Energy from fuels: R1.3.5 Fuel cells IB DP Chemistry Study Notes - New Syllabus 2025

Reactivity 1.3.5 — Fuel Cells

Fuel cells are electrochemical devices that convert the chemical energy of a fuel (typically hydrogen or methanol) and an oxidizing agent (usually oxygen) directly into electrical energy through redox reactions. Unlike combustion engines, fuel cells do not involve thermal energy conversion; hence, they offer higher efficiency and lower emissions.

Key Features of a Fuel Cell:

• Continuous supply of fuel and oxidant maintains energy output as long as they are provided.
• Operates under relatively low temperatures compared to combustion systems.
• Produces electricity, water, and sometimes heat as by-products.
• No combustion involved — therefore, no greenhouse gases are emitted from the process itself (especially in hydrogen fuel cells).

Main Components:

• Anode: Site of oxidation (fuel loses electrons).
• Cathode: Site of reduction (oxidant gains electrons).
• Electrolyte: Allows ion transfer between the electrodes but blocks electron flow.
• External Circuit: Electrons flow through this, producing electric current.

Electrode Half-Equations in Fuel Cells

Fuel cells rely on redox reactions at electrodes to convert chemical energy into electrical energy.

• Oxidation occurs at the anode (fuel loses electrons).
• Reduction occurs at the cathode (oxidant gains electrons).
• Electrons flow through the external circuit, powering electrical devices.

Types of Fuel Cells

1. Hydrogen-Oxygen Fuel Cell (in Acidic Medium)

Uses hydrogen gas as the fuel and oxygen as the oxidant. Water is the only product, making this a clean energy source.

Half-Equations of Hydrogen-Oxygen Fuel Cell:

• Anode (oxidation): $ \text{H}_2(g) \rightarrow 2\text{H}^+(aq) + 2e^- $
• Cathode (reduction): $ \frac{1}{2}\text{O}_2(g) + 2\text{H}^+(aq) + 2e^- \rightarrow \text{H}_2\text{O}(l) $

Overall Reaction:

$ \text{H}_2(g) + \frac{1}{2}\text{O}_2(g) \rightarrow \text{H}_2\text{O}(l) $

Key Notes:

• Only water is produced — no CO₂.
• Efficient and environmentally friendly when hydrogen is sourced sustainably.

Advantages

• Produces only water — no greenhouse gases if hydrogen is sustainably sourced.
• High efficiency (~60%) compared to internal combustion engines.
• Quiet operation — minimal moving parts.
• Suitable for clean environments (e.g., space, hospitals).

Disadvantages

• Hydrogen is difficult to store — requires high pressure or cryogenic tanks.
• Currently limited infrastructure for hydrogen refueling.
• Hydrogen production may still rely on fossil fuels (e.g., steam reforming).
• Platinum catalysts increase cost.

2. Methanol Fuel Cell (Direct Methanol Fuel Cell – DMFC)(in Acidic Medium)

Uses liquid methanol (CH₃OH) as the fuel. These are easier to store and transport than hydrogen.

Half-Equations:

• Anode (oxidation): $ \text{CH}_3\text{OH}(aq) + \text{H}_2\text{O}(l) \rightarrow \text{CO}_2(g) + 6\text{H}^+(aq) + 6e^- $
• Cathode (reduction): $ \frac{3}{2}\text{O}_2(g) + 6\text{H}^+(aq) + 6e^- \rightarrow 3\text{H}_2\text{O}(l) $

Overall Reaction:

$ \text{CH}_3\text{OH}(aq) + \frac{3}{2}\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(l) $

Key Notes:

• Emits CO₂ — not as clean as hydrogen fuel cells.
• More practical storage and transport (liquid fuel).

Advantages

• Liquid methanol is easier and safer to store than hydrogen gas.
• Simple refueling process — ideal for portable devices and backup systems.
• No need for high-pressure tanks or cryogenic systems.
• More compact suited for small-scale and consumer applications.

Disadvantages

• Emits CO₂: Not fully carbon-neutral.
• Lower efficiency (~30–40%) than hydrogen fuel cells.
• Toxicity and flammability: Methanol is poisonous if ingested or inhaled, and spills pose fire hazards, making safe handling and storage important.
• Catalyst reliance: Still requires precious metal catalysts, increasing cost.

Advantages of Fuel Cells:

• High efficiency (up to 60%) compared to internal combustion engines.
• Environmentally friendly — hydrogen fuel cells emit only water vapor.
• Quiet operation — no mechanical engine noise.
• Can run continuously as long as fuel and oxygen are supplied.

Disadvantages of Fuel Cells:

• Hydrogen is difficult to store and transport safely (requires high pressure or cryogenic conditions).
• Hydrogen production may still rely on fossil fuels (unless using electrolysis with renewable energy).
• Fuel cells can be expensive due to precious metal catalysts (e.g., platinum).
• Methanol emits CO₂ — though less than fossil fuels, it is not fully carbon-neutral.

Applications:

• Electric vehicles (e.g., hydrogen-powered cars like the Toyota Mirai)
• Backup power systems (e.g., hospitals, satellites)
• Portable energy sources for electronics and military