Design of materials for bio-inspired energy conversion and storage

Abstract

Innovative designs of materials play a vital role to achieve breakthrough in next-generation technolo-gy. Natural organisms often hint at the advanced design of materials with desirable functionalities. For exam-ple, biological energy transduction and storage systems such as photosynthesis and respiration can be an op-timal model for the development of man-made energy devices. Cellular metabolism comprises highly opti-mized energy transduction machineries that operate by a series of redox-active components for storing ener-gies from nutrients or sun light, which are transduced into high energy intermediates for cellular works such as chemical synthesis, transport, and movement. In this thesis, bio-inspired design and synthesis of functional materials for sustainable energy conver-sion and storage applications including artificial photosynthesis and rechargeable batteries were attempted based on the understanding of cellular energy transduction mechanisms in nature. The first part of this thesis including Chapter 1 and 2 deals with the development of integrated photocatalytic assemblies by adopting plasmonic metal nanostructures which can greatly enhance the light harvesting process due to localized sur-face plasmon resonance, mimicking the function of light-harvesting antenna in photosystems. The second part including Chapter 3 and 4 presents fundamental studies and rational design strategies to utilize naturally oc-curring redox cofactors, inspired by their redox cycling in electron transport chains, as promising alternatives to conventional inorganic electrode materials in rechargeable batteries. Chapter 1 proposes and demonstrates an innovative scheme to fabricate elaborate core-shell nanohy-brid architecture, in which the coupling between plasmonic resonator and photosensitizer is controlled in a sub-nanometer scale through a simple, solution-based process. The simplicity of our scheme owes largely to the multi-purpose polydopamine (PDA) nanolayers inspired by mussel adhesion. PDA not only facilitates the formation of metal nanoparticles that support surface plasmons, but also serves as a scaffold to incorporate photosensitizers around metal cores as well as an adhesive between nanohybrids and the substrates. According to our simulation and experimental data, the core-shell configuration greatly enhances light absorption in pho-tocatalytic systems to augment artificial photosynthesis. Furthermore, material-independent surface chemistry of PDA makes our approach to be widely applicable to various substrates independent of their chemical com-position and shape. The design flexibility allows the synthesis of assorted sets of plasmonic light harvesting assemblies with desired optical properties, providing an effective platform for plasmon-enhanced solar energy conversion applications. Chapter 2 describes a new strategy to make a robust and versatile platform utilizing aluminium (Al) nanostructures for plasmon-enhanced solar to chemical energy conversion. The use of Al nanostructures is highly advantageous for the practical realization of plasmonic light harvesting, because of the broad plas-monic spectral range, the compatibility with CMOS technology, and the great abundance of the material. However, oxidation and corrosion of Al critically limit its applicability for light harvesting applications, in particular, solution-based photocatalytic reactions. The stability of Al nanostructures was firstly examined in photocatalytic environments, and PDA nanolayers were introduced to protect the plasmonic nanostructures against corrosion. The PDA nanolayers which can encapsulate molecular photocatalysts effectively suppress the degradation of Al nanostructures and enable the photo-active molecules to access the plasmonic resonanc-es from the metal surfaces. The resulting Al-dye nanohybrids exhibited serface-enhanced Raman scattering, photocurrent enhancement, and increased yields of biomimetic cofactor regeneration due to the resonant ener-gy transfer from the plasmonic Al nanostructures. Chapter 3 presents an advanced bio-inspired strategy to design high performance energy devices based on the understanding of the basic components and their operating principles of respiration. Universal biologi-cal redox centers, flavins, which accommodate two electrons in the nitrogen atoms of conjugated diazabutadi-ene during cellular metabolism, were demonstrated as molecularly tunable cathodes, exhibiting a reversible reactivity with two lithium ions and electrons per formula unit. Analysis of both the ex situ characterizations and density-functional theory (DFT)-based calculations revealed that the redox reaction occurs via two suc-cessive single-electron transfer steps, which has an analogy to the proton-coupled electron transfer mechanism of flavoenzymes. Tailored flavin analogues obtained via chemical substitution on the isoalloxazine ring show fine tunability of electrochemical properties, exhibiting a gravimetric capacity of $174 mAh g^{-1}$ and an average redox potential of 2.65 V whose expected energy density is comparable to that of $LiFePO_4$. Lastly, Chapter 4 deals with a novel and facile design of bio-organic electrodes to achieve high energy and power densities combined with excellent cyclic stability. Non-covalent nanohybridization of electroactive aromatic molecules with single-walled carbon nanotubes (SWNTs) by exploiting $\pie - \pie$ interactions leads to a rearrangement of electroactive molecules from bulk crystalline particles into molecular layers on conductive scaffolds. The nanohybrid electrode in the form of a flexible, free-standing paper (free of binder/additive and current collector) results in ultrafast kinetics delivering $510 Wh kg^{-1}$ within 30 minutes ($204 mAh g^{-1} \approx$ 98% of theoretical capacity) and $272 Wh kg^{-1}$ of energy even within 46 seconds. Moreover, the stable anchorage of electroactive molecules on SWNTs enables above 99% capacity retention upon 100 cycles, which was hardly achieved for organic electrodes. Our approach can be extended to other aromatic organic electrode systems, bringing organic redox chemicals a step closer to practical cathodes in rechargeable batteries.