Fuel cells produce voltage by using hydrogen and oxygen as raw materials, and releases water as a waste product.

Fuel cells can be alkaline or acidic, depending on the ion that moves across the electrolyte.


Acidic fuel cells



Half equations:

\(2H^+_{(aq)} + 2e^- ⇌ H_{2 (g)} \qquad E^θ = 0.00 V \)

\(\frac{1}{2}O_{2 (g)} + 2H^+_{(aq)} + 2e^- ⇌ H_2O_{(l)} \qquad E^θ = +1.23 V \)

Overall:

\(H_{2 (g)} + \frac{1}{2}O_{2 (g)} ⟶ H_2O_{(l)} \qquad E^θ_{cell} = +1.23 V \)


Alkaline fuel cells



Half equations:

\(2H_2O_{(l)} + 2e^- ⇌ H_{2 (g)} + 2OH^-_{(aq)} \qquad E^θ = -0.83 V \)

\(\frac{1}{2}O_{2 (g)} + H_2O_{(l)} + 2e^- ⇌ 2OH^-_{(aq)} \qquad E^θ = +0.40 V \)

Overall:

\(H_{2 (g)} + \frac{1}{2}O_{2 (g)} ⟶ H_2O_{(l)} \qquad E^θ_{cell} = +1.23 V \)


Non-hydrogen fuels

Methanol and other hydrogen-rich fuels can be used in fuel cells.