Loav · VCE Chemistry · Unit 3 / Area of Study 2
U3A2-05 · 2026 curriculum · Expanded original teaching guide · Study guide
1. One device, two operating modes
A rechargeable or secondary cell stores energy chemically and can undergo repeated discharge and recharge. During discharge, a spontaneous redox reaction delivers electrical energy to a connected load: the cell operates galvanically. During recharge, an external power source drives the reverse chemical change: it operates electrolytically. The electrical energy used to charge is converted into stored chemical energy, with some energy dissipated as heat and other losses.
Always define anode by oxidation and cathode by reduction. During galvanic discharge the anode is negative and the cathode positive. During electrolytic recharge the anode is positive and the cathode negative. For the same physical electrodes in a typical rechargeable cell, the electrode that was the negative discharge anode becomes the negative charging cathode. The electrode that was the positive discharge cathode becomes the positive charging anode. The physical terminal signs can remain the same while the reaction-based anode/cathode names swap.
This resolves a common apparent contradiction. “The anode changes sign” compares the electrode role across galvanic and electrolytic modes. “The negative electrode stays negative” follows one physical electrode in a rechargeable device. State which you mean. Some battery descriptions use anode/cathode names conventionally based on discharge even when discussing charging; in a chemistry answer, explicitly label operating mode and the reaction at each electrode.
2. What makes reversal feasible?
Recharging requires the products of discharge to remain available in appropriate contact with the electrodes and electrolyte. The materials must support substantially reversible reactions without unacceptable structural damage or side reactions. A suitable charging voltage must oppose and exceed the cell’s discharge potential sufficiently to drive reverse current, while a controlled charging system limits damaging conditions. Voltage, current, temperature and state of charge all matter.
Simply connecting any spent primary cell to a power source does not make it safely rechargeable. Escaped products, irreversible changes, corrosion or competing reactions can prevent restoration. Even secondary cells gradually lose usable capacity because real cycles are not perfectly reversible. Separators prevent direct electrical contact between electrodes while permitting suitable ionic transport; internal electronic contact would create a short circuit instead of useful current through the load.
Worked example 1: reverse the chemistry and relabel
Consider an idealised teaching model, not a recommendation for constructing a rechargeable battery. During discharge: negative electrode Zn(s) → Zn²⁺(aq) + 2e⁻; positive electrode Cu²⁺(aq) + 2e⁻ → Cu(s). Overall Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). During ideal reverse operation: negative electrode Zn²⁺(aq) + 2e⁻ → Zn(s), now cathode; positive electrode Cu(s) → Cu²⁺(aq) + 2e⁻, now anode. Overall Zn²⁺(aq) + Cu(s) → Zn(s) + Cu²⁺(aq). Actual aqueous zinc deposition can compete with hydrogen formation, so algebraic reversal alone does not establish a practical rechargeable design.
3. Green hydrogen from renewable electricity
Hydrogen is an energy carrier: energy is required to produce it and can later be released through oxidation, for example in a fuel cell. In renewable-powered polymer electrolyte membrane (PEM) electrolysis, photovoltaic cells or wind turbines provide electrical energy for splitting water. “Green” refers to the production pathway and energy source, not to a different hydrogen molecule or an absence of all environmental impacts.
A PEM electrolyser contains catalytic electrodes separated by a proton-conducting polymer membrane. At the positive anode, water is oxidised: 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e⁻. Protons travel through the membrane toward the negative cathode; electrons travel through the external power circuit, not through the membrane. At the cathode, 4H⁺(aq) + 4e⁻ → 2H₂(g). Adding and cancelling gives 2H₂O(l) → 2H₂(g) + O₂(g). These equations are written in acidic form as required for this context.
The membrane helps separate hydrogen and oxygen while permitting proton transport. Catalysts improve electrode reaction rates and reduce the extra operating voltage needed to achieve a useful reaction rate; they do not eliminate the energy needed to split water. A suitable power system must manage variable solar or wind output. Water supply, membrane and catalyst manufacture, equipment lifetime, compression, storage and transport contribute to the overall resource and emissions assessment.
4. Artificial photosynthesis couples light to chemistry
Artificial photosynthesis imitates the broad natural strategy of using sunlight to drive otherwise unfavourable chemical transformations. For the water-splitting context here, a light absorber generates separated charges that are delivered to a water-oxidation catalyst and a proton-reduction catalyst. The oxidation catalyst supports 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e⁻, while the reduction catalyst supports 4H⁺(aq) + 4e⁻ → 2H₂(g). The same overall water-splitting equation results.
