Studies On Novel Electrode Mateirals For Electrochemical Conversion And Storage Of Solar Energy

The limited resource and environment issues related to fossil fuels haveprompted the power generation by renewable energies. Solar energy is clean andinfinite, and solar electricity is expected to meet a considerable portion of globalpower desire. Among photovoltaic technologies, new-generation inexpensive onessuch as dye-sensitized solar cells （DSCs） have aroused world-wide research interests.While current high-efficiency DSCs are predominantly based on Ru complexes, pureorganic DSCs have provided noble metal-free alternatives with their efficienciespending for improvement. On the other hand, solar energy is known as intermittentand its non-stability has placed enormous demands on the grid system. Efficientenergy storage system is necessary for the future power generation from renewableenergies into the grid. Rechargeable lithium batteries, with their outstanding energydensities, are attractive candidates for such applications given that lower costs ofelectrode materials could be realized.Towards cost-effective solar electrochemical conversion and storage, thisdissertation has dealt with three potentially low-cost electrochemical technologiesincluding DSCs, organic rechargeable lithium batteries and rechargeable magnesiumbatteries, with particular focus on the design of novel electrode materials. The mainresults and conclusions include（1） The efficiency of a solar cell depends on short-circuit density, open-circuitvoltage (V_OC), and fill factor. While accomplishments have been made by moleculardesign of organic dyes to increase short-circuit densities of DSCs, studies on V_OCareby far less. In order to study the relationship between molecular structure of organicdyes and the V_OCof DSCs, a series of triphenylamine-based organic sensitizers wereengineered and compared. The origin of V_OCvariance was exmined in terms ofband-edge movement of TiO₂conduction band （CB） and interfacial chargerecombination. Firstly, the influence of adsorption behavior was investigated. Thetwo dyes with cyanoacrylic acid as anchoring group （TC-dyes） adopt a standing adsorption mode and exert larger surface dipole potential on TiO₂than theircounterparts bearing rhodanine-3-acetic acid （TR-dyes） which lie along the surface.TR-dyes exhibit greater extent of charge recombination than TC-dyes because of thelow surface-blocking efficiency of the dye layer and the intimacy between theI₃^-bound dyes and TiO₂. Secondly, two ionic dyes （TI dyes） featuring a1,1-diphenylvinyl auxiliary electron donor and N-alkyl indolium carboxylic acidacceptors have been synthesized to investigate the generally low V_OCvalues DSCsbased on organic ionic dyes. TI dyes have shown much lower V_OCvalues than that ofnon-ionic ones, suffering from serious interfacial charge recombination anddownward shift of the TiO₂CB. Through combined experimental and computationalanalysis, it was concluded that the electronic distribution over ionic dye molecules isintrinsically not ideal for the construction of efficient sensitizers. Thirdly, introductionof rationally modified3,4-propylenedioxythiophene units into triphenylamine dyeswas found to enhance light capturing, suppress dye aggregation, and remarkablyretard charge recombination in dye-sensitized solar cells. V_OCvalues for these dyes（⁸00mV） are much higher than that for a thiophene congener （720mV） undersimilar conditions, as a result of self-passivation benefited from theirthree-dimensional branched structures.（2） Organic carbonyl compounds are potentially low-cost andhigh-energy-density cathode materials for rechargeable lithium batteries, generaldesign rules of which are not yet established. By embedding pre-aromatic1,2-dicarbonyl moieties into extended conjugated systems, a series of organiccarbonyl-based molecular cathode materials integrating all known stabilizing factorsand enabling up to four-electron reduction were designed and synthesized.Remarkably, two new carbonyl electrodes, pyrene-4,5,9,10-tetraone and1,10-phenanthroline-5,6-dione, have delivered a reversible capacity of360mA h g^-1and an average working potential of2.74V, respectively, providing insights indesigning high-energy organic positive electrodes for energy storage. Theoreticalmodeling revealed that molecular orbital profiles and energetics can be applied for theprediction of carbonyl utilization and modulation of redox potentials. The influenceof heteroaromatic building blocks on the electrochemical behavior of organic electrode materials was further demonstrated in a systematic way. The structuralincorporation of heteroaromatics improves the cell performance of carbonyl-basedmolecular cathode materials in rechargeable lithium batteries in terms of specificgravimetric capacity, working potential, rate capability, and cyclability. In particular,benzofuro[5,6-b]furan-4,8-dione has achieved an energy density of up to539W hkg^-1, a power density of up to3278W kg^-1, and a capacity retention of86%after100discharge charge cycles. Functionalization with heteroaromatic structures, in parallelto various families of organic functional electronic materials, are therefore recognizedas a versatile strategy to improve the performance of organic electrodes.（3） Layered chalcogenides are typical ion intercalation material but so faroperate moderately in rechargeable magnesium batteries. The passivation of metallicmagnesium in some electrolyte systems has also restricted the battery performance.With the advent of nanoengineering, highly exfoliated graphene-like MoS₂（G-MoS₂）was synthesized via solvo-thermal conditions with an average layer number of≤4,and ultrasmall magnesium nanoparticles （N-Mg） with an average diameter of2.5nmwere prepared through ionic liquid-assisted reduction. These two novel nanomaterialsshow significantly superior electrochemical performance to their micro-structuredcounterparts. Rechargeable magnesium batteries with G-MoS₂as cathode and N-Mgas anode were fabricated, showing a high working potential of1.8V and a reversiblespecific capacity of170mA h g^-1, of which95%was preserved after50discharge charge cycles. The magnesium storage mechanism was preliminarilystudied by theoretical modeling, which explains the enhanced capacity of G-MoS₂over bulk MoS₂. These results highlight the G-MoS₂N-Mg combination as one ofthe most successful configurations for rechargeable magnesium batteries, and suggestthat rational morphological design of electrode materials should be a promisingpathway towards high performance magnesium batteries.