Spin-controlled electrochemistry using chiral electrodes: Effects on water electrolysis

Hydrogen is the ecologically ideal energy vector. Efficient photo-electrochemical production of hydrogen from water could be the optimal solution to the energy storage problems related to renewable sources. However, in the water splitting reaction the electric potential required to initiate the process significantly exceeds the thermodynamic limit. By controlling the spins of the electrons that are transferred from the solution to the anode, and ensuring that they are coaligned, the threshold voltage for the process can in theory be decreased to that of the thermodynamic voltage. In the present study, by using TiO2 anodes coated with chiral materials, we explore what are the effects of having a spin-polarized current on water electrolysis. The spin-polarization arises from exploiting what is known as Chiral Induced Spin Selectivity effect by using chiral molecules as spin filters. When using chiral molecules instead of a non-chiral analogue, the hydrogen production from water is enhanced, the threshold voltage is reduced and the by-product formation of hydrogen peroxide is suppressed. Figure: control of hydrogen peroxide production. UV-vis spectra from the titration of the used electrolyte (Na2SO4) with o-tolidine of bare TiO2 and TiO2 electrodes coated with (A) self-assembled Zn-porphyrins of either achiral (A-Zn) or chiral (S-Zn) and (B) TPyA molecules. (C) When the electrons transfer to the anodes is non spin specific the spins of the unpaired electrons on the two radicals are aligned antiparallel, hence the interaction is on a singlet surface that correlates with the production of hydrogen peroxide (H2O2). (D) When the electron transfer to the anode is spin specific, the spins of the two electrons are aligned parallel to each other, hence the two radicals interact on a triplet surface that forbids the formation of H2O2 and facilitates the production of oxygen in its ground state.