The key distinction from a solar-panel-powered electrolyser is how light absorption and charge transfer are integrated with the chemical system. Do not claim that all artificial photosynthesis must produce glucose, or that a catalyst itself supplies the necessary energy. The light provides energy; catalysts provide reaction pathways. Effective designs need efficient light capture, separation of charges before they recombine, catalytic selectivity, durability and safe product separation. The course requires these general principles rather than memorisation of a particular named commercial cell.
Worked example 2: test a production claim
An original idealised PEM calculation supplies 1930 C, with 100% current efficiency. Using F = 96 500 C mol⁻¹ gives n(e⁻) = 0.0200 mol. Four electrons correspond to two H₂, so n(H₂) = 0.0100 mol; four electrons correspond to one O₂, so n(O₂) = 0.00500 mol. At SLC their ideal volumes are 0.248 L and 0.124 L. The 2:1 volume ratio follows stoichiometry at matching temperature and pressure. These numbers alone do not show energy efficiency: charge is not energy, and operating voltage is also needed to calculate electrical energy input.
5. Separate charge recovery, energy recovery and lifetime
A cell may return nearly the same charge used during charging but still return substantially less electrical energy. Charge measures transferred electrons; energy also depends on voltage. For constant operation, E = VIt = VQ. Charging often requires a higher operating voltage than the voltage delivered during discharge because of internal resistance and electrode-reaction losses. Thus a charge-efficiency statement cannot substitute for an energy-efficiency statement.
Capacity describes how much charge a cell can deliver under specified conditions, often in ampere-hours; 1 A h = 3600 C. It is not an energy unit. Two cells with equal ampere-hour ratings can supply different energy at different voltages. Power also matters: a cell with adequate stored energy may not deliver a desired current safely. Practical design balances energy, power, cycle life, charging conditions, mass, material availability and end-of-life recovery.
Worked example 3: evaluate a complete storage chain
Challenge, original synthetic performance data. A renewable-electricity hydrogen system receives 100.0 kJ of electricity at the electrolyser input. Under a consistently defined chemical-energy convention, 70.0% is retained as hydrogen chemical energy. Compression and storage leave 90.0% of that energy available; a fuel cell converts 55.0% of the delivered hydrogen chemical energy to useful electricity. Find useful output and overall efficiency.
After electrolysis: 100.0 × 0.700 = 70.0 kJ. After storage: 70.0 × 0.900 = 63.0 kJ. Useful output: 63.0 × 0.550 = 34.65 kJ, reported as 34.7 kJ. Overall efficiency = 0.700 × 0.900 × 0.550 = 0.3465, or 34.7%. The efficiencies multiply; averaging 70%, 90% and 55% would not describe successive energy transfers.
If the renewable generator converted 20.0% of incident solar energy into the initial 100.0 kJ, its incident sunlight input was 100.0/0.200 = 500 kJ. The solar-to-returned-electricity efficiency is then 34.65/500 = 6.93%. Both 34.7% and 6.93% can be valid because they have different system boundaries. These invented figures illustrate accounting, not a claim about any actual commercial technology. They assume the stated storage-stage factor already accounts for its energy penalty; if compression instead draws a separate electricity supply, that extra input must be added explicitly.
Hydrogen’s chemical energy may be reported using different conventions depending on whether product water is vapour or liquid. Use consistent conventions when comparing energy efficiencies; the same hydrogen mass can otherwise appear to have inconsistent efficiencies. Emissions also require a defined boundary and functional unit, such as emissions per kilogram of delivered hydrogen or per kilowatt-hour returned. Renewable operating electricity alone does not quantify mining, manufacturing, water treatment, replacement or transport impacts.
6. Explain innovation without assuming it guarantees superiority
Artificial photosynthesis can integrate light absorption with water-splitting chemistry and potentially avoid some separate components. Fewer named conversion stages do not automatically mean higher measured efficiency. Charges may recombine before reaching catalysts, only part of sunlight may be absorbed usefully, and catalysts or photoabsorbers may degrade. A fair comparison with photovoltaic-powered PEM electrolysis needs equal light input conditions, the same product-energy convention and evidence over a relevant operating lifetime.
