[ materials ]
For example , knowing that fuel cell technology requires the membrane with expected attributes , it ’ s important to not only understand its intrinsic native properties but also its functionality subjected to the fuel cell operating conditions ( from beginning of life to the end of life ).
The intrinsic chemical properties of PFSA directly influence it proton conductivity capabilities , but its effectiveness in a fuel cell stack will be challenged as a function of water management effectiveness .
Let ’ s now consider the broader implications of the membrane behaviour within the context of operational conditions over the lifespan of the stack . When in operation , the fuel cell design elements are subjected to the challenging boundary conditions ( stressors ) and complex effects coming from the interface interactions alongside the presence of coexisting failure mechanisms .
Apart from the operating conditions such as temperature , pressure , relative humidity , reactant concentration and utilization , and current density , key stressors include load cycling , mechanical forces , reactant contaminants ( impurities ), and environmental factors . While each stressor or material functionality can be examined independently , they are interconnected ; changes in one stressor invariably impact others .
Imagine deteriorating GDL ; its structure starts to decrease due to the degradation of backing material . Mechanical stress leads to the decreased GDL stability in water management due to the changed material ’ s hydrophobicity , causing the GDL to eventually corrode and loose its conductivity . What can this mean for the membrane ? In the case of water shortage accompanied by the insufficient heat management , the membrane and ionomer start to dry and , thus , increasing the local membrane resistance and ultimately causing its damage , which in turn affects cell performance and longevity .
It is incredibly important to understand the degradation mechanisms and possible consecutive failure modes that can affect neighbouring materials and , thus , stack as a whole ( see Table 1 ). Investigating the commonly reported failure modes ( patterns ) reported for the materials can provide insights into potential materials and design strategies and their relevant impacts on cell performance .
Table 1 . Failure modes and potential consecutive effects in fuel cell components
Failure modes Causes Further possible consecutive failures
Decrease in GDL structure
Degradation of backing material ( breakage of fibers , hydrophobic coating ); carbon corrosion
Shortage of water and insufficient heat management → Membrane and ionomer drying → dry- and hot-spots in PEM → increase of local membrane resistance and its damage → decrease of overall cell performance , stability and durability .
Decrease in water management stability
Mechanical stress ( compression + localized compression - peripheral , ribs ); change in materials hydrophobicity ( erosion by fluid circulation - gas and water circulation , freeze-thaw )
Conductivity loss Corrosion ( due to oxidizing conditions , electrochemical degradation , operating conditions - high current densities , high temperature ( T ), high relative humidity ( RH ))
Excess of water * → Water accumulation in MPL and GDL – flooding ( under high current densities ) → hindered pathway for gases to reach catalyst layer , reactants starvation → flooding of catalyst layer → decreased number of reaction sites , carbon support corrosion → decreased platinum performance and ORR efficiency → low electric conversion efficiency → low electric power density * At cold start-up → repeated freeze-thaw cycling → membrane degradation , gaps between membrane electrodes → loss of contact resistance → degradation of internal structure of MPL and GDL by ice → blockage of internal pores hinders gas supply to CL
Hydrogen Tech World | Issue 15 | April 2024 23