Ionomer Loading Research Articles

Layered PGM-Free Electrode for Improved Mass Transport

Increased interest in platinum group metal-free (PGM-free) catalysts as a low-cost replacement for PGM catalysts for the oxygen reduction reaction (ORR) has resulted in significant progress in activity and durability over the past decade (1-4). However, PGM-free catalysts still require high catalyst loadings to generate an acceptable level of performance due to their low active site density and turnover frequency. The thick catalyst layers suffer from both high ionic resistance and gas-transport limitations. Consequently, improvements to the morphology of PGM-free electrodes is as important to enhancing the ORR performance as is catalytic activity. Previously, Yin et al. were able to improve PGM-free electrode performance by developing a new fabrication technique for PGM-free membrane electrode assemblies (MEAs) and by optimizing ionomer loading and equivalent weight (5). Previous studies by Komini Babu et al. suggested that more substantial advancements in fuel cell performance could be accomplished by improving water management in the conventional PGM-free electrode (6, 7). Conventionally thick PGM-free electrodes suffer from mass transport resistance and challenging water management, limiting high catalyst utilization to only near the polymer membrane (6). Therefore, in order to increase catalyst utilization improved water management can be achieved by decreasing hydrophilicity in the catalyst layer (CL) near the microporous layer (MPL), which can be achieved by reducing the ionomer content or by replacing the low equivalent weight (EW) ionomer with a high-EW. In this work, we introduce PGM-free cathodes with improved high-current performance (mass transport-controlled range of current densities) by depositing two catalyst layers with varied ionomer loading (IL) or equivalent weight (EW) in order to decrease hydrophobicity near the MPL. For example, the EW effect was studied by preparing a MEA with two cathode layers consisting of 55 wt.% IL with EW of 1100 near the MPL and EW of 830 near the membrane. A different MEA with varying IL layers was prepared using 830 EW ionomer with 55wt. % IL near the membrane and 35 wt.% IL near the MPL. Our preliminary results (Figure 1) show that the MEA with the layered PGM-free cathode demonstrated improved performance when compared to the conventional single layer MEA, with the highest performance observed from the IL layered MEA. Acknowledgments This research was supported by DOE-EERE’s Fuel Cell Technologies Office (FCTO) through Electrocatalysis Consortium (ElectroCat). References G. Wu, K. L. More, C. M. Johnston and P. Zelenay, Science, 332, 443 (2011).H. T. Chung, D. A. Cullen, D. Higgins, B. T. Sneed, E. F. Holby, K. L. More and P. Zelenay, 484, 479 (2017).U. Martinez, S. Komini Babu, E. F. Holby, H. T. Chung, X. Yin and P. Zelenay, Advanced Materials, 1806545 (2019).U. Martinez, S. K. Babu, E. F. Holby and P. Zelenay, Current Opinion in Electrochemistry (2018).X. Yin, L. Lin, H. T. Chung, S. Komini Babu, U. Martinez, G. M. Purdy and P. Zelenay, ECS Transactions, 77, 1273 (2017).S. K. Babu, H. T. Chung, P. Zelenay and S. Litster, Journal of The Electrochemical Society, 164, F1037 (2017).S. Komini Babu, H. T. Chung, P. Zelenay and S. Litster, ACS Applied Materials & Interfaces, 8, acsami.6b08844 (2016). Figure 1

Comprehensive Characterization of PGM-Free PEM Fuel Cells Using AC and DC Methods

