Can electrodeposited thin film rival nanoparticle ensemble as oxygen reduction reaction catalyst?

Yihua Liu, Carlos M. Hangarter, Ugo Bertocci, Vladimir P. Oleshko,

Leonid A. Bendersky, Thomas P. Moffat

In hydrogen fuel cells, sluggish kinetics of Oxygen Reduction Reaction (ORR) at the cathode currently accounts for up to 70% of energy loss.  Combined with high cost of Pt-based catalysts, currently the most active ORR electrocatalyst, these limitations have made Pt utilization one of the main research themes.  Although nanoparticle ensembles are an effective scheme in Pt utilization, due to “size effect” nanoparticles are shown to have substantially lower intrinsic catalytic activity toward ORR compared to extended surface of the same material.  Recent studies suggest that thin films would be a viable alternative to nanoparticle ensemble for fuel cell catalyst.  This is justified by the fact that thin films not only have an extend surface, but also show comparable levels of Pt utilization to nanoparticles when the film thickness is on an order of 10 nm.  To engineer thin-film catalyst into fuel cells, electrodeposition stands out as a low-cost and scalable production scheme compared to high vacuum techniques. 

In this presentation, electrodeposition and ORR performance of Pt100-xNix thin films will be illustrated.  To date, Pt100-xNix is among the most active class of ORR electrocatalyst experimentally shown and theoretically predicted.   In contrast to overpotential deposition of both constituents, the proposed electrodeposition scheme is that Ni is underpotentially deposited concurrent with overpotential deposition of Pt.  More significantly, this work takes aim at settling the role of alloy composition in determining the intrinsic catalytic activity of Pt100-xNix thin films.  Significant dealloying was evident for films with x > 45 and substantial shrinkage of the film thickness accompanied dealloying for films with x > 55. A maximum ORR specific activity of 2.8 mA/cm2 at 0.900 RHE was observed for alloys between Pt45Ni55 and Pt55Ni45, This represents an enhancement factor of 4.7 compared to electrodeposited Pt thereby matching the best published results reported for Pt-Ni nanoparticles or thin films. A peak ORR mass activity of 0.78 A/mgPt at 0.900 V RHE was observed for alloy film compositions between Pt38Ni62 and Pt45Ni55. In comparison to electrodeposited Pt, these films exhibit a ten-fold improvement in mass activity.