For fuel cells to become viable and cost effective energy sources for the Navy, the application of the catalyst becomes a critical step in the cost reduction efforts. This article describes part of the materials analysis that Navy ManTech has conducted through its Centers of Excellence as part of a more comprehensive attempt to address this issue.

Fuel cells and related components use precious metals (such as palladium) as catalysts and hydrogen purification membranes. In addition to their high cost, a major problem with these metals is the poor integrity and longevity of these thin film coatings that are a vital part of fuel cell operation. In order to ensure consistent membrane coatings and catalyst surfaces, a simple, highly reliable technology is needed that is capable of economically producing uniform thin film deposits on geometrically intricate substrate structures. For these applications, deposition of palladium and palladium alloys is typically done using the electroless plating method. The final step in the process is a reduction of the palladium solution using hydrazine. Based on the results from numerous preliminary tests done by the Center for Advanced Mineral and Metallurgical Processing (CAMP) for Navy ManTech, it was shown that hydrazine concentration has a prominent effect on the palladium deposition morphology.

The initial tests were done to develop a potential operating range of hydrazine concentration. No parts were plated during these tests, which allowed the palladium to form in its preferential state. It was from these tests that different plating morphologies were observed. At low hydrazine concentrations the time to plate was considerably longer than at high concentrations, but palladium films could be made. At high hydrazine concentrations the palladium precipitated out of the solution very quickly producing microparticles. At intermediate concentrations various plating morphologies could be seen. Using scanning electron microscopy, three types of plating morphologies are shown in Figure 3-1; coherent films, tree and branch formations, and microparticles.

In an effort to optimize plating conditions and the concentration of hydrazine that causes morphology changes, three statistical test matrices were developed. These matrices were used to evaluate changes in hydrazine concentration, time, and agitation. The coatings were then evaluated with a mercury porosimeter and a scanning electron microscope.

The first of the three matrices was prepared using 35 wt% hydrazine and varying stirring speeds, reaction times, and reductant volumes. For most of these samples a good coating was not obtained presumably due to the high hydrazine concentration. It was found that high reductant concentration in the plating bath caused the palladium to precipitate out almost immediately. A slower reaction rate was necessary to produce the coherent layer necessary for hydrogen purification.

From Figure 3-2 it can be seen that a high reductant concentration can also produce abnormal layer orientations. These rosettes are believed to be caused by unplanned reaction rate and agitation speed. Further investigation is planned to determine whether this type of plating could be beneficial for hydrogen purification due to increased surface area if deposited on a coherent layer.

A second matrix was run with a one to 9 dilution of the 35 wt% hydrazine and varying stirring speeds, reaction times, and reductant volumes. Again, this matrix appeared to produce poor coatings depending on the volume of reductant added to the bath and reaction time. The dilution helped to slow the reaction so that the palladium had a chance to form a layer rather than forming small particles. However, the desired coatings were not seen using the Feico desktop scanning electron microscope (Phenom). This is perhaps due to a low reductant concentration, however, several acceptable coatings were observed with a large hydrazine volume and a long reaction time.

The third matrix, run with a one to four dilution of the 35 wt% hydrazine, appeared to be the most optimal hydrazine concentration. The only samples that did not produce a good palladium layer were those that were reacted for 15 minutes or less, presumably because the palladium did not have enough reaction time to form a consistent layer on the stainless steel matrix.

Figure 3-3 shows how the palladium formed around, and closed pores between stainless steel particles. This phenomenon may increase hydrogen purification flux due to the increased palladium surface area. This will be the subject of future research.

As a result of these tests, the optimum hydrazine concentration has been determined for future testing. Additional experiments will evaluate the effect of temperature and substrate media size. The samples that are plated with palladium using the next test matrix will be evaluated in a helium leak testing apparatus to determine the integrity of the membrane. Following these tests, the membranes will be tested in a hydrogen purification test apparatus to determine how well the coating performs in their intended environment. It is planned to then evaluate the best performers using reformed hydrocarbon fuel to further validate the deposition parameters and hydrogen purification ability.