Electrical characterization of electrode material
Gelatin powder was soaked in distilled water and left at room temperature for 10 min. The amounts of gelatin powder and distilled water are 30 g and 120 g, respectively. Then, glycerol and NaCl were added. The amount of glycerol was 15 g. The amount of NaCl was 0, 1.25, 2.5, 3.75, and 5 g, which corresponded to the mass fraction relative to the amount of gelatin and glycerol of 0, 2.7, 5.3, 7.7, and 10 wt%, respectively. The mixed materials were dissolved at 80 °C and stirred at 200 rpm for 1 h using a hot stirrer (AS ONE, CHPS-170DF). The solution was then blade-casted on a plastic substrate using a film applicator (Industrial Physics, TQC Sheen) and an applicator coater (Zehntner, ZUA 2000), forming an electrode layer with a thickness of 143 μm after curing at room temperature for 24 h at 23 °C (67% RH). The thickness of the layer was measured using a laser displacement sensor (OPTEX FA, CDX-L15). Four samples were prepared for each material composition (mass fraction of NaCl was 0, 2.7, 5.3, 7.7, and 10 wt%). The dimensions of the samples were 10 mm in length and 10 mm in width. The electrical resistance of every specimen was measured using a digital multimeter (Keithley, 2100). The resistivity was then calculated based on the measured resistance by considering the dimensions of the sample.