Fuel Cells

Fuel cells are galvanic cells that continuously convert chemical energy in the fuel to electrical energy. Unlike primary cells, their reactants are continuously supplied externally rather than stored internally.

In typical fuel cells, the reactants include a fuel and oxygen gas. Similar to primary galvanic cells, the cathode is where the oxidising agent (\(\ce{O2}\)) is reduced, serving as the positive terminal, while the anode is where the fuel is oxidised, serving as the negative terminal.

Use this page to revise the following concepts within fuel cells:

The common design features of fuel cells

Fuel cells can continuously generate electrical energy as long as fuel is supplied, making them ideal for applications requiring uninterrupted power, such as hospitals. There are some special features that enable this continuous operation.

Fuels in fuel cells are often gas or liquid, so that a continuous flow of the fuel can be supplied. Typical fuels include hydrogen (\(\ce{H2}\)), methane (\(\ce{CH4}\)), and methanol (\(\ce{CH3OH}\)).

Similar to galvanic cells where the two reactants are separated, the electrodes in the fuel cells physically separate the fuel and oxidising agents to prevent a direct reaction, allowing controlled electron transfer through the external circuit.

To maximise the conversion of chemical energy in the fuel to electrical energy, the oxidation/reduction reactions must happen at a high rate efficiently. This requires some design features, such as:

  • Using porous electrodes: They provide a large surface area for the reactions to occur, facilitating better interaction between the fuel and the electrode.
  • Using catalysts: Catalysts are used to lower the activation energy of the oxidation/reduction reaction, enabling faster and more efficient reactions. These catalysts are typically coated onto the surfaces of porous electrodes.

The electrolyte of the fuel cell ensures the completion of the circuit and maintains the electroneutrality of the cathode and anode. The types of electrolytes vary depending on the type of fuel cell but they all allow certain types of ions to move across.

  • Acidic or alkaline electrolytes (e.g. phosphoric acid or potassium hydroxide) where \(\ce{H^{+}}\) or \(\ce{OH^{-}}\) can move freely.
  • Polymer membranes are used in modern designs for their efficiency and selectivity, to allow certain ions to move across the membrane e.g., proton exchange membranes (\(PEM\)).
  • Ceramic oxides (e.g. zirconium oxide) are used in high-temperature fuel cells, such as solid oxide fuel cells (\(\ce{SOFCs}\)).

The choice of electrolyte affects the operating temperature, efficiency, and type of fuel used in the cell.

The sustainability of fuel cells

The sustainability of fuel cells lies in their potential to address key environmental challenges while providing efficient energy solutions to the current generation. Both energy efficiency and renewability of fuel cells impact the sustainability of fuel cells.

Energy efficiency of fuel cells

Fuel cells have the same overall reactions as combustion reactions in fire power plants and combustion engines. However, fuel cells have a higher efficiency because they convert chemical energy directly into electrical energy without the intermediate step of heat generation. This reduces energy losses and increases overall energy efficiency. Generally speaking, fuel cells can achieve efficiencies of up to 60% for electricity generation, and this value could be even higher when combined with heat recovery systems.

Renewability of fuel cells

If a renewable source of fuel is used, fuel cells could be a renewable energy solution. Common renewable fuels include biofuels that are produced from biomass, e.g. biogas, bioethanol and biodiesel, or H2 that is sourced in renewable ways.

Case study: fuel cells using H2

Hydrogen can be considered as a renewable fuel if it is produced from electrolysis of water using renewable energy sources, like wind, solar, or hydro power.

Using hydrogen gas generated from renewable energy in fuel cells offers several advantages:

  • Similar to other fuel cells, it has a higher energy efficiency in converting chemical energy to electrical energy.
  • Similar to combustion engines that use hydrogen as fuel, using H2 for fuel cells has no carbon emissions.
  • It reduces the reliance on fossil fuels, lowering greenhouse gas emissions, and supports global efforts toward achieving carbon neutrality.