The heart of a hydrogen car is the fuel cell, whose parts are illustrated in Fig. 3.51. Hydrogen is forced into the channels in the anode plate and is then spread out uniformly in the diffusion layer. This layer has been described as a wet rag whose moisture content must be carefully controlled to keep the proton exchange

ANODE BIPOLAR PLATE

HYDROGEN IN

CHANNELS

OXYGEN IN

CHANNELS

CATHODE BIPOLAR PLATE

Fig. 3.51 Schematic of a fuel cell. It is not to scale. The catalyst layers and the PEM are only 10’s of microns thick, while the diffusion layers are 100’s of microns thick. The bipolar plates are of macroscopic dimensions
membrane (PEM) from drying out without dripping. The PEM is a plastic layer like plastic wrap made of a special material called Nafion® made by Dupont Chemical. It has the magical property of allowing hydrogen ions (H+) to pass through but not electrons. It is the platinum catalyst layer that dissociates hydrogen gas (H2) and ionizes it into two hydrogen ions (H+). This is an even more magical property. The catalyst layer consists of platinum nanoparticles thinly deposited on carbon paper which has to be rough to present a large surface area and porous to let the water through. The electrons, being blocked by the PEM, are drained off into a wire to form the electric current that is the output of the cell. When the H+ ions reach the other side, they encounter another catalyst layer, which could be platinum or iridium. Meanwhile, oxygen (O2) from air is pushed into the cathode plate and diffusion layer to meet the hydrogen ions in the catalyst layer. Therefore, the O2 is dissociated into atoms (O) and picks up electrons from the wire that has gone through the load to become negative ions (O-). Each O — then combines with two H+s to form H2O. Hydrogen and oxygen have been combined to form water and electricity. All in all, the fuel cell is a serendipitous invention, but it has problems.

Each fuel cell generates only 0.6-0.7 V, so as many as 100 of them have to be connected in a series to form a stack with a useful voltage output. Platinum is a precious metal used in jewelry and in catalytic converters. Its price drives the price of fuel cells to about $73/kW, twice the commercially viable value.49 Cyclic opera­tion of PEMs degrades their performance. PEMs have to be heated to at least 60°C from a battery before they can even start, and they need about 100°C to operate reliably. The water in the cell must not boil or freeze under all driving conditions. Corrosion of the bipolar plates is a problem; they cannot be made of a metal that can corrode and contaminate the system with iron or chromium. A carbon com­pound has to be used. Besides the electric motor, the car has to have a system to pressurize the gases. And the fuel cell has to last for 300,000 miles.

Currently, the whole shebang is too large to fit inside a car but can be used in trucks. No large-scale production and testing has been done. What can be gained is a fuel-cell efficiency of 80% times another 80% efficiency of the electric motor, giving a maximum efficiency of 64% in the conversion of hydrogen energy to mechanical energy. This compares favorably with the efficiency of gasoline-driven cars, about 15%, but the energy in producing the hydrogen has not yet been counted. If that part is 40% efficient, the net efficiency is 64 x 40= 26%, still higher than burning natural gas in a gas engine. However, the real gain will be when hydrogen is pro­duced in fission or fusion plants with no use of fossil fuels or emission of GHGs.