(*i*) Haeckelites**

The name 'Haeckelite' has been proposed to designate a three-fold coordinated network generated by a periodic arrangement of pentagons, hexagons and heptagons. Starting from a planar Haeckelite array, tubular structures are obtained by applying the same wrapping procedure as for the usual nanotubes, which are rolled up sheets of graphene.

Haeckelite carbon nanotubes are structures obtained by rolling up a flat and periodic three coordinated network containing an equal number of pentagons and heptagons, and an arbitrary number of hexagons. These are interesting systems that have been proposed theoretically a few years after the prediction that a graphitic planar structure composed exclusively of pentagons and heptagons could be metallic.

Pentagon and heptagons are important structural defects in carbon nano structures. Pentagons are certainly present in fullerenes and carbon onions, in nanotube caps, nanocones, and in the conical cap of nanohorns. The presence of heptagons is suspected in some negatively-curved parts of multiwall nanotubes and in ideal Y-type tube connections. Pentagons and heptagons are very often grouped in pairs or in the form of Stone–Wales defects, which are associations of two such pairs.

(*ii*) Hydrogen as fuel**

Hydrogen fuel is a zero-emission fuel which uses electrochemical cells, or combustion in internal engines, to power vehicles and electric devices. It is also used in the propulsion of spacecraft and can potentially be mass-produced and commercialized for passenger vehicles and aircraft.

Hydrogen is the first element on the periodic table, making it the lightest element on earth. Since hydrogen gas is so light, it rises in the atmosphere and is therefore rarely found in its pure form, H2. In a flame of pure hydrogen gas, burning in air, the hydrogen (H2) reacts with oxygen (O2) to form water (H2O) and releases heat. Other than water, hydrogen combustion may yield small amounts of nitrogen oxides.

Combustion heat enables hydrogen to act as a fuel. Nevertheless, hydrogen is an energy carrier, like electricity, not an energy resource. Energy firms must first produce the hydrogen gas, and that production induces environmental impacts. Hydrogen production always requires more energy than can be retrieved from the gas as a fuel later on. This is a limitation of the physical law of the conservation of energy.

Benefits

Produced Domestically: Hydrogen can be produced domestically from several sources, reducing our dependence on petroleum imports.

Environmentally Friendly: Hydrogen produces no air pollutants or greenhouse gases when used in fuel cells; it produces only nitrogen oxides (NOx) when burned in ICEs.

Challenges:

Fuel Cost & Availability: Hydrogen is currently expensive to produce and is only available at a handful of locations, mostly in California.

Vehicle Cost & Availability: Fuel cell vehicles are currently far too expensive for most consumers to afford, and they are only available to a few demonstration fleets.

Onboard Fuel Storage: Hydrogen contains much less energy than gasoline or diesel on a per-volume basis, making it difficult for hydrogen vehicles to go as far as gasoline vehicles between fillups about 300 miles. Technology is improving, but the onboard hydrogen storage systems do not yet meet size, weight, and cost goals for commercialization.

(*iii*) Reverse Osmosis**

Reverse Osmosis (RO) is a water purification technology that uses a semipermeable membrane. This membrane-technology is not properly a filtration method. In RO, an applied pressure is used to overcome osmotic pressure, a colligative property, that is driven by chemical potential, a thermodynamic parameter. RO can remove many types of molecules and ions from solutions and is used in both industrial processes and in producing potable water. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be "selective," this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as the solvent) to pass freely.

In the normal osmosis process, the solvent naturally moves from an area of low solute concentration (High Water Potential), through a membrane, to an area of high solute concentration (Low Water Potential). The movement of a pure solvent is driven to reduce the free energy of the system by equalizing solute concentrations on each side of a membrane, generating osmotic pressure. Applying an external pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis. The process is similar to other membrane technology applications. However, there are key differences between reverse osmosis and filtration. The predominant removal mechanism in membrane filtration is straining, or size exclusion, so the process can theoretically achieve perfect exclusion of particles regardless of operational parameters such as influent pressure and concentration. Moreover, reverse osmosis involves a diffusive mechanism so that separation efficiency is dependent on solute concentration, pressure, and water flux rate.

Reverse osmosis is most commonly known for its use in drinking water purification from seawater, removing the salt and other effluent materials from the water molecules.