VOLUME I: ANOSTRUCTURED NITROGEN-CONTAINING CARBON
1.3 Shortcomings of Current PEM Fuel Cells
Fuel cells have several economical obstacles to overcome before they can be commercialized. One setback is that they are competing against a very cheap and well- developed source of energy in gasoline combustion engines. However, as the demand for oil increases, and its supply decreases, fuel cells will become a more economical option.
The world’s production of oil is expected to peak before 2050, and as early as next decade [5], therefore it is important to consider alternative energy sources now.
Although current prototype fuel cell systems are expensive, according to principles of mass production this cost will drop once large-scale production is initiated. Most experts in the automotive industry, agree that the cost of building a H2 powered PEM fuel cell vehicle would be close to the cost of building a gasoline powered automobile if done on a larger scale.
Before large-scale production of PEMFC’s could begin there are other obstacles that must be overcome. One very important issue is that a source of fuel for the cells and the corresponding infrastructure must be developed. Since fuel cells are merely energy conversion devices, they are only as clean and efficient as the upstream fuel source. An extensive amount of research is being carried out, particularly by catalysis researchers, on the production of fuels for fuel cells, but this complicated issue will not be discussed here (VOLUME II: of this thesis deals with hydrogen production).
In addition to fuel source issues, the reliance of PEMFC’s on scarce materials poses a problem. Because of the low temperature of operation, catalysts play an important role in the electrodes and represent one of the biggest challenges to
commercialization. Currently, it appears that the low availability of platinum will hinder the inauguration of PEMFC’s as replacements for ICE’s. State-of-the-art PEMFC’s use platinum supported by carbon black as electrocatalysts in both the anode and the cathode.
The lowest loading of platinum achievable (before performance is reduced) using state- of-the-art methods is 0.05 mg/cm2 in the anode and 0.4 mg/cm2 in the cathode [3]. The cathode requirements present a more difficult challenge compared to the anode, as evident from the high platinum loadings. Estimates show that the maximum possible production of platinum in the world would barely be high enough to allow 10-20% of the automobiles being produced annually to be powered by PEM fuel cells [6-8].
Furthermore, even if breakthroughs in platinum-based catalysts are achieved, there are other concerns with relying on platinum-based materials. One particular concern is that most of the world’s platinum is mined in unstable regions, such as Africa and the Ural Mountains region in the former Soviet Union [9]. Additionally, beyond the geopolitical and availability shortcomings of platinum, it is still not an ideal material for use in a PEM fuel cell cathode because of poor performance compared to what may theoretically be attainable.
A large amount of research has focused on reducing platinum loadings in PEMFC electrodes. However, it seems unlikely that required platinum loadings could be reduced by another order of magnitude by simply developing better preparation techniques.
Figure 3 shows a magnified drawing of a PEMFC electrode. Particles of platinum are supported by carbon black and covered by a thin film of Nafion. Only the platinum particles that are in contact with the so-called “triple boundary phase” are electrochemically active. This means platinum must be connected electrically to the
external circuit through the conductive carbon support, it must be connected to the electrolyte through the proton conductive film, and must be exposed to the reactant all at the same time. The largest difficulty with platinum utilization is from it being covered by too thick of a layer of Nafion for fast H2 or O2 diffusion [10], or with too large of platinum particles leaving a high percentage of platinum atoms in subsurface layers [11].
Moreover, sintering of platinum particles occurs during normal operation, causing the active surface area to decrease with time [12]. Platinum can also be poisoned by impurities in the reactant stream, thus rendering it inactive. This is especially a problem for carbon monoxide, often found in hydrocarbon derived fuel streams, which can hurt activity in concentrations as low as 10 ppm [13]. In state-of-the-art fuel cell technology it is estimated that between 25% to 50% of the platinum is in contact with the triple boundary phase [14], and thus electrochemically active (before deactivation). Therefore, while optimizing platinum usage could potentially yield a 4-fold reduction in the required platinum loadings, world-wide platinum supplies will still be drained.
As discussed in the previous section, the use of platinum as the PEMFC cathode catalyst also contributes extensively to inefficiencies in the cell because of poor activity.
The slow ORR kinetics typically contribute the most out of all sources to inefficiencies in a H2 fueled PEMFC operating at maximum power [3].
Because of the problems facing current PEMFC technology, researchers are focusing on improving many aspects of the current technology, from anodes with better carbon monoxide resistance, to better performing electrolyte membranes. However, the use of platinum in the cathode of PEM fuel cells is an issue that must be resolved since it drastically hurts the efficiency of the cell, and limits the possibility for wide-scale
production. Alternative cathode catalysts to platinum has been the objective of many researchers over the past four decades [15]. Still, an acceptable replacement for platinum-based cathodes has yet to be developed. This volume of work focuses on bettering the understanding of alternative catalysts to platinum for use in a PEM fuel cell cathode.
Figure 3: Drawing of a PEMFC electrode demonstrating the triple boundary phase.