Proton exchange membrane fuel cell and direct methanol fuel cell

Proton exchange membrane fuel cell

Proton Exchange Membrane Fuel Cell (PEMFC) uses perfluorosulfonic acid solid polymer as electrolyte, platinum/carbon or platinum nail/carbon as electrocatalyst, hydrogen or purified reformate gas as fuel, air or pure oxygen as oxidant, and graphite or surface-modified metal plate with gas flow channel as bipolar plate. The solid polymer material is the electrolyte. The electrolyte is a polymer material that can conduct ions, with catalytic porous electrodes bonded on both sides. The hydrogen reaches the anode through a pipe or a gas guide plate. Under the action of the anode catalyst, the hydrogen molecules dissociate into positive charges. The hydrogen ions and the negatively charged electrons are released, the hydrogen ions pass through the proton exchange membrane to the cathode, and the electrons reach the cathode through the external circuit. The working principle of the proton exchange membrane fuel cell is shown in Figure 1.

Proton exchange membrane fuel cell and direct methanol fuel cell
Figure 1 – The working principle of a proton exchange membrane fuel cell

The hydrogen in the anode catalytic layer undergoes an electrode reaction under the action of the catalyst:

H2→2H++2e

The electrons generated by the electrode reaction reach the cathode through the external circuit, and the hydrogen ions reach the cathode through the proton exchange membrane. Oxygen reacts with oxygen ions and electrons at the cathode to form water:

1/2O2+2H++2e→H2O

The resulting water does not dilute the electrolyte, but is exhausted with the reaction off-gas through the electrodes.

Proton exchange membrane fuel cells have the advantages of low operating temperature (about 80 ℃), fast start-up, high specific power, simple structure and convenient operation, and are recognized as the preferred energy source for electric vehicles and stationary power stations. Inside the fuel cell, the proton exchange membrane provides a channel for the migration and transport of protons, so that the protons pass through the membrane from the anode to the cathode, and form a loop with the electron transfer of the external circuit to provide current to the outside world. Therefore, the performance of the proton exchange membrane plays a very important role in the performance of the fuel cell, and its quality directly affects the service life of the cell. But there are still a series of problems to be solved. Chief among them is the manufacturing cost, since both membrane materials and catalysts are very expensive. However, research is being carried out to reduce costs, and once mass production is possible, the economic benefits of price comparison will be fully demonstrated.

Another big problem is that the batteries require pure hydrogen to work because they are highly susceptible to contamination by carbon monoxide and other impurities. This is mainly due to the fact that highly sensitive catalysts must be used when they operate at low temperatures. When they work with membranes that can work at higher temperatures, a more tolerant catalyst system must be created to work.

Direct methanol fuel cell

Direct Methanol Fuel Cell (DMFC) belongs to a kind of proton exchange membrane fuel cell, and its working principle is basically the same as that of the above-mentioned proton exchange membrane fuel cell. The methanol aqueous solution and steam methanol can be directly used as the fuel supply source, and there is no need to recombine methanol, gasoline and natural gas through a reformer and then extract hydrogen for power generation. The fuel of the direct methanol fuel cell is methanol (gaseous or liquid), and the oxidant is still air or pure oxygen. Methanol is converted into Co2, protons and electrons at the anode. Like a standard PEM fuel cell, the protons pass through the PEM and react with oxygen at the cathode. The electrons reach the cathode through an external circuit and do work.

Its anode and cathode catalysts are Pt-Ru/C (or Pt-Ru black) and PVC, respectively

Its anodic reaction is

CH3OH+H2O→CO2↑+6H++6e

Its cathodic reaction is

3/2O2+6e+6H+→3H2O

The fuel cell operates at a temperature of 120°C, slightly higher than standard proton exchange membrane fuel cells, and its efficiency is around 40%. The disadvantage is that when methanol is converted to hydrogen and CO2 at low temperature, it requires more platinum catalyst than conventional PEM fuel cells. However, the fuel cell can conveniently use liquid fuel and operate without reforming. The technology used in direct methanol fuel cells is still in the early stages of its development, but has successfully shown the potential to be used as a power source for mobile phones and laptops, with future potential for designated end-users.