In particular, RbBr was chosen as both the anion and cation fluoresce at higher energy (>12 keV) avoiding fluorescence overlap from ambient Ar (Figure ) and excessive photon absorption, which allows XRF measurements to be carried out under ambient conditions. These considerations are especially important when looking towards future in-situ/operando application of XRF measurements. In evaluating the generality of monovalent metal halide salts, OECTs employing aqueous 100 mM RbBr preformed nearly identically to the more commonly employed NaCl electrolytes at equivalent concentrations. In addition, XPS performed on both RbBr and NaCl exposed samples revealed similar ion uptake, implying RbBr as a suitable approximation of other monovalent metal halide salts (Figure ).