Combinatorial Catalyst Screening

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 1).

Figure 1. (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.

Figure 1. (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 2). The modified electrochemical cell can be used to do cyclic voltammetry of particular areas of the library.

Figure 2: 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.

Figure 2: 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 + 4 e- –> 4 H2O

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.