An ultra-high rate solid-state electrochemical capacitor (EC) is studied for 60 Hz AC line-filtering applications. The combination of vertically-oriented graphene nanosheet electrodes and a hydroxide ion-conducting tetraethylammonium hydroxide-polyacrylamide polymer electrolyte enabled an interdigitated planar EC with capacitive behaviour at rates up to 1000 V s(-1) and a response time of less than 1 ms. This performance level was maintained for over 3 weeks without capacitor packaging. The EC responded capacitively at 120 Hz at a temperature of 120 degrees C, demonstrating outstanding high-temperature resiliency. Thermal and chemical characterizations of the polymer electrolyte revealed that electrochemical changes caused by high-temperature are reversible. Two competing factors were identified that govern solid-state capacitor performance at elevated temperatures.