IB Chemistry - Fuel cells

IB Chemistry home > Syllabus 2025 > Redox processes > Fuel cells

A fuel cell is an electrochemical cell in which the components are steadily fed into the system as electrical energy is drawn from the system. Syllabus reference

Reactivity 3.2.7 - Secondary (rechargeable) cells involve redox reactions that can be reversed using electrical energy.

  • Deduce the reactions of the charging process from given electrode reactions for discharge, and vice versa.


  • Include discussion of advantages and disadvantages of fuel cells, primary cells and secondary cells.

Tools and links

  • Reactivity 2.3 - Secondary cells rely on electrode reactions that are reversible. What are the common features of these reactions?

Secondary cells

Secondary electrochemical cells, commonly known as rechargeable batteries, are energy storage devices that can be electrically recharged after use. Unlike primary cells, which are intended for single use, secondary cells can undergo multiple charge-discharge cycles. This capability is due to the reversible nature of the chemical reactions occurring within the cell.

The most common types of secondary electrochemical cells include lithium-ion, nickel-cadmium, nickel-metal hydride, and lead-acid batteries. Each type has its specific advantages and applications. For instance, lithium-ion batteries are known for their high energy density and are widely used in portable electronics and electric vehicles. Nickel-cadmium cells, though suffering from memory effect, offer robust performance in extreme temperatures. Nickel-metal hydride cells, with a similar form factor to nickel-cadmium, provide higher capacity and are more environmentally friendly. Lead-acid batteries, the oldest type, are cost-effective and reliable, commonly used in automotive and industrial applications.

The efficiency of secondary cells is a critical aspect, typically characterized by their energy density (amount of energy stored per unit mass or volume), cycle life (number of charge-discharge cycles before capacity falls below a certain threshold), and self-discharge rates. Advances in materials science and electrochemistry are continually improving these characteristics, leading to more efficient, durable, and environmentally sustainable secondary cells.


Primary cells

Primary electrochemical cells, commonly referred to as primary batteries, are a type of battery designed for single-use and cannot be recharged. These cells convert chemical energy into electrical energy through irreversible chemical reactions, meaning that once the reactants are exhausted, the battery can no longer produce electricity and must be replaced.

There are various types of primary cells, each with unique chemistries and applications. The most common types include alkaline batteries, zinc-carbon batteries, and lithium primary cells. Alkaline batteries, known for their long shelf-life and stable output, are widely used in household devices like remote controls and clocks. Zinc-carbon batteries are less expensive but have a lower energy density and shorter shelf life, making them suitable for low-drain applications like simple toys or flashlights. Lithium primary cells, with their high energy density and ability to operate in a wide range of temperatures, are ideal for critical applications in medical devices, military, and aerospace.

Primary cells are characterized by their energy density, shelf life, and temperature range of operation. They are typically chosen for applications where battery replacement is infrequent or where charging is impractical. Despite the advantages of rechargeable batteries, primary cells remain popular due to their convenience, reliability, and the ability to store them for long periods without significant loss of charge.


Fuel cells

Fuel cells are devices that convert the chemical energy of a fuel, often hydrogen, and an oxidizing agent, such as oxygen, directly into electricity, heat, and water. Unlike traditional combustion-based power sources, they do this through an electrochemical process, not combustion, making them inherently more efficient and environmentally friendly. The most common chemical reaction in fuel cells involves hydrogen and oxygen, with the general reaction being: 2H2 + O2 → 2H2O + electricity + heat.

There are several types of fuel cells, each with its own advantages and disadvantages. Proton exchange membrane (PEM) fuel cells, characterized by their low operating temperature and quick start-up, are ideal for automotive and portable applications. However, they require pure hydrogen fuel and have a relatively short lifespan. Solid oxide fuel cells (SOFC), operating at high temperatures, are highly efficient and can use a variety of fuels, but they face challenges in terms of thermal management and long start-up times. Alkaline fuel cells (AFC), used in space applications, offer high efficiency and operate at relatively low temperatures, but are sensitive to CO2 and require pure hydrogen and oxygen. Molten carbonate fuel cells (MCFC) are suitable for large-scale stationary power generation due to their high efficiency and fuel flexibility, but they have high operating temperatures and complex material requirements.

The advantages of fuel cells include high efficiency, low environmental impact, and the versatility in terms of fuel used (especially for types that can utilize fuels other than hydrogen). On the downside, fuel cells face challenges such as high costs, durability issues, and the need for infrastructure for hydrogen production, storage, and distribution, especially for those types that require pure hydrogen.

Methanol fuel cells

Methanol fuel cells are a type of fuel cell that use methanol as their primary fuel source. These cells operate on the principle of converting the chemical energy of methanol directly into electrical energy through an electrochemical process. The most common type of methanol fuel cell is the Direct Methanol Fuel Cell (DMFC), which is known for its ability to use methanol in a liquid form, making it more convenient and energy-dense compared to gaseous fuels.

The basic chemical reaction in a DMFC involves methanol (CH3OH) and water at the anode, producing carbon dioxide (CO2), protons (H+), and electrons. The overall reaction can be summarized as: CH3OH + H2O → CO2 + 6H+ + 6e-. At the cathode, oxygen from the air reacts with the protons and electrons to form water. This reaction generates electricity, heat, and water vapor as by-products.

Methanol fuel cells offer several advantages, including ease of fuel storage and transportation due to methanol's liquid state at room temperature, relatively simple fuel cell design, and lower operating temperatures compared to some other types of fuel cells. However, they also have some disadvantages such as lower efficiency compared to other fuel cell types, the production of CO2 as a by-product, and the need for careful handling of methanol, which is a toxic and volatile substance.


ColSol Testing