Converting and Storing Sunlight – a Russian Solution

Russian physicists have developed a unique device – a photoelectrochemical supercapacitor condenser, able to directly convert sunlight energy into electricity and accumulate it with high density in double electrical layer of electrodes, made of various nanomaterials. New device can be an element of autonomous solar electric generating stations, or illumination systems, being a cheap alternative to foreign solutions.

Existing photovoltaic facilities, working on sunlight, directly transform solar energy into electricity by means of semiconducting solar elements. This energy is partly stored in accumulators for regular supply of users. Common accumulators for autonomous solar electric generating stations are leak-proof VRLA (Valve Regulated Lead Acid) batteries. However, 1 kilowatt per hour (kWh) of energy in this case costs about $0.3-$0.35. Scientists from Lebedev Institute of Physics suggest a solution, which reduces energy costs and extends energy accumulator’s lifetime. New device combines a semiconducting solar element and an energy accumulator made of nanoporous materials.

A photoelectrochemical supercapacitor (PES) consists of a multicomponent photoelectrode, which can be made of various semiconducting materials, positively and negatively charged nanoporous electrodes, a porous separator between electrodes, and electrolyte. Sunlight is absorbed by a photoelectrode, producing electron-hole pairs – a standard process for sunlight capturing. Then photoelectrons are quickly separated from holes and head for nanoporous cathode, while holes travel to anode. Excessive negative charge on a cathode is compensated by positive electrolyte particles.

As a result, a double electrical layer forms between a nanopore’s wall and electrolyte along an extended surface of a cathode. During a charging process photo-generated electrons accumulate in cathode’s electric capacity, making its electric potential shift further to negative value range. Similar process takes place at a positive electrode – a double electric layer of photoholes and negatively charges electrolyte particles forms. During charging active material of a positive electrode gets oxidized, and during discharge it gets reduced.

Possible materials for electrodes are nanoporous carbon materials with predetermined average nanopore size. Nanoporous structure of such materials makes their surface area as large as 1400–1600 square meters, and electric capacity – up to 1500 Farad in some electrolytes. In other words, each cubic centimeter of such a condenser can accumulate 1 000 000 times more energy, than a common condenser. Estimated price of 1 kWh of energy, produced by PES, is $0.1, which is 3–3.5 times lower, than existing accumulators have.

Structure and materials of each PES depend on natural environment and geographical location of exploitation site, as well as on required output parameters and operation regimes. Scientists have created a theoretical model in order to consider all possible PES exploitation factors, and calculations showed that specific discharge energy of this device couldn’t be less than 12 Wh/kg, energy efficiency reached 10%, and cyclic life exceeded 7 000 cycles – over 15 years of continuous operation.

Researchers plan to build and test first samples of photoelectrochemical supercapacitors in 2.5 years, and to build prototypes of commercial samples in 4 years.

Kizilova Anna