$D_i{}^R$	relative gas-diffusion coefficient (m2/s)	${\sigma }_{ion}$	proton conductivity (S/m)
$D_i{}^{eff}$	effective diffusion coefficient (m2/s)	${\tau }^{ele}$	tortuosity of carbon
$D_i{}^{bulk}$	molecular diffusion coefficient (m2/s)	${\tau }^{ion}$	tortuosity of ionomer
$D_i$	gas-diffusion coefficient (m2/s)	$\rho$	density (kg/m3)
$D_i^k$	Knudsen diffusion coefficient (m2/s)	$\mu$	dynamic viscosity (kg/(m s))
$R$	universal gas constant (J/(mol K))	${\rho }^{eff}$	effective density
$T$	Temperature (K)	$\sigma$	surface tension
$r$	average pore radius (m)	$\theta$	contact angle
$M_i$	relative molecular mass (kg/mol)	${\varphi }_{ele}$	electronic potential (V)
$V^{ele}$	carbon volume fraction	${\varphi }_{ion}$	ionic potential (V)
$V^{ion}$	ionomer volume fraction	$\gamma$	concentration index
$s_{lq}$	liquid water saturation	${\eta }_c$	cathode overpotential (V)
$\overrightarrow{u}$	mixed gas velocity vector (m/s)	$\alpha$	transport coefficient
$S$	source term (kg/(m3 s), W/m3…)	Subscripts
$p_g$	gas pressure (Pa)	$i$	gas species
$c_p$	constant pressure heat capacity (J/(mol K))	$ele$	electron
$K^e{{}^f}^f$	effective thermal conductivity (W/(m K))	$ion$	proton
$Yi$	gas species mass fraction	$lq$	liquid water
$K$	intrinsic permeability (m2)	$g$	gas phase
$p_{ca}$	capillary pressure (Pa)	$c$	cathode
$J_c$	cathode electrochemical reaction rate (A/m2)	$H_{2}O$	water
$Ac$	active specific surface area (m−1)	$O_2$	oxygen
$C_{O_2,ref}$	oxygen reference concentration (mol/m3)	$N_2$	nitrogen
Greek letters
$\epsilon$	porosity
$\tau$	tortuosity
${\kappa }_{ele}^{eff}$	effective electrical conductivity (S/m)
${\kappa }_{ion}^{eff}$	effective ionic conductivity (S/m)
${\sigma }_{ele}$	electron conductivity (S/m)