  Tungsten oxide and its associated bronzes (compounds of tungsten oxide and an
alkali metal) are well known for their interesting optical and electrical
characteristics. We have modified the transport properties of thin WO$_3$ films
by electrolyte gating using both ionic liquids and polymer electrolytes. We are
able to tune the resistivity of the gated film by more than five orders of
magnitude, and a clear insulator-to-metal transition is observed. To clarify
the doping mechanism, we have performed a series of incisive operando
experiments, ruling out both a purely electronic effect (charge accumulation
near the interface) and oxygen-related mechanisms. We propose instead that
hydrogen intercalation is responsible for doping WO$_3$ into a highly
conductive ground state and provide evidence that it can be described as a
dense polaronic gas.
