This paper is devoted to the modeling and numerical optimization of proton-exchange membrane (PEM) water electrolysers for operation at elevated pressures (up to 130 bars). The model takes into account different geometrical parameters of the PEM cell, the kinetics of the hydrogen and oxygen evolution reactions, the electro-osmotic drag of water molecules, the permselectivity of the solid polymer electrolyte and associated gas cross-over phenomena. The role of various operating parameters (such as pressure, temperature, current density, flow rate of water) on cell efficiency, faradaic yield and heat produced during water electrolysis is evaluated and discussed. The model is also used for the purpose of optimizing the performances of PEM cells. In particular, optimal values of some critical operating parameters (current density, rate of water supplied to the anodes) are recommended.Keywords Proton-exchange membrane (PEM) Á Water electrolysis Á High pressure Á Mathematical modelingMolar density of gas, liquid, hydrogen, oxygen. t g , t l (m s -1 ) Velocity of gas and liquid. t 0 (m s -1 ) Velocity of cooling water P g , P l , P c (Pa) Pressure of gas, liquid and capillary pressure K p (m 2 ) Permeability of porous current collector R p (m)Average pore radius of porous current collector k rl , k rg Relative permeabilities of liquid and water s (adim.)Saturation of porous current collector s fc (adim.) Saturation at the current collector-flow channel interface s cl (adim.) Saturation at the current collector-catalytic layer interface h c (deg.)Wetting angle of porous current collector e (adim.) Porosity of gas diffusion layer R bc (m) Critical bubble radius N g , N l , N (mol m -2 s -1 ) Flux of gas, liquid, total flux N ga , N la (mol m -2 s -1 ) Gas and liquid fluxes at current collector-anodic layer interface N gc , N lc (mol m -2 s -1 ) Gas and liquid fluxes at current collector-cathodic layer interface C H (mol m -3 )Hydrogen concentration in the gas phase R p (m) Effective pore radiusTotal, useful, and parasitic current densities n p (adim.) Electro-osmotic drag coefficient P s (Pa) Pressure of saturated water vapor h m (m) Membrane thickness h (m) Gas diffusion layer thickness h fc (m) Flow channel thickness DT (°C) Temperature difference along the membrane-electrode assembly (MEA) S MEA (m 2 ) Active area of MEA i ex (A m -2 ) Exchange current density q m (Ohm -1 m -1 ) Specific ionic conductivity of solid polymer electrolyte Physicochemical parameters C pw = 4,217 (J kg -1 K -1 ) Specific heat of liquid water at T = 100°C l l = 2.822 9 10 -2 (Pa s) Dynamic viscosity of liquid water at T = 100°C and P = 13 MPa l H = 1.09 9 10 -5 (Pa s) Dynamic viscosity of hydrogen at T = 100°C and P = 13 MPa l O = 2.47 9 10 -5 (Pa s) Dynamic viscosity of oxygen at T = 100°C and P = 13 MPa r = 0.05884 (Pa m) Surface tension of water at T = 100°C and P = 13 MPa U H = 1.67 9 10 -3 (mol mol -1 ) Water solubility of hydrogen at T = 100°C and P = 13 MPa U O = 1.85 9 10 -3 (mol/mol) Water solubility of oxygen at T = 100°C and P = 13...

Electrochemical hydrogen pumps are electrochemical devices which are used for hydrogen purification and pressurization purposes. In such cells, gaseous hydrogen is oxidized at the anode and released at the cathode. Results presented in this paper are related to the characterization and optimization of a proton-exchange membrane (PEM) hydrogen pump using electrochemical impedance spectroscopy (EIS). Only the case of hydrogen purification is considered here. Current-voltage characteristics have been measured. The kinetics and efficiency of the cell have been investigated using EIS. The roles played by the cell structure (in particular by the ion-exchange polymer content in the electro-catalytic layers) and by different operating parameters (the cell temperature, the relative humidity and the partial pressure of hydrogen) on the overall process efficiency have been evaluated and are discussed.

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