Alkaline water electrolysis enhanced by radio frequency alternating magnetic field

This PhD aimed at studying the influence of a radio frequency alternating magnetic field (AMF) on the water splitting reaction in alkaline media (KOH, 1 mol/L). The objective of this approach was to heat the catalyst at a very local scale to enhance water electrolysis, without heating the electrolyte and the other cell component, especially to limit their degradation and provide the energy exactly where it is needed: on the catalyst active sites. The heat stems from (a priori) hysteresis losses of ferromagnetic nanoparticles submitted to an AMF. In such respect, the work consisted in a first place of finding non-PGM materials (especially FeNi and Ni-based) which are fast water splitting catalysts, as well as magnetically sensitive materials. Thus, several electrochemical tests (cyclic voltammetry, durability tests, Identical Location TEM) were conducted, as well as magnetic characterization (VSM, SAR), without an AMF application in first instance, then with it. The magnetic field proved to influence both the charge-transfer kinetics and the mass transport kinetics (cyclic voltammetry, chronopotentiometry, Tafel, open-circuit voltage analysis), and that the heating came also from eddy current generation in the electrode support (non-graphitized carbon felt). No influence on the thermodynamics was concluded. In parallel, a bibliographic survey permitted to account for the various effects which can occur in an electrochemical system submitted to a magnetic field. Thus, Lorentz, Kelvin, spin polarization, as well as Soret, Marangoni and Maxwell stress effects were evaluated in the system. No influence of a Lorentz or spin polarization effects was observed, but the others are likely to intervene. Then, post mortem analyses (ILSEM, XRD, ETEM) allowed to study the influence of the temperature and of a reductive/oxidant atmosphere on the best catalyst (FeNi₃[at]Ni). Finally, preliminary experiments were conducted to take benefit from the eddy current as main heating source. The efficiency of the system is also discussed. (author)