Composite nanostructured solid-acid fuel-cell electrodes via electrospray deposition

Abstract

Stable, porous, nanostructured composite electrodes were successfully fabricated via the inexpensive and scalable method of electrospray deposition, in which a dissolved solute is deposited onto a substrate using an electric field to drive droplet migration. The desirable characteristics of high porosity and high surface area were obtained under conditions that favored complete solvent evaporation from the electrospray droplets prior to contact with the substrate. Solid acid (CsH 2 PO 4 ) feature sizes of 100 nm were obtained from electrosprayed water-methanol solutions with 10 g L À1CsH 2 PO 4 and 5 g L À1 Pt catalyst particles suspended using polyvinylpyrrolidone (PVP). Alternative additives such as Pt on carbon and carbon-nanotubes (CNTs) were also successfully incorporated by this route, and in all cases the PVP could be removed from the electrode by oxygen plasma treatment without damage to the structure. In the absence of additives (Pt, Pt/C and CNTs), the feature sizes were larger, $300 nm, and the structure morphologically unstable, with significant coarsening evident after exposure to ambient conditions for just two days. Electrochemical impedance spectroscopy under humidified hydrogen at 240 C indicated an interfacial impedance of $1.5 U cm 2 for the Pt/CsH 2 PO 4 composite electrodes with a total Pt loading of 0.3 AE 0.2 mg cm À2. This result corresponds to a 30-fold decrease in Pt loading relative to mechanically milled electrodes with comparable activity, but further increases in activity and Pt utilization are required if solid acid fuel cells are to attain widespread commercial adoption.

Carbon nanotubes as electronic interconnects in solid acid fuel cell electrodes

“…Again, reproducibility is high, and the results for the cell with Pt but no CNTs match previously reported results. 15 Moreover, the enhancement obtained using the CNT-modified carbon paper occurs despite the potential presence of residual PVP, which has been shown to have a slight negative impact on activity. 15 Overall, the low impedance values obtained in the presence of the CNTs directly translate into five-and three-fold improvements in the catalyst utilization, respectively.…”

“…14 Deviations from the 'ideal' value of the mass-normalized activity of the well-defined structure provide a measure of Pt utilization in composite structures. Third, the catalyst-mass-normalized activity in the finest composite structures we have been able to prepare to date, obtained using the method of electrospray, 15 is substantially lower than the ideal value. This low relative activity suggests that Pt in the composite structure is poorly utilized, which, in turn, may be the result of poor electronic connectivity between Pt nanocatalyst particles and the exterior current collector.…”

“…As a control, a solution of CsH 2 PO 4 and PVP (but no Pt) was electrosprayed on the CNT-overgrown carbon paper to assess the possible contribution of CNTs and/or Ni nanoparticles to any observed catalytic activity. As in our previous work, 15 the electrospray chamber was operated at ambient pressures, the temperature was fixed at 140 1C (to facilitate solvent evaporation), the voltage between the capillary tip and the substrate was 4.5 kV, the tip to substrate distance was 2.5 cm, the nitrogen sheath gas velocity was 67 cm min À1 , the liquid flow rate was 1 mL h…”

“…Composite electrodes comprising CsH 2 PO 4 and Pt were deposited on carbon paper substrates by electrospray using methods described by Varga et al 15 Both neat (as-purchased) and CNTovergrown carbon paper substrates were employed. In addition, two different electrospray solutions were utilized, the compositions of which are detailed in Table 2.…”

“…The quantity of material deposited onto the substrate was determined from a comparison of the pre-and post-deposition weights, as measured using a microbalance, from which the Pt loading was computed. In continued analogy to our earlier study, for control electrodes not containing CNTs, an oxygen plasma treatment step (150 W power, 13.56 MHz frequency, 13.6 mTorr Ar + O 2 mixture, 4 min exposure) was employed in order to remove PVP, 15 which could otherwise coat the Pt catalyst particles and prevent electrochemical access. In the case of the CNT-containing electrodes, the oxygen plasma treatment step was omitted to avoid possible damage to the carbon structures.…”