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Electrochemical investigations were conducted during the second year of the project on Layer-by-Layer (LbL) assembled nanoparticle arrays that were synthesized and characterized using HAADF-STEM and FFT during the first year.   These investigations have identified a previously unknown Pt structure synthesized by the PI's group that exhibits substantially enhanced electrocatalytic oxygen reduction reaction (ORR) activity relative to state-of-the-art commercial systems.  This new Pt structure exists as an ordered but non-crystallographic assembly of atoms that bridges the transition from atomic clusters to single crystals with lattice structure.  Such a novel arrangement of Pt atoms presents a unique opportunity for new research in the fields of electrocatalysis and colloid chemistry that may enable significant improvements in the efficiency of sustainable energy alternatives to fossil fuels. 

Fig. 1 shows the average ORR mass activity at 900 mV vs. RHE in HClO₄ for each particle size in Table 1 obtained from the average of 1, 5, and 10 Layer-by-Layer (LbL) assembled electrodes on glassy carbon.  The small standard deviation in this data is a reflection of relatively constant Pt loading per layer obtained using this approach.  The dotted and dashed lines in Fig. 1 represent the Mass-Averaged Distribution (MAD) of the (111) and (100) face atoms in truncated, cubo-octahedral nanoparticles for the sizes indicated based on work by Van Hardeveld and Hartog.¹ MA for the different particle sizes investigated is compared to the MAD of the atoms in the predominant crystal faces of truncated cubo-octahedral particles as determined by Van Hardeveld for perfectly sized particles.  Peak MA for the particle sizes tested is near the peak MAD of the (111) facet.  Such a coincidence is consistent with the existing hypothesis regarding the correspondence of ORR structure sensitivity in highly dispersed systems with planar single crystal studies.  However, the PI's work indicates that the loss in MA is actually caused by a crystal to cluster transition and the nearly discontinuous peak in activity observed in Fig. 1 correlates to structural re-arrangements associated with the onset of this transition.   Peak MA occurs on particles having an amorphous atomic structure not previously observed with Pt.

Table 1: Synthesis Conditions and Particle Characteristics

Figure 1.  Average MA [A/g] plotted versus particle size (diamonds). 

In the case of larger, crystalline particles, a relatively smaller loss in MA is observed as the particle size increases, consistent with the change in the MAD of (111) face atoms for truncated cubo-octahedral structures. The MA of a commercial Vulcan-supported Pt catalyst (TKK) was measured under similar conditions to be 36.7±1.9 A/g at 0.9 V. As may be observed from Fig. 1, the single-crystals and transitional particles, when incorporated into LbL assembled electrodes, exhibit higher mass activities than the commercial C-Pt catalyst. The specific activity (SA) of these structures (based on ECA measurements) was also found to be substantially higher than the commercial catalyst. An average value of 90.5±7.3 mA/cm²_Pt was obtained for the commercial catalyst, as compared to an average value of 230±21 mA/cm²_Pt  for LbL-assembled electrodes of the 2.5 nm crystalline particles.  Such activity enhancement with LbL electrocatalyst assembly was hypothesized in the original proposal for this work. An even higher average SA value of 260±67mA/cm²_Pt  was measured for LbL-assembled electrodes of the 1.7 nm transitional particles.

The PI has also demonstrated that similar activity gains are obtained from the transitional particles when they are assembled via the LbL technique on a Vulcan support. Fig. 2(a) is a TEM image of the 1.7 nm transitional Pt particles synthesized by the PI and supported on Vulcan while Fig. 2 (b) is that of commercial Vulcan-supported Pt catalyst (TKK)². The scale of Figs. 2 (a) and (b) are chosen to be identical. The advantage of the PI's LbL electrode fabrication technique as regards electrocatalyst monodispersity is evident in these images.

Figure 2.  (a) Vulcan supported electrocatalyst. (b) Commercial system from Tanaka Kikinzoku Kogyo, Japan (TKK)².

A critical aspect of attaining the activities described in an operating fuel cell is the ability to remove the stabilizing ligand in electroless fashion. Methods for accomplishing this have been developed by the PI with this ACS funding and have been recently published.  Fig. 3 depicts how NaOH and HCl may be used to activate the LbL assembled catalyst for this purpose.

Figure 3.Pt/Snratio for 10 (dashed) and 20 (solid) layer electrodes assembled ongraphite–epoxy composite and accelerated with NaOH (filled circles) and HCl (open diamonds).

<>Next Steps.

1.The impact of electrolytic and electroless activation on the structure of the LbL assembled electrocatalysts (both unsupported and on carbon support,respectively) that span the crystal to cluster transition will be characterizedin detail over time to evaluate the durability of the structures synthesized.

2.Additional funding will be pursued to conduct electrochemical ad-atominvestigations and structural analyses (SANS and SAXS) to characterize thesurface chemistry (bond coordination) of the transitional particles synthesizedand provide more detailed understanding of the fundamental mechanismsunderlying the ORR reaction.

References

  (1)       VanHardeveld, R.; Hartog, F.Surface Science1969,15, 189.

  (2)       Ferreira,P. J.; la O, G. J.; Shao-Horn, Y.; Morgan, D.; Makharia, R.; Kocha, S.;Gasteiger, H. A.Journal of theElectrochemical Society2005,152, A2256.