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| Fig. 1: Basic anatomy of a PEMFC. (Source: Wikimedia Commons.) |
Increasingly strict regulation of fossil fuel-based internal combustion engines has given rise to an economic climate that incentivizes innovation in the alternative energy space. Moreover, from an environmental standpoint, the inevitable depletion of fossil fuels has made the alternative automotive power sources an area of increased interest. At this time, there are two primary alternative energy sources to power vehicles: electricity from batteries, and fuel from hydrogen fuel cells. Through different processes, batteries and fuel cells convert chemical energy into electricity useful for powering machines such as automobiles. A conventional battery is filled with the chemical fuel needed to produce electricity, and therefore die after prolonged use. As long as there is a constant flow of the fuel, however, a hydrogen fuel cell will continue to produce power. In essence, a fuel cell uses a chemical reaction paired commonly with hydrogen fuel in order to produce electricity.
While there are many types of fuel cells, different both in the process of conversion, and the fuels used, one of the most popular, and potentially most useful in powering vehicles, is the proton exchange membrane fuel cell (PEMFC), which, unlike internal combustion engines, produces no harmful emissions when using hydrogen as fuel. [1] As seen in figure 1,there are three fundamental hardware elements of a PEMC to assist in the process of converting chemical energy into electricity. First, there is an anode, which is a negatively charged component of the fuel cell. The anode is a conductor of the free electrons associated with the hydrogen fuel, in order to be channeled through an external circuit. On the other end of the fuel cell is a cathode, which is the positively charged component of the cell, which is used as the oppositely charged end of the circuit, allowing charge to flow through some load. Furthermore, the cathode connects a catalytic component of the cell to the anode, so the hydrogen can combine with oxygen at the end of the process to form the byproduct of the reaction, water. The electrolyte separates the anode and cathode, and is made of a material that only conducts positively charged ions, forcing the electrons to flow through an external circuit into the load, producing electric power. On one end of the cell, pressurized hydrogen gas enters the cell, is comes into contact with the catalyst and subsequently the electrolyte, which in turn separates the free hydrogen electrons and forces them into an external conductive circuit. On the other side of the cell, pressurized oxygen gas is forced into the cathode. [2] These oxygen molecules have a negative charge, and attract the hydrogen ions through the electrolyte, forming water molecules as the byproduct of the conversion. While such a process in a single cell produces small amounts of electricity, adding multiple cells into a single array, or stack, can produce enough electricity to power larger machines, such as cars. [3]
While, in theory, an ideal hydrogen fuel cell would produce no harmful emissions or pollutants, in practice there are several factors that pose risks to the environmental impact of hydrogen fuel cells. In contemporary technology, the hydrogen used in the fuel cell process is sourced from commercial sources. As such, the hydrogen used in the process comes primarily from the reformation of hydrocarbons, such as methane, and methanol. Therefore, while the fuel cell itself does not produce harmful emissions, the constituent materials needed for the process are primarily produced through harmful processes. To mitigate these concerns, the implementation of filtration systems proves costly. Furthermore, air is a plentiful and cheap way to fuel the cell stack. However, there are many pollutants within the air, including nitrogen and carbon oxides, which contaminate the fuel cell, leading to degraded performance, and a shortened lifespan. Organic compounds from the pressurized air can degrade the catalytic surfaces and the electrolyte, inhibiting the transportation of electrons in the external circuit, and positively charged ions through the electrolyte. More specifically, contaminants cause two large problems with fuel cell efficiency: they reduce the ability of ions and electrons to move throughout the materials, and increase the resistance of the components, leading to marginal ohmic losses, Various levels of pollution in the air have large consequences on the output of the fuel cell, and filtering to reduce the harm is costly. [1]
While developments in fuel cell technology represent a significant step towards creating a viable means to power devices on a large scale, issues of fuel contamination and component degradation continue to prove problematic in scaling fuel cells. The cheapest production methods of hydrogen fuel to feed the cell rely on natural gas, proving five times cheaper than hydrogen production through wind-based electrolysis. [4] The future viability of hydrogen fuel cells will depend on advances in hydrogen production, and cheaper filtration methods to prevent degradation of fuel cells.
© Scott White. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] X. Cheng et al., "A Review of PEM Hydrogen Fuel Cell Contamination: Impacts, Mechanisms, and Mitigation." J. Power Sources 165, 739 (2007).
[2] S. Gamburzev and J. Appleby, "Recent Progress in Performance Improvement of the Proton Exchange Membrane Fuel Cell (PEMFC)" J. Power Sources 107 5 (2002).
[3] D. L. Trimm and Z. L. Önsan. "Onboard Fuel Conversion For Hydrogen-Fuel-Cell-Driven Vehicles." Catal. Rev. 4, 31 (2001)t.
[4] M. Granovskii, I. Dincer and M. A. Rosen, "Life Cycle Assessment of Hydrogen Fuel Cell and Gasoline Vehicles," Int. J. Hydrogen Energy 31, 337 (2006).