In a two-catalyst water-splitting system, the water-oxidation catalyst manages the removal of four electrons for each O₂ formed; the proton-reduction catalyst combines two electrons with two protons for each H₂. Light-driven charge separation must connect these complementary processes. Four electrons cannot produce four H₂ molecules: count the balanced half-equations. Separating the gases is both a safety function and a way to avoid product recombination. A catalyst is regenerated during normal cycles, but that does not mean real materials never degrade or require replacement.
Keep the water and membrane descriptions compatible. A PEM is a solid polymer electrolyte that conducts protons under appropriate hydrated operating conditions. It is not simply a beaker of bulk aqueous salt, but its use does not imply absence of water. Acidic half-equations are a bookkeeping model for the proton-involving electrode chemistry. Do not replace H⁺ with oxide ions just because the membrane is solid: different electrolyser types transport different ions.
Likewise, the ability to reverse a written battery equation is necessary but insufficient evidence for safe recharge. The actual material structure, product retention, electron and ion pathways and side-reaction suppression must permit repeated reversal. Losing active material can reduce capacity; higher internal resistance can reduce useful power and increase heating. These mechanisms explain why recycling and design for durability matter even where the electricity source is renewable. The task is to reason from supplied cell chemistry, not memorise a list of commercial battery compositions.
Practice
- A physical electrode oxidises a metal during discharge. Name its role and sign in discharge and in ideal recharge.
- Explain why replacing the external circuit of a PEM electrolyser with a proton-conducting membrane would not provide the required electron pathway.
- A claimed water-splitting equation is H₂O(l) → H₂(g) + O₂(g). Correct it, then give the acidic anode half-equation.
- A company calls hydrogen “zero-impact” because wind electricity is used. Give a balanced evaluation using two additional pieces of evidence needed.
- A synthetic secondary cell is charged at 1.50 A for 40.0 min at 4.00 V, then discharges at 1.20 A for 45.0 min at 3.20 V. Calculate charge recovery and electrical energy recovery.
- An artificial photosynthesis model successfully transfers 0.0400 mol electrons between the water-oxidation and proton-reduction processes, with no side reactions. Find both gas amounts and the amount of water consumed.
- A new catalyst allows a PEM electrolyser to produce the same hydrogen amount in the same time at the same current but lower voltage. Explain what improves, what stays fixed, and what cannot yet be concluded.
Worked answers
- Discharge: negative anode, because oxidation releases electrons. Recharge: negative cathode, because reduction restores the metal. The same physical electrode changes reaction role, not necessarily terminal sign.
- The membrane conducts protons, not electrons. Electrons must be withdrawn from the anode and supplied to the cathode by the external powered circuit. Ionic conduction and electronic conduction perform different functions.
- 2H₂O(l) → 2H₂(g) + O₂(g). Acidic anode: 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e⁻. Check four H atoms, two O atoms and zero net charge on both sides.
- Wind-powered electrolysis can lower operational fossil-fuel emissions. Assess actual electricity sourcing over time, embodied impacts of equipment/catalysts, water demand and compression/storage energy. A supported conclusion distinguishes lower impacts from no impacts; comparative lifecycle data are needed.
- Charging Q = 1.50 × 2400 = 3600 C; discharge Q = 1.20 × 2700 = 3240 C. Charge recovery = 3240/3600 × 100 = 90.0%. Charging E = 4.00 × 3600 = 14 400 J; discharge E = 3.20 × 3240 = 10 368 J. Energy recovery = 10 368/14 400 × 100 = 72.0%. The different voltages explain why the two percentages differ.
- The acidic oxidation half-equation produces one O₂ per four electrons: n(O₂) = 0.0100 mol. Reduction produces one H₂ per two electrons: n(H₂) = 0.0200 mol. Overall 2H₂O(l) → 2H₂(g) + O₂(g), so water consumption is 0.0200 mol. Electron and proton cancellation must precede the net water calculation.
- The same current and electron yield imply the same hydrogen production rate under the stated efficiencies. Lower voltage reduces power VI and energy input for that amount, improving the stated electrical-to-hydrogen energy efficiency. It does not alter the net equation or required two electrons per H₂. Overall environmental superiority still requires evidence about catalyst manufacture, lifetime, scarcity, water supply and the electricity source.
