$A_{cl}$	catalyst area, ${\mathrm{m}}^2$
$c_i^{\left(g\right)}$	molar concentration of species i, mol m−3
$C_{i,ref}^{\left(g\right)}$	reference molar concentration of species i, mol m−3
$c_p^{\left(g\right)}$	specific heat capacity, J kg−1 K−1
$c_1,c_2,c_3,c_4$	constants for saturation pressure of water, -, K−1, K−2, K−3
$D^{\left(c\right)}$	capillary diffusion, m2 s−1
$D_i$	diffusivity of species i, m2 s−1
$E_{cell}$	cell voltage, V
$E_a$	activation energy, J mol−1
$E_{rev}$	reversible cell potential, V
$F$	Faraday constant, C mol−1
$h_j$	height of layer j, m
$H_{O_2}^{\left(l\right)},H_{O_2}^{\left(p\right)}$	Henry’s constant for air-water and air-polymer interfaces, Pa m3 mol−1
${\eta }_{a,c}$	relative humidity, %
$i$	current density, A m−2
$j_{a,c}^{ref}$	anode and cathode volumetric reference exchange current density, A m−3
$J$	volumetric reference current density, A m−3
$L$	length of channel, m
$m_{H_{2}O}$	interphase mass transport, kg m−3 s−1
$M^{\left(g\right)}$	mean molecular mass of the gas phase, kg mol−1
$M_i$	molecular mass of species, kg mol−1
$M^{\left(m\right)}$	equivalent weight of the dry membrane, kg mol−1
$n_d$	electroosmotic drag coefficient
$p_{H_{2}O}^{sat}$	saturation pressure of water, Pa
$R$	gas constant, J mol−1 K−1
$s$	liquid saturation
$S$	source term
${\mathrm{T}}_0,{\mathrm{T}}_1,{\mathrm{T}}_2$	constants, K
$T$	temperature, K
$V$	volume, m3
${\omega }_i^{\left(g\right)}$	mass fraction of species i
$Greeksymbols$
$\mathsf{\alpha }$	transfer coefficient
${\mathsf{\beta }}_{\mathrm{m}}$	membrane modification coefficient
$\mathsf{\gamma }$	volume fraction
$\mathsf{\delta }$	thickness of film, m
$ϵ$	porosity
$\mathsf{\eta }$	overpotential, V
$\mathsf{\theta }$	wetting angle
$\mathsf{\kappa }$	permeability, m2
$\mathsf{\lambda }$	membrane water content
$\mathsf{\mu }$	dynamic viscosity, kg m−1 s−1
$\mathsf{\xi }$	stoichiometry
$\mathsf{\rho }$	density, kg m−3
$\mathsf{\tau }$	surface tension, Pa
$\mathsf{\sigma }$	total stress tensor, Pa
$\mathsf{\varphi }$	potential, V
$Superscripts$
$\mathrm{sat}$	saturation
$\left(\mathrm{g}\right)$	gas phase
$\left(\mathrm{m}\right)$	membrane
$\left(\mathrm{l}\right)$	liquid phase
$\left(\mathrm{s}\right)$	solid
$\mathrm{in}$	inlet
$\mathrm{ref}$	reference
$\left(\mathrm{c}\right)$	capillary
$Subscripts$
$\mathsf{\alpha },\mathsf{\beta }$	index for species
a	anode
$\mathrm{c}$	cathode
$\mathrm{cc}$	current collector
$\mathrm{cl}$	catalyst layer
$\mathrm{ff}$	flow field
$\mathrm{gdl}$	gas diffusion layer
${\mathrm{H}}_2$	hydrogen
${\mathrm{H}}_2\mathrm{O}$	water
$\mathrm{i}$	species i
$\mathrm{j}$	functional layer j
$\mathrm{m}$	membrane
${\mathrm{N}}_2$	nitrogen
${\mathrm{O}}_2$	oxygen
$\mathrm{pot}$	potential
$\mathrm{ref}$	reference
$\mathrm{o}$	standard condition