17.5 Batteries and Fuel Cells

Batteries and Fuel Cells

Batteries and fuel cells both convert chemical energy into electrical energy using redox reactions. Electrons flow from the anode to the cathode through an external circuit, while ions move through the electrolyte to complete the circuit internally. Understanding how these devices work ties together the core electrochemistry concepts from this unit.

Components of Common Batteries

Every battery has the same basic parts working together:

• Anode (negative electrode): This is where oxidation happens. The anode releases electrons into the external circuit. In an alkaline battery, the anode is made of zinc.
• Cathode (positive electrode): This is where reduction happens. The cathode accepts electrons from the external circuit. In an alkaline battery, the cathode is manganese dioxide.
• Electrolyte: A medium that allows ions to flow between the anode and cathode, completing the internal circuit. It can be a liquid, gel, or solid. Alkaline batteries use potassium hydroxide ($KOH$) as the electrolyte.
• Separator: A physical barrier that prevents the anode and cathode from touching directly (which would short-circuit the battery) while still allowing ions to pass through. Lithium-ion batteries, for example, use a thin polyethylene membrane.

A helpful way to remember the setup: oxidation at the anode (a for anode, a for "away" go the electrons), reduction at the cathode.

Electrochemistry in Batteries and Fuel Cells

Redox reactions drive the energy conversion in both batteries and fuel cells. A few key concepts connect here:

• Cell potential (also called electromotive force, or emf) determines the voltage a battery or fuel cell can produce. It depends on the specific materials used for the anode and cathode.
• The Nernst equation relates cell potential to the concentrations of reactants and products. As a battery discharges and reactant concentrations drop, the cell potential decreases, which is why batteries "die" gradually.
• Faraday's laws of electrolysis describe the quantitative side of electrochemical reactions. They let you calculate how much material is consumed or produced based on the amount of charge that flows.

Fuel Cells vs. Combustion Engines

Fuel cells and combustion engines both use chemical fuels, but they convert that energy in very different ways.

The big takeaway: fuel cells skip the intermediate step of converting energy to heat, which is why they're more efficient. Combustion engines lose a large portion of their energy as waste heat.

Advantages and Limitations of Rechargeable Batteries

Rechargeable (secondary) batteries can be charged and discharged many times, unlike single-use (primary) batteries. Common types include lithium-ion and nickel-metal hydride.

Advantages:

• Reusable: Can be recharged hundreds or even thousands of times, reducing waste compared to disposable batteries.
• Cost-effective long term: The higher upfront cost pays off because you aren't constantly buying replacements.
• Higher energy density: Rechargeable batteries like lithium-ion store more energy per unit volume than most non-rechargeable alternatives, making them ideal for phones and laptops.

Limitations:

• Higher initial cost compared to a single disposable battery.
• Charging time: You need to wait for the battery to recharge rather than just swapping in a fresh one.
• Limited charge cycles: Performance degrades over time. A lithium-ion battery might last 500–1,000 full charge cycles before its capacity drops significantly.
• Safety concerns: Damaged or improperly used rechargeable batteries can overheat or, in rare cases, catch fire.
• Environmental trade-offs: Mining the materials (lithium, cobalt) raises environmental and ethical concerns, and proper recycling infrastructure is still developing.