Haile Group SURF2008

SURF 2009: Electrospray preparation of CsH₂PO₄-based porous, fuel-cell electrodes

Direct Mentor: Áron Varga <avarga@caltech.edu>

Course Requirements:
Chemistry or Physics lab course (Ph 5) Electricity and Magnetism course (Ph 1b)

Academic Standing:
Caltech students only, juniors preferred.

Background

The compound CsH₂PO₄ is an attractive material for fuel cell applications because of its high proton conductivity (~ 10^-2 S/cm) at moderate temperatures (~ 250C). Fuel cell membranes as thin as 25 mm can be made from this material and the resulting fuel cells yield power outputs that are competitive for commericial applications. Further increasing the power output (which will lower the cost per unit power output) requires that attention be directed to the electrodes (rather than the electrolyte). The electrodes must fulfill several functions, including ion transport, electron transport, gas transport and electocatalysis. It is unusual for a single material to perform each of these functions and according fuel cell electrodes are commonly porous composites comprising the electrolyte material (for ion transport), an electronic conductor and a catalyst.

Description

We have recently shown that it is possible to prepare a highly porous framework of CsH₂PO₄, into which the additional components can be incorporated, via the process of electrospraying. The electrospray method relies on electrostatic forces to draw a liquid from a solution, creating micro- or nanoscale droplets which, in turn, create micro-or nanoscale particles. In a typical configuration, a liquid is supplied to a metal capillary which is biased by a high voltage relative to a substrate placed some distance away. The liquid (here, an aqueous solution of CsH₂PO₄) emerging at the tip experiences Coulombic forces and charged liquid droplets are extracted, which are then electrostatically accelerated towards the substrate. As the solvent evaporates during flight, the charge is concentrated, inducing a break-up of the droplet into smaller secondary droplets. This process can occur multiple times, until the solvent completely evaporates, leaving solid nanoparticles which are ultimately deposited onto the substrate (here, carbon paper).

A very large number of material and process parameters can be manipulated to control the outcome of the electrospray process. These include the solution composition (which impacts the surface tension, the electrical resistivity and the droplet-to-solid volume ratio) as well as the chamber temperature and flow rate of background gases. The SURF student would carry out a systematic study of the impact of these various parameters on the resulting electrode structure, where the structural characteristics will be observed by scanning electron microscopy. Because the experimental system is entirely constructed and an initial exploration of the relevant parameter space has already been carried out, it is anticipated that this project will yield important results relatively quickly.

Evaluation of the performance of these electrode structures in solid acid fuel cells electrodes is considered beyond the scope of this ten week SURF project.

Additional Information

Interested students should review the following references prior to contacting either Prof. Haile or the research mentor, Áron Varga. These papers will also be relevant for preparing the SURF research proposal.

S. M. Haile, C. R. I. Chisholm, K. Sasaki, D. A. Boysen and T. Uda, “Solid acid proton conductors: From laboratory curiosities to fuel cell electrolytes,” Faraday Discussions134, 17-39 (2007).

Y. K. Taninouchi, T. Uda, Y. Awakura, A. Ikeda and S. M. Haile, “Dehydration Behavior of the Superprotonic Conductor CsH₂PO₄ at Moderate Temperatures: 230 to 260°C,” J. Mater. Chem. 17, 3182-3189 (2007).

A. Jaworek, “Electrospray droplet sources for thin film deposition,” J. Mater. Sci. 4, 266–297 (2007).

W. Gu, P. E. Heil, H. Choi and K. Kim, “Comprehensive model for fine Coulomb fission of liquid droplets charged to Rayleigh limit,” Appl. Phys. Lett. 91, 064104 (2007).

Additional papers may be posted.

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