Supercapacitor
Since capacitors' energy content increases with the square of the voltage, researchers were looking for a way to increase the electrolyte's breakdown voltage. In 1994 using the anode of a 200V high voltage tantalum electrolytic capacitor, David A. Evans developed an "Electrolytic-Hybrid Electrochemical Capacitor". These capacitors combine features of electrolytic and electrochemical capacitors. They combine the high dielectric strength of an anode from an electrolytic capacitor with the high capacitance of a pseudocapacitive metal oxide (ruthenium (IV) oxide) cathode from an electrochemical capacitor, yielding a hybrid electrochemical capacitor. Evans' capacitors, coined Capattery, had an energy content about a factor of 5 higher than a comparable tantalum electrolytic capacitor of the same size. Their high costs limited them to specific military applications.
Two capacitor paradox
The two capacitor paradox or capacitor paradox is a paradox, or counterintuitive thought experiment, in electric circuit theory. The thought experiment is usually described as follows: 
Two identical capacitors are connected in parallel with an open switch between them. One of the capacitors is charged with a voltage of formula_1, the other is uncharged. When the switch is closed, some of the charge formula_2 on the first capacitor flows into the second, reducing the voltage on the first and increasing the voltage on the second. When a steady state is reached and the current goes to zero, the voltage on the two capacitors must be equal since they are connected together. Since they both have the same capacitance formula_3 the charge will be divided equally between the capacitors so each capacitor will have a charge of formula_4 and a voltage of formula_5. At the beginning of the experiment the total initial energy formula_6 in the circuit is the energy stored in the charged capacitor: 
At the end of the experiment the final energy formula_8 is equal to the sum of the energy in the two capacitors 
Thus the final energy formula_8 is equal to half of the initial energy formula_6. Where did the other half of the initial energy go?