60th Annual Report on Research 2015

Polymer electrolyte membrane fuel cells (PEMFCs) and direct formic acid fuel cells (DFAFCs) are a highly promising and well-developed technologies that will likely support a future hydrogen economy. To address cost and performance setbacks in the pursuit of widespread adoption of fuel cell technology, the US Department of Energy has recently broadcast a need for nanoscaled catalysts and for the discovery of catalyst systems with reduced precious metal loading. A major strategy in this regard is the development and application of sub-5 nm diameter bimetallic catalysts instead of bulk platinum (Pt or large Pt particles). Clusters of NPs offer important advantages over isolated NP architectures. In particular, an interparticle gap with decreasing nanoscale dimensions will permit increased readsorbtion of chemical intermediates in a fuel cell half reaction and hence a more efficient power output. Furthermore, the potential drop in an interparticle gap can approach 300 mV suggesting that fuel cell reactions may be catalyzed with less applied overpotential and less interference or blocking from other ions. Controlling the density and number of NPs in a cluster is particularly challenging and hence the understanding of the role of Pt-based clusters in fuel cell catalysis is largely unknown.

 Background:

The immediate needs that exist in responsible-energy technologies, the cost and material challenges surrounding precious metals, and the necessity in achieving the full potential of platinum-based catalysts demand highly practical and comprehensive approaches for studying and applying these catalysts. The importance of interparticle active sites on Pt NP catalysts was demonstrated in 2013. It was shown that as the interparticle distance in neighboring Pt NPs approaches 1 nm the specific activity of Pt NPs for the oxygen reduction reaction can be increased 6 fold compared to that of a commercial Pt/C catalyst. The challenge in this scientific pursuit however lies in the controlled synthesis of uniformly dispersed Pt-based NP clusters with a controlled NPs population. Further opportunities for the advancement in this research area will result from the integration of the core-shell strategy for tuning the structural and electronic properties of the Pt surface active sites.

Achievements:

This research project has established a highly versatile method for the preparation of a self-assembled polystyrene-block-poly(4-vinylpyridine) diblock copolymer template capable of metal ion-loading for NP synthesis (Scheme 2). We have shown that selection of the block copolymer template determines the length scale of the array of NP clusters while the distribution of metal anions co-dissolved in a common loading bath specifies for the composition of the NPs (Scheme 2, step iii). Using this approach, our group has studied two different bimetallic NP systems in great detail, and importantly, shown determined important structure-property relationship between the arrays of NP clusters and their electrocatalytic activity. 

In the case of the PtAu system (Figure 1), the block template synthesis yields a PtAu nanostructure comprised of a Pt-rich shell is located on a PtAu alloy core. The PtAu core-shell bimetallic NPs were found to have a very high density of electrochemically active Pt surface sites.  Accordingly, the activity of Pt-rich core-shell PtAu nanocatalysts for the electrocatalytic oxidation methanol was approximately 2-4 fold that of a monometallic Pt benchmark catalyst (ETEK) and only 28% less than that of the PtRu bimetallic benchmark catalyst. The Au that is present in the PtAu NPs was beneficial to Pt-rich electrocatalysts as the carbonaceous poisoning I_f/I_b metric was greatly enhanced relative to both benchmark materials (~ 2-3 fold increase; Figure 3B). In the case of the PtIr system (Figure 2), the block template synthesis yields an array of alloyed PtIr NP. Many of the bimetallic PtIr NPs isolated from the block copolymer template approach are much higher in activity than typical industry/research standards. The most active array of Pt₈₈Ir₁₂ NPs had mass activities for formic acid oxidation that were four-fold higher that a PtRu industrial research standard (Figure 3C). Overall, we anticipate that this block copolymer template method of preparing highly active clusters of low-diameter, Pt-contianing NPs will have a positive impact on several emerging areas in nanoscience such as energy devices, solar fuels, and heterogeneous catalysis.