VOLUME I: ANOSTRUCTURED NITROGEN-CONTAINING CARBON
1.2 How a PEM Fuel Cell Works
Although fuel cells operate by the same principles as a battery, PEMFC’s use a replenishable fuel and a source of oxygen (usually air) to produce energy. PEMFC’s typically operate off of pure hydrogen as the fuel, but can also produce electricity directly from fuels such as methanol or formic acid. Figure 1 shows a schematic demonstrating how a PEMFC works. Hydrogen enters at the anode side, where it reacts to form protons and electrons on the anode catalyst. Alternatively, in a Direct Methanol Fuel Cell (DMFC), an equimolar methanol and water mixture can be used as fuel for the anode, where they react to form protons, electrons, and carbon dioxide. In a formic acid PEM fuel cell, formic acid (HCOOH) reacts at the anode to form two protons and carbon dioxide. The solid polymer electrolyte has low permeability to the reactants and electrons; however, protons can travel across the electrolyte to the cathode. Typically, a dense acidic polymer, such as Nafion, is used as the electrolyte to achieve this function.
Electrons are forced through an external circuit to produce electricity before reaching the cathode. At the cathode, protons and electrons react with oxygen to form water. The reduction of oxygen in the cathode is the most challenging reaction in a PEM fuel cell, as will be discussed in the following sections.
Currently, few materials possess the necessary properties for use as a fuel cell
electrode. The electrode of interest in this study is the cathode. Here are the five basic attributes that a PEMFC cathode must have for successful operation of the cell:
i.) High electrochemical activity for the Oxygen Reduction Reaction (ORR) – The cathode catalyst must be active for oxygen reduction, where oxygen reacts with protons and electrons to form water. The reaction is shown here:
O2 + 4 H+ + 4 e- → 2 H2O (1.2 V vs. NHE)
Thermodynamically, this reaction could occur at a voltage as high as 1.2 V vs.
Normal Hydrogen Electrode (NHE), if a catalyst with infinite activity existed.
ii.) Chemical stability – Obviously to function as a catalyst, the material must be stable in the cathode environment for an extended period of time. This is not trivial considering the low pH of the electrolyte, the high oxidizing potential under normal operation, and the active oxygen intermediates that form during the reaction. Therefore, oxidation and/or dissolution of the catalysts and support is often an issue.
iii.) Electrical conduction – the electrodes of the fuel cell must be able to conduct electrons to produce a usable current.
iv.) Proton conduction/mobility – protons must travel from the H2 adsorption sites to the anode/electrolyte interface, through the electrolyte to the cathode, and then meet up with activated oxygen species. Typically, the electrodes are doped with a proton conducting polymer (such as Nafion) that serves as a binding agent and connects the catalyst layer to the electrolyte.
v.) Morphology – At low current densities, having a higher surface area catalyst
allows for more available active sites and a higher kinetic current. At high current densities, good porosity allows for better mass transfer of oxygen into the cathode, while water must be transferred out. Therefore, having a mix of hydrophobic and hydrophilic pores is desired.
If the cathode lacks any of these properties, then it will have a detrimental effect on the efficiency and maximum power of the cell. For instance, resistance from slow reaction kinetics contribute significantly to voltage losses, as will be discussed in the following paragraph. While the kinetics for the oxidation of hydrogen are fast and contribute minimally (< 5% of the kinetic losses) [3], the same cannot be said for the cathode reaction. In the case of the ORR, the kinetics are exponentially dependant on the voltage in the cathode, as is the case for electron transfer reactions [4]. As current is drawn from the cell, the voltage drops from the open circuit value, and the kinetic current increases exponentially. However, since the kinetics for the ORR are slow over all known catalysts, a large potential drop always occurs before the current is measurable.
The worse the catalytic activity, the larger the potential drop incurred, meaning less efficiency and power. Similarly, resistance to proton conduction through the membrane or electrical resistance in the electrodes will decrease the voltage proportionally to current. With regards to morphology, poor mass transfer of oxygen into the cathode increases the mass transfer resistances, and inability to transfer water out of the cathode causes flooding, which completely cuts off the catalyst accessibility to oxygen.
Figure 2a shows individual contributions for potential losses due to kinetics, ohmic resistances, and mass transfer in a typical PEM fuel cell. Figure 2b shows a typical
characteristic curve of current versus potential for a PEM fuel cell, where the potential drop from the theoretical voltage of 1.2 V is a summation of the losses in Figure 2a.
Figure 2c shows the power density curve for a PEM fuel cell, which is obtained from the data in Figure 2b using the equation:
P = I * V
The efficiency of the conversion of chemcical energy into electrical energy was obtained from the equation:
ε = (Ecell / Etheoretical) * 100%.
From this series of graphs it is apparent how the required properties previously are necessary for high power output and high efficiency. The red line in the figures shows the effect of have better ORR kinetics. Better kinetics would mean the kinetic current takes off with less of a voltage drop. Correspondingly, the cell would operate with a higher electrical potential at equivalent currents. From Figure 2c, it is apparent that the improved kinetics improve both the power density of the fuel cell, and efficiency.
If there are large ohmic resistances in the cell, then the voltage will drop steeply with increasing current. Correspondingly, efficiency and power density would be lost. If mass transfer is poor in the cathode, then the onset of mass transfer voltage losses will become significant at lower currents, and again, the same negative effects would result.
Therefore, all the required properties discussed effect the power density and efficiency of the cell.
H2 → 2 H+ + 2 e-
Figure 1: Schematic drawing of how a PEM fuel cell works.
(a)
(b)
(c)
Figure 2: Characteristic curves for a typical PEM fuel cell.