Passage V

Liquid H₂O can be broken down into hydrogen gas（H₂）and oxygen gas (O₂) by electrolysis according to the following chemical equation:

2H₂O → 2H₂ + O₂

A scientist performed an experiment to study the elec­trolysis of H₂O using electricity generated from sunlight.

Experiment

Steps 1-5 were performed daily for 12 months:

1. A tank fitted with 2 electrodes一an anode (where O₂ would be produced) and a cathode (where H₂ would be produced)—was assembled. Each electrode was suspended in an inverted plastic tube, and each tube was marked to allow gas volume to be measured.

2. Four liters (4.0 L) of a 25% by mass aqueous solution of sodium hydroxide (NaOH) was added to the tank. As a result, the tubes were completely filled with the solution.

3. At 8:00 a.m., a rectangular solar cell was attached to the electrodes and placed next to a particular south-facing window for 8 hr. (Figure 1 shows the apparatus at the initiation of electrolysis.)

4. Eight hours later, the solar cell was detached from the electrodes, and the amount of H₂ that had been produced was measured.

5. The tank, tubes, and electrodes were cleaned and dried for reuse.

Figure 2 shows the total volume of H₂ produced (in L) in each month of the experiment. Table 1 shows the aver­age solar irradiance (power per unit area), in watts per square meter (W/m²), at the location of the solar cell during each month of the experiment.

Figures and table adapted from Sergii Bespalko, "Hydrogen Production by Water Electrochemical Photolysis Using PV-Module. " @2011 by Sergii Bespalko.