Haile Group SURF2008

SURF 2008: Synthesis of CsH₂PO₄ Nanoparticles

Background

The compound CsH₂PO₄ is the electrolyte of choice for solid acid fuel cells. Unlike other sulfate and selenate containing solid acids, cesium dihydrogen phosphate is stable under both reducing and oxidizing atmospheres, both of which occur in the fuel cell environment. In solid acid fuel cells (as in many other types of fuel cells) the electrolyte material is incorporated into the electrodes, which are themselves composite components that include an electronically conducting phase and a catalyst in addition to the proton conducting phase. We hypothesize that the electrochemical activity in solid acid fuel cells will be improved via the use of nanoparticulate CsH2PO4 in the composite electrodes. The objective of this SURF project is accordingly the synthesis of nano-CsH2PO4.

Description

While many different approaches can be pursued to meet this objective, we have selected here a method based on water-in-oil microemulsions (or inverse micelles). In such a microemulsion, water, oil (an alkane such as hexane) and a surfactant molecule are brought together in solution. The water and oil being otherwise immiscible would normally separate into two distinct liquids. The surfactant, however, changes everything. One end of the long-chain molecule is hydrophobic and is attracted to the water, whereas the other end is hydrophyllic and is attracted to the oil. Depending on the concentrations of water, oil and surfactant in the mixture, the surfactant molecule traps small water droplets which are encased by a layer of surfactant molecules and disperses in the oil. The droplets are just a few nanometers in size and therefore invisible to the eye and the solution appears clear. If we further modify the soultion such that there is some CsH2PO4 dissolved into the aqueous phase, we ideally generate nanoscale reactors for the precipitation of CsH2PO4 via the evaporation of the water. The specific tasks that the student will undertake generally fall into the following categories.

Proof of principle - synthesis of NaH2PO4

The most widely examined surfactant is sodium bis(2-ethylhexyl)sulfosuccinate, known more commonly as Na(AOT). In this ionic compound, the sodium cation is charge balanced with a sulphonate end to molecule. In principle, the Na can be exchanged with Cs to avoid possible contamination by Na in the final product CsH2PO4. Before proceeding with the the lengthy exchange process, the student will first establish whether the procedure can be used to synthesize the chemically analogous compound NaH2PO4.

Establish the range of water-oil-surfactant-NaH2PO4 concentrations which produce a clear solution indicating that a microemulsion has formed. Hexane will be used here initially, again, because the solution behavior with hexane (in the absence of NaH2PO4) has been well documented.

Use gentle evaporation to remove both the water from the solution. The hexane will also likely evaporate, but the surfactant will not. Ideally, the result will be nanoparticle NaH2PO4 coated with surfactant molecules.

X-ray diffraction will be used to characterize the resulting powder.

In principle, the surfactant coated NaH2PO4 can be redispersed in hexane. The colloidal dispersion will either be sent out to a commercial laboratory for particle size analysis, or dilute droplets will be dried on a substate for in-house examination using electron microscopy or atomic force microscopy.

The path to CsH2PO4

If the synthesis of NaH2PO4 nanoparticles by the methods outlined above is successful, then the next step is to obtain a surfactant molecule in which the Na is replaced with Cs and the sulfonate end-group in the AOT is replaced with a phosphonate end-group.

Replacing Na with Cs may be tedious but is straightforward by ion exchange methods that are available in house.

Replacing the -SO3 with -PO3 on the molecule requires purchasing the appropriate surfactant molecule and will not be performed in house. This is likely beyond the scope of the ten week SURF project.

Evaluation of the performance of nanoparticle solid acids in fuel cell electrodes is also 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 mentors, Ali Samarat (post-doctoral scholar) and Mary Louie (graduate student). These papers will also be relevant for preparing the SURF research proposal.

J. Eastoe, M.J. Hollamby and L. Hudson, "Recent advances in nanoparticle synthesis with reversed micelles," Advances in Colloid and Interface Science 128-130 5-15 (2006).

C. Destree and J.B. Nagy, "Mechanism of formation of inorganic and organic nanoparticles from microemulsions," Advances in Colloid and Interface Science 123-126 353-367 (2006).

D.G. Shchukin and G.B. Sukhorukov, "Nanoparticle Synthesis in Engineered Organic Nanoscale Reactors," Advanced Materials 16 671-681 (2004).

V. Marciano, A. Minore and V. T. Liveri, "A simple method to prepare solid
nanoparticles of water-soluble salts using water-in-oil microemulsions," Colloid Polym Sci 278 250-252 (2000).

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 Discussions (cover image) 134, 17-39 (2007).

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