Batteries and Fuel Cells

Batteries

Battery: A battery is a device consisting of one or more electrochemical cells that convert stored chemical energy into electrical energy. Batteries are essentially galvanic cells designed for practical applications.

Components: A battery consists of two half-cells (anode and cathode) separated by an electrolyte.

• Anode: Site of oxidation (negative electrode in a battery).
• Cathode: Site of reduction (positive electrode in a battery).
• Electrolyte: Facilitates ion transport between electrodes.

Types of Batteries: Batteries are broadly classified into primary and secondary batteries based on their rechargeability.

Primary Batteries

Definition: Primary batteries are cells that cannot be recharged once the reactants are exhausted. The redox reaction proceeds irreversibly, and the battery is discarded after use.

Characteristics:

• Non-rechargeable.
• Generally have a longer shelf life than secondary batteries.
• Often simpler and less expensive to manufacture.
• Used in low-drain devices like remote controls, clocks, and flashlights.

Examples:

• Dry Cell (Leclanché Cell):
• Anode: Zinc metal ($Zn$)
• Cathode: Carbon rod surrounded by Manganese dioxide ($MnO_2$) and powdered carbon.
• Electrolyte: Paste of Zinc chloride ($ZnCl_2$) and Ammonium chloride ($NH_4Cl$) with starch.
• Reactions:

Anode: $Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-$

Cathode: $2MnO_2(s) + H_2O(l) + 2e^- \rightarrow Mn_2O_3(s) + 2OH^-(aq)$ (simplified reaction)

• Voltage: Approximately 1.5 V.
• Alkaline Battery:
• Similar to dry cells but uses Potassium hydroxide ($KOH$) as the electrolyte (an alkaline medium).
• Anode: Zinc metal ($Zn$)
• Cathode: Manganese dioxide ($MnO_2$)
• Electrolyte: Potassium hydroxide ($KOH$) paste.
• Advantage: Provides a more stable voltage and longer service life than dry cells.
• Mercury Cell:
• Anode: Zinc amalgam ($Zn(Hg)$)
• Cathode: Mercuric oxide ($HgO$)
• Electrolyte: Potassium hydroxide ($KOH$) paste containing $NaCl$.
• Voltage: Approximately 1.35 V.
• Advantage: Maintains a very constant voltage throughout its life. Often used in watches and calculators.
• Lithium Cells:
• Use lithium as the anode and various cathode materials.
• Offer high energy density and long shelf life.
• Examples: $Li-MnO_2$ (button cells), $Li-SOCl_2$.

Secondary Batteries

Definition: Secondary batteries (rechargeable batteries) are cells where the redox reaction is reversible. The battery can be discharged (providing power) and then recharged by passing an electric current through it in the opposite direction, reversing the chemical reaction.

Characteristics:

• Rechargeable.
• More expensive initially but cost-effective over their lifetime due to multiple reuses.
• Used in applications requiring repeated use, like mobile phones, laptops, electric vehicles.

Examples:

• Lead Storage Battery (Lead-Acid Battery):
• Used in cars.
• Anode: Lead metal ($Pb$).
• Cathode: Lead dioxide ($PbO_2$).
• Electrolyte: Sulfuric acid ($H_2SO_4$) solution.
• Discharge Reactions:

Anode: $Pb(s) + SO_4^{2-}(aq) \rightarrow PbSO_4(s) + 2e^-$

Cathode: $PbO_2(s) + SO_4^{2-}(aq) + 4H^+(aq) + 2e^- \rightarrow PbSO_4(s) + 2H_2O(l)$

• Charging Reactions: The reverse of the discharge reactions.
• Voltage: Approximately 2.1 V per cell (usually 6 cells in series for 12V).
• Nickel-Cadmium (Ni-Cd) Battery:
• Anode: Cadmium ($Cd$).
• Cathode: Nickel(III) oxide hydroxide ($NiO(OH)$).
• Electrolyte: Potassium hydroxide ($KOH$) solution.
• Advantages: Relatively inexpensive, can be recharged many times.
• Disadvantages: Cadmium is toxic, and these batteries suffer from "memory effect".
• Nickel-Metal Hydride (Ni-MH) Battery:
• An improvement over Ni-Cd, using a hydrogen-absorbing alloy as the anode instead of cadmium.
• Higher energy density and less environmental impact than Ni-Cd.
• Lithium-ion Battery:
• Currently the most popular rechargeable battery technology for portable electronics and electric vehicles.
• Anode: Graphite (intercalated with lithium ions).
• Cathode: Metal oxides containing lithium (e.g., $LiCoO_2$, $LiMn_2O_4$, $LiFePO_4$).
• Electrolyte: Organic solvent containing lithium salt.
• Advantages: High energy density, low self-discharge rate, no memory effect.

Fuel Cells

Fuel Cell: A fuel cell is an electrochemical device that converts the chemical energy of a fuel (like hydrogen) and an oxidant (like oxygen) directly into electrical energy through a redox reaction. Unlike batteries, fuel cells require a continuous supply of fuel and oxidant to operate.

Working Principle: Fuel cells are essentially galvanic cells that operate continuously as long as fuel and oxidant are supplied.

Advantages over Batteries:

• Continuous Operation: Can operate as long as fuel is supplied, unlike batteries which have a finite amount of reactants.
• High Efficiency: Generally more efficient than combustion engines.
• Environmentally Friendly: Many fuel cells, especially hydrogen fuel cells, produce only water as a byproduct, making them very clean.
• No Exhaust Gases: Do not produce greenhouse gases or air pollutants directly.

Components:

• Anode: Where the fuel is oxidized.
• Cathode: Where the oxidant is reduced.
• Electrolyte: Conducts ions between the electrodes.
• Fuel Supply System: Provides the fuel (e.g., $H_2$).
• Oxidant Supply System: Provides the oxidant (e.g., $O_2$ from air).

Examples of Fuel Cells:

• Hydrogen-Oxygen Fuel Cell:
• Anode Reaction: $H_2(g) \rightarrow 2H^+(aq) + 2e^-$ (in acidic electrolyte) or $H_2(g) + 2OH^-(aq) \rightarrow 2H_2O(l) + 2e^-$ (in alkaline electrolyte).
• Cathode Reaction: $O_2(g) + 4H^+(aq) + 4e^- \rightarrow 2H_2O(l)$ (in acidic electrolyte) or $O_2(g) + 2H_2O(l) + 4e^- \rightarrow 4OH^-(aq)$ (in alkaline electrolyte).
• Overall Reaction: $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$
• Electrolyte Types: Acidic electrolyte (e.g., proton-exchange membrane or PEM), alkaline electrolyte, molten carbonate, solid oxide.
• Applications: Electric vehicles, backup power systems, portable power sources.
• Methanol-Oxygen Fuel Cell: Uses methanol ($CH_3OH$) as fuel.
• Natural Gas Fuel Cell: Uses natural gas (primarily methane, $CH_4$) as fuel, often after reforming it to produce hydrogen.

Key Considerations for Fuel Cells:

• Fuel Source: Availability and cost of fuel.
• Electrolyte Stability and Conductivity: The electrolyte must efficiently transport ions and remain stable under operating conditions.
• Catalyst: Catalysts (often platinum) are usually required at the electrodes to facilitate the redox reactions, especially for hydrogen oxidation and oxygen reduction.
• Durability and Cost: Making fuel cells durable and cost-effective for widespread use is an ongoing challenge.