ABSTRACT
Owing to the rising energy demands, the depletion of fossil fuel reserves, and environmental pollution problems, a growing demand for high-efϐicient and low-cost renewable energy has sparked signiϐicant interest in the commercialization of fuel cell technology as a replacement for combustion-based energy sources [1, 2]. Among the multitude of fuel cell technologies available, proton exchange membrane fuel cells (PEMFCs) have become increasingly important because they operate at relatively low temperatures and have short startup and transient-response times compared with other types of fuel cells that operate at higher temperatures (200-800°C) [3]. PEMFCs are devices that directly convert chemical energy stored in fuel molecules into electric energy. During operations, oxygen gas
is fed from the cathode and electrochemically reduced, while fuel (including hydrogen, methanol, formic acid, and ethanol) with low standard redox potential is electrochemically oxidized at the anode. Accordingly, based on the type of fuels used, PEMFCs can be further categorized into direct hydrogen fuel cell (DHFC), direct methanol fuel cell (DMFC), direct formic acid fuel cell (DFAFC), and so on [4]. The high cost of the PEM fuel cell technology is one of the major challenges hindering its commercialization. Among the components in a PEM fuel cell, platinum (Pt)-based electrocatalysts and their associated catalyst layers contribute over 55% of the total cost [5, 6]. Unfortunately, Pt is currently the only efϐicient electrocatalyst (which is very hard to replace) in practical PEM fuel cells due to its outstanding catalytic and electrical properties and superior resistant characteristics to corrosion. As the demand for Pt grows, the price of Pt has increased by more than three times from $600 per oz in 2001 to $1800 per oz in 2011 over the last decade [7]. It is predicted that the demand for Pt will continue to grow rapidly in future [8]. Given the climbing price of Pt, we must ϐind ways to reduce the Pt loading (particularly in the cathode catalyst layer) without compromising fuel cell performance in order to meet the cost requirements for fuel cell commercialization. In other words, the design of novel Pt catalyst requires not only reducing the amount of Pt used but also enhancing catalytic activity and durability [6, 9]. Interestingly, it has been established that the catalytic activity and durability of platinum nanostructures depends highly on their morphology (size, shape, and dimensionality), and therefore the design and synthesis of well-controlled shapes and sizes of platinum nanostructures is crucial for their applications, especially in the ϐield of catalysis. Great effort has been made to increase the ratio of surface area to volume by reducing the size of Pt nanoparticles. Studies have also shown that by altering the surface structure of a bulk single crystal one can manipulate the catalytic properties of a Pt catalyst [10, 11]. For example, platinum nanocubes were found four times more active than polyhedral (truncated cubic) Pt NPs for oxygen reduction reaction (ORR), indicating a dominant effect of NP shape on the ORR in PEMFC reaction conditions [12]. It has also been established that a rough surface containing terraced, stepped, and kinked sites is generally more active than a ϐlat, low-index surface [13]. The reaction kinetics of platinum surface can also be tuned by changing the dimensionality. One-dimensional (1D) Pt nanowires
and nanotubes exhibited much enhanced activity and durability, for ORR, than commercial Pt/C catalyst, which is made of Pt nanoparticles [14-17]. By taking into account both size and surface structure, one can tailor the catalytic activity and durability of a Pt catalyst at the nanoscale by controlling the shape of Pt nanocrystals during their chemical syntheses. In this review, we will focus on the syntheses of Pt nanocrystals with well-deϐined and controllable shapes and their use as electrocatalysts in PEM fuel cell applications.
