Figure 4 Block diagram of the system for gas measurement
The analysis is carried out by an automatic measurement routine. The
time-resolved data is sequentially acquired in series of cycles, each of
which consists of three steps (Figure 5):
The pressure in the electrochemical cell is set to 1.1 bar with Ar from
the integrated tank of the GC. In this step, the hydrogen concentration
drops.
With all valves closed, the water splitting reaction is started by
switching on the solar simulator. The episode may last for 300 s but can
be expanded if photoelectrodes with low efficiencies are investigated.
Pressure and hydrogen concentration in the cell increase linearly.
3. A valve in the GC is opened and allows the gas mixture in the
electrochemical cell to relax through the sampling valve. After a
certain volume flow, which correlates with the volume of the metal
tubes, the sample is taken. The GC measurement is started and runs
parallel to steps 1 and 2.
After the automatism is terminated by the user, the data of the GC, the
pressure sensor and the potentiostat are loaded into an Excel® script
which calculates the molar amounts of hydrogen and oxygen, the Faraday
efficiencies and, if applicable, the STH efficiency. Leakage is detected
by a continuous pressure drop in the cell during an idle period and can
be corrected to a limited extend.