In the past decade, a lot of efforts have been done to develop a platinum group metal-free (PGM-free), cathodic materials for proton exchange membrane fuel cells (PEMFCs) as alternative to classic Pt/C cathodes [1, 2]. At present, Fe-N-C systems are considered as one of the most promising catalysts for oxygen reduction reaction (ORR) with peak power density of about 0.5 W cm-2 [3]. The key problems in Fe-N-C electrodes relate to ORR mechanism which can proceeds through a combination of 2- and 4-electron pathways depending on operating current density, and to mass transport processes occurring in relatively thick electrodes structure. Herein, we report comprehensive study of Fe-N-C electrocatalyst integrated into cathodic catalyst layers of PEMFC with various composition configurations using AC and DC methods. We fit the physics-based impedance model, which takes into account oxygen transport in the cathode catalyst layer (CCL), gas diffusion layer (GDL) and flow-field channels to the experimental impedance spectra, and obtain main kinetic (Tafel slope) and transport parameters (proton conductivity and oxygen mass transport coefficients) of PGM-free PEMFCs in application-relevant conditions. Electrochemical evaluation has been performed using a segmented cell system and a test station developed at Hawaii Natural Energy Institute. Membrane electrode assemblies (MEAs) consisted of Fe-N-C (with varied catalyst and ionomer loadings) cathode gas diffusion electrodes (GDE), Pt anode GDE (0.1 mgPt cm-2, Alfa Aesar) and Nafion XL. The MEA was operated under galvanostatic control of the whole cell current, 80°C, 304 kPa back pressure, 100% relative humidity for the both electrodes and varied cathode stoichiometry. Details of the impedance model used in the present work have been reported in [4]. Fig. 1 a) shows a representative SEM image of the Fe-N-C GDE cross-section. The cathode thickness was found to be 120-140 μm for a sample with 3.0 mg cm- 2 catalyst loading (Fig. 1 a), while a catalyst content to 1.5 mg cm-2 caused appropriate decrease in the thickness to 70 μm. The independence of electrode thickness from ionomer content (35 vs 45 wt. %) may indicate the preferential distribution of Nafion® inside of large pores of Fe-N-C catalyst synthesized by sacrificial support method. The results obtained from AC-impedance and IV data clearly showed that the largest problem of all the electrodes is high absolute value of the kinetic Tafel slope, which varies from 100 to 250 mV/dec for current densities in the range of 0.025 to 0.4 A cm- 2. This parameter is responsible for faster decay of the cell polarization curve as compared to Pt/C-based PEMFCs. The mean proton conductivity σ p and CCL oxygen diffusivity Dox increase with the cell current density by a factor of ten (Fig. 1 b, c). Both effects take place also in Pt/C systems [5]; however, the absolute value of Dox in the Fe-N-C electrodes is an order of magnitude higher, than in the Pt/C cathodes (0.5-2.5×10- 4 cm2 s-1) [6]. The growth of σ p with current partly is due to accumulation of liquid water in the CCL; however, it may also be due to ionization of acidic molecules on carbon surface at lower cell potentials. A comparison of different CCL configurations shows that each catalyst loading value requires its own optimum ionomer content in the catalyst layer. Detailed analyses of the PGM-free PEMFC performance and its correlation with AC-impedance results will be presented and discussed. ACKNOWLEDGEMENTS We gratefully acknowledge ONR (N00014-18-1-2127), DOE EERE (DE-EE0008419) and Hawaiian Electric Company. The authors are thankful to Rohan Gokhale for preparing some GDE samples, Tina Carvalho for assisting with SEM, Günter Randolf and Jack Huizingh for valuable help in the system operation. REFERENCES S.T. Thompson, A.R. Wilson, P. Zelenay, D.J. Myers, K.L. More, K.C. Neyerlin, D. Papageorgopoulos, Solid State Ionics 319, 68-69 (2018).U. Martinez, S.K. Babu, E.F. Holby, P. Zelenay, Current Opinion Electrochem, 9, 224-232 (2018).A. Serov, K. Artyushkova, E. Niangar, C. Wang, N. Dale, F. Jaouen, M.-T. Sougrati, Q. Jia, S. Mukerjee, P. Atanassov, Nano Energy, 16, 293-300 (2015).A. Kulikovsky, J. Electroanal. Chem., 669, 28-34 (2012).T. Reshetenko, A. Kulikovsky. J. Electrochem. Soc., 163, F1100-F1106 (2016).T. Reshetenko, A. Kulikovsky, J. Electrochem. Soc., 164, F1633-F1640 (2017). Figure 1

Multiscale Modeling, Design, and Efficiency Analysis of High-Pressure Hydrogen Water-Splitting System

