15.2 Energy harvesting and storage applications

Molecular electronics is revolutionizing energy harvesting and storage. From solar cells using organic molecules to nanogenerators converting vibrations into electricity, these technologies offer exciting possibilities for sustainable power generation.

Advanced materials like carbon nanotubes and redox-active molecules are transforming energy storage. Supercapacitors and flow batteries built with these materials promise higher capacity, faster charging, and longer lifespans than traditional batteries.

Solar Energy Harvesting

Molecular Photovoltaics and Dye-Sensitized Solar Cells

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• Molecular photovoltaics harness solar energy using organic molecules and polymers
  • Utilize light-absorbing organic compounds to generate electrical current
  • Offer advantages such as flexibility, low cost, and ease of fabrication compared to traditional silicon-based solar cells
• Dye-sensitized solar cells (DSSCs) use a photosensitive dye to absorb light and generate electrons
  • Consist of a photoanode (typically TiO2) coated with a light-absorbing dye, an electrolyte, and a counter electrode
  • Dye molecules absorb photons and inject electrons into the conduction band of the semiconductor, generating electrical current
  • Ruthenium-based dyes and organic dyes (porphyrins, phthalocyanines) are commonly used in DSSCs

Organic Solar Cells

• Organic solar cells (OSCs) use organic semiconductors, such as conjugated polymers and small molecules, to convert sunlight into electricity
  • Consist of an active layer sandwiched between two electrodes, where the active layer is composed of electron donor and acceptor materials
  • Light absorption in the active layer generates excitons (bound electron-hole pairs), which dissociate at the donor-acceptor interface and transport to respective electrodes
• OSCs offer advantages such as low-cost manufacturing, flexibility, and tunable optical and electronic properties through molecular design
  • Bulk heterojunction (BHJ) architecture, where donor and acceptor materials are blended to form an interpenetrating network, enhances exciton dissociation and charge transport
  • Examples of donor materials include P3HT (poly(3-hexylthiophene)) and PTB7 (polythieno[3,4-b]thiophene-co-benzodithiophene), while common acceptor materials are fullerene derivatives (PC61BM, PC71BM) and non-fullerene acceptors (ITIC, Y6)

Thermoelectric and Piezoelectric Energy Harvesting

Thermoelectric Materials

• Thermoelectric materials convert temperature gradients into electrical energy (Seebeck effect) or vice versa (Peltier effect)
  • Seebeck effect: When a temperature difference is applied across a thermoelectric material, charge carriers (electrons or holes) diffuse from the hot side to the cold side, generating a voltage
  • Peltier effect: When an electrical current is passed through a thermoelectric material, heat is absorbed or released at the junctions, creating a temperature gradient
• Efficient thermoelectric materials have a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity
  • Examples include bismuth telluride (Bi2Te3), lead telluride (PbTe), and silicon-germanium (SiGe) alloys
• Nanostructured thermoelectric materials, such as quantum dots and nanowires, can enhance the thermoelectric figure of merit (ZT) by reducing thermal conductivity while maintaining electrical properties

Piezoelectric Nanogenerators

• Piezoelectric nanogenerators harvest mechanical energy from ambient vibrations, human motion, or acoustic waves and convert it into electrical energy
  • Piezoelectric materials generate an electric charge when subjected to mechanical stress or strain due to the displacement of ions in the crystal lattice
  • Examples of piezoelectric materials include zinc oxide (ZnO), lead zirconate titanate (PZT), and polyvinylidene fluoride (PVDF)
• Nanogenerators based on piezoelectric nanowires or nanofibers offer high sensitivity and flexibility
  • Vertically aligned ZnO nanowire arrays grown on flexible substrates can generate voltage and current when subjected to mechanical deformation
  • PVDF nanofibers, produced by electrospinning, exhibit high piezoelectric coefficients and can be integrated into wearable devices for energy harvesting from human motion

Energy Storage Systems

Supercapacitors

• Supercapacitors, also known as electrochemical capacitors or ultracapacitors, store energy through the formation of electrical double layers at the electrode-electrolyte interface
  • Consist of two high-surface-area electrodes separated by an ion-permeable separator and an electrolyte
  • Energy storage occurs through the adsorption and desorption of ions at the electrode surface, resulting in high power density and rapid charge-discharge cycles
• Nanostructured electrode materials, such as carbon nanotubes, graphene, and metal oxides, enhance the surface area and capacitance of supercapacitors
  • Carbon-based materials offer high conductivity, chemical stability, and tunable pore structure for efficient ion transport
  • Pseudocapacitive materials, such as transition metal oxides (RuO2, MnO2) and conducting polymers (polyaniline, polypyrrole), undergo fast surface redox reactions, providing additional charge storage

Redox Flow Batteries and Molecular Charge Storage

• Redox flow batteries (RFBs) store energy in two separate liquid electrolytes containing dissolved redox-active species, which are pumped through an electrochemical cell
  • Anolyte and catholyte are stored in external tanks and flow through the cell, where redox reactions occur at the electrodes to charge or discharge the battery
  • Examples of RFB chemistries include vanadium redox (VRB), zinc-bromine (ZBB), and organic redox flow batteries using quinones or TEMPO-based molecules
• Molecular charge storage involves the use of redox-active organic molecules or metal complexes for energy storage
  • Redox-active molecules, such as quinones, viologens, and ferrocene derivatives, undergo reversible redox reactions to store and release charge
  • Can be used in solid-state or flow battery configurations, offering the potential for high energy density, low cost, and environmental sustainability compared to traditional inorganic battery materials
• Molecular engineering of redox-active compounds allows for tuning of electrochemical properties, solubility, and stability
  • Functionalization of molecules with solubilizing groups (e.g., sulfonate, carboxylate) improves solubility in aqueous electrolytes
  • Molecular design strategies, such as the incorporation of multiple redox centers or the use of pi-conjugated systems, can enhance charge storage capacity and redox potential