Let’s not forget isotope effect when replacing H2O with D2O for operando neutron studies – Kinetics and transport considerations in PEM water electrolyzer

Operando neutron imaging techniques are at the forefront of advanced characterization strategies to investigate core phenomena in electrochemical energy devices such as proton exchange membrane water electrolyzers (PEMWEs). The replacement of H 2 O with D 2 O is a practical approach to minimizing attenuation and incoherent scattering effects during neutron imaging, though its impact on the PEMWE performance has not been well-studied. Herein, we comprehensively analyzed PEMWE cells fed with five D 2 O/H 2 O mixtures using electrochemical methods and operando neutron imaging. While the PEMWE characterization highlighted an overpotential of around 190 mV at 1 A cm -2 in the case of using D 2 O instead of H 2 O, the kinetic and ion conductivity differences of these isotopes were confirmed by separate electrochemical studies. Moreover, the water profile inside the PEMWE cell was calculated for the studied D 2 O/H 2 O concentrations, further supporting the results obtained by the electrochemical tests. The neutron data showed a more than two times faster H 2 O permeation compared to D 2 O at 1500 mA cm -2 in PEMWE cells, which perfectly aligned with faster ionic conduction and higher OER performance of H 2 O-fed cells. Also, more stationary D 2 O molecules in the membrane area, even at higher currents, were detected, explaining the lower diffusion of D 2 O through the membrane and the higher electrochemical stability of the D 2 O-fed PEMWE cell. These results were translated to the higher kinetic energy demanded for D 2 O bond breakdown and reduced electro-osmotic drag for D 2 O due to its stronger bond to the membrane and higher viscosity. • The kinetic and transport properties of H 2 O and D 2 O in a PEM water electrolyzer were investigated using electrochemical and operando neutron radiography methods. • Fully-H 2 O operated PEMWE cells showed 190 mV less overpotential at 1 A cm -2 , and four times higher ionic conductivity in the membrane than their D 2 O counterpart. • Faster OER kinetics of the IrO x catalyst in the H 2 O-based electrolyte were measured, while more chemically stable OER activity was associated with the D 2 O-based electrolyte. • More water permeation through the membrane in H 2 O-fed PEMWE cells and more stagnant water inside the membrane in D 2 O-fed PEMWE cells were observed by neutron radiography. • Dynamic phenomenon studies in neutron radiography, e.g. replacing H 2 O with D 2 O, must take the isotope effect into account.