Nickel-Based Rechargeable Cells

Introduction

Nickel–cadmium and nickel–metal-hydride cells share a nickel oxyhydroxide positive electrode in alkaline electrolyte. They differ mainly in the negative electrode: cadmium metal in NiCd and a hydrogen-storing alloy in NiMH. This comparison shows how replacing one electrode changes environmental and performance properties while preserving a related positive-side redox process.

Core explanation

During discharge, a simplified nickel positive-electrode reduction is NiOOH + H2O + e− → Ni(OH)2 + OH−. Nickel in oxyhydroxide is reduced as the cell supplies current. The reverse reaction occurs on charging. Real nickel electrode phases can vary with state of charge and manufacturing history, so NiOOH/Ni(OH)2 is a useful model rather than a complete phase diagram.

In a nickel–cadmium cell, the negative electrode is cadmium. A simplified discharge oxidation is Cd + 2OH− → Cd(OH)2 + 2e−. Multiplying the nickel reaction by two and adding gives Cd + 2NiOOH + 2H2O → Cd(OH)2 + 2Ni(OH)2. Hydroxide cancels from the net equation even though alkaline electrolyte is essential for ion transport and electrode kinetics. Cadmium's toxicity makes collection and recycling especially important.

In nickel–metal-hydride chemistry, the negative material is represented as MH, a hydrogen-storing alloy. Its discharge oxidation can be written MH + OH− → M + H2O + e−. Adding one nickel positive half-reaction gives MH + NiOOH → M + Ni(OH)2. The exact alloy composition varies and should not be replaced by an imaginary pure elemental “M” in a materials specification; M is a bookkeeping symbol for the host.

Both systems are rechargeable because their intended electrode reactions can be driven backward under controlled charging. They are often discussed as roughly 1.2 V nominal cell families, but loaded voltage depends on current, temperature and state. NiMH avoids cadmium but still requires materials processing and can exhibit self-discharge, heat generation and overcharge concerns. No chemistry is automatically superior in every device.

The terms anode and cathode refer to oxidation and reduction at the moment of operation. On discharge, the negative cadmium or hydride electrode is the anode and nickel oxyhydroxide positive electrode is the cathode. On charging, reaction directions reverse, so formal anode/cathode labels by reaction direction switch even though “positive electrode” remains the same physical side. Using positive and negative electrode names can avoid confusion in rechargeable-cell descriptions.

Step-by-step reasoning

Write the nickel reduction half-reaction first. Choose cadmium or hydride oxidation according to cell type, then balance electrons. Cancel OH− and H2O to obtain the net discharge equation. Identify which material is actually oxidized, and reverse the arrows for charging. Keep nominal voltage and capacity separate from equation stoichiometry.

Visual explanation

Draw a common NiOOH positive plate connected by two alternative diagrams to either Cd or MH negative plate. Under NiCd, show two nickel units per cadmium to balance two electrons; under NiMH, show one nickel unit per MH electron. Color OH− as an internal electrolyte species that cancels from the overall net reaction.

Real-world analogy

Two rechargeable devices can share the same charging port and positive-side mechanism but use different storage modules on the negative side. Changing the module affects materials and performance without changing the role of the shared component. The analogy is limited because electrochemical half-reactions must still balance ions and electrons exactly.

Real-world example

Rechargeable NiMH cells have been used as replacements for some primary cylindrical cells. A user sees repeated charge–discharge cycles, while the chemistry moves hydrogen into and out of an alloy and cycles the nickel oxyhydroxide/hydroxide pair. The stated nominal voltage is lower than a fresh alkaline primary cell, so device compatibility depends on operating requirements.

Why?

The nickel positive electrode accepts electrons on discharge, while cadmium or hydride negative material supplies them. Alkaline electrolyte supports charge balance and interfacial reactions even when OH− cancels algebraically in the total equation. Reversibility depends on maintaining electrode structure and avoiding damaging side reactions.

Common misconception

“Metal hydride” in NiMH does not mean free hydrogen gas is the cell's negative electrode. Hydrogen is stored in an alloy. Also, because OH− cancels from the net reaction, it is wrong to conclude the alkaline electrolyte is unnecessary.

Worked example

Question: Combine MH + OH− → M + H2O + e− with NiOOH + H2O + e− → Ni(OH)2 + OH− for NiMH discharge.

Reasoning: One electron is released by the hydride oxidation and consumed by nickel oxyhydroxide reduction, so no multiplier is needed. Add equations and cancel one electron, OH− and H2O from opposite sides. The remaining reactants are MH and NiOOH; products are M and Ni(OH)2.

Answer: MH + NiOOH → M + Ni(OH)2.

Quick check

1. What positive-electrode couple is shared by NiCd and NiMH cells? Answer: The NiOOH/Ni(OH)2 couple in a simplified alkaline description.

Exam focus

Balance nickel's one-electron reaction against cadmium's two-electron or hydride's one-electron reaction. Label discharge direction clearly. Distinguish the physical positive electrode from formal cathode/anode naming during charge versus discharge.

Advanced insight

Battery cycling changes particle structure, alloy composition and local electrolyte distribution. Two materials with the same net half-reaction can have different rates and longevity because of morphology and transport, reinforcing that a balanced equation is only one level of device understanding.

Summary

Nickel-based rechargeable cells reduce NiOOH to Ni(OH)2 on discharge. NiCd oxidizes cadmium to Cd(OH)2; NiMH oxidizes hydrogen stored in an alloy. Both use alkaline electrolyte, but their negative materials and associated practical concerns differ.

Practice questions

1. What is the NiCd negative-electrode discharge product in the simplified model? Answer: Cd(OH)2. 2. How many NiOOH units balance one Cd oxidation? Answer: Two, because Cd releases two electrons and each NiOOH consumes one. 3. What does M represent in an NiMH equation? Answer: A hydrogen-storing alloy host, not necessarily a pure single metal. 4. Does cancellation of OH− in the net equation make electrolyte unnecessary? Answer: No. It still conducts ions and participates in half-reaction environments.