To increase the energy density of hydrogen fuel, a common practice is through the production of high pressure hydrogen (up to 70 MPa).1-4 Furthermore, to ensure green or renewable hydrogen, water electrolysis from solar or wind resources is an attractive pathway, yet such technology has limitations on efficiency, especially when trying to deliver high-pressure hydrogen. Increasing electrolysis efficiency at the cell and system level requires the use of advanced membranes that are thin and have high proton conductivity. However, such thin membranes are more prone to mechanical failure and increased hydrogen crossover when under a large pressure gradient. An intriguing possibility to solve this is to couple the electrolysis directly with an electrochemical compressor. In this fashion, a thin membrane can be used in electrolysis as a lower pressure differential is required. Moreover, the electrochemical compressor has less issues related to noise and scalability that are inherent with mechanical compression. Thus, a combined, optimized system could be deployed hydrogen refueling stations or at larger installations. In this work, we will explore the tradeoffs and opportunities for electrolysis and compressor system including detailed analysis of the operating physics of both devices using multiscale mathematical modeling. The two devices are modeled together by utilizing the output conditions from electrolysis submodel as the input for hydrogen compressor submodel. Of importance is examining the possibilities for advancements in the cell components such as strong, conducting membrane materials that inhibit crossover and optimizing for multiphase flow. Such latter phenomena are accounted for using a multiscale modeling approach where the microstructure effect on transport properties is considered. The structural information such as pore-size distribution and tortuosity is obtained from X-ray computed tomography (CT) data, which is also used to validate some of the model predictions under operando conditions. Based on the X-ray data, the microstructure of the porous transport layer has been reconstructed and two-phase fluid simulations have been carried out to predict effective transport properties that are incorporated into the marcoscale model. Lastly, the porous electrode parameter optimization such as ionomer loading, porosity, and thickness, is introduced to identify critical areas for improvement at the cell and system levels for better efficiency and less energy consumption. Acknowledgements The authors would like to thank Melissa Bosch, Jonne Konink, Martijn Mulder, and Denise Swanborn for insightful discussions, and Bryan Pivovar and Kelly Meek for their work on the EOD determination. This work was jointly funded by a CRADA with HyET Hydrogen and under the HydroGen Consortium of the U.S. Department of Energy, Fuel Cell Technologies Office under contract number DE-AC02-05CH11231. M. Bernt and H. A. J. J. o. T. E. S. Gasteiger, 2016, 163, F3179-F3189.U. Babic, M. Suermann, F. N. Büchi, L. Gubler and T. J. Schmidt, Journal of The Electrochemical Society, 2017, 164, F387-F399.M. Carmo, D. L. Fritz, J. Mergel and D. Stolten, International Journal of Hydrogen Energy, 2013, 38, 4901-4934.K. E. Ayers, E. B. Anderson, C. Capuano, B. Carter, L. Dalton, G. Hanlon, J. Manco and M. J. E. T. Niedzwiecki, 2010, 33, 3-15.

Analysis and Optimization of Transport Losses in Hydroxide Exchange Membrane Fuel Cells

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

Investigation of the effect of carbon-covering layer on catalyst layer in polymer electrolyte membrane fuel cell in low relative humidity condition

The application field of the fuel cell system has recently expanded to portable devices and unmanned aircraft systems, which require development of high performance fuel cell systems that function in versatile environments including near-dry operation conditions. In such low relative humidity condition, hydration of membrane electrode assembly in the fuel cell system is essential in prevention of performance degradation caused by decreased ion conductivity of the Nafion® membrane. Herein, we successfully improved the device performance of the membrane electrode assembly in low relative humidity condition by depositing a functional carbon-covering layer onto the catalyst layer on the cathode side. The carbon-covering layer plays an effective role of retaining generated water in the membrane electrode assembly without interfering with the electrochemical reaction of the catalyst as confirmed by the electrochemical impedance spectroscopy and cyclic voltammograms. Moreover, we have investigated the optimized Nafion® ionomer loading and the thickness of the carbon-covering layer.

