acidic in use. The most common membrane material is perfluorinated sulfonic acid (PFSA or Nafion). However, vanadium ions can penetrate a PFSA membrane and destabilize the cell. A 2021 study found that penetration is reduced with hybrid sheets made by growing tungsten trioxide nanoparticles on the surface of single-layered graphene oxide sheets. These hybrid sheets are then embedded into a sandwich structured PFSA membrane reinforced with polytetrafluoroethylene (Teflon). The nanoparticles also promote proton transport, offering high Coulombic efficiency and energy efficiency of more than 98.1 percent and 88.9 percent, respectively. Operation The reaction uses the half-reactions: () () Other useful properties of vanadium flow batteries are their fast response to changing loads and their overload capacities. They can achieve a response time of under half a millisecond for a 100% load change, and allow overloads of as much as 400% for 10 seconds. Response time is limited mostly by the electrical equipment. Unless specifically designed for colder or warmer climates, most sulfuric acid-based vanadium batteries work between about 10 and 40 °C. Below that temperature range, the ion-infused sulfuric acid crystallizes. Round trip efficiency in practical applications is around 70–80 %. Proposed improvements The original VRFB design by Skyllas-Kazacos employed sulfate (added as vanadium sulfate(s) and sulfuric acid) as the only anion in VRFB solutions, which limited the maximum vanadium concentration to 1.7 M of vanadium ions. Around 2010 a team from Pacific Northwest National Laboratory proposed a mixed sulfate-chloride electrolyte, that allowed for the use in VRFBs solutions with the vanadium concentration of 2.5 M over a whole temperature range between −20 and +50 °C. Based on the standard equilibrium potential of the V(+5)/V(+4) couple it is expected to oxidize chloride, and for this reason chloride solutions were avoided in earlier VRFB studies. The surprising oxidative stability (albeit only at the state of charge below ca. 80%) of V(+5) solutions in the presence of chloride was explained on the basis of activity coefficients. Nevertheless, because of a high vapor pressure of HCl solutions, such mixed electrolytes have not been widely adopted. Another variation is the use of vanadium bromide salts. Since the redox potential of Br2/2Br- couple is more negative than that of V(5+/4+), the positive electrode operates via the bromine process. However, due to problems with volatility and corrosivity of Br2, they did not gain much popularity (see zinc-bromine battery for a similar problem). A vanadium/cerium flow battery