Gas-diffusion electrocrystallization is a process electrochemically driven at porous gas-diffusion electrodes, in which a triple phase boundary is established between a liquid solution, an oxidizing gas, and an electrically conducting electrode. The liquid solution containing dissolved metal ions (e.g., CuCl, ZnCl) flows through an electrochemical cell equipped with a gas diffusion electrode, making contact with its electrically conducting part (typially a porous layer). The oxidizing gas (e.g., pure O, O in air, CO, etc.) percolates through a hydrophobic layer on the gas diffusion electrode, acting as a cathode. After the gas diffuses to the electrically conducting layer acting as an electrocatalyst (e.g., hydrophilic activated carbon), the gas is electrochemically reduced. For instance, by imposing specific cathodic polarization conditions (e.g., −0.145 VSHE O is reduced, to HO in a 2 electron (2 e) transfer process and HO in a 4 electron (4 e) transfer process. OH are also produced in the process. As this happens, abrupt local pH and local electrolyte redox potential changes arise within the cathode porosity. As the hydroxyl ions spread to the bulk electrolyte, systematic pH increases become consistently manifest in the electrolyte bulk. In due course, low amounts of HO are generated. In steady state, a reaction front is fully developed throughout the hydrodynamic boundary layer. This creates local saturation conditions at the electrochemical interface, where metal ions precipitate in metastable or stable phases depending on the operational variables. When oxygen is the oxidizing gas, the mechanism for gas-diffusion electrocrystallization has been explained as an oxidation-assisted alkaline precipitation using gas-diffusion electrodes.