Measurement of Ionic Conductivity of PEM Water Electrolyzer Electrodes

In this paper, the hydrogen pump technique is used to study the proton-transport resistance of polymer electrolyte membrane water electrolyzer electrodes. Three catalyst coated membranes made by sandwiching two membranes together were prepared, with two of them including an intermediate pseudo catalyst layer (PCL) with 35 and 55 %wt. ionomer loadings. The proton-transport resistance was calculated by subtracting the overall resistance of the cell without a PCL from that with a PCL. The effect of the ionomer loading on the PCL proton conductivity was studied. As expected, the proton conductivity increased with increasing ionomer loading. The results are in line with the expectation based on the literature data and show that the hydrogen pump technique can be used to obtain the proton-transport resistance of the electrodes.

PGM-Free Electrode Development and Optimization Using H2 Limiting Current

PEMFCs with platinum group metal (PGM) catalysts can meet performance targets, but the main obstacle for widespread commercialization is the high cost. Alternatively, precious group metal-free (PGM-free) catalyst can meet cost targets and have the potential to reach DOE performance goals. However, successful PGM-free PEMFC development must include device-level studies and microstructure optimization, especially for minimizing the concentration overpotential in the thick PGM-free electrode layers. To date, limited work has been done to decouple the gas phase transport losses from those associated with stability1, active-site density2 and ion transport at the MEA level. Elucidating the voltage loss contributions or resistances stemming from gas transport and proton conduction at the MEA level along with subsequent correlation to MEA performance will help identify phenomena currently limiting the inception of PGM-free electrode performance and identify paths to improved electrode optimization. Baker et al. and Beuscher first suggested the use of limiting current for mass transport characterization in 2006.3,4 The technique has been developed over the past decade for operando measurement, deconvolution, and quantification of the discrete contributions to the concentration overpotential, often expressed as a mass transport resistance, arising from constituent materials and cell components.5,6 However, this method can be difficult to apply to PGM-free based electrodes, where ohmic contributions may dominate over transport phenomena. In this condition, hydrogen could be used as the molecular probe for mass transport resistance instead of oxygen. Spingler et al. first attempted the use of hydrogen as a probe molecule for PGM based electrodes.7 In this work, we integrated a platinum black (PtB) sensor layer into the MEA., enabling in situ determination of mass transport resistance throughout the entire PGM-free catalyst layer. These experiments were performed using a 5 cm2 differential cell, operated at differential conditions in all experiments such that the reactant concentration gradient down the channel was negligible. The total transport resistance ( ) is written in Equation 1 and was measured for various PGM-free electrodes with different ionomer loadings. Rtotal = nFCH2, channel / id Here, we will present 1) a limiting current measurement approach to characterize in situ mass transport resistances in fuel cell materials through bulk catalyst layer at an MEA device level; 2) PGM-free catalyst performance with different ionomer loading in the cathode; 3) how to balance mass transport and ionomer loading to achieve the best FC performance. The results from this experimental approach not only offer the guidance of transport properties and the role of ionomer loading played in the PGM-free electrodes but also increase our understanding of achieving an optimized performance for a PGM-free catalyst at an MEA level. Improved device-level understanding of mass transport limitations in the catalyst coated layer can help accelerate deployment of low cost, PGM-free PEMFCs and be useful to guide ongoing research and development in both catalyst synthesis and membrane electrode assembly fabrication.

