ABSTRACT
Fuel cells, the electrochemical direct energy-conversion power sources that generate electricity with high efciency at mild operating conditions, have been heralded as ideally useful to secure sustainable supply of clean energy.1 Recently, their performance has been signicantly improved by advances in fuel-cell electrocatalysts that brought this technology close to applications. Applying fuel cells in portable electronics and power vehicles is peculiarly desired since they ensure negligible carbon footprint upon energy conversion.2 At the heart of such applications, there is a longstanding issue with the oxygen-reduction reaction (ORR) at the cathode, whose fairly slow kinetics causes the fuel-cell efciency to fall down into the range of 50%–60%, much lower than the thermodynamically calculated value of 83% at 298 K. To accelerate the ORR, most fuel cells use platinum particles supported on porous carbon supports (Pt/C), the most active electrocatalysts known to date.3 However, such superior activity is far too compromised by the high cost and low abundance of Pt to be well appreciated in commercializing fuel cells on a large scale. By 2015, realistic Pt-based electrocatalysts should accomplish the Pt mass activity roughly four times higher that of the standard Pt/C to accord with the latest protocol of the US Department of Energy.4-7
In addition to several partly successful approaches that have been studied to resolve high Pt content and insufcient activity and stability (cf. review1), in the quest of cost-effective ORR electrocatalysts, a strategy has been explored that rests on the active center of nitrogen-coordinated transition metals, that is, MNx (M = Fe, Co; x = 2, 4); examples include nonprecious transition-metal macrocycles and nitrogen-doped carbon materials.8,9 However, quite a few of these nonprecious electrocatalysts show the activity close or equivalent to the typical Pt/C in acid media-even with
4.1 Introduction .......................................................................................................................... 125 4.2 Tunable ORR Activity of Pt Monolayer Electrocatalysts ..................................................... 126 4.3 Mixed PtML Electrocatalysts: Surface Ensemble Effect on the ORR Activity ..................... 128 4.4 Tailored Smooth Surfaces for Favorable Core-Shell Interaction ......................................... 130 4.5 Br Treatment of Pd3Co/C ...................................................................................................... 132 4.6 Low-Index Facets: Tetrahedral Pd Cores.............................................................................. 132 4.7 Ultrathin Pd Nanowire Cores ............................................................................................... 135 4.8 Hollow Core-Induced Contraction of Pt Monolayer ............................................................. 136 4.9 Stability of Pt ML: Self-Healing Mechanism ....................................................................... 138 4.10 Remarks and Outlook ........................................................................................................... 141 Acknowledgments .......................................................................................................................... 142 References ...................................................................................................................................... 142
their components, composition, and structures being well optimized. Another notorious drawback long has prevented their practical applications in fuel cells, largely due to the loss of the metal center that not only degrades the ORR activity but also perhaps causes dysfunction of the polymeric membrane.10 Ultimately, noble metal-based ORR nanocrystals have been considered as a primary solution to the trade-off between activity, stability, and cost-effectiveness.
