The GO sheets were synthesized from graphite flake powders using
a modified Hummers method [15]. The obtained GO aqueous solution
was concentrated (to 20 mg/mL) by centrifugation. A self-built 3D
micro-extrusion system was used to print graphene-based interdigitated
electrodes. The printing system consists of a desktop XYZ motor
(Technodigm, Model: DR3331T-EX), a temperature-controlled substrate
holder (100 °C used here), and a high-precision displacement
pump (Technodigm,Model: PDP 1000). The printing headmounted on
the precision XYZ motor (20 μm resolution) prints along the preprogrammed
tracks with an adjustable speed (50 mm/s used). The
printing head includes a piston, a syringe and a changeable microneedle
(inner diameter of 210 μmused). The displacement pumpdrives
the piston with a controllable speed (0.001 mm/s) to extrude GO
suspension fromthe syringe onto the substrate. To obtain the all-carbon
solid-state PMSC, printed GO patterns were chemically reduced into
rGO using hydroiodic acid (HI) followed by coating a layer of polyvinyl
alcohol (PVA)-H2SO4 gel electrolyte on top. PVA-H2SO4 gel electrolyte
was prepared bymixing H2SO4 solution (1 M, 10 mL) with PVA powder
(1 g), and subsequent heating at 90 °C with vigorous stirring. The
printed electrodeswere examined by a field-emission scanning electron
microscope (JEOL, JSM-6700F). The electrochemical measurements
were performed using CHI 660D Electrochemical Work Station (CH
Instruments).