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

ηelectrolyzer E × Q
=
Volt × Current
(2.4)
Where E is the calorific value of hydrogen (J/ml) and Q is the hydrogen flow rate (ml/s).
The efficiency of an electrolyzer is found to decrease as the current density and
corresponding H2 production rate of the cell increases. This means that the required
electrical potential increases as the current density increases. This is due to the
irreversibility occurring in the cell, which can be divided as the activation losses, ohmic
losses, mass transport and concentration losses and increasing crossover of products
through membrane in the high reaction rates and the operating pressures. (Onda et al.
2003).
Typical PEM electrolysis cell voltage is reported to be around 2V and the
commercial electrolyzers have an efficiency ranging from 65% to 80% (Barbir 2004).
Although an electrolyzer can be operated at higher efficiencies (up to 95%), this condition
requires a lower cell voltage which also lowers the current passes through the electrolyte
and the hydrogen production rate (Grigoriev et al. 2006). This dilemma could be overcome
with the utilization of an electrolyzer stack with the high efficiency. Although 70% of the
hydrogen production cost in PEM electrolyzers is due to the cost of electricity, increasing
the efficiency was reported to compensate the relatively high capital expense of a PEM
electrolyzer (Grigoriev et al. 2006). The analysis reported by Larminie and Dicks 2003
shows that an electrolyzer needed to be optimized by considering the efficiency which in
turn affected the unit production cost and also the utilization of electrolyzer which
ultimately affected the initial investment of the device.
Operating temperature of an electrolyzer is another important parameter on the
system design. From electrolysis thermodynamic equations, it is expected that as the
temperature increases the cell voltage should decrease. Yim and coworkers investigated the
temperature effect on PEM electrolyzer with a 4.0 mg/cm 2 Pt loaded electrodes for both
anode and cathode. It was found that voltage decreased from 1900mV to 1700mV as the
temperature increased from 50 o C to 80 o C at 500mAmp/cm 2 current density (Yim et al.
2004). However, it is known that lifetime of solid polymer electrolytes decreases and
product crossover through membrane increases with the increasing temperature. Thus,
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