GDL is to help remove water, a product, flooding can occur when water effectively blocks the GDL. This limits the reactants ability to access the catalyst and significantly decreases performance. Teflon can be coated onto the GDL to limit the possibility of flooding. Several microscopic variables are analyzed in the GDLS such as: porosity, tortuosity and permeability. These variables have incidence over the behavior of the fuel cells. Efficiency The maximal theoretical efficiency applying the Gibbs free energy equation ΔG = −237.13 kJ/mol and using the heating value of Hydrogen (ΔH = −285.84 kJ/mol) is 83% at 298 K. The practical efficiency of a PEMs is in the range of 50–60% . Main factors that create losses are: Activation losses Ohmic losses Mass transport losses Bipolar plates The external electrodes, often referred to as bipolar plates or backplates, serve to distribute fuel and oxygen uniformly to the catalysts, to remove water, to collect and transmit electrical current. Thus, they need to be in close contact with the catalyst. Because the plates are in contact with both PEM and catalyst layers, the backplate needs to be structurally tough and leakage-resistant in case of structural failure due to fuel cell vibration and temperature cycling. As fuel cells operate in wide ranges of temperatures and in highly reductive/oxidative environments, plates must have high surface tolerances over wide ranges of temperatures and should be chemically stable. Since the backplate accounts for more than ¾ of the fuel cell mass, the material must also be lightweight to maximize the energy density. Materials that fulfill all of these requirements are often very expensive. Gold has been shown to fulfill these criteria well, but is only used for small production volumes due to its high cost. Titanium nitride (TiN) is a cheaper material that is used in fuel cell backplates due to its high chemical stability, electrical conductivity, and corrosion resistance. However, defects in the TiN coating can easily lead to corrosion of the underlying material, most commonly steel. To perform their main function of distributing gas and fuel, these plates often have straight, parallel channels across its surface. However, this simple approach has led to issues such as uneven pressure distribution, water droplets blocking gas flow, and output power oscillations. Innovative approaches, such as nature-inspired fractal models and computer simulations are being explored to optimize the function of these bipolar plates. Metal-organic frameworks Metal-organic frameworks (MOFs)