Hydrogen Utilization: Fuel Cells - Hydrogen Generation, Storage, and Utilization - page 154

9.2 TYPES OF FUEL CELLS
Fuel cells come in many varieties; however, they all work in the same general
manner. As mentioned earlier, a typical fuel cell is made up of three adjacent
segments: the anode, the electrolyte, and the cathode. Two chemical reactions
occur at the interfaces of the three different segments. The net result of the
two reactions is that the fuel is consumed, water or carbon dioxide is gener-
ated, and an electric current is produced, which can be used to power electri-
cal devices, normally referred to as the load. Fuel cells are usually classified
by the electrolyte employed in the cell, as summarized in Table 9.1. Direct
methanol fuel cell (DMFC), with methanol directly fed to the anode, is
beyond this classification, and we will mainly discuss the fuel cells based on
hydrogen as fuel.
9.2.1 Alkaline Fuel Cell (AFC)
The AFC has the highest electrical efficiencies among fuel cells, but works
only with the pure gases, which is a restraint for wide applications [8]. The
AFC is also one of the first fuel cells used in space, and the first technologi-
cal AFC was developed by the Bacon group at the University of Cambridge
[9]. The initial electrodes used in AFC are Ni-based catalysts, sometimes
activated with novel metals. Pt-carbon electrodes are now widely used for
both the anode and cathode. The Pt-based alloys have been proved to have
higher activity than bare Pt for oxygen reduction due to a higher exchange
current density. Nowadays, various Pt-based alloys have been synthesized,
and their sizes, morphologies, and compositions have been widely explored
[10]. The alkaline KOH electrolyte is used in AFC, which has an advantage
over acid fuel cells in that oxygen reduction kinetics are much faster in it
than in acid electrolyte [11]. The circulating KOH also provides a good
barrier against gas leakage and can be used as a cooling liquid in the cell or
stack. In addition, since the high open circuit voltage of the KOH electrolyte-
based AFC cell may induce the carbon oxidation and produce carbonates
that are deleterious to the cell performance, circulating the KOH, instead of
using a stabilized matrix, can avoid the buildup of carbonates [12].
9.2.2 Proton Exchange Membrane Fuel Cell (PEMFC)
The PEMFC (originally referred to as the solid polymer electrolyte fuel cell)
was the first type of fuel cell to find an application—power source for
NASA's Gemini space flights in the 1960s [13]. This technology was dormant
for about 20 years thereafter. More recently, the California Environmental