Role of Bismuth in the Electrokinetics of Silicon Photocathodes for Solar Rechargeable Vanadium Redox Flow Batteries

Abstract

The ability of crystalline silicon to photoassist the V 3 + /V 2 + cathodic reaction under simulated solar irradiation, combined with the effect of bismuth have led to important electrochemical improvements. Besides the photovoltage suppliedb yt he photovoltaics, additional decrease in the onset potentials, high reversibility of the V 3 + /V 2 + redox pair,a nd improvement in the electrokinetics were attained thanks to the addition of bismuth. In fact, Bi 0 deposition has shown to slightly decrease the photocurrent, but the significant enhancement in the charge transfer,r eflected in the overall electrochemical performance clearly justifiesi ts use as additive in ap hotoassisted system for maximizing the efficiency of solar charge to battery.The electrical conversion and storage of solar energy is ac rucial target for assuringt he world energy supply in the long term. Because of the current maturity of the photovoltaic( PV) technology,s ystems devoted to PV electricity or to photoelectrochemical (PEC) approaches for water splitting have been strongly developed in the last years. [1,2] However,b ecause of the sluggish kineticsa ssociated to the water oxidationr eaction [3] and of inherent limitations in the H 2 economy,t he possibility of storing energy by coupling PV systems to other kind of redox pairs has recently attracted more attention, giving rise to the so-calleds olar-powered electrochemical energy storage (SPEES), [4] in which two systems, aP Va nd ab attery,a re integrated in as ingle device with potential advantages such as chargingb yusing solar light.Among other technologies, the vanadium redox flow batteries (VRFBs) are ar obust alternative to be integrated into SPEES with several advantages:d ecoupling of energy and power, use of the same elementi nb oth half-reactions and possibility of large-scale development. Previously,s ome studies reported the combination of PEC systems in VRFBs, such as TiO 2 [5] and TiO 2 / WO 3 photoanodes, [6] and CdS thin films in tandem with dyesensitized solar cells. [7] Particularly in the second case, an unbiaseds ystem reaching ap romising state of charge was achieved,a lthought he long-term stability in acid electrolyte and the low chargec urrent attained by the semitransparent CdS film were substantial limitations. These systems also have significant restraints associated to the low photon absorption in comparison to other absorbers. Crystalline silicon has also been integrated as light absorber in aq uinone/bromine biasfree redox flow battery. [8] For that purpose,surface-buried junctions were used as photoelectrodes in both half-reactionsa ttainingt he necessary charge voltage upon illumination (photovoltage % 1V)i nasystem involvingaPEC cell and aR FB. Despite the promising results, it should be pointed out that the associated cell voltage for theseo rganic redox couples can only attain am aximum cell voltage of % 0.89 V, which is lower than that of VRFBs. [9] One important aspectt oc onsider in the design of this kind of SPEES is the overall cell voltage re...

Design principles for efficient photoelectrodes in solar rechargeable redox flow cell applications

“…Poor reactivity of the bare c-Si sample (Fig. 1a, grey) can be attributed to a deactivation of the surface by oxidation or silanol group formation, as confirmed by previous studies 2,24 . Interestingly, while the sample with Pt and TiO 2 interlayer showed almost unchanged results compared to the result from the sample without the TiO 2 in Fig.…”

“…1 and Supplementary Note 1), thereby indicating that this is not related to the chemical affinity difference between the catholyte and anolyte. It has been accepted that conventional redox reactions in RFB are non-selective with respect to the electrode material 22 unless intermediate catalytic reactions take place, such as metal hydride in the acidic supporting electrolyte 24 . Despite the fact that the redox reaction of the ferri-/ferrocyanide involves a simple single-electron transfer reaction, an apparent material dependency was observed for both cases (with and without protection layer).…”

Unravelling the practical solar charging performance limits of redox flow batteries based on a single photon device system

“…So far, nearly all studies have employed front-side illumination despite the inevitable optical-loss by the electrolyte. 10,19,20,42,43 Only a few studies have been performed using back-side illumination, which allows direct photon absorption on a dry surface. 9,18,44 Döscher 35 and Seger 33 demonstrated a method to model solar water splitting technologies by using a detailed balance between the PEC device and optical losses by water with various overpotentials of the catalysts.…”

Electrocatalysis at Electrodes for Vanadium Redox Flow Batteries

“…Silicon has been studied as a photocathode material in a solar rechargeable VRFB [467]. Bismuth was found to effect a significant electrochemical improvement of the negative electrode reaction.…”