Peter Nicholas Pintauro

Due to human health concerns, there is increasing pressure to replace perfluorosulfonic acid (PFSA) membranes and binders in proton-exchange membrane (PEM) fuel cells with an ionomer that is more environmentally friendly. Pemion^®from Ionomr Innovations, Inc., a non-fluorinated ionomer with high conductivity and good chemical stability, is a promising alternative to PFSA materials like Nafion™ [1,2]. In the present study, Pemion^® was examined as the cathode binder in sprayed powder electrode MEAs, where the membrane and anode binder were Nafion™. In an initial set of experiments, sprayed powder cathodes were prepared with neat Pemion^® and with mixtures of Pemion^® + poly(acrylic acid) (henceforth abbreviated as PAA), and Pemion^® + polyethylene oxide (PEO), where the I/C ratio was fixed at 0.2 and where the PAA or PEO content in the binder was very high, e.g., Pemion^®/PAA weight ratios of 1/1, 1/2, and 1/3. For all blended binders, a portion of water soluble PAA or PEO was removed from the cathode by a hot water soaking step before a fuel cell test. Further extraction of PAA or PEO was carried out in-situ by lowering and then raising the temperature of the MEA multiple times in the fuel cell test fixture (between 30^oC and 80^oC) over a period of a few hours.

When fuel cell electrodes are prepared by a conventional spraying or slot-die coating method, there is little or no control over the multi-scale organization of catalyst particles and polymer binder. Electrode structural features such as particle and binder interconnectivity, macro-porosity, and micro-porosity are difficult to control, but are critically important when high-performance nanocatalysts are used. Nanofiber electrospinning is an electrode fabrication technique that can address fuel cell electrode morphology issues for next-generation MEAs. As a commercial fabrication method, electrospinning has been shown to be scalable, robust, and cost-effective, especially for the creation of non-woven mats of sub-micron-diameter polymer fibers. Electrospinning can also be used to prepare particle/polymer fiber networks with intra- and inter-fiber porosity. Such fiber mats have been used as the electrode material in Li-ion batteries, H₂/Br₂ redox flow batteries, and H₂/air proton-exchange membrane fuel cells [1-4]. Recent work on nanofiber electrodes at Vanderbilt has focused on inks containing pre-formed electrospun catalyst/binder fibers. With such inks, nanofiber electrode MEAs can be made using commercial roll-to-roll fabrication lines. In this talk, experimental results will be presented on nanofiber mat electrode MEAs with Pt/C catalyst and Nafion™ binder. Procedures for preparing inks with nanofibers and then using the inks to create CCMs or GDEs will be presented. Fuel cell performance and durability results with the resulting MEAs will be contrasted with fuel cell data where the electrodes are made directly from electrospun mats, i.e., by the direct transfer of a mat from the electrospinner to a GDL or membrane. The effects of catalyst loading and fuel cell operating conditions (backpressure, feed gas flow rate, and feed gas relative humidity) on fuel cell power output before and after metal dissolution and carbon corrosion accelerated stress tests will also be discussed. As an example of nanofiber electrode performance: an MEA with a Nafion 211 membrane and TKK Pt/C anode and cathode catalyst (0.1 mg/cm² anode and 0.22 mg/cm² cathode) operating at 100% RH, 80^oC, and 3 atm. total pressure generated 1.0 W/cm² at 0.7 V. Acknowledgment This work was funded by the U.S. Department of Energy’s Hydrogen and Fuel Cell Technologies Office, for the Fuel Cell Performance and Durability Project, contract No. DE-EE0007653. References C. Self, M. Naguib, R. E. Ruther, E. C. McRen, R. Wycisk,G. Liu, J. Nanda, and P. N. Pintauro, “High Areal Capacity Si/LiCoO2 Batteries from Electrospun Composite Fiber Mats”, ChemSusChem, 10, 1823 – 1831 (2017). Saadi, X. Fan, S. S. Hardisty, P. Pintauro, and D. Zitoun, “Ultralow Platinum Loading for Redox-Flow Battery by Electrospinning the Electrocatalyst and the Ionomer in Core-Shell Fibers”, Journal of Energy Storage, 59, 106430-106435 (2023). Slack, M. Brodt, D. A. Cullen, K. S. Reeves, K. L. More, and P. N. Pintauro, “Impact of Polyvinylidene Fluoride on Nanofiber Cathode Structure and Durability in Proton Exchange Membrane Fuel Cells”, Journal of the Electrochemical Society, 167, 054517 (2020). Waldrop, J. J. Slack, C. Gumeci, J. Parrondo, N. Dale, K. Shawn Reeves, D. A. Cullen, K. L. More, and P. N. Pintauro, “Electrospun Nanofiber Electrodes for High and Low Humidity PEMFC Operation”, Journal of the Electrochemical Society, 170, 024507 (2023).

