A fuel cell is an electrochemical device that directly converts chemical energy of oxidant and externally supplied fuel to electrical energy. Comparing with heat engines, fuel cells have higher conversion efficiencies. As an environmental aspect, currently fuel cell emissions are less adverse than heat engine (5). There are number of fuel cell technologies such as alkali fuel cell (AFC), direct methanol fuel cell (DMFC), proton exchange membrane fuel cell (PEMFC), phosphoric acid fuel cell (PAFC), solid oxide fuel cell (SOFC) and molten carbonate fuel cells (MCFC). PEMFC use a quasi- solid electrolyte unlike other fuel cell, which is based on a polymer backbone with side chains possessing acid based groups (3). PEMFC is the new type of fuel and it's believed to be the best type of fuel cell. This fuel cell was first developed at General Electric in the 1960s for spacecraft and military applications. It has attracted considerable attention due to their potential for clean, environmentally friendly and power generation.
PEMFC occurs through a direct electrochemical reaction and takes place silently without combustion. It uses a solid polymer membrane (a thin plastic film) as an electrolyte. The membrane is made light and small as possible (50µm) to catalyse the reaction. The porous carbon electrodes contains a noble-metal catalyst typically platinum used to separate the hydrogen protons and electrons. The polymer membrane need hydrogen (protons), water, and oxygen from air to function and does not require corrosive fluids like most fuel cells.
Figure 1 - Electrochemical reaction within a Proton Exchange Membrane Fuel Cell (4)
This type of fuel is hydrogen, while the oxidant is oxygen. Hydrogen fuel is supplied through field flow plates to the anode on one side of the fuel cell, while oxygen, usually from the air is supplied to the cathode on the other side of the cell. At the anode, a platinum catalyst causes the hydrogen to split into hydrogen ions (protons) and negatively charged electrons. PEMFC allows only the positively charged ions to pass through to the cathode. The negatively charged electrons must travel along an external circuit to the cathode, creating an electrical current. At the cathode, the electrons and positively charged hydrogen ions combine with oxygen to form water, which flows out of the cell. The reactions at the electrodes are (2):
Anode Reactions:
2H2 => 4H+ + 4e-
Cathode Reactions:
O2 + 4H+ + 4e- => 2 H2O
Overall Cell Reactions:
2H2 + O2 => 2 H2O
By using a platinum catalyst the splitting of the hydrogen molecule becomes fairly easy. On the other hand the splitting of stronger oxygen molecule becomes rather difficult that causes significant activation loss. Up to now platinum catalyst is still the best selection for oxygen reduction reaction. An additional significant source of losses is the resistance of the membrane to proton flow, which minimised by making it as thin as possible (around 50µm). (3).
The advantages of PEMFC compare to other fuel cell are that they produce more power for a given weight or volume of fuel cell. The high power density features makes them lightweight and compact. PEMFC operates at low temperatures, around 60-100°C (1). Low temperature process allows the fuel cell to start rapidly (less warm-up time) and causes less wear on the material construction, resulting in better durability. As a result to these characteristic they have been favoured for applications such as portable power, vehicles, and backup power applications. Another advantage is that the electrolyte is a solid rather than a liquid. The sealing of the cathode and anode gases is easier and therefore less costly to manufacture than other types of fuel cell. In addition, the solid electrolyte has less trouble with corrosion leading to longer cell and stack life.
Low operating temperature can also be disadvantage as temperature around 100°C are not high enough to perform useful cogeneration (2). The problem with the PEMFC is that the component pieces are expensive. The precious metal catalysts (usually platinum), gas diffusion layers, and bipolar plates make up 70 percent of a system's cost (6). When a car is made using this system, the total cost of the fuel cell vehicle is 10 times that of a traditional car with an internal combustion engine (7). Therefore research must find an alternative catalyst.
Non - platinum catalyst
To improve the performance of PEMFC it requires high durability and activity of electro catalyst. As previously mentioned, the platinum catalyst comprises a large portion of the PEMFC's cost and it's easily poisoned by carbon monoxide. One of the researcher stated that platinum catalyst accounts for 55% of the total cost (10). Therefore the platinum catalyst for PEMFC commercialisation is not realistic. Researcher must discover non-noble catalyst that is feasible for the PEMFC. One of the ways for cutting down the Pt consumption in PEM-systems is to reduce metal loadings (12).
Researchers have been using cheaper materials such as nitrogen, iron, and carbon/palladium as a catalyst for oxygen reduction reaction to replace platinum catalyst. However these materials have been shown too slow for practical. Another researcher has shown that ruthenium - platinum catalyst has been shown particularly effective at oxidizing carbon monoxide to form carbon dioxide which is a less harmful fuel contaminant (11).
Gold catalyst is approximately half the cost of platinum on a weight for weight basis (13). Recent have shown that gold - platinum catalysts have excellent stability, high durability and activity (14) which therefore could be a promising candidate for electro catalyst for PEMFC.
Platinum with its high stability and activity and despite of its cost remains the best catalyst for hydrogen PEMFC (15). Other potential catalysts such as gold, metal oxides, and carbon are still early stage of development and do not offer same activity as platinum.
The applicants of PEMFC
PEMFC has many different applications such as distributed power, transportation, portable, and stationary applications. Buses and cars are suitable for this type of fuel cell because of its fast start up time, favourable power to weight ratio and low sensitivity to orientation.
Figure 2 - Total number of PEMFC units installed globally by application (8)
Due to rapid technologies advances the PEMFC in many applications is now on the verge of commercialisation especially for portable markets as seen from the graph above producing the most units. The portable application such as mobile phones are very good applicants because of its compactness. Transportation is mostly likely to be last of all applications to reach full commercialisation due to low energy density of hydrogen. It is difficult to store enough hydrogen on-board to allow vehicles to travel the same distance as gasoline-powered vehicles before refuelling (9). However people can simply see the potential of PEMFC for transportation as it gives friendly environment energy.
Even though there are promising prospects of PEMFC, there are still remaining problems for it to replace energy system successfully and economically. The three major problems to each common application are cost, various technology problems and low hydrogen purity. Other problems for this fuel type are scale- up from single cells to cells stacks and CO poisoning of platinum catalyst.
Figure 3 - Relative prospects of PEMFC in various applications based on the current status of PEMFC technology (7):
Application
Prospect
Main reason
Competition
Comments
Transportation
Bus, RV, Lightweight vehicle
The most positive
More space for equipment of the fuel processor
None
PEMFC-based hybrid system desired
Passenger car
Positive
Health benefits for people
ICE-based hybrid system without PEMFC
PEMFC-based hybrid system desired
Powered bicycle
Less positive
Inconvenient for hydrogen supplies
Battery
Batteries or hybrid system desired
Leisure applications, (Sailing yacht)
Positive
Bottled LPG is wide spread
DMFC, DBFC
Used as a APU
Stationary
Stationary power generator (middle scale)
Less positive
Challenge of stable hydrogen supplies with high purity
MCFC, SOFC
MCFC, SOFC desired
Uninterrupted power supply (small scale)
Positive
Possible for long blackout periods
Battery
Hybrid system desired
Portable
Portable computer
The least positive
Impossible hydrogen supplies as the liquid
Batteries, DMFC, DBFC
DMFC or DBFC desired
According to this researcher (7) portable computer is the least positive prospect for PEMFC due to impossible hydrogen supplies. Buses, RV, and lightweight vehicle are the most positive prospect for PEMFC because of the large space for equipment of a fuel processor.
