(written by: Willy Yanto Wijaya)
There are many types of fuel cells, which are mostly classified based on the types of the electrolytes or fuels being used. Here, we will discuss briefly several most common types of fuel cells, as well as their characteristics.
1. PEMFC (Proton Exchange Membrane Fuel Cell)
PEMFC has a solid polymer membrane as an electrolyte with the working temperature range 50-120°C which is quite low compared with other types of fuel cell. PEMFC also has high power density, it can vary its output quickly to meet shift in power demand. PEMFC also has low weight and volume with good power-to-weight ratio. These characteristics make PEMFC suitable for mobile and automotive applications. The efficiency of PEMFC ranges between 40-60%. However, PEMFC is sensitive to fuel impurities, such as CO poisoning. Besides, efforts should also be made to decrease the cost of expensive platinum catalyst (and thus expensive cost of its output electricity)
2. DMFC (Direct Methanol Fuel Cell)
DMFC has the same electrolyte as PEMFC, while the fuel input is methanol. The temperature range of this DMFC is slightly higher than PEMFC, ranging 90-120°C. Currently, DMFC is limited in the power it can produce, but it can still store a high energy content in a small space. This means DMFC can produce a small amount of power over a long period of time. These characteristics of DMFC make it suitable for tiny to mid-sized applications, to power cellular phones and laptops.
3. SOFC (Solid Oxide Fuel Cell)
SOFC uses a hard, non-porous ceramic compound as the electrolyte, and operate at high temperature; about 800-1000°C. High temperature operation eliminates the need for precious metal catalyst, and SOFC is also tolerant to CO poisoning. Several advantages of SOFC include high efficiencies (60-65%), long term stability, fuel flexibility, low emissions, and cost. SOFC is well-suited for stationary applications; medium-to-large scale, on-site power generation. However, due to its high operating temperature, SOFC requires longer start-up time as well as some mechanical/ chemical compatibility issues.
4. MCFC (Molten Carbonate Fuel Cell)
MCFC operates at 600-750°C, using molten alkali carbonate mixture (such as NaHCO3) as the electrolyte. MCFC has high efficiency (50-60%), tolerant to carbon poisoning, able to use non-precious metals as catalyst thus reducing cost. However, MCFC faces some problems related to durability due to its high operating temperature and the corrosive nature of its electrolyte.
5. PAFC (Phosphoric Acid Fuel Cell)
PAFC uses molten phosphoric acid (H3PO4) as its electrolyte, with working temperature 150-200°C. With this temperature condition, compared with PEMFC, PAFC is more tolerant to impurities. PAFC can tolerate a CO concentration of about 1.5%. PAFC, with efficiency about 40%, are commercially available, widely installed in many facilities, suited for stationary applications. However, PAFC solidifies at a temperature of 40°C, making startup difficult and restraining PAFC to continuous operation.
6. AFC (Alkaline Fuel Cell)
The electrolyte of AFC is solution of potassium hydroxide, with typical operating temperature about 70°C. It has been used by NASA to produce power and drinking water for astronauts since 1960. AFC can reach high efficiency up to 60-70%. However, AFC can be poisoned easily by small quantities of carbon dioxide, that’s why AFC typically operate on pure oxygen (causing cost increase), and they are mostly used in controlled aerospace or underwater applications.
A schematic of typical Proton Exchange Membrane Fuel Cell (PEMFC)