(Invited) Fuel Cell Electrodes – from Imaging to Quantification

Understanding how the composition, microstructure, and spatial distribution of the phases in fuel cell catalyst layers (carbon support, metal catalyst, ionomer and pores) affect the fuel cell performance and degradation is of crucial importance. This can only be achieved if the correlation between microstructural and compositional parameters, layer properties and performance are systematically investigated. Even though imaging using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) is typically performed for failure analysis of the catalyst layers to provide information on layer thickness changes, delamination, defects, pin-holes, Pt particles size distribution, etc., there are rarely attempts to systematically quantify microscopy and spectroscopy data and use it to correlate the microstructural and compositional parameters to the layers’ properties and performance. Recently advanced 3D imaging techniques such as electron tomography (ET), focused ion beam scanning electron microscopy (FIB-SEM), X-ray computed tomography (x-CT), and fast scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS) have been employed to provide answers on spatial morphology of the catalyst layers and compositional changes. These techniques show immense promise in providing valuable information from the nano- to macro-scale to understand phenomena underlying fuel cell operation and degradation. Still, utilization of the obtained 3D data sets to provide practical and useful microstructural and compositional parameters requires efforts and further development in data processing, quantification and mathematical analysis. In this work, we are presenting a comprehensive approach for 2D and 3D imaging and quantification of fuel cell electrodes, at several scales, and the correlation to their properties and performance: 2D imaging and analysis of catalyst layers (beginning of life and end of life) using STEM-EDS will be presented with a novel, practical approach to quantify a number of parameters, such as Pt loading, loss and distribution, ionomer loading and I/C ratio, layer porosity, and oxygen evolution reaction (OER) agglomerate size. The method enables 2D visualization of component distribution at a whole layer scale, as well as at an agglomerate scale. Quantification of changes after degradation of fuel cells offers very valuable learning about possible processes occurring during degradation - information that was not available until now. Challenges with the method and potential development will be presented as well. 3D imaging and quantification of catalyst powders and catalyst layers on multiple scales will be presented, correlatively utilizing ET, FIB-SEM, and x-CT microscopy. Spatial distribution of all phases – Pt catalyst, carbon support, ionomer and pores will be revealed. A number of microstructural parameters will be quantified from the 3D data sets. Direct numerical simulation of the 3D data sets to obtain effective transport properties such as electrical and thermal conductivity, and gas diffusivity will also be reported. The approach shows promise for improved modeling, design and optimization of the fuel cell catalyst layers. Finally, the talk will conclude with the new capabilities and possible prospects for advanced imaging and quantification for further fuel cell development and commercialization.

Mitigation of Humidification Requirements for Proton Exchange Membrane Fuel Cells Using Inorganic Ligand Surface Chemistry on Platinum and Platinum Alloy Electrocatalysts

Computational and experimental investigations have previously revealed that the surface chemistry of metal nanoparticles used as fuel cell electrocatalysts can influence both wetting by water as well as the morphology of deposited solid polymer electrolytes. However, the impact of nanoparticle surface chemistry on practical fuel cell operation at high current load is unknown. Mass-transport and ohmic overpotentials are found to dominate performance at high load, hindering the impact of significant recent advances in electrocatalytic activity during practical operation. Prior investigations with operating fuel cells have typically focused on altering the distribution and content of ionomer in the electrode as well as the introduction of hydrophilic agents but often neglect the role of metal nanoparticle surface chemistry. In this investigation, we contrast the electrocatalytic and practical fuel cell performance of commercial carbon-supported Pt nanoparticles to carbon-supported and unsupported Pt and Pt-alloy nanoparticles synthesized via a metal-redox scheme that utilizes stabilizing inorganic ligands to alter the nanoparticle surface chemistry. 119Sn Mossbauer spectroscopy, X-ray Absorption, and X-ray Scattering are used to characterize ligand surface chemistry and structure. The catalysts are also characterized with respect to their Pt surface atom coordination using electrochemical stripping of Bi and Ge adatoms. In addition, the surface wettability of the various catalyst components (support, ionomer, metal nanoparticles) is differentiated using dynamic contact angle analysis. We determine that hydrogen/air fuel cell operation at high load is well-correlated to the surface chemistry of the nanoparticles. For a given surface atom coordination and ORR mass activity, catalysts produced via hydrophilic surface ligands permit the electrodes to operate under substantially drier conditions. Comparable single cell maximum power of 0.5 W/cm2 is realized at 40% inlet gas humidification for nanoparticles synthesized utilizing hydrophilic surface ligands versus 80% when an industrial precipitation approach is used for Pt nanoparticles ~3 nm, independent of ionomer loading. A remarkable swap in polarization behavior with humidification is observed. Correlations of fuel cell humidification requirements to alternate nanoparticle morphologies with varying surface atom coordination, inorganic ligand structures, and wetting behavior are described. Figure 1

Cathode catalyst layer design with gradients of ionomer distribution for proton exchange membrane fuel cells

