Over the years there has been meaningful contribution on the development of cationic OH- conducting solid polymer electrolytes (SPEs) known as alkaline anion exchange membranes (AAEM). These AAEMs use liquid fuels of small organic molecules such as methanol, ethanol, formic acid and others, which is of great advantage due to their higher volumetric and gravimetric energy densities. However, fundamental and extensive analytical studies must be conducted to understand the processes that occur in these membranes in operando. OH- transport, anion exchange dynamics in the membranes, the effect of carbonation and swelling of the polymer upon hydration are some of the fundamental investigations needed to be cultivated in the field. Ex-situ and in-situ characterization is currently being conducted using different techniques and instrumentation found at our facilities. Our collaboration with the Coates group has granted us the opportunity to study the most novel and promising AAEMs found in literature.

Figure 1: Schematic representation of an alkaline anion exchange membrane fueled by methanol.

With future energy needs growing, it has become essential for alternative energy conversion technologies to be explored, one of which being fuel cells. Factors limiting further application of fuel cells are that known electrocatalysts for both the anode and cathode are expensive, not very efficient, intolerant to fuel contaminants, and unstable over time. This project focuses on the high throughput screening of possible electrocatalysts for fuel cell applications.
As a means for investigating fuel cell electrocatalysts, modified and traditional combinatorial methods are employed to make libraries of materials to look for specific and desired properties. Magnetron sputtering is used to deposit multi-element compositions onto 3” Si wafers or 5 mm glassy carbon slugs. (Figure 2).

Figure 2. (left) The sputtering chamber used to deposit the combinatorial libraries. (right) The sample holders inside the chamber that allow deposition of 3 different films at one time.

Libraries can be studied using a few different methods, including a fluorescence assay, and a modified electrochemical cell. Both facilitate the study of libraries on 3” Si wafers without breaking them apart. The fluorescence assay works by the addition of indicators that fluoresce with a pH decrease. When particular catalyst areas oxidize the fuel, the local pH decreases, leading to fluorescence. The greater the fluorescence, the more active the catalyst. These areas can then be mapped onto a phase diagram to determine the composition (Figure 3). The modified electrochemical cell can be used to do cyclic voltammetry of particular areas of the library.

Figure 3. The fluorescence intensity increases in the active region as more positive potentials are applied. On the right the MeOH fluorescence onset potentials are displayed on a ternary phase diagram projection.

The sputtered glassy carbon can help test electrocatalysts using a different technique, rotating disk electrode voltammetry, or RDE. RDE allows for the determination of mechanistic information, enabling the differentiation between different reaction pathways.
The glassy carbon combinatorial libraries are of particular interest for studying cathode materials, or materials that can catalyze the oxygen reduction reaction, seen below in base.

4H⁺ + 2O₂ + 4e^– → 4H₂O

Although this reaction would be ideal, many catalysts will only catalyze this reaction through to an intermediate—H₂O₂. Not only does this reduce the number of electrons consumed at the cathode, but H₂O₂ can also oxidize and destroy the membrane in the fuel cell. When looking for new catalysts, then, we want to find ones that will catalyze the full reaction and will do so at potentials that maximize the voltage of the fuel cell.
Once a promising catalyst is discovered, we collaborate with the DiSalvo group and work to synthesize the catalyst for further investigation with our fuel cell test station. Besides the empirical nature of this work, some useful correlations between the reactivity and catalyst structures may be obtained and therefore can possibly facilitate the searching process.
The nature of this project is highly collaborative and involves many different techniques. Among the techniques used are different electrochemical methods, including RDE, CV and fuel cell testing; Differential Electrochemical Mass Spectrometry (DEMS); electron microscopies (TEM and SEM); X-ray techniques (XPS and XRD); and deposition techniques, primarily sputtering.

We are involved in extensive efforts to discover new materials for fuel cell applications. Ordered intermetallic compounds have been shown to exhibit remarkable electrocatalytic activity for both fuel oxidation and oxygen reduction in terms of onset potential and current density. They need to be prepared in nanoparticle form, so as to have increased surface area, which is the most effective way of increasing activity per amount of precious metals. We have prepared nanoparticles of ordered intermetallic phases by reducing metal oxides or salts using various powerful reducing agents. Our group has designed, synthesized and characterized a series of Pt/Pd-based intermetallic nanoparticles, and investigated on their structure and catalytic activity. For example, Pt₃Co core-shell ordered intermetallic particles exhibited over 200% increase in mass activity compared to disordered Pt₃Co alloy and commercially available Pt nanoparticles, and showed minimal loss of activity and structure after 5,000 cycles¹.

Figure 4. ORR polarization curves for Pt/C, Pt₃Co/C (annealed at 400°C) and Pt₃Co/C (prepared at 700°C) in O₂-saturated 0.1 M HClO₄ at room temperature, with rotation rate, 1,600 rpm and sweep rate, 5 mV s⁻¹. b, The Koutecky–Levich plots from ORR data for Pt₃Co/C-700 at different potentials. The inset in b shows the rotation-rate-dependent current–potential curves. c, Comparison of mass activities for Pt/C, Pt₃Co/C-400 and Pt₃Co/C-700 at 0.85 and 0.9 V. d, Comparison of specific activities (Ik).

Figure 5. The chemical microstructure of the surface of the Pt-rich Pt₃Co/C-700 nanoparticles was examined using electron energy loss spectroscopic (EELS). It can be seen that its structure is highly ordered with 5 nm-Pt shell, which is 2–3 atomic layers.

Nanoparticle synthesis is also the final step of our combinatorial material exploration, because nanoparticles are what will be used in fuel cell stacks to maximize surface area and therefore mass activity.

Citations

1. Wang, D.; Xin, H. L.; Hovden, R.; Wang, H.; Yu, Y.; Muller, D. A.; DiSalvo, F. J.; Abruña, H. D. Nature Materials 2013, 12, 81–87.
2. Yu, Y.; Wang, D.; Abruña, H. D.; Muller, D. A. Microsc. Microanal. 2012, 18, 1306-1307.
3. Chen, H., Wang D., Yu, Y, Newton K., Muller D, Abruña H. D., DiSalvo J. F.;, J. Am. Chem. Soc. (2012), 134(44), 18453-18459.
4. Wang D., Yu Y.,. Xin H. L., Hovden R., Ercius P., Mundy J. A., Chen H., Richard J. H., Muller D. A., DiSalvo F. J., Abruña H. D.; Nano Lett. (2012), 12(10), 5230-5238.