorosulfonic acid (PFSA) ionomer, e.g., Nafion® is employed as both the proton exchange membrane (PEM) and ionomer. However, perfluorinated materials are not without challenges as their manufacture utilizes controlled substances and potentially environmentally hazardous chemical feedstocks, which add to their relatively high cost. In contrast, fluorine-free hydrocarbon-based analogues may be prepared from ubiquitous, readily available chemical feedstocks, and afford much-reduced gas permeability. In this presentation, highly performing (power densities of >1 W cm-2), sulfo-phenylated poly(phenylene)PPB-H+ solid polymer electrolyte will be reported as a complete substitute for incumbent PFSA materials in PEMFCs, serving both as the electrode catalyst binder, and membrane.

Influence of Carbon Support Type, Presence of Platinum and Ionomer Content on Agglomeration in Catalyst Layers

Polymer electrolyte fuel cell (PEFC) energy conversion systems have found application in transportation applications due to their high power density, low operating temperature, high energy conversion efficiency, and speed of refueling.1 One of the key components of a PEFC is the membrane electrode assembly (MEA), which consists of a proton exchange membrane (PEM), such as a perfluorosulfonic acid (PFSA), sandwiched between two catalyst layers (CLs) serving as anode and cathode. Several studies of CL microstructure−performance relationships suggest that the electrode microstructure characteristics, such as the extent of the catalyst−ionomer interaction, continuity of the ionomer phase, and pore size distribution are critical for performance.2-4 However, successful implementation of novel materials for PEFC electrodes is hindered by the lack of a fundamental understanding of the complex, multicomponent nature of the CL, the interplay of its components, and their independent and cooperative effects on the effective properties and performance.In this work, ultra-small angle X-ray scattering (USAXS) was employed to investigate the effects of carbon support type, presence of platinum, and ionomer loading on catalyst agglomeration in CLs. Particle size distributions (PSDs), obtained from fitting the scattering data using the maximum entropy (MaxEnt) method, were used to determine the size of carbon aggregates and agglomerates from carbon-ionomer and platinum-carbon-ionomer CLs. Three types of carbon supports were chosen for the investigation: high surface area carbon (HSC), Vulcan XC-72, and graphitized Vulcan XC-72. CLs with a range of perfluorosulfonic acid (PFSA) ionomer loadings (0.2-1.0 ionomer to carbon ratio, I/C) were studied in order to evaluate the effect of ionomer on the CL agglomerate structure. The CL agglomerate structure will be correlated with electrochemical performance to reveal the interplay of the components, and their independent and cooperative effects on performance. References B.D. McNicol, D. A. J. Rand, K. R. Williams, J. Power Sources 100 (2001) 47–59.F. C. Cetinbas, X. Wang, R. K. Ahluwalia, N. N. Kariuki, R. P. Winarski, Z. Yang, J. Sharman, and D. J. Myers, J. Electrochem. Soc., 164, (2017) F1596–F1607.Y.-C. Park, H. Tokiwa, K. Kakinuma, M. Watanabe, M. Uchida, J. Power Sources 315 (2016) 179-191.S. Holdcroft, Chem. Mater., 26 (2014) 381−393. This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office under the auspices of the Fuel Cell Performance and Durability Consortium (FC-PAD). The Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science User Facility. Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, also under contract DE-AC-02-06CH11357.