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

voltages. This concept is also valid for the electrolysis cells connected in series. The
serially connected cells are called as “stack” and they could be operated at high voltages
which are proportional to the number of cells. To connect cells in series, the anode side of
one cell should connect with the cathode of another one. This can be achieved by wiring
each cell with next one in the stack. In this way, current can pass from one cell to the next
one but for a higher current rate (which is usually the case for electrolysis) the current
distribution problems may occur. The other way to pass the current between cells is to
construct an electric conductive plate which is called as bipolar plate. The name of the
bipolar comes from this unwired stacking configuration where one side of the plate acts as
the anode of the cell while the other side behaves as the cathode for the adjacent cell. Other
duties of bipolar plates are that they have to supply water to the anode gas diffusion layer
while dispelling the oxygen gas from electrode and also it has to dispel hydrogen gas from
the cathode gas diffusion layer. These are major duties of a bipolar plate in an electrolyzer
but also it has to be a good heat conductor to prevent the high temperatures inside the cell
and it has to be made from a durable and high strength materials since the other parts of the
cell are made up of low mechanical strength materials. A bipolar plate should have low
permeability values for both oxygen and hydrogen to ensure that they are separate.
Metals can be used as bipolar plates since they are abundant and cheap although the
most common material used for bipolar plates is graphite since it is a good thermal and
electrical conductor like metals. Moreover, it is easy to machine the flow channels on
graphite blocks with respect to other metals. Graphite is also less permeable to hydrogen
than most of the metals.
Graphite bipolar plates constitute almost 88% of the electrolyzer and in particular
coated metal bipolar plates constitute 81% by mass of a stack since the other parts are very
thin (Li and Sabir 2004).
As department of energy (DOE) points out that, one of the main obstacles in front
of the hydrogen economy is the low power density (according to internal combustion
engines) of fuel cells and electrolyzers. Reducing the weight of bipolar plates can increase
the power density significantly as it can be understood from its total weight sharing (DOE-
HVR 2006).
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