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Burning fossil fuels to generate electricity has led to concerns over air quality and climate change. This has driven the need for clean and renewable energy sources. Supercapacitors, batteries and solar cells offer an eco-friendly alternative for energy storage and conversion systems, which is likely to be vital for sustainable development.
Graphene shows excellent potential in energy storage and conversion, with 2D free-standing graphene films and powders being particularly promising. Aerogels also have their part to play.
What is an Aerogel?
An aerogel is a term used to describe a group of synthetic materials with a specific geometric structure. Aerogels are porous and ultralight, and exhibit extremely low density and low thermal conductivity.
The material is derived from a gel and the liquid component is replaced by air. The air content is around 99.98% air by volume, making it one of the world's lightest solid material. It is produced by removing the liquid constituent of the gel through supercritical drying. The liquid is slowly dried off without causing the solid matrix of the gel to collapse via capillary action (as seen in evaporation).
Aerogels were created as the result of a bet. In 1931, Samuel Stephens Kistler and Charles Learned strived to see who could replace the liquid in 'jellies' with gas, without causing shrinkage.
Read more about aerogels and their properties here
Reduced Graphene Oxide Aerogels
Aerogels founded on graphene-oxide/reduced graphene oxide are a relatively new but promising composite material. They have a specific architecture that stops the accumulation of graphene layers with highly developed surfaces.
These aerogels have high porosity, low material density, high specific area and the ability to conduct an electric current. This makes them very attractive, with possible uses in numerous applications from supercapacitors to sensors.
There are several approaches to fabricating reduced graphene oxide (rGO)-based 3D materials, including self-gelation of CO containing systems, hydrothermal-assisted formation and crosslinking of the structure. However, all need freeze-drying or supercritical drying to prevent the stacking of graphene oxide/reduced graphene oxide sheets during drying.
Freeze drying leads to the formation of a large number of macropores. During the drying process, ice crystals are formed that displace part of the material from its volume, causing local structural irregularities and the formation of macropores in place of crystals after they are removed from the material during the drying process. The size of the crystals is dictated by how quickly the material freezes.
Reduced graphene oxide aerogels possess unique physicochemical properties. In addition to their high specific surface area, extremely low density and high porosity, they exhibit a unique structure and good electrical conductivity.
rGO for Photovoltaics
As a natural energy source, solar radiation is one of the most competitive sources of the future; not only is it non-polluting, it is available in abundance and is the cheapest source of electrical power in regions with very high potential. However, it requires energy storage systems or high-voltage direct-current power lines. Worldwide, the installed photovoltaic capacity exceeded 515 GW in 2018, meeting approximately 2% of the global energy demand.
Photovoltaics involved the conversion of light to electricity using semiconducting materials that exhibit the photovoltaic effect. The best-known method is by using solar cells to convert energy from the sun into a flow of electrons, which can be used to power equipment or recharge batteries. Such systems employ solar modules consisting of several solar cells that generate electrical power.
There are further benefits to using solar power; its operation generates no pollution and no greenhouse gases, and the silicon required for solar cells is available in large quantities in the Earth's crust.
Carbon-based materials have been used widely so far as the electrode materials. The major advantages are their good electrical conductivity, high specific surface area, porous structure and excellent biocompatibility.
Research in 2019 investigated the use of rGO twinned with polyaniline nanotubes as a potential counter electrode material for a quasi-solid-state dye-sensitized solar cell. The aerogel showed excellent electro-catalytic activity towards the reduction of triiodide ions found in the electrolyte. The excellent catalytic activity of the aerogel-based electrodes was demonstrated with cyclic voltammetry-based analyses.
In 2019, other researchers found that rGO and carbon nanoparticles can aid the perovskite crystallization process. Perovskite is widely used in photovoltaics, with the presence of carbon in perovskite layers enhances the electrical property of the device.
Other Uses of Reduced Graphene Oxide Aerogels
Reduced graphene oxide aerogels could also be used as an active electrode for supercapacitors, electrochemical devices for storing electric energy on the surface of highly porous materials with organic/inorganic electrolyte.
In 2020, researchers in Dublin developed rechargeable batteries and energy storage devices where the cathode was made from graphene-based aerogels.
They could also be used in rechargeable lithium-ion batteries as an anode material. Here, they can facilitate multidimensional electronic transport routes and reduce transport spaces between the electrode and electrolyte, offering increased performance.
References and Further Reading
Graphene Oxide/Reduced Graphene Oxide Aerogels, https://www.intechopen.com/books/graphene-oxide-applications-and-opportunities/graphene-oxide-reduced-graphene-oxide-aerogels
Mao J et al (2019) Graphene aerogels for efficient energy storage and conversion, Energy and Environmental Science https://pubs.rsc.org/en/content/articlelanding/2018/ee/c7ee03031b#!divAbstract
Mohan et al (2019) Polyaniline nanotube/reduced graphene oxide aerogel as efficient counter electrode for quasi solid state dye sensitized solar cell, Solar Energy https://www.sciencedirect.com/science/article/abs/pii/S0038092X19304931
Graphene Flagship partners produce environmentally-friendly graphene inks https://www.graphene-info.com/graphene-flagship-partners-produce-environmentally-friendly-graphene-inks
He R et al (2019) Carbon‐based perovskite solar cells: From single‐junction to modules, Carbon Energy https://onlinelibrary.wiley.com/doi/full/10.1002/cey2.11