A revisit to Redox/Hydrolysis: PCET

1. The Initial Misunderstanding: Redox vs. Hydrolysis

During a discussion about hydrogen production from a metal-water reaction, I incorrectly interpreted the process as hydrolysis rather than redox. My reasoning:

Hydrolysis involves water splitting a bond (e.g., M–X + H₂O → M–OH + H–X), often without formal electron transfer.

I viewed the metal-water reaction as water "cleaving" the bond between the metal and its electron, redistributing electron density to form H₂ and MOH:
2M + 2H₂O → 2MOH + H₂

My mistake was corrected by a scholar, as it involves:

Oxidation: Metal loses electrons (M → M⁺ + e⁻).
Reduction: Water’s protons gain electrons (2H₂O + 2e⁻ → H₂ + 2OH⁻).

2. Discovering Proton-Coupled Electron Transfer (PCET)

The correction prompted me to explore deeper, leading to PCET, a framework unifying proton (H⁺) and electron (e⁻) transfers.

PCET (proton-coupled electron transfer) is when protons and electrons move together or sequentially, often across different molecular sites, to drive efficient chemical reactions like those in batteries or photosynthesis.

The reaction can be reinterpreted via PCET:

Water autoionization: H₂O ⇌ H⁺ + OH⁻.
Electron transfer: M → M⁺ + e⁻.
Proton reduction: H⁺ + e⁻ → H⋅ (hydrogen radical).
Radical coupling: 2H⋅ → H₂.
Hydroxide binding: M⁺ + OH⁻ → MOH.

3. Linking PCET to Lithium Battery Degradation

PCET principles explain key degradation mechanisms in lithium-ion batteries (LIBs), which resulted in electrolyte decomposition, gas formation, etc.

4. Mitigation via Solid-State Batteries (SSBs)

SSBs (e.g., Li₇La₃Zr₂O₁₂ electrolytes) address PCET-linked degradation:

No liquid electrolytes → Fewer proton sources, minimizing PCET side reactions.

Stable interfaces → Less SEI(solid electrolyte interface) growth, longer cycle life.

Safety → No flammable gases (e.g., H₂) from PCET.