Invited: To be or Not to be Pseudocapacitive

Electrochemical capacitors (ECs), so-called supercapacitors are energy storage device that combine a high power density with long cycle life. Their main drawback is their moderate energy density that usually hardly exceeds 5 Wh/kg. This limitation becomes even worse when reported as volumetric energy density, a critical parameter in many applications. Indeed, ECs often have a given volume due to standardization of the size of electrochemical energy storage devices, and the challenge is to optimize the energy stored in such volume. Carbon based device are currently commercialized, but due to their low density and high porosity, the capacitance cannot lead to very high energy density. Their major advantage is the possibility to operate such device with cell voltage up to 2.7V in organic based electrolyte and even higher with ionic liquids. The optimization of the cell capacitance C using carbon electrodes is a dilemma since high porosity is required to enhance electrode/electrolyte interaction but an increase in porosity often translates in a decrease in the density of carbon electrodes.

The use of oxide or nitride based pseudocapacitive materials as electrodes also leads to a dilemma since the cell capacitance is usually enhanced but at the expense of the cell voltage, since most of these alternative electrodes can only be operated in aqueous electrolytes. An option is to couple different oxide or nitride based electrodes in order to enhance the cell voltage, playing on HER and EOR overpotentials at the negative and positive side respectively. The main advantage of using such alternative materials to carbon is that they combine fast and reversible surface redox reactions which provide the pseudocapacitive properties and make the electrodes "look like" carbon, but with usually much higher capacitance.

However, upon the past 5 years, many studies have targetted the use of electrode materials that definitely do not exhibit pseudocapative behavior. A typical example is the use of Ni(OH)2, a well known alkaline battery material for positive electrode in NiMH cells for example, which is now presented as a pseudocapacitive electrode for supercapacitor. The origin of that probably lies in the use of Ni(OH)2 in hydrid device using a negative carbon electrode (capacitive) and a positive faradaic electrode (Ni(OH)2). Many authors now believe that battery type electrode can be turned in supercapacitor type electrode simply by making composite electrode with various carbons such as graphene or carbon nanotubes. The situation is of course again a dilemna since the resulting composite electrode usually does not fulfill the requirement of pseudocapacitive materials which are concommitant to their electrochemical properties: long term cycling efficiency, power capability, etc...

In this communication, the aim will be to clearly show the difference between battery type electrodes and pseudocapacitive materials, based for example on the capacitance calculation which gives tremendously high values in the case of Ni(OH)2 but these values are dependant on the width of the potential window used for the calculation, unlike most of the pseudocapacitive materials identified up to now. Strategies to improve volumetric energy and power densities will also be detailed.