Separator for fuel cell

1. A separator for a fuel cell, the separator being used in a fuel cell having a membrane electrode assembly, wherein

the membrane electrode assembly includes

an electrolyte membrane, and

electrode layers that are joined to opposite sides of the electrolyte membrane in a thickness direction,

the separator for a fuel cell comprises:

a separator base that is arranged on an outer side of the membrane electrode assembly in the thickness direction and has conductivity,

a plurality of crest sections that are provided in the separator base and protrude toward the membrane electrode assembly, and

a plurality of trough sections that are provided in the separator base and recessed in a direction opposite to a direction in which the crest sections protrude,

the crest sections and the trough sections are alternately arranged in a direction along a plane of the membrane electrode assembly and extend parallel to each other,

regions surrounded by the respective trough sections and a corresponding electrode layer each constitute a passage that supplies oxidation gas or fuel gas to the electrode layer,

a thin film having conductivity is placed at least on a top surface of each crest section in surfaces of the crest sections and the trough sections that face the corresponding electrode layer,

the thin film on the top surface of each crest section has a groove that connects the passages on opposite sides of the crest section to each other,

each trough section has a flow resistance increasing portion on a downstream side of the groove in a flow direction of the gas, and

the flow resistance increasing portion reduces a cross-sectional flow area of the passage such that the cross-sectional flow area at the flow resistance increasing portion is smaller than that at a section to which the groove is connected.

2. The separator for a fuel cell according to claim 1 , wherein

the groove includes a plurality of branch groove sections in the thin film on the top surface of each crest section,

of the passages on the opposite sides of the crest section, the branch groove sections are branched at a starting point closer to the passage of which the cross-sectional flow area is not reduced by the flow resistance increasing portion, and

of the passages on the opposite sides of the crest section, ends of the branch groove sections that are farther from the starting point are connected to, while being separated from each other, the passage of which the cross-sectional flow area is reduced by the flow resistance increasing portion.

3. The separator for a fuel cell according to claim 1 , wherein

each trough section includes two side walls that face each other in an arrangement direction of the crest sections and the trough sections, and

each flow resistance increasing portion is configured by a pair of protrusions that are provided at facing sections on the side walls of a corresponding trough section and protrude toward each other.

4. The separator for a fuel cell according to claim 1 , wherein, in the thin films on the crest sections located on opposite sides of the trough section in which the flow resistance increasing portion is provided, the grooves are provided at positions facing each other in an arrangement direction of the crest sections and the trough sections.

5. The separator for a fuel cell according to claim 1 , wherein, in each adjacent pair of the trough sections, combinations of the flow resistance increasing portion and the groove that is connected to the passage of which the cross-sectional flow area is reduced by the flow resistance increasing portion are located at different positions in the flow direction of the gas.

6. The separator for a fuel cell according to claim 1 , wherein

the separator base is made of metal,

a first thin film is placed over the entire surfaces of the crest sections and the trough sections that face the corresponding electrode layer,

the first thin film has conductivity and a corrosion resistance higher than that of the separator base, and

the thin film is provided, as a second thin film, on a section of the first thin film that is placed at least on the top surface of each crest section.

7. The separator for a fuel cell according to claim 1 , wherein each flow resistance increasing portion is configured by a protrusion provided on a wall of a corresponding trough section.