Cathode catalyst layer (CCL) design is critically important for improved performance, durability and stability of proton exchange membrane (PEM) fuel cells. In this study, CCL is designed to consist of two sub-layers with different loadings of short-side chain (SSC) ionomer and platinum (Pt) catalyst for each sub-layer; and such CCLs are manufactured and characterized through morphological, microstructural, physical, and electrochemical characterizations, and the performance of single scaled-up cells containing such CCL designs is measured. The results show that the two-layer design results in a higher porosity and mean pore size, and hence higher air permeability, and provides distinctively higher catalyst activity and Pt utilization for the CCL than the conventional single-layer design under the same overall loadings of the ionomer and Pt for the entire CCL. It is also found that a higher ionomer loading in the first sub-layer (faced to the membrane) is beneficial to cell performance, and the Pt loading in this sub-layer plays a considerably important role as well. Overall, it is demonstrated that with a proper ionomer/Pt-gradient design, it is possible to achieve favorable morphological and microstructural characteristics that lessen ionic resistance while improving mass transport capability, catalyst activity, and Pt utilization, hence overall cell performance.

Electrospun carbon nanofiber catalyst layers for polymer electrolyte membrane fuel cells: Structure and performance

Carbon nanofiber-based fuel cell catalyst layers are prepared by electrospinning random and orthogonally-aligned structures using various structural and compositional parameters. Specifically, the influence of the level of fiber alignment, Platinum (Pt) loading, ionomer loading and distribution, deposition methods, and fiber support carbonization temperature on the support microstructure and fuel cell performance are studied and characterized by physicochemical methods. The effects of these controlled fabrication parameters were evaluated in-situ, specifically in the activation, ohmic, and mass transport overpotential regions. SEM/TEM observations reveals continuous fibrous conductive network formed by ionomer bridges between the fibers. It is found that ionomer/carbon of 0.158 with a thickness of ∼60 nm achieved the highest current density under normal operating conditions. Thicker ionomer layer resulted in high oxygen transport resistance, decreasing performance at high potentials. Carbon nanofiber (CNF) samples with 157 μg cm−2 Pt loading and I/C 0.158 achieve 5× higher current density compared to conventional V50 carbon black at 600 mV. Although the effect of orthogonally organized fibers is not clear, the results prove the advantage of using CNF as a catalyst support with tailorable material properties. Further process optimization may advance the CNF/Pt structure, while developing an understanding of the role of microstructure and organization.

Understanding Binary Interactions in Fuel-Cell Catalyst-Layer Inks

The catalyst layer (CL) in a fuel cell comprises the majority of the stack cost. Therefore, for mass commercialization of fuel cells, it is critical to understand the formation process of the CL, thereby informing the design and optimization of next-generation CLs. CLs are made from precursor inks that are colloidal dispersions of a catalyst (typically platinum on carbon), an ion-conducting polymer (traditionally perfluorinated sulfonic acid or PFSA, the most widely used of which is Nafion), and a solvent traditionally composed of different low-weight alcohols and water. Ink properties directly impact the resultant morphology, transport, and electrochemical properties of the CL.[1] While there has been much work on ink composition, the research to date has been almost entirely empirical, resulting in limited understanding of fundamental species interactions in the ink and their effect on the resultant CL structure. Furthermore, though the structure of nafion in solution has been extensively studied,[2-4] there has been little work on applying this information to decribe interactions with other ink components. In this study, we create a series of inks composed of commerical PFSA dispersions of different equivalent weights and side chains, a variety of solvents, and model compound particles. Effects of solvent composition, ionomer loading, and processing conditions are investigated on both the solution phase and the resulting cast solid layers including ionomer thin films and micrometer layers with and without solid particles. Using microscopy, rheology, and dynamic light scattering, we explore the aggregation behavior and fundamental forces governing the behavior of these ionomer/particle, particle/solvent, and solvent/ionomer interactions. Using these insights, design metrics and governing design rules are elucidated, which can help in fabrication of next-generation CLs. Acknowledgements This work was mainly funded under the Fuel Cell Performance and Durability Consortium (FC PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231. S.B also acknowledges support from National Science Foundation Graduate Research Fellowship under Grant No. DGE 1106400.

