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Double layer electrolytic capacitors already enable to replace backup batteries in many applications.

Modern high-capacity double layer capacitors (also known as supercapacitors) feature a very high energy/volume ratio, compared to usual electrolytic capacitors. Their capacity is so high, that they are able to replace backup batteries in many designs. On the market there are available miniature types as well as physically big types with capacities of tens to hundreds of Farads.

One of the biggest advantages of capacitors in comparison to batteries is their long lifetime, because their electrodes don´t undergo degradation neither after many thousands of cycles. On the other side, even modern batteries have a limited lifetime and a limited number of cycles, because energy storage in batteries is related to chemical changes of electrodes during charge/discharge (change from a solid to a liquid form, crystallization,…), what causes degradation of electrodes.
In our offer, you can find small “coin-type” double layer capacitors suitable for backup power supply of memories. For example a capacitor with a 1F (1000mF=1 000 000 uF) capacity charged at 5V can store energy of 12.5 Jouls (E=1/2*C*U²), what is 12.5 Wattseconds. This energy is in most cases sufficient to ensure a safe backup of memories at a voltage dropout. Such energy is also sufficient for a short time power supply of low power devices. This can be very useful for example for a safe write of data to EEPROM and a safe switch off of a device.

Supercapacitors instead of batteries? - [Link]

Here is an interesting project which uses capacitors to store energy instead of chemical,sit uses an different type of capacitors called Goldcap capacitors,GoldCap capacitors offer an interesting alternative power source when compared to conventional disposable or even rechargeable batteries. They can be charged very rapidly and can also deliver a high peak output current. Their voltage rating however is quite low so a little electronic assistance is necessary to raise the output voltage to a more useful level.PP3 (6F22) type 9 V batteries are often used in small portable equipment that require very little current and may only be used intermittently. Under these conditions its often the case that the battery is flat just when you urgently need to use the equipment. NiCd rechargeable cells are not a good choice in these applications because their self-discharge characteristics are much worse than dry cells and often there is no charge left after a long time in storage.

Superfast Rechargable Battery - [Link]