Electrochemical Atomic-level Controlled Syntheses of Electrocatalysts for the Oxygen Reduction Reaction - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 163

solution, where part of the Ni atoms from the alloy are galvanically displaced
by Pd and a Pd reach shell is formed on the WNi core. In the next steps,
performed in the same cell, a Pt ML layer is deposited on the top of as ob-
tained Pd/WNi nanostructures by using the Cu UPD strategy. As obtained
Pt ML /Pd/WNi electrocatalysts are deposited directly on the gas diffusion layer
and after rinsing the electrode is dried and assembled with a Nafion 211
(25 mm thick) membrane and an anode with Pt loading of 100 mgcm 2 ,by
hot pressing at 130 1C for 1 min. As prepared membrane electrode assembly
is first activated and then tested at real fuel cell operating conditions.
Figure 6.9, presents the polarization curves measured on the MEA with size
of the electrodes of 25 cm 2 .
The Pt mass activity and the total PGM mass activity, as estimated from
the iR compensated curve (plotted in black in Figure 6.9) at 0.9 V, are 1.1 A
mg 1 and 0.42 A mg 1 , respectively. These values indicate that the electro-
catalysts of interest possessed activity that meets the DOE targets. In add-
ition, the accelerated stability test cycles (not shown here), performed on the
same MEA at 80 1CinH 2 /N 2 atmosphere, showed no degradation and even
better performance after 15 000 cycles.

d n 9 r 4 n g | 4

6.4 Conclusions
The Pt ML core-shell type electrocatalysts for the ORR are the most advanced
class of fuel cell catalysts that possess a potential to boost the implemen-
tation of the PEMFCs in the electric vehicles. This type of electrocatalysts
offer ultra-low Pt content, complete Pt utilization, very high activity and
excellent performance stability surpassing the US DOE targets for 2020. In
this chapter we reviewed for first time the electrochemical strategies for
atomic layer deposition of Pt ML on different transition metal surfaces, in-
cluding nanoparticles. The electrochemical ALD strategies are divided in two
groups, based on the surface specific reaction that is used to limit the de-
position of Pt only to maximum monolayer coverage. In the 'displacement
driven' strategy a UPD ML of less noble element (Cu, Pb or H) is first de-
posited on the surface of interest, and then is galvanically displaced by Pt.
Based on the stoichiometry of the displacement reaction the amount of Pt
can be controlled at sub-monolayer level. This strategy is also known in the
literature as surface limited redox replacement. In the second group, named
'adsorption driven', once a Pt ML is electro-deposited on the Au substrate a
layer of strongly adsorbed non-metallic elements or molecules (H ads or CO)
is formed and hinders the growth of a second ML. This electrochemical ALD
strategy has been introduced in the literature in the last couple years, and
has been demonstrated to work only for deposition of Pt ML on single- and
poly-crystalline Au surfaces.
The advantages of the electro-deposition techniques over conventional
chemical methods for synthesis of electrocatalysts for the oxygen reduction
reaction are also discussed herein. It is demonstrated that the SLRR of Cu
UPD ML, pre-deposited on electro-deposited Pd, PdAu, PdIr, NiW

.