Platinum catalyst in fuel cells

A fuel cell is a power generation device that converts energy into electricity with very high efficiencies. when run on hydrogen and air, hydrogen and oxygen molecules combine to provide electricity with water as the only byproduct. The key to making a fuel cell work is a catalyst, which facilitates the reaction of hydrogen and oxygen. The most common, but expensive, catalyst is platinum. Currently, the amount of platinum catalyst required per kilowatt to power a fuel cell engine is about 0.5 to 0.8 grams. The platinum is the only preferred catalyst and is the major contributor to the cost of such fuel cells. The reasons for the higher activity of Pt alloys for oxygen reduction are due to the measured improvement in the stability to sintering, surface roughening due to removal of some base metal which increases the Pt surface area, preferential crystal orientation, a more favourable Pt-Pt interatomic distance, electronic effects and oxygen adsorption differences due to modified anion and water adsorption.

Direct methanol fuel cell

The direct methanol fuel cell (DMFC) is a variant of the proton exchange membrane (PEM) fuel cell and uses aqueous methanol directly without prior reforming. In the DMFC methanol is converted to carbon dioxide and hydrogen at the anode. The hydrogen then reacts with oxygen, as in a standard PEM fuel cell. Conventional materials for DMFCs include platinum-ruthenium (Pt-Ru) for the electrode electrocatalysts and carbon in various forms as the electrocatalyst support. Electrocatalysts with high activity for methanol oxidation are essential for improved performance of DMFCs. Such catalysts are generally prepared as unsupported metal colloids or nanocomposites with the metal nanoparticles supported on an electrically conducting carbon of high surface area. Mixed metal Pt-containing catalysts are presently used for methanol oxidation. Researchers at University of Minnesota, U.S.A., have developed a Pt-Ru/graphitic carbon nanofibre (GCNF) nanocomposite which exhibits high relative performance as a DMFC anode catalyst.

Membrane electrode assembly (MEA)

The MEA is the key component in a Proton exchange membrane fuel cell where hydrogen and air react electrochemically to generate electrical power. It is a five layer structure containing at the centre the proton exchange membrane electrolyte which separates the electrode structures to prevent reactant gas mixing and the formation of an electrical short. Each electrode consists of a gas diffusion substrate with the platinum based (Pt) electrocatalyst layers located between the membrane and the substrate. The electrocatalyst can be deposited in the case of a PEMFC either on the gas diffusion substrate or on the proton conducting membrane electrolyte using techniques such as screen printing, flexographic printing, gravure printing, spraying or rolling and calendering. Electrocatalyst layers are typically from 5 to 20 μm thick with the complete MEA being around 400 to 500 μm thick. The MEA layers are normally bonded together by hot pressing catalysed substrates to the membrane or, in the case of catalysed membranes, by compressing the gas diffusion substrate to the membrane during stack assembly.

Proton exchange membrane fuel cell (PEMFC)

This type of fuel cell can be chosen for a wide range of applications such as portable, stationary and transportation due to high power density and relatively low temperature of operation. Present day PEMFC typically operates at close to 80ºC although there is a desire to move to higher temperatures close to 150ºC to mitigate the effects of carbon monoxide (CO) poisoning at the anode. The membrane electrode assembly (MEA) is the key component where hydrogen and air react electrochemically to generate electrical power. The MEA is typically located between a pair of flow field plates to give a single cell. The flow field plates are designed to distribute the reactant gases across the face of the MEA and also to collect the electrical current from the MEA. Sufficient unit cells are connected electrically to generate the desired power output. Depending on the application a PEMFC system may contain from tens to a few thousand MEAs to produce from a few watts to several hundred kilowatts of power.

Operation of biological fuel cells

Unlike chemical fuel cells, biological fuel cells operate under mild reaction conditions, namely ambient operational temperature and pressure. They also employ neutral electrolyte and use inexpensive catalysts. In biological fuel cells, the catalyst is either a microorganism as simple as Baker’s yeast or an enzyme. Biological fuel cells convert the chemical energy of carbohydrates, such as sugars and alcohols, directly into electric energy. As most organic substrates undergo combustion with the evolutionof energy, bio-catalysed oxidation of organic substances by oxygen at the two electrode interfaces provides a means for the conversion of chemical energy into electrical energy. In normal microbial catabolism, a substrate such as carbohydrate is oxidized initially without participation of oxygen, while its electrons are taken up by an enzyme-active site, which acts as a reduced intermediate.

Types of biological fuel cells

Microorganisms can be used in four ways for producing electrical energy: (i) Microorganisms can produce electrochemically active substances through fermentation or metabolism. For the purpose of energy generation, fuels are produced in separate microbial bio-reactors and transported to the anode of a conventional fuel cell, (ii) In the second configuration, the microbiological fermentation process proceeds directly in the anodic compartment of the fuel cell,(iii) In the third configuration, electron-transfer mediators shuttle electrons between the microbial bio-catalytic system and the electrode. The mediator molecules accept electrons from the biological electron transport chain of the microorganisms and transport them to the anode of the biological fuel cell, (iv) In the fourth configuration, the metal-reducing bacterium having cytochromes in its outer membrane and the ability to communicate electrically with the electrode surface directly result in a mediator-less biological fuel cell.

Biological fuel cells

Algae and bacteria were among the first organisms used in biological fuel cells. First biological fuel cell used Clostridium butyricum as a biological material to generate hydrogen by glucose fermentation. In 1963,biological fuel cells were already commercially available for use as a power source in radios, signal lights and other appliances at sea. However, these fuel cells were not a commercial success and soon disappeared from the market. With the successful development of technical alternatives, e.g. solar photovoltaics for the energy supply on space flights, biological fuel cells suffered a short setback.