Low-Cost Nanomaterials for High-Performance Polymer Electrolyte Fuel Cells (PEMFCs) - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 351

25-40 % is achievable under practical conditions [ 11 ]. The significance of this
problem can easily be understood from the sharp rise of CO 2 levels in the atmo-
sphere from the 280 ppm of pre-industrial era, to 393 ppm in 2012 suggesting a
37 % increase from 1750s [ 12 ]. More importantly, while it took 215 years for
initial 50 % of this increase in CO 2 content, it only took 33 years for the next 50 %
and this rate of increase is only expected to grow further as observed from the
Keeling curves [ 13 , 14 ]. Thus, a clean energy conversion and/or storage device
with almost no environmental impact and higher practical efficiency (ca. 70 % at
least) is an urgent need for sustainable development. In this respect, electro-
chemical devices due to their intrinsic ability to convert chemical energy directly
into electrical energy offer a unique opportunity to tackle many of these challenges
in terms of innovative materials, processes and devices.
2 What Are Low-Cost Nanomaterials
The objective of this discussion is to identify relevant low-cost nanomaterials with
high potential for next generation energy storage and conversion applications. The
classification of such nanomaterials should be as simple as possible. For this reason
the following material categories were defined based on their abundance, recy-
clability, cost-effective production methods as well as their intensive applications in
various energy-related technologies. The material categories are (a) carbon-based
nanomaterials; (b) nanocomposites; (c) metals and alloys; (d) nanopolymers and
some of their hybrids. Carbon-based nanomaterials are one of the most widely
employed candidates in various energy technology applications and have predicted
to play a vital role in hydrogen storage and electrical energy storage.
3 Applications in Electrochemical Power Sources
Electrochemical power sources are devices that convert chemical energy stored in
materials (or fuels) directly into electricity. Major electrochemical energy con-
version and storage devices that are considered for the future energy needs are
batteries, fuel cells and supercapacitors. While all these three technologies have
different, material dependant, reactions for storage and conversion, the basic
energy providing steps take place at the electrode-electrolyte interface as the case
with any electrochemical system. Scheme 1 shows one major classification of
electrochemical power sources with the exception of fuel cells which are post-
poned for a later discussion. Apart from the above categories, there are also
overlapping systems such as metal-air and redox flow batteries where a battery
electrode is combined with a fuel cell electrode (i.e. half-cell reaction) to realize
the benefits of both the systems as illustrated by metal-air rechargeable batteries
using Zn (or Fe) as the anode and air as the cathode [ 15 , 16 ].