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Date	November 2019	Marks available	3	Reference code	19N.3.hl.TZ0.20
Level	HL	Paper	3	Time zone	TZ0
Command term	Outline	Question number	20	Adapted from	N/A

Question

A proton-exchange membrane (PEM) fuel cell uses pure hydrogen gas as the fuel and a proton exchange membrane as the electrolyte.

A dye-sensitized solar cell (DSSC) uses light energy to produce electricity.

Deduce the half-equations for the reactions occurring at the electrodes.

Anode (negative electrode):

Cathode (positive electrode):

[2]

Calculate the cell potential, E^θ, in V, using section 24 of the data booklet.

[1]

b(i).

Suggest how PEM fuel cells can be used to produce a larger voltage than that calculated in (b)(i).

[1]

b(ii).

Suggest an advantage of the PEM fuel cell over the lead-acid battery for use in cars.

[1]

Outline the functions of the dye, TiO₂ and the electrolyte in the operation of the DSSC.

Dye:

TiO₂:

Electrolyte:

[3]

d(i).

Suggest an advantage of the DSSC over silicon-based photovoltaic cells.

[1]

d(ii).

Markscheme

Anode (negative electrode):
H₂ (g) → 2H⁺ (aq) + 2e⁻ ✔

Cathode (positive electrode):
O₂ (g) + 4H⁺ (aq) + 4e⁻ → 2H₂O (l) ✔

NOTE: Accept any correct integer or fractional coefficients. Award [1 max] for M1 and M2 if correct half-equations are given at the wrong electrodes OR if incorrect reversed half-equations are given at the correct electrodes.

(+)1.23 «V» ✔

NOTE: Do not accept “-1.23 «V»”.

b(i).

connect several fuel cells in series
OR
increase pressure/concentration of reactant/hydrogen/oxygen ✔

NOTE: Do not accept changes in [H⁺]/pH as they do not affect cell potential in this case.
Do not accept reference to quantity for “concentration”.

b(ii).

liquid in cell is less/not corrosive
OR
does not contain lead/toxic chemicals
OR
larger energy density/charge capacity/current per unit mass
OR
does not have to be charged prior to use / is always ready for use «as long as fuel is available» ✔

Dye:
absorbs photons/light
OR
releases electrons ✔

TiO2:
conducts current/electricity
OR
semiconductor ✔

Electrolyte:
reduces/regenerates «the oxidized» dye ✔

d(i).

Any one of:
cheaper/ease of manufacture
OR
plentiful and renewable resources «to construct DSSC cells» ✔

use light of lower energy/lower frequency/longer wavelength
OR
use of nanoparticles provides large surface area for exposure to sunlight/sun/light
OR
can absorb better under cloudy conditions ✔

operate at lower «internal» temperatures
OR
better at radiating heat away «since constructed with thin front layer of conductive plastic compared to glass box in photovoltaic cells» ✔

better conductivity ✔

more flexible/durable ✔

NOTE: Accept “lower mass/lighter «so greater flexibility to integrate into windows etc.»” OR “greater power-conversion efficiency «with latest DSSC models»”.

d(ii).