## 1 Introduction  
The goals set by the European Commission in the recently launched Green Deal forces member states of the European Union to reduce their greenhouse gas emissions by at least 55% by the year 2030 [1]. Meeting this target will require increasing the binding renewable energy target in the EU's energy mix to 40%. However, the efficient use of renewable energy resources, such as solar, wind, or tidal, requires mitigation or fulfilling the mismatch between its inherent intermittent energy production and the electricity demand. The current progress in photovoltaic technology (PV) has led solar energy to be foreseen as the foremost source of renewable energy. According to the NREL chart of the highest confirmed conversion efficiencies for research cells [2], their up-to-date values range from 25.5% for perovskite and 26.7% for crystalline SI cells through 29.5% for perovskite/Si tandem cells, 29.1% for GaAs single-junction, and 39.3% for multijunction cells, all measured under non concentrated sunlight conditions. Although there is still a lot of room to optimize the efficiency of solar cells, the main problem is their variable output, depending on the conditions of sunlight. They can produce excess energy at some times,and then none at others. Many kinds of energy storage systems have been employed to compensate for the load on the power grid. One of them is redox flow batteries (RFB) that have emerged as relevant candidates to address sustainable energy generation. RFBs have a reasonably good energy density, flexible modular design and operation, excellent scalability, moderate maintenance costs, and long-life cycling. Their most attractive characteristics are the decoupled nature of their volumetric power and energy densities and their low self-discharge rate, making them suitable for long-duration storage [3]. The emerging technology that combines PV and RFB in one system is the solar redox flow battery (SRFB) [4]. It is a promising energy generation and storage technique that is cost-effective and balanced, with the electricity demand way of energy production. The system has been engineered in two architectures [5], (1) where the photoelectrode is integrated directly with RFB and the redox couples of RFB are shared with the photovoltaic cell (PVC) [6; 7; 8] (Figure 1); and (2) where PVC with an electrochemical module of RFB are stacked together, but each operates autonomously [9; 10; 11] (Figure 2). Figures 1 and 2 show schematically the structure of SFRBs with photoanodes built on the basis of an n-type semiconductor, which introduces electrons to the working electrode. For semiconductor-liquid junction cells (Figure 1), energy-level matching between semiconductors and redox species is crucial, as it determines the photovoltage of such cells. On the other hand, the stacked system of PV and RFB cells is integrated by their internal electrical junction only, and the redox couple of PV is thus independent of the RFBs redox couples. This configuration is easiest to realize and optimize because the difficulties in the overall device design and voltage matching can be greatly reduced. Recently, the all-vanadium redox flow batteries (VRFB) get the most scientific and industrial attention [12; 13] and are considered a promising candidate for large-scale energy storage systems [14]. Thanks to the ability of vanadium to exist in solution in four different oxidation states, vanadium ions in sulfuric or hydrochloric acidic-water solution are used at both compartments, as catholyte VO2+/VO2+ (O1/R1 in Figures 1 and 2) and V2+/V3+ (R1/O1 in Figures 1 and 2) as anolyte. In VRB, the H+ ion is exchanged (X+ in Figures 1 and 2) through the ion exchange membrane. The membrane is usually made from Nafion polymer. The best current densities were noticed for Nafion N212 [15]. During the charge/discharge cycles in RBFs, catholyte and anyolite are pumped into the stack of electrochemical cells from reservoirs.