Hydrogen-treated TiO as an electrocatalyst has shown to boost the capacity of high-performance all-vanadium redox flow batteries (VRFBs) as a simple and eco-friendly strategy. The graphite felt-based GF@TiO :H electrode is able to inhibit the hydrogen evolution reaction (HER), which is a critical barrier for operating at high rate for long-term cycling in VRFBs. Significant improvements in charge/discharge and electron-transfer processes for the V /V reaction on the surface of reduced TiO were achieved as a consequence of the formation of oxygen functional groups and oxygen vacancies in the lattice structure. Key performance indicators of VRFB have been improved, such as high capability rates and electrolyte-utilization ratios (82 % at 200 mA cm ). Additionally, high coulombic efficiencies (ca. 100 % up to the 96th cycle, afterwards >97 %) were obtained, demonstrating the feasibility of achieving long-term stability.

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Solar-powered vanadium redox-flow batteries (VRFB) have emerged as an attractive alternative to large-scale and efficient energy storage and conversion. However, due to the stringent charging voltage requirements of vanadium-based systems (1.4-1.7 V), common photobatteries, applying standard photovoltaics with nonoptimized photovoltages, cannot be completely charged bias-free, i.e. by only using bias-free solar energy, or if they can be, only at unpractical low current densities of just a few mA cm −2 . In response to this critical challenge, the present study aimed to design and test a compact device combining a highphotovoltage silicon multijunction solar cell with an all-vanadium continuous-flow battery. In particular, we applied a monolithic triple junction solar cell, which can provide photovoltage of up to 2.2 V. Additionally, we have introduced the concept of increased illumination intensity for the solar VRFB. As a first demonstration, a complete bias-free solar charging at 25 mA cm −2 (300 mW cm −2 illumination) is reported. Moreover, we investigated the influence of the operation parameters of the redox-flow battery itself: the membrane type and the vanadium concentration in the electrolyte (i.e. storage capacity). The presented results provide evidence that the low-cost thin-film silicon based solar VRFB can be considered as an outstanding alternative for practical energy storage and conversion usage. A maximum bias-free solar conversion efficiency of 12.3% was achieved during charging, combined with promising and competitive energy efficiencies for the complete charge-discharge process that can guarantee an overall solar-to-electricity conversion efficiency of >10%.

A new positive electrolyte formulation for all-vanadium redox flow batteries (VFRB) with a significant improvement in electrochemical properties is reported in this communication. A 5 KDa poly(ethyleneimine)-based dendrimer (dPEI) demonstrates enhanced performance when used as an additive for positive electrolytes in VFRB. A thermal stability test at 40 8C shows that the dPEI additive prevents the precipitation of VO 2 + over 720 h. The optimum concentration of the dPEI additive was as low as 0.6 % (weight per volume of electrolyte), leading to a cost-effective solution in terms of a 38 % increase in energy density (18 Wh L À1) with respect to the additive-free system (ca. 13 Wh L À1). Moreover, this innovative electrolyte performance remains stable at 88 % in energy efficiency over a wider operational temperature (up to 60 8C) while retaining the energy density.A gradual increase in dispersed and intermittent power generation (i.e. renewable energy) has accompanied the depletion of fossil fuel energy reserves. As a consequence, large-scale electrical energy storage (EES) has become an objective of particular socio-economic relevance as well as the focus of interest because of the demand for efficient, dependable, and cleaner electricity.[1] A broad set of different storage technologies have been developed. [2] In contrast with other secondary batteries, vanadium-based redox flow batteries (VFRB) are an appealing alternative for energy storage due to several attractive features including long cycle life, low maintenance cost, low cross contamination between electrolytes, short response time, and low self-discharge.[3] Furthermore, their most significant feature is that VFRB are well-suited for large-scale application because of the independence of the electricity storage capacity and power generation capacity. [3] Although multi-MW h VFRB systems have been demonstrated, [4] existing VFRB technology still displays some drawbacks, limiting it to the precommercial phase and preventing it from breaking into the industrial marketplace. In particular, the stability and solubility of VO 2 + species are very limited, especially at temperatures above 40 8C.[5] However, the precipitation of VO 2 + species can be avoided by reducing the vanadium concentration to below 1.5 m, but this greatly decreases the energy density of VFRB (ca. < 25 W h L À1 ). [6] In the last decade, there have been great efforts to improve the solubility and stability of electrolyte solutions with a wide range of additives. [7] Unfortunately, improvements of VFRB related to long cycle life at temperatures up to 60 8C and improvements to highenergy-density electrolytes still remain a major challenge. A mixture of sulfate and hydrochloric acid employed as solvent can considerably improve the thermal stability of VO 2 + ions.[8]Here, we report the effect of the new positive electrolyte formulation containing a 5 KDa poly(ethyleneimine)-based dendrimer (dPEI) with a 100 %-functionalized NH 2 unit, which constitutes a cost-effective way to improve...