Stabilization of the reaction temperature
The rate k of a chemical reaction depends on the reaction temperature. For many chemical reactions an exponential relationship ofk = A exp[–E A/(RT )] is found, whereas the processes within the semiconductor usually result in a negative temperature coefficient. For the example of WO3 photoanodes it was measured that the photocurrent increases by 64 % in a temperature interval of 25 to 65 °C.[13]
Commercial photoelectrochemical cells are usually not equipped with systems for temperature control (e.g ., double jackets). We found that the temperature in an idle cell under illumination with an LED solar simulator rises by 3 °C in 10 min. For Xe arc lamp-based systems the temperature increase is probably higher due to their stronger IR emission. The changing temperature can limit the reliability of measurement data, especially for long measurements under illumination (e.g ., voltammetry, impedance spectroscopy).
In the developed system (Figure 2) for temperature control, the PEC cell is connected to a closed loop of silicone tubes. A peristaltic pump, which allows the use of acidic or alkaline electrolytes without corrosion, circulates the electrolyte with a flow rate of 60 ml min–1. Heat exchange occurs through the silicon tubes which are wound through the fins of an aluminum heat sink on a voltage-controlled Peltier element. The temperature is measured with an electronic sensor in an electrolyte reservoir situated between the cell and the Peltier element. An automatic control of the Peltier element with the sensor data is possible but has not yet been realized.