| Light absorption |
One or more light absorbers
are needed (see tandem and z-scheme configurations). |
Either one (photocathode
or photoanode) or two photoactive electrodes. The individual photoelectrodes
can also be multicomponent. |
Tailored photovoltaic cells
can be designed (from single- to multijunction cells), to provide
the necessary cell voltage. |
| Charge carrier collection |
No need for carrier collection,
but photogenerated holes and electrons need to reach the respective
surface sites. |
Charge
carrier trapping
at defect sites at the electrode/electrolyte interface hinders charge
carrier collection. |
Rapid charge carrier collection
is achieved in the PV cell. |
| Charge transfer (reaction) |
Both reactions proceed on
the same particles. Preferably different sites for the two half reactions.
Back reactions are possible. The rates of the two half reactions have
to match. |
Slow charge
carrier transfer
to the substrate or mediator from the electrode surface, compared
to the timescale of charge carrier recombination. |
A separate electrochemical
interface is responsible for the chemical reaction. Well-known stable
and active electrocatalysts can be employed. |
| Nano aspects |
A high surface area is necessary
to provide enough active sites for the reaction. High probability
of surface recombination. |
A high surface area is necessary
to provide enough active sites for the reaction. High probability
of surface recombination. |
Nanostructured electrocatalysts
can be used, without the detrimental surface recombination in the
light absorber. |
| Stability |
Intermediate stability,
because of the presence of the solid/liquid interface. |
Very difficult to achieve
reasonable stability, because of the presence of current flow and
the electrode/electrolyte interface. Different protective coatings
seem to ensure certain improvements. |
The stability is dictated
only by the stability of the electrolyzer, as PV panels are stable
for ages. Examples on the order of hundreds of hours are available. |
| Cost |
Cheap experimental setup
or device, but expensive multifunctional catalyst materials are needed. |
More expensive and sophisticated
cell designs are necessary, especially in the case of continuous flow
processes. If a cocatalyst is employed, large amounts are needed because
of the identical surface area of the light absorber and the electrochemical
interface. |
Relatively
expensive system
cost. Much smaller electrochemically active area is needed (compared
to the size of the PV) and thus less electrocatalysts, membranes,
etc. have to be used. It is also possible to select high-performance
PV cells with a smaller area (under concentrated light) and electrodes
with a larger area. |
