Electrochemical Studies of Water Splitting in Organic Media

Abstract

The development of new routes towards the generation of environmentally friendly solar fuels, such as molecular hydrogen (H2) for use in fuel cells, is a grand challenge facing the scientific community. In this regard, water splitting represents one of the most promising, yet challenging, options. In nature, water splitting occurs under primarily non-aqueous conditions. This provides a blue print for the development of efficient systems for water oxidation (WOR) and hydrogen evolution (HER), profiting from the changes structure-reactivity in those complex environments. Herein, we investigate these two processes, ocurring in mainly non-aqueous environment. Another aspect addressed in this work, is the inmobilization of catalitycally active nanoparticles towards WOR, at the surface of an electrode. In first instance, a layer-by-layer methodology was used to prepare thin films consisting of bilayers of negatively charged citrate-stabilized IrOx nanoparticles (NPs) and the positively charged poly(diallyldimethylammonium chloride) (PDDA) polymer. The IrOx films obtained were amorphous, with the NPs therein being well dispersed and retaining their as-synthesized shape and sizes. UV/vis spectroscopic and electrochemical studies confirmed that electrochemically addressable surface coverage of IrOx NPs increased linearly with the number of bilayers. Moreover, the electrodes obtained exhibit an “ideal” electrochemical response, which resembles the highly reversible waves observed in hydrous iridium oxide films (HIROFs). Taking one step further, the concept of layer-by-layer inkjet printing was implemented for the fabrication of pH sensing electrodes based on IrOx. Those electrodes exhibited good performance, with a linear and near-Nernstian pH response. Water oxidation catalyzed by IrOx NPs in water/acetonitrile mixtures using [RuIII(bpy)3]3+ as oxidant was studied as a function of the water content, the acidity of the reaction media and the catalyst concentration. Under acidic conditions and at high water contents (80 % (v/v)) the reaction is slow, but its rate increases as the water content decreases, reaching a maximum at approximately equimolar proportions (≈ 25 % H2O (v/v)). At high water fractions, water is present in highly hydrogen-bonded arrangements and is less reactive. As the water content decreases, water-clustering gives rise to the formation of water-rich micro-domains, which are considerably more reactive towards oxygen production. Further analysis via electrochemical measurements at PDDA-IrOX modified electrodes, demonstrated that those changes in reactivity are correlated with decrease in the overpotential for WOR. Based on a straightforward thermodynamic analysis, those changes in reactivity were associated with favorable kinetics rather than a lowering in the thermodynamic barriers. Finally, the light-driven HER catalyzed by Pt in acidified acetonitrile solutions and using tetrathiafulvalene (TTF) as both, sensitizer and electron donor, was studied. Kinetic studies indicated that the reaction starts by strong and fast adsorption of TTF-H+ on the catalyst surface, followed by slow photoreduction of protons. On the other hand, the addition of water to the mixture of reaction leads to evolution of O2 by water oxidation, inhibiting HER. In both cases, no decomposition of the electron donor/acceptor was observed, indicating that both reactions are reversible and therefore the donor/acceptor potentially recyclable.