(Invited) Microstructural Modeling of PEFC Catalyst Layer Performance and Durability

Improving our understanding of the role that microstructure has on the performance and durability of polymer electrolyte fuel cell (PEFC) cathodes is critical to advancing this technology. Modeling is an important tool in interpreting imaging and experimental characterization of microstructure effects because it allows us to quantify the impact that specific microstructure characteristics have and also allows us to visualize processes and quantities that are otherwise inaccessible in the physical system. Various continuum-level and volume-averaging approaches are available to incorporate idealized microstructural effects, but they unfortunately are generally limited to highly idealized geometries and decoupled and simplified transport and reaction models. Herein, we will present recent advances in directly modeling transport, reaction, and degradation mechanisms in catalyst layers where a spatially-resolved multi-dimensional representation of the microstructure is used to capture the impact of anisotropic and heterogeneous features. In these works, the microstructure geometry is often generated through 3D computed tomography (CT), stochastic reconstruction, or a combination thereof. For example, high resolution scanning transmission electron microscope CT (STEM-CT) images can be combined with X-ray nano-CT images to stochastically reconstruct the catalyst layer microstructure with both electrode and catalyst length scales resolved. In addition, correlative methods of incorporating different imaging modalities can be used to resolve the properties and distribution of multiple material phases, including spatial ionomer loading heterogeneity in the cathode. In addition to advances in the field, this work will highlight progress in our group in applying microstructural modeling to both Pt/Pt-alloy and Pt group metal-free (PGM-free) catalyst cathodes. The modeling domains we consider in our work span from Pt particles embedded in high surface area carbon supports to transport and reaction across the electrode thickness. The modeling results range from characterizing microstructural effects on the effective transport properties for the bulk electrode to particle-scale modeling of catalyst metal dissolution and redistribution. Figure 1

Understanding Binary Interactions and Aging Effects in Catalyst Layer Inks for Controlled Manufacturing

The effect of perfluorosulfonated ionomer loading in carbon-based colloidal inks is investigated under static and dynamic operating conditions. The results show that both the solvent-type and polymer loading play a role in ink stability, aggregation, and aging. Dynamic light scattering and shear and creep rheological experiments show that the addition of a polymer to carbon inks can act as a stabilizing agent which increases steric repulsion and decreases the inks overall viscosity. As the polymer loading is increased above 30wt% a decrease of approximately 30 MPa-1 in compliance is observed. A decrease in compliance at high polymer loading suggests weak interactions between the underlying carbon materials and indicates that a critical polymer loadings may exist for different shear processing routes.

Steady state and transient simulation of anion exchange membrane fuel cells

We present a new model for anion exchange membrane fuel cells. Validation against experimental polarization curve data is obtained for current densities ranging from zero to above 2 A cm−2. Experimental transient data is also successfully reproduced. The model is very flexible and can be used to explore the system's sensitivity to a wide range of material properties, cell design specifications, and operating parameters. We demonstrate the impact of gas inlet relative humidity (RH), operating current density, ionomer loading and ionomer ion exchange capacity (IEC) values on cell performance. In agreement with the literature, high air RH levels are shown to improve cell performance. At high current densities (>1 A cm−2) this effect is observed to be especially significant. Simulated hydration number distributions across the cell reveal the related critical dependence of cathode hydration on air RH and current density values. When exploring catalyst layer design, optimal intermediate ionomer loading values are demonstrated. The benefits of asymmetric (cathode versus anode) electrode design are revealed, showing enhanced performance using higher cathode IEC levels. Finally, electrochemical reaction profiles across the electrodes uncover inhomogeneous catalyst utilization. Specifically, at high current densities the cathodic reaction is confined to a narrow region near the membrane.

Impedance of a PEM fuel cell cathode with nonuniform ionomer loading: Analytical and numerical study

We report a physics-based model for impedance of the cathode catalyst layer (CCL) with nonuniform Nafion loading in a PEM fuel cell. The general system of equations for the perturbation amplitudes is derived and solved analytically for the cases of either fast oxygen transport, or fast proton transport in the CCL. The expressions for the respective CCL impedances are obtained. In the case of large cell current, we analyse the numerical solutions for the CCL impedance.

