Fuel cells are devices that generate electricity from a fuel source and oxygen in an electrochemical process. Hydrogen and natural gas are the two most common fuel types that are used in fuel cells.
The Opposite of Electrolysis
Let’s illustrate how fuel cells work by using hydrogen as the fuel source.
Electrolysis is a method to use electricity to drive otherwise non-spontaneous cheimcal reactions. Electrolysis of water is the splitting of water into its two components, hydrogen and oxygen. In a fuel cell, the opposite happens. In other words, hydrogen and oxygen are forced together and forms water and electricity.
Above is an illustration over what happens in a hydrogen fuel cell. The same principle behind the hydrogen fuel cell is the foundation of other types of fuel cells as well: Electricity is generated in response to the combination of oxygen and a fuel source.
Similar to Batteries
Even the battery is built on the same type of electrochemical reactions as fuel cells are. The obvious difference is that a fuel cell requires an outside source of fuel, while batteries acts as energy storages themselves, where chemical energy is stored internally and can be spent/recharged on demand.
The core of the fuel cell technology is the membrane that separates the air and fuel (anode, electrolyte and cathode). It is difficult to make a membrane with the right properties we need to construct a high-performing fuel cell.
In particular, a long enough lifetime of the membrane has been difficult to achieve. For mobile applications, such as for passenger cars, the fuel cell should be usable for at least 10 years. Estimations show that today’s fuel cells only have a lifetime of somewhere betewen 4000-5000 hours – the equivalent of driving 125 000 miles with a speed of 25-40 mph (or 200 000 km and 40-50 km/h).
Lifetime is also a great challenge when it comes to stationary applications of fuel cells. If we assume that these fuel cells should be usable for 15 years with a total annual operating time of 6000 hours, the fuel cell needs to have a lifetime of 90,000 hours. There is currently no evidence pointing to the fact that this can be achieved.
Maintenaining performance when the temperatures gets close to the freezing point is another major technical challenge. Since water is a byproduct of hydroge fuel cells during operation, ice can easily be formed at freezing temperatures, which unfortuantely can lead to damage of the thin membranes in the fuel cell. A car should be able to perform well during conditions below 32° F (0° C).
Fuel Cell Stacks
A single fuel cell can produce direct current electricity at a voltage of approximately 0.5-0.7 V. In order to get higher voltages, several fuel cells can be tied together in series known as a fuel cell stack. The amount of current produced is proportional to the surface area of ??the membrane. Therefore, fuel cells are easy to scale – fuel cells can be stacked together for higher voltage rates, while larger surface area provides higher current.
Regenerative Fuel Cells
Just as you have rechargeable batteries, fuel cells can also be designed to run both ways. These are referred to as regenerative fuel cells that can either operate in fuel cell mode or electrolysis mode, depending on whether it is consuming or producing hydrogen (or other fuel types). Regenerative fuel cells have lower efficiency rates those designed to either be pure fuel cells or in electrolysis mode.