Hierarchical Nanostructures for Fuel Cells and Fuel Reforming - Hierarchical Nanostructures for Energy Devices - page 87

correspond to 72-94 g of platinum for a 85 kW fuel cell stack in a 75 kW net
automotive fuel cell system. 14,15
Despite this significant reduction compared to the PEM fuel cell tech-
nology of the 1990s, a further reduction is required for commercially suc-
cessful PEMFCs in automotive applications. Estimates have shown that the
amount of platinum has to be further reduced five-fold, mostly due to cost 16
and limitations of Pt supply. 17 It has been demonstrated that these limi-
tations can be met by reducing the Pt loading-specific power density below
0.2 g Pt kW 1 at cell voltages of Z0.65 V, keeping hydrogen-to-electricity
eciencies above 55%. 12 To achieve this, the fuel cell power density has to
be improved to 0.8-0.9 W cm 2 at Z0.65 V by reducing mass transport
limitations at high current densities and by simultaneously decreasing the
Pt-loading to 0.15 mg Pt cm 2 without compromising cell performance.

d n 3 r 4 n g | 4

5.2.1.2 Nanostructures in Low-temperature Polymer Electrolyte
Membrane Fuel Cells
Reducing the catalyst loading while improving or at least maintaining the
fuel cell performance is a very demanding challenge, which can be achieved
by increasing the Pt utilization and the catalytic activity. In PEMFCs, the
reaction kinetics of the oxygen reduction on the Pt catalyst of the cathode
side is the rate-limiting step, creating a large overpotential for the oxygen
reduction reaction (ORR). 18,19 Decreasing the overpotential of ORR is widely
seen as the most critical step towards increasing the PEM fuel cell eciency.
Three approaches can lead to this goal: improve the catalyst layer structure,
increase the catalytic activity for ORR, and increase the utilization of Pt by a
higher dispersion of catalyst in the electrode. 12,20
The second approach, increasing the catalytic activity of Pt for ORR, has
been elaborated by many studies. 20-23 One possibility is to substitute Pt with
Pt-transition metal alloys for enhanced catalytic performance.
The utilization of Pt can be enhanced by increasing the reactive surface
area. 20,24 The catalyst can be deposited on highly conductive, high-surface
area substrates, for example micro- and nano-structured carbon particles.
Carbon black has commonly been used as a catalyst support for MEAs in
PEMFCs. 20,24 By uniformly incorporating highly dispersed Pt nanoparticles
into the carbon electrodes, the catalytically active surface area per mass of
catalyst can be dramatically increased.
The main focus of this section is the third approach, improving the
catalyst layer structure by using hierarchical nanostructures. Since the dis-
covery of carbon nanotubes in 1991, 25 carbon nanotubes (CNTs) as well as
carbon nanofibers (CNFs) have created great excitement in the nanoscience
and nanotechnology community due to their unique chemical, structural,
mechanical, and electromechanical properties. 26
Compared to Vulcan XC-72R, a widely used carbon support with a con-
ductivity of 4.0 S cm 1 and a specific surface area of 237 m 2 g 1 , 27 CNTs and

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