Supercapacitors Handout PDF

Frontiers in Chemistry CHEM1008 Electrochemistry and Devices Lecture 3 Supercapacitors Darren Walsh Room A09, GSK Carbon Neutral Laboratory for Sustainable Chemistry Email: [email protected]  Lecture 3 Learning Outcome...

Frontiers in Chemistry CHEM1008 Electrochemistry and Devices Lecture 3 Supercapacitors Darren Walsh Room A09, GSK Carbon Neutral Laboratory for Sustainable Chemistry Email: [email protected]  Lecture 3 Learning Outcomes 1. To understand how capacitors store electrical energy and how various parameters such as plate area affect the “capacitance” of the devices 2. To understand the difference between a capacitor and a supercapacitor, i.e., how using very large electrodes and electrolytic solution makes the capacitance very large 3. To be able to calculate the amount of energy stored in a supercapacitor 4. To understand how the mode of operation of supercapacitors gives them certain advantages and disadvantages compared to batteries 5. To appreciate the applications in which supercapacitors can be used  Capacitors o Capacitors are devices that store electrical energy, but in a different way to that in batteries o The electrical energy is stored electrostatically, at the surface of electrodes o Except in some special cases, no redox reactions occur  Capacitors o Capacitors store charge on two oppositely-charged, electrically-conducting plates o The plates can be charged using a power supply such as a battery o The capacitance (symbol C) has units of farads (F) and tells us how many coulombs (units C) of charge are stored per unit voltage (V) between the plates (in units of volts, V) o Also depends on the area (A) of the plates, the distance (d) between them and the electrical properties of the medium between them  Example 1 What is the capacitance of a 100 cm2 (0.01 m2) capacitor containing an evacuated 2.0 mm gap between the electrodes?  Example 2 What is the capacitance of that 100 cm2 (0.01 m2) capacitor containing a 2.0 mm gap containing glass (εr = 7) between the electrodes? So what is a supercapacitor? o In supercapacitors, the dielectric (insulating) separator is replaced with an ionic solution – an electrolyte When charged, the electrolyte ions adsorb onto high surface area electrodes 1. Why do we use high surface area electrodes 2. Why do we use electrolytes in between the plates? So a supercapacitor is a little like 2 capacitors  Charging/discharging supercapacitors o The voltage of a capacitor (and supercapacitor) increases and decreases when we charge and discharge it o This is because there is no “chemistry” happening – remember that the voltage in batteries is controlled by ∆G of the net reactions o In supercapacitors we are simply pushing and pulling charge from the surfaces  How much energy can we store in a supercapacitor? o The energy stored in a supercapacitor is given by: E = ½ CV2 o We have already discussed how to increase C o How big we can make V depends on the stability of the electrolyte – if charged too high, we would electrolyse the electrolyte (break it down) and ruin the device o The limit for aqueous electrolytes is about 1 V  Example How much energy is stored in a 310 pF capacitor with voltage of 5.0 V? E = ½ CV2 = ½ × 310 × 10–12 F × 5.0 V2 = 3.9 nJ What about is a 1.1 F supercapacitor with a voltage of 1.0 V? E = ½ CV2 = ½ × 1.1 F × 1.0 V2 = 0.55 J A big advantage of supercapacitors over batteries is their lifetime can be fully charged and discharged in seconds sometimes can be charged/discharged up to 106 times Compare this to charge times up to hours and lifetimes of thousands of cycles for typical batteries Where do supercapacitors fit compared to other energy-storage devices? o Because they only store charge on their electrode surfaces, supercapacitors (or “electrochemical capacitors”) store less energy (per unit weight) than batteries o However, they are higher-power devices than batteries (remember that power is how quickly energy is released) o This is shown in the “Ragone” plot on the left Where are supercapacitors used? Applications where we need short bursts of energy o Doors on airplanes o Back-up for memory functions in mobile devices o Actuators in digital cameras o Regenerative braking in cars and trains o Energy smoothing in power-delivery systems