Cation-exchange, anion-exchange, and bipolar membranes play crucial roles in a variety of electrochemical processes and devices, including chloralkali cells, electrodialysis separations for water purification, proton-exchange membrane and hydroxide-exchange membrane (alkaline) fuel cells, redox flow batteries, and processes for direct air capture of CO₂. The incorporation of polymeric nanofibers into such membranes provides an attractive and tunable method of creating materials with new nano-morphologies and highly desirable properties. The impregnation of an ionomer solution into a pre-formed nonwoven porous mat of electrospun polymer fibers is a well-known method of making reinforced proton-exchange membranes. The use of simultaneous dual-fiber electrospinning or the electrospinning of polymer blends can be used to intermix/incorporate/co-locate dissimilar and incompatible polymers on the nanoscale. Although less studied in the literature, these methods offer many interesting possibilities for new membrane structures with targeted and unique transport and mechanical properties. In this review talk, the use of dual fiber and blended polymer fiber electrospinning for membrane fabrication will be presented for the following: (1) Nanofiber reinforced cation (proton) exchange membranes, (2) Electrospun Nafion^TM/PVDF dual fiber and single-fiber membranes for H₂/Br₂ redox flow batteries, (3) Composite anion exchange membranes, and (4) Bipolar membranes with a 3D nanofiber junction. Materials and methods for membrane fabrication will be described and the properties of the membranes will be discussed.

In recent years, electrospinning has been gaining popularity as a robust and scalable technique for the fabrication of non-woven mats of sub-micron diameter polymer fibers. Although not as well studied, the technique can also be used to prepare particle/polymer fiber networks with high intra- and inter-fiber porosity. Such fibrous mats can be used as porous electrodes in fuel cells and batteries, where a high electrode/electrolyte interfacial area is needed. In this talk, particle/polymer electrospinning will be presented as a platform technique to fabricate a variety of electrode and membrane-electrode-assembly (MEA) products with unique and desirable compositions, architectures, and performance characteristics. Laboratory results will be presented for: (1) hydrogen/air fuel cell cathodes with Pt/C and PGM-free catalysts, (2) Pt/C hydrogen electrodes in a H₂/Br₂ redox flow battery, and (3) Si anodes for lithium-ion batteries. High particle content fibers containing Pt/C or Pt-alloy catalyst powder and a Nafion®perfluorosulfonic acid binder have shown great promise as electrodes in H₂/air fuel cells. During electrospinning, Nafion ionomer migrates to the outer edges of the fiber, effectively creating a core-shell fiber morphology, with a low I/C ratio in the fiber interior. Additionally, the interior of the fiber contains nm-size pores and the outer surface of the Nafion shell exhibits enhanced hydrophobicity, due presumably to an accumulation of the polytetrafluoroethylene backbone chains (-CF₂-CF₂-) of Nafion. The resulting structure is advantageous in that the majority of catalyst particles are in a low I/C environment, nanopores in the fiber interior trap electro-generated water via capillary condensation, and the hydrophobic surface of the fibers facilitates electrogenerated water removal from the mat during fuel cell operation. The fuel cell power output using fiber mat anodes and cathodes is very high for a low Pt loading (0.10 -0.20 mg/cm²), near or above 1.0 W/cm² for H₂/air feeds in the range of 60-100% relative humidity.^1,2 Additionally, the fiber mat cathodes exhibit excellent durability, with minimal power loss after accelerated voltage cycling tests. The hydrophobicity of fibers can be enhanced by adding polyvinylidene fluoride (PVDF) directly to the Nafion binder. Such blended binders in fiber/particle hybrid electrodes extend the operational lifetime of fuel cells with iron-based PGM-free cathode MEAs. As an extension of the fuel cell electrode work, electrospun Pt/C-Nafion nanofiber mat hydrogen electrodes have been made and tested for a H₂-Br₂ redox flow battery.³ The hydrophobic surface coating of core-shell fibers effectively blocks bromine species from poisoning the hydrogen electrode. In a 6.6 M HBr electrolyte lab-scale redox flow battery, the fiber mat electrodes worked extraordinarily well, with: (i) high power at low Pt loading, e.g., 650 mW/cm² at 0.70 Vfor a loading of 0.11 mg_Pt/cm² (6.4 kW/g_Pt) and 550 mW/cm² at 0.65 V for a loading of 0.048 mg_Pt/cm² (11.5 kW/g_Pt), (ii) no loss in power for more than 170 charge/discharge cycles, and (iii) a roundtrip current efficiency of 95%at both 0.2 and 0.3 A/cm². For Li-ion battery applications, single and dual fiber anode mats have been prepared and evaluated, where the fibers contain a polymer binder (e.g., polyacrylic acid, polyamidimide, or polyvinylidene fluoride), Si nanoparticles as the electrochemically active material, and carbon powder as an electron conductor.⁴ Here, the gravimetric and areal capacities of the fiber anode are considerably higher than those of a conventional carbon particle anode. Inter-fiber voids allow for volume changes in Si during lithiation/delithiation, thus minimizing capacity fade during charge/discharge cycling (up to 600 cycles). Acknowledgments This work was funded by the U.S. Department of Energy, Contract No. DE-EE0007653 from the Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability Consortium (Fuel Cells Program Manager: Dimitrios Papageoropoulos) and Contract No. DE-EE0007215 from the Vehicle Technology Office. References: J. Slack, M. Brodt, D. A. Cullen, K. S. Reeves, K. L. More, and P. N. Pintauro, Journal of the Electrochemical Society,167 (2020) 054517. K. Waldrop, J. J. Slack, C. Gumeci, J. Parrondo, N. Dale, K. S. Reeves, D. A. Cullen, K. L. More, and P. N. Pintauro, Journal of the Electrochemical Society, 170 (2023) 024507. K. Saadi, X. Fan, S. S. Hardisty, P. Pintauro, and D. Zitoun, Journal of Energy Storage, 59, (2023) 106430. E. C. Self, M. Naguib, R. E. Ruther, E. C. McRen, R. Wycisk, G. Liu, J. Nanda, and P. N. Pintauro, ChemSusChem, 10 (2017) 1823.

