The boom in multifunctional, flexible, and portable electronics and the increasing need of low-energy cost and autonomy for applications ranging from wireless sensor networks for smart environments to biomedical applications are triggering research efforts towards the development of self-powered sustainable electronic devices. Within this context, the coupling of electronic devices (e.g. sensors, transistors) with small size energy storage systems (e.g. micro-batteries or micro-supercapacitors) is actively pursued.
Micro-electrochemical supercapacitors are attracting much attention in electronics for their capability of delivering short power pulses with high stability over repeated charge/discharge cycling.
For their high specific pseudocapacitance, electronically conducting polymers are well known as positive materials for hybrid supercapacitors featuring high surface carbon negative electrodes. The processability of both polymer and carbon is of great relevance for the development of flexible miniaturised devices.
Electronically conducting polymers are even well known to feature an electronic conductivity that depends on their oxidation (p-doped state) and that it is modulated by the polymer potential. This property and the related pseudocapacitive response make polymer very attracting channel materials for electrolyte-gated (EG) transistors.
Here, we propose a novel concept of “Trans-capacitor”, an integrated device that exhibits the storage properties of a polymer/carbon hybrid supercapacitor and the low-voltage operation of an electrolyte-gated transistor.
About 3 μm thick tungsten trioxide film electrodes consisting of partly sintered, 40-80 nm in diameter, particles
deposited on conducting glass substrates exhibit high photon-to-current conversion efficiencies for the photooxidation of
water, exceeding 70% at 400 nm. This is facilitated by a ca. 40% film porosity resulting in high contact area with the
electrolyte. It is shown that the activity of the WO3 electrodes towards photooxidation of water is enhanced by addition
of even small amounts of halide (Cl-, Br-) ions to the acidic electrolyte. Photoelectrolysis experiments performed either
in acidic electrolytes containing chloride or bromide anions or in a 0.5 M NaCl solution, under simulated 1.5 AM solar
illumination, demonstrated long term stability of the photocurrents. Oxygen remains the main product of the
photoanodic reaction even in a 0.5 M NaCl solution, a composition close to the sea water, with chlorine accounting for
ca. 20% of current efficiency.