Gas generation in battery systems, particularly in aqueous electrolytes, presents significant challenges for safety, efficiency, and longevity. The primary gases evolved are hydrogen and oxygen, resulting from water decomposition during charging or overcharging. This process is fundamentally linked to the electrochemical stability window of water, which is theoretically 1.23 V but can vary based on electrode materials, pH, and temperature. In contrast, organic electrolytes exhibit different gas generation mechanisms, primarily involving solvent decomposition rather than water splitting.

Aqueous electrolytes, such as those in lead-acid and zinc-ion batteries, are particularly prone to gas evolution due to the presence of water. The hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) occur when the applied voltage exceeds the thermodynamic stability limit of water. In lead-acid batteries, HER proceeds at the negative electrode (Pb) via:
2H⁺ + 2e⁻ → H₂ (acidic media)
while OER occurs at the positive electrode (PbO₂):
2H₂O → O₂ + 4H⁺ + 4e⁻
These reactions become prominent during overcharge, leading to water loss and requiring periodic maintenance in flooded lead-acid systems.

Zinc-ion batteries with aqueous electrolytes face similar challenges. HER at the zinc electrode is a major side reaction, especially at high charging rates or low pH:
Zn²⁺ + 2e⁻ → Zn (desired reaction)
2H₂O + 2e⁻ → H₂ + 2OH⁻ (competing HER)
This not only reduces Coulombic efficiency but also poses safety risks due to hydrogen accumulation.

The actual onset potentials for HER and OER deviate from theoretical values due to overpotentials. For example, HER on lead surfaces begins around -0.65 V vs. SHE in sulfuric acid, while OER on PbO₂ starts near 1.5 V vs. SHE. These overpotentials are influenced by electrode material, surface morphology, and electrolyte composition. Catalysts can further alter these thresholds. Platinum group metals lower HER overpotentials, while iridium or ruthenium oxides facilitate OER. However, their use in commercial batteries is limited by cost.

Gas recombination systems are critical for mitigating water loss in sealed lead-acid batteries. Oxygen generated at the positive electrode diffuses to the negative electrode, where it reacts with lead to form PbO, which then reacts with H₂SO₄ to regenerate water:
O₂ + 2Pb → 2PbO
PbO + H₂SO₄ → PbSO₄ + H₂O
This oxygen recombination cycle suppresses hydrogen evolution but does not eliminate it entirely. Advanced designs incorporate hydrogen recombination catalysts, such as palladium-doped separators, to further reduce gas emission.

In contrast, organic electrolyte systems (e.g., lithium-ion batteries) generate gases through different pathways. Common gases include CO₂, CO, CH₄, and C₂H₄, resulting from solvent reduction at the anode or oxidation at the cathode. For instance, ethylene carbonate decomposes at low potentials:
EC + 2e⁻ → C₂H₄ (gas) + CO₃²⁻
Unlike aqueous systems, these reactions do not involve water splitting but rather direct decomposition of organic molecules. The gas composition depends on the specific electrolyte formulation and operating conditions.

Temperature significantly impacts gas generation in both systems. In aqueous electrolytes, higher temperatures accelerate HER and OER kinetics, while in organic electrolytes, thermal runaway can trigger violent solvent decomposition. Pressure buildup from gas evolution is more predictable in aqueous systems due to the well-defined stoichiometry of water splitting (2:1 H₂:O₂ ratio), whereas organic systems produce complex gas mixtures with varying expansion factors.

Quantitative differences in gas evolution rates highlight key distinctions. A lead-acid battery at 2.4 V/cell (overcharge) may produce 0.3 mL/Ah of gas, primarily H₂ and O₂. In comparison, a lithium-ion cell undergoing electrolyte decomposition might generate 0.1-0.5 mL/Ah of mixed gases, depending on the state of charge and temperature.

Mitigation strategies differ between the two systems. Aqueous batteries rely on recombination chemistry and pressure-regulated vents, while organic electrolyte systems employ electrolyte additives (e.g., vinylene carbonate) to form stable solid-electrolyte interphases that suppress gas-forming reactions. Material selection also plays a role—zinc anodes with alloying elements (Bi, In) exhibit higher HER overpotentials, while lithium-ion cathodes with protective coatings reduce oxidative gas generation.

Understanding these mechanisms is essential for battery design. Aqueous systems require careful balancing of recombination efficiency and pressure management, whereas organic systems demand precise control of electrode potentials to avoid electrolyte breakdown. Future developments may bridge these approaches, such as hybrid electrolytes that combine the safety of aqueous systems with the wide voltage window of organic systems.

The study of gas generation remains critical for advancing battery technology, particularly in improving energy efficiency and safety across different chemistries. By addressing the fundamental electrochemical processes, researchers can develop better mitigation strategies tailored to each system's unique challenges.