The alkaline electrolyser represented by the simplified diagram above is essentially a stack of cells connected electrically in parallel. There are 15 cells, which increases the current capability and gas production by that factor without altering the operating voltage. A real alkaline electrolyser is much more complicated:
(a) a barrier is required to separate the gases generated by each cell (a mix of hydrogen and oxygen is too dangerous to store), a barrier that at the same time allows an electrical conduction path (the barrier is represented by the soft grey lines in the diagram);
(b) a mechanism is needed to deliver deionised water to maintain the optimal ion concentration because H2O is continuously removed during electrolysis;
(c) the pressure difference between the two gases needs to be managed (twice the volume of hydrogen gas is produced) – the pressure needs to be carefully controlled with appropriate pressure relief valves and channels available for safely routing the vented gases;
(d) the gas must be dried and possibly purified.
An applied voltage of 1.229 V corresponds to electrical to chemical energy conversion that is 100% efficient. The work done on the electrons round the external circuit driven by this potential difference gives the electrons the exact amount of energy required to reconfigure the molecular bonds. A higher voltage gives the electrons more energy, but will not result in more gas being released because one electron will only ever free from solution one atom of hydrogen. The extra energy ends up as heat hence more energy is needed to split water than is recoverable later when the gas is consumed – the round-trip efficiency is therefore reduced. Generally speaking, the efficiency is the ideal voltage divided by the actual voltage measured across the cell.
But 1.229 V is not sufficient in practice. Though the energised electron could travel from the electrode into the solution, this happens very, very slowly because there are surface effects that need to be considered. A higher-than-threshold voltage must be applied to overcome the barrier effects and drive the process. There is a barrier on each electrode (two half-cell reactions take place in every electrolytic cell), hence the problem is doubly bad! The total extra voltage required is referred to as the surface overpotential, and the value is very dependent on the nature of the surface and the material that forms the surface. Materials with the lowest overpotentials act as catalysts, and the best choices are noble metals such as platinum. The overpotential decreases with temperature and it is therefore preferable to run the electrolyser at higher temperature (80oC is typical). A second effect that further increases the external voltage that must be supplied is the joule resistance of the electrolyte. The ohmic losses can be minimised by putting the electrodes as close together as possible (1-3 mm is typical), but cannot be entirely eliminated (you should note also that as the spacing is decreased, bubbles have an increasingly negative impact). There are also material transport effects which influence the voltage, but these come into play only at very high current flows (concentration polarisation) and are neglected in the modelling performed here. For a practical alkaline electrolyser, an efficiency in the range 50-80% is typical. The lower efficiency is not always a significant problem - the waste heat is available at the electrolyser operating temperature which is convenient for domestic heating.
Task
Using the model above, obtain current readings for all voltages and generate a plot showing how the efficiency varies directly with current and indirectly with the hydrogen production rate.
The model uses an equation for the total voltage made up of the ideal value plus the two activation overpotentials combined and the resistance losses. The general form that takes temperature variation into account is
V = Vo + k1 log ((k2 + k3 /T + k4 /T2) I/A + 1) + (k5 + k6 T) I/A.
Either natural logarithms or log to the base 10 can be used in the model but the constants should be consistent with the choice made. Temperatures are in K and A is the total electrode area.
A much more sophisticated model is possible, but this basic model illustrates very well the operation at the higher temperature of a standard KOH electrolyser. In the model above, 0.25 m2 total area and a separation of 1 mm were selected. Platinum electrodes are assumed. The constants are chosen to match empirical data for the leading electrolysers of this type. The results are pretty much the best that can be achieved with this class of equipment and do not take into account losses up and down the line relating to factors such as electrical conversion and gas compression. For more detail, go to Ref. 1 , Ref. 2 and Ref. 3.