Next-Gen Energy: H2-O2 and methanol (fuel cells, solar cells), Lubricants (Flash point, Fire point, acid value)

Fuel Cells, Solar Cells & Lubricants

1. Fuel cells (continuous galvanic cells)

Fuel + oxidant fed continuously; no recharging needed, only fuel supply. General anode reaction: fuel oxidized releasing electrons; cathode: $O_2 + 4H^+ + 4e^- \to 2H_2O$.

(a) H₂–O₂ fuel cell (alkaline, ~1.23 V theoretical):

• Anode: $2H_2 + 4OH^- \to 4H_2O + 4e^-$; cathode: $O_2 + 2H_2O + 4e^- \to 4OH^-$.
• Overall: $2H_2 + O_2 \to 2H_2O$ — only water as by-product. Used in Apollo spacecraft; needs pure H₂ storage.
  (b) Methanol fuel cell (DMFC):
• Anode: $CH_3OH + H_2O \to CO_2 + 6H^+ + 6e^-$; cathode: $\frac{3}{2}O_2 + 6H^+ + 6e^- \to 3H_2O$.
• Liquid fuel = easy storage for portable/laptop power; lower efficiency (methanol crossover, CO poisoning of Pt catalyst).

Fuel-cell advantages: 60–70% efficiency (vs ~35% thermal), silent, modular, clean. Limits: H₂ storage, catalyst cost (Pt), infrastructure.

2. Solar cells (photovoltaic)

p–n junction silicon: photons with $E \ge$ band gap (1.1 eV for Si) create electron–hole pairs; built-in junction field sweeps them apart → DC current.

• Single cell ~0.5–0.6 V; modules string cells to 12/24 V. Efficiency: mono-Si ~20–24%, poly ~16–18%.
• Advantages: no fuel, no pollution, low maintenance. Limits: intermittent, area-hungry, storage needed, manufacturing energy cost.

3. Lubricants — why needed

Reduce friction, wear, heat; seal, protect from corrosion, carry away debris. Good lubricant: right viscosity, stable at working T, non-corrosive.

4. Flash point, fire point, acid value

• Flash point: lowest T at which oil vapours momentarily flash (catch fire briefly) with a test flame. Abel/Pensky–Martens closed-cup apparatus.
• Fire point: lowest T at which vapours burn continuously ≥ 5 s. Fire point ≈ flash point + 5–10 °C.
• Low flash/fire point → fire hazard, volatile loss — lubricating oils need HIGH values (transformer oil > 140 °C).
• Acid value: mg of KOH to neutralize free fatty acids in 1 g of oil:

($V$ = KOH volume mL, $N$ = normality, $W$ = oil weight g). High acid value = rancid/degraded oil, corrodes machinery — fresh edible/transformer oils need LOW values.

Numerical: 5 g oil needs 2 mL of 0.1 N KOH → acid value $= 56.1 \times 2 \times 0.1/5 = 2.24$ mg KOH/g.

Exam Points

• H₂–O₂ and methanol cell reactions + 2 advantages (5 marks favourite).
• Solar-cell working principle + band-gap idea (3 marks).
• Flash vs fire point definitions + apparatus; acid-value formula + one numerical (3–5 marks).