Fuel Cell Technology.

IPM Technology

What is a fuel cell?

A fuel cell is an electrochemical conversion device that converts chemical energy stored in a fuel (e.g. hydrogen or methanol) directly into electricity without inefficient combustion. This process is facilitated by a PGM based catalyst, and occurs silently and very efficiently. For example, a diesel engine is about 20% efficient while a fuel cell is typically 50% efficient (electrical efficiency).

There are various types of fuel cell, named after the type of electrolyte that they use:

  • PEMFC: Proton Exchange Membrane or Polymer Electrolyte Membrane
  • DMFC: Direct Methanol Fuel Cell (utilises a PEM electrolyte)
  • PAFC: Phosphoric Acid Fuel Cell
  • SOFC: Solid Oxide Fuel Cell
  • MCFC: Molten Carbonate Fuel Cell
  • AFC: Alkaline Fuel Cell

A distinct advantage of fuel cells is that they can use a variety of fuels from a number of sources. For example, a PEMFC can use hydrogen from renewables (via electrolysis of water) or hydrogen from the reformation of hydrocarbon fuels such as natural gas. A DMFC can use chemical methanol or biomethanol from locally produced biomass.

However, only the PEMFC, DMFC and PAFC directly utilise PGM-containing catalysts (while some reformation technology also contains PGMs), and hence are of interest within the South African context from a platinum beneficiation perspective.

Components of a Fuel Cell System.

A fuel cell system is comprised of a number of different components that together make the complete system that enables a fuel to be efficiently turned in electricity. The central component is the membrane electrode assembly (MEA). This comprises the membrane, catalyst layers and gas diffusion layers (GDLs). In order to seal the MEA gaskets are added. Bipolar plates are used to assemble the individual cells into the stack and provide channels for the fuel and air to pass.

A complete fuel cell system

Polymer Electrolyte Membrane.

The polymer electrolyte membrane is a specially treated material that looks something like ordinary kitchen plastic wrap and conducts only positively charged ions and blocks the electrons. The PEM is the key to the fuel cell technology; it must permit only the necessary ions to pass between the anode and cathode (in this case H+ ions). Other substances passing through the electrolyte would disrupt the chemical reaction, and potentially poison the catalyst (e.g. carbon monoxide).

Catalyst Layers.

A layer of catalyst is added on both sides of the membrane—the anode layer on one side and the cathode layer on the other. Conventional catalyst layers include nanometer-sized particles of platinum dispersed on a high-surface-area carbon support. This supported platinum catalyst is mixed with an ion-conducting polymer (ionomer) and sandwiched between the membrane and the GDLs. On the anode side, the platinum catalyst enables hydrogen molecules to be split into protons and electrons. On the cathode side, the platinum catalyst enables oxygen reduction by reacting with the protons generated by the anode, producing water. The ionomer mixed into the catalyst layers allows the protons to travel through these layers.

Gas Diffusion Layers.

he GDLs sit outside the catalyst layers and facilitate transport of reactants into the catalyst layer, as well as removal of product water. Each GDL is typically composed of a sheet of carbon paper in which the carbon fibers are partially coated with polytetrafluoroethylene (PTFE). Gases diffuse rapidly through the pores in the GDL. These pores are kept open by the hydrophobic PTFE, which prevents excessive water build-up. In many cases, the inner surface of the GDL is coated with a thin layer of high-surface-area carbon mixed with PTFE, called the microporous layer. The microporous layer can help adjust the balance between water retention (needed to maintain membrane conductivity) and water release (needed to keep the pores open so hydrogen and oxygen can diffuse into the electrodes).

A single cell within a fuel cell stack

A fuel cell stack

Fuel Processing.

Fuel processing is the process of taking a primary fuel, such as diesel, natural gas, LPG or methanol, and converting it to a gaseous mixture that is suitable as a fuel for a specific type of fuel cell. The process itself can be simple, such as purification of the primary fuel feed.

More often than not the process involves several chemical reactors such as reformers, shift reactors and heat recovery. The key drivers in fuel processor design are effective coupling with the fuel cell subsystem to maximize efficiency through the use of waste heat and unutilized oxidant and fuel.

Many types of fuel processors contain catalysts based on metals mined in South Africa, such as rhodium. Effective and efficient fuel processing will enable a multi-fuel refuelling infrastructure to be rolled out which in turn will enable fuel cell deployment.