Question bank — Carbon capture, hydrogen economy (electrolysis, fuel cells)

False — combustion of ${\text{H}}_2$ yields only water, but the making of the ${\text{H}}_2$ matters; only if the electrolysis electricity is renewable is the whole loop low-carbon.

False — they are the same redox reaction $2{\text{H}}_2\text{O}\rightleftharpoons 2{\text{H}}_2+{\text{O}}_2$ run in opposite directions; only the sign of $\Delta G$ and of the voltage flips. See Gibbs Free Energy and Spontaneity.

False — the anode is defined as where oxidation happens; it is positive in electrolysis but negative in a galvanic/fuel cell. Track oxidation, not the terminal sign.

True — ${\text{CO}}_2$ is a mild acid anhydride, forming acid in water, so a basic amine neutralises it; see Acids, Bases and Anhydrides.

False — a fuel cell converts chemical energy directly to electricity and is not a heat engine, so Carnot does not apply; its ceiling is $\Delta G/\Delta H$.

False — the capture reaction is reversible; heating the loaded amine releases pure ${\text{CO}}_2$ and regenerates the amine for reuse. That temperature swing is the core of the process.

True — both use steam reforming of methane (Steam Reforming and Industrial H2); blue simply adds carbon capture to the ${\text{CO}}_2$ produced.

False — 1.23 V is the thermodynamic minimum; real cells need ~1.8–2.0 V because of overpotential and internal resistance.

True — moles of ${\text{H}}_2\propto$ charge $=I\times t$ (see Faraday's Laws of Electrolysis), so double the current means double the charge per second and double the gas.

The error: a positive $E_{\text{cell}}^{\circ }$ means spontaneous, but for electrolysis the cell reaction has $E_{\text{cell}}^{\circ }=-1.23\text{ V}$ (you must supply that voltage). Splitting water is non-spontaneous; $+1.23$ V belongs to the fuel-cell (reverse) direction.

Two errors: the cathode is reduction, not oxidation, and it releases hydrogen, not oxygen. Oxygen leaves at the anode via $2{\text{H}}_2\text{O}\to {\text{O}}_2+4{\text{H}}^{+}+4e^{-}$.

The error: air is only ~0.04% ${\text{CO}}_2$, so DAC must process enormous volumes of gas per tonne captured — this dilution makes it the hardest and most energy-intensive route.

The error: cooling favours absorption (the forward exothermic reaction). Regeneration needs heating, which by Le Chatelier's Principle drives the reaction backward to release ${\text{CO}}_2$.

The error: overpotential is kinetic — it comes from the sluggishness of electrode reactions and cannot be reasoned away; only better catalysts and electrode design reduce it.

The error: limestone ${\text{CaCO}}_3$ is a solid; the point of mineral capture is to lock carbon into a stable solid, which is why it counts as permanent storage.

Absorption is exothermic, so by Le Chatelier's Principle low temperature shifts equilibrium toward capture; high temperature reverses it, releasing pure ${\text{CO}}_2$ and regenerating the amine.

Hydrogen goes from ${\text{H}}^{+}$ (charge $+1$) to neutral atoms, and ${\text{H}}_2$ has two H atoms — so $2{\text{H}}^{+}+2e^{-}\to {\text{H}}_2$ needs one electron per proton, two per molecule.

Because $E^{\circ }=-\Delta G^{\circ }/nF$ (Electrochemistry — Standard Electrode Potentials); the electrical work per electron must at minimum equal the free-energy uphill climb of splitting water.

An engine must first turn chemical energy into heat and is then Carnot-limited; a fuel cell skips the heat step and converts chemistry straight to electricity, so no Carnot ceiling applies.

The ${\text{H}}_2$ molecule carries no memory of its origin; if the electrolyser ran on coal power, the ${\text{CO}}_2$ was simply emitted at the power plant instead — the emission moved, it did not vanish.

Removing ${\text{CO}}_2$ from a concentrated, high-pressure pre-combustion stream is far easier than scrubbing dilute flue gas, and the leftover ${\text{H}}_2$ burns cleanly to water.

No — the half-reactions rewrite in terms of ${\text{OH}}^{-}$, but the net cell reaction is still $2{\text{H}}_2\text{O}\to 2{\text{H}}_2+{\text{O}}_2$ and the minimum voltage is still 1.23 V; only the ionic bookkeeping differs.

Nothing useful — with no oxidant at the cathode the reduction half-reaction cannot proceed, so no sustained current flows; both reactants are required for the redox loop to close.

Infinitesimally slowly — 1.23 V is the equilibrium point where forward and reverse rates balance; net production requires driving above it, which is why real cells run higher.

No — CCS is defined by permanent storage; if ${\text{CO}}_2$ escapes from a reservoir it re-enters the atmosphere and the capture is undone, which is why geological integrity is essential.

No — capture is never 100% efficient and methane leakage upstream adds emissions, so blue ${\text{H}}_2$ is lower-carbon than grey but not truly zero like ideal green ${\text{H}}_2$.

No — strong heating reverses the reaction (${\text{CaCO}}_3\to \text{CaO}+{\text{CO}}_2$), releasing the ${\text{CO}}_2$ again; storage only holds while the solid stays cool and undisturbed.