1D Pd-Based Nanomaterials as Efficient Electrocatalysts for Fuel Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 314

1 Introduction
Due to the increasing worldwide energy demand and environmental concerns, much
effort has been devoted to the seeking for efficient and clean energy sources to
replace the traditional fossil fuels such as gasoline and diesel [ 1 ]. Fuel cell, a device
that converts the chemical energy stored in fuels into electricity through electro-
chemical reactions efficiently without pollution, has been attracting increasing
research interest as a new power source for portable applications due to their high
energy-conversion efficiency and relatively low operating temperature [ 2 , 3 ]. Based
on the operating temperature and the type of used electrolyte, fuel cells are usually
classified into phosphoric-acid fuel cells (PAFCs), solid oxide fuel cells (SOFCs),
alkaline fuel cells (AFCs), molten-carbonate fuel cells (MCFCs), polymer exchange
membrane fuel cells (PEMFCs), and direct-methanol fuel cells (DMFCs) [ 4 ]. While
all types of fuel cells work on the similar principle: hydrogen or other fuels oxidation
at anode and oxygen reduction at cathode. Among different types of fuel cells, the
PEMFCs and DMFCs are especially promising for automotive and portable elec-
tronic applications owing to the low operation temperatures [ 5 , 6 ]. It should be noted
that despite extensive research progress, there are still many scientific and tech-
nological challenges to realize the widespread commercialization of fuel cells. For
instance, the reactions on both anode and cathode need electrocatalysts with high
catalytic performance. To improve the efficiency and durability, and to reduce the
cost of fuel cells, the conventional Pt catalysts have to be replaced by novel nano-
structured electrocatalysts with high electrocatalytic activity, high stability, and
low-cost. For fuel cells, platinum has been regarded as the best electrocatalyst
because of the highest electrocatalytic activity among the metal catalysts for elec-
trooxidation of small organic fuels and for oxygen reduction [ 7 ]. However, with
platinum as an anode catalyst, its surface is usually heavily poisoned by CO
intermediates produced during the oxidation of organic fuels, resulting in the
lowering of catalytic performance. On the other hand, the state-of-the-art cathode
electrocatalysts, Pt nanoparticles (2-5 nm) supported on amorphous carbon mate-
rials (Pt/C), usually suffer from poor durability caused by the fast and significant loss
of platinum electrochemical surface area (ECSA) over time during fuel cell oper-
ation. Moreover, the high price and the limited global supply of Pt largely drive up
the cost of fuel cells. To reduce the cost and minimize the self-poisoning of catalysts,
Pt-based electrocatalysts alloyed with other transition metals (Fe, Co, Ni, Cu, Mn,
Ir) with controlled surface composition and structures have been extensively studied
in recent years [ 8 - 17 ]. Compared to pure platinum, Pt-based electrocatalysts exhibit
higher performance and a reduced sensitivity toward CO poisoning because of the
so-called bifunctional mechanism [ 18 - 20 ] or ligand effect [ 21 , 22 ]. For instance,
Pt-Ru alloys have shown enhanced electrocatalytic activity for fuel cell anode
reactions and the enhancement could be well explained by the ligand effect and the
bifunctional mechanism [ 23 , 24 ]. Based on the ligand effect, the presence of
ruthenium was proposed to alter the electronic properties of Pt, resulting in less
strongly CO adsorption on the catalyst surface. While according to the bifunctional