Due to structural changes in silicon during lithiation/delithiation, most Li-ion battery anodes containing silicon show rapid gravimetric capacity fade upon charge/discharge cycling. Herein, we report on a new Si powder anode in the form of electrospun fibers with only poly(acrylic acid) (PAA) binder and no electrically conductive carbon. The performance of this anode was contrasted to a fiber mat composed of Si powder, PAA binder, and a small amount of carbon powder. Fiber mat electrodes were evaluated in half-cells with a Li metal counter/reference electrode. Without the addition of conductive carbon, a stable capacity of about 1500 mAh/g (normalized to the total weight of the anode) was obtained at 1C for 50 charge/discharge cycles when the areal loading of silicon was 0.30 mg_Si/cm², whereas a capacity of 800 mAh/g was obtained when the Si loading was increased to ~1.0 mg_Si/cm². On a Si weight basis, these capacities correspond to >3500 mAh/g_Si. The capacities were significantly higher than those found with a slurry-cast powdered Si anode with PAA binder. There was no change in fiber anode performance (gravimetric capacity and constant capacity with cycling) when a small amount of electrically conductive carbon was added to the electrospun fiber anodes when the Si loading was ≤1.0 mg_Si/cm².

MEAs with nanofiber mat electrodes containing Pt/C catalyst and Nafion binder were fabricated and evaluated. The electrodes were prepared by electrospinning a solution of catalyst powder, salt-form Nafion (with Na⁺, Li⁺, or Cs⁺ as the sulfonic acid counterion), and a carrier polymer of either polyethylene oxide or poly(acrylic acid). The carrier polymer was extracted prior to MEA testing by a hot water soaking step. The resulting fibers were 15%–17% porous, with a core–shell-like morphology (a coating of primarily Nafion on the fiber surface). MEAs with anode/cathode catalyst loadings of 0.1 mg_Pt cm⁻² each and a Nafion 211 membrane produced high power at both high and low relative humidity (RH) conditions in H₂/air fuel cell tests, e.g., a maximum power density of 919 mW cm⁻² at 100% RH and 832 mW cm⁻² at 40% RH for a test at 80 °C and 200 kPa_abs. The presence of nm-size pores within the fibers trapped water via capillary condensation during low RH feed gas testing, thus maintaining a high proton conductivity of the Nafion binder in the anode and cathode while minimizing/eliminating ionic isolation of catalyst particles in low water content, poorly conductive binder.