(Energy Technology Division Walter van Schalkwijk Award in Sustainable Energy Technology Address) Exploring New Electrode Designs with Nanofibers
Authors: Pintauro Peter N. , Fan Xiaozong , Waldrop Krysta , Slack John , Self Ethan , Waugh John , Wycisk Ryszard , Saadi Kobby , Zitoun David 
Published: February 2024

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 mgPt/cm2 (6.4 kW/gPt) and 550 mW/cm2 at 0.65 V for a loading of 0.048 mgPt/cm2 (11.5 kW/gPt), (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.

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