hydrogen production from water using solar cells powered nafion ...

hydrophobic substances in electrodes will show worse performance due to the high
resistance between the membrane and reactant water. Hydrophilic additives, such as
Nafion ® , are used to enhance the anode performances of electrolyzers (Ioroi et al. 2002).
At the beginning of PEM technology, the catalyst loading for both anode and
cathode sides were as high as 28 mg/cm 2 of platinum (Larminie and Dicks 2003) where as
0.2mg/cm 2 or less is used now (Kim et al. 1998). Due to increased catalyst activity precious
raw materials became only a small portion of both electrolyzer and fuel cells.
The kinetics of hydrogen on platinum is well known and shows that hydrogen
evolution is the most efficient over platinum based catalyst material. Also, high current
densities could be achieved at low overpotential with almost no mass transport limitation.
Thus Pt is the most suitable catalyst for hydrogen generation on the cathode side of a PEM
electrolysis cell. But there are some restrictions about platinum such as its sensitivity to
poisoning gases like CO, COS and H2S above 10 ppm. (Levin et al. 2004). The models
developed by Choi and coworkers shows that the overpotential of Pt coated cathode
electrode is as small as 0.17 V at 1Amp/cm 2 for PEM electrolysis under standard conditions
(Choi et al. 2004). Experimental works usually does not mention anode or cathode
overpotential directly because of the experimental difficulties except Millet’s found that the
cathode overpotential changed from 0.15 V to 0.1 V with respect to Pt loading and
temperature (Millet et al. 1992).
Different support and catalyst materials are used to investigate their effects on the
total cell voltage (Grigoriev et al. 2006, Yim et al. 2003). There have been a lot of studies
on electrodes and high efficient electrocatalysts for fuel cells and electrolyzers but they
mainly focuses on anode electrode since the main energy losses occurs in water
dissociation reaction.
Stable and active oxygen electrode is one of the key issue for the manufacture of
electrolyzers. It is known that for oxygen reduction, platinum does not work well in the
electrolyzer anode (Petersson et al. 2006). Thus, unlike fuel cell, the anode of the
electrolyzer has different electrocatalyst, such as IrO2, RuO2, SnO2 or their combinations
(Grigoriev et al. 2006). These catalyst is usually mixed with Ta2O5, TiO2 or SnO2 to
stabilize the structure (Rasten et al. 2003).
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