Effect of crosslinked poly(vinyl alcohol)/sulfosuccinic acid ionomer loading on PEMFC electrode performance

In this study, crosslinked poly(vinyl alcohol)/sulfosuccinic acid (PVA/SSA) ionic polymer and Nafion® N-115 membranes are used as electrolytes to fabricate different types of membrane electrode assemblies (MEAs) for proton exchange membrane fuel cells (PEMFC). Each MEA is fabricated with various crosslinked PVA/SSA ionomer loadings (0.04–1.5 mg cm−2) in catalyst layer based on volumetric ratio between catalyst and ionomer. Properties of each catalyst layer are evaluated using scanning electron microscopy, cyclic voltammetry, and hydrogen pumping technique. I–V characteristics, impedance, and incubation behaviour of each cell are evaluated in fuel cell testing fixture. Incubation of cell is effective when ionomer loadings are below 0.1 mg cm−2. At ionomer loading of 0.04 mg cm−2, PEMFC demonstrates highest power densities than that observed with other loadings of ionomer due to higher electrochemical active surface area of the electrode. Hydrogen pumping studies reveals that the PVA/SSA ionic polymer is a potential anode material for PEMFC electrodes while ionomer loadings are below 0.1 mg cm−2.

Impedance Spectroscopy Study of the PEM Fuel Cell Cathode with Nonuniform Nafion Loading

We report modeling and experimental study of impedance of the PEM fuel cell cathode with nonuniform ionomer loading. A physics–based model for the high–frequency impedance is developed and analytical solution for impedance is derived. Assuming that the CCL proton conductivity σp exponentially decays from the membrane surface, we fit the model to experimental spectra of the cell measured at the open circuit conditions. Fitting gives the characteristic scale of the σp decay, the average CCL proton conductivity and the double layer capacitance.

Resolving Electrode Morphology's Impact on Platinum Group Metal-Free Cathode Performance Using Nano-CT of 3D Hierarchical Pore and Ionomer Distribution.

This article reports on the characterization of polymer electrolyte fuel cell (PEFC) cathodes featuring a platinum group metal-free (PGM-free) catalyst using nanoscale resolution X-ray computed tomography (nano-CT) and morphological analysis. PGM-free PEFC cathodes have gained significant interest in the past decade since they have the potential to dramatically reduce PEFC costs by eliminating the large platinum (Pt) raw material cost. However, several challenges remain before they are commercially viable. Since these catalysts have lower volumetric activity, the PGM-free cathodes are thicker and subject to increased gas and proton transport resistances that reduce the performance. To better understand the efficacy of the catalyst and improve electrode performance, a detailed understanding the correlation between electrode fabrication, morphology, and performance is crucial. In this work, the pore/solid structure and the ionomer distribution was resolved in three dimensions (3D) using nano-CT for three PGM-free electrodes of varying Nafion loading. The associated transport properties were evaluated from pore/particle-scale simulations within the nano-CT-imaged structure. These characterizations are then used to elucidate the microstructural origins of the dramatic changes in fuel cell performance with varying Nafion ionomer loading. We show that this is primarily a result of distinct changes in ionomer's spatial distribution. The significant impact of electrode morphology on performance highlights the importance of PGM-free electrode development in concert with efforts to improve catalyst activity and durability.

Analyzing the influence of high electrode potentials on intrinsic properties of catalyst coated membranes using impedance spectroscopy

Abstract Catalyst layers (CLs) with varying ionomer contents are produced using a stencil coating and screen printing technique. The optimum ionomer content of 31−34 wt% confirms the findings of other groups and performance is found to be independent of production technique. A new CL impedance transition line model is developed and fitted to in-situ data. The results indicate that the protonic contact resistance between CL and membrane is an important factor for the used transfer-decal process, especially for CLs with low ionomer loading. When subjected to potentials higher than 1.2 V, an increased performance is observed for low ionomer loading CLs. It is found that by applying the high potential to the electrode a significantly increased proton conductivity is counteracting and superimposing the loss of electrochemical surface area (ECSA) due to carbon corrosion. After aging, the performance of the 15 wt% CL is at the same level as the 31−34 wt% ionomer content CLs at the beginning of life, even though the ECSA is reduced due to carbon corrosion or platinum dissolution. The findings indicate that for the optimization of the ionomer loading, either the changing wetting properties or the redistribution of ionomer during lifetime have to be taken into account.