Controllable Synthesis of Metal Nanoparticles for Electrocatalytic Activity Enhancement - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 234

reduction of O 2 to H 2 O is much preferred in a fuel cell device and the
2-electron reduction pathway leading to the formation of H 2 O 2 , a strong
oxidizer, should be avoided.
Currently, the state-of-the-art electrocatalysts for ORR in polymer elec-
trolyte fuel cells (PEFCs) are carbon supported Pt NPs. Despite their ac-
ceptable activity, these catalysts do have the stability issues that need to be
resolved. Furthermore, Pt is limited in natural reserve and is costly to use.
Therefore, to minimize the Pt usage or ideally to develop more advanced
catalysts without the use of any noble metals for ORR has attracted tre-
mendous interests. 32,33

d n 9 r 4 n g | 5

9.4.1.1 Pt-based Alloy NP Catalysts
In spite of the great progress achieved in last decade, 34 the ORR activity of
the pure Pt catalysts is still limited. During the ORR, the formation of the
adsorbed oxygenated species (e.g.,O ads ,OH ads , OOH ads ), originating from
the aqueous electrolyte and/or the reaction itself, is considered as the de-
cisive process for determining the catalyst performance. As a result, a bal-
ance between adsorption and desorption of O ads ,OH ads and OOH ads is
believed to be the key to successful development of a viable ORR catalyst.
Recent density functional theory (DFT) calculations indicate the Pt-O
binding energy of Pt(111) surface is 0.2 eV away from optimal energy
level. 35-38 Theoretical studies further show that the addition of an early
transition metal (M) to Pt can downshift the d-band center of the Pt catalyst,
leading to a lower degree of adsorption of oxygenated species and increases
the number of active sites accessible to oxygen. 39,40 Importantly, volcano-
type relationships between the oxygen adsorption energy (or the d-band
center) and the ORR activity for various PtM alloys have been also estab-
lished. 39,41,42 In parallel to the theoretical studies, extensive efforts have
been directed towards the preparation of multi-metallic PtM electrocatalysts
with enhanced activities for the ORR. Among the catalysts studied, Pt alloyed
with Fe, Co, Ni, and Cu show much improved ORR activity relative to that of
other transition metals and Pt itself, which is in good agreement with DFT
calculations. 40,43,44
One of the most promising bimetallic systems developed in our group is
the FePt alloy. FePt NPs are considered an important material for a range of
magnetic (data storage, high performance magnets) and electrocatalytic
applications. 6,45 Although preliminary studies demonstrate that FePt NPs
are indeed ORR active, 46 there exists a serious concern when using these
alloy NPs to catalyze ORR in acidic solution: the alloy structure is chemically
unstable and the Fe components of the FePt NPs are subject to fast dis-
solution. The easy dissolution of Fe and the formation of the Pt skeleton
structure make FePt NPs even less stable than Pt NPs in acidic environ-
ments. As synthesized, these FePt NPs have the chemically disordered solid
solution structure with Fe and Pt arranged randomly in the face centered
cubic (fcc) fashion (Figure 9.7a), the FePt NPs are often denoted as fcc-FePt.

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