The initial difference in the HFR between the cells (in Figure ) may be attributed to a difference in contact resistance due to the different methods of compression in each cell as discussed in the previous section. The fact that the HFR only increased in the ME-cell may be because the bolts providing a static compression did not compensate for the 53 ± 3% decrease in membrane thickness measured by μ-CT. This meant the cell compression reduced as the membrane thinned, increasing the contact resistance. Since the LE-cell applied a constant pneumatic pressure, contact resistance should remain unchanged even when the membrane thickness decreased.