Final Report Summary - MUAPPEN (Multi-junction nano-materials with coated highly ordered structure and their Application in energy generation and Energy storage)
This research project focuses on synthesis of multi-junction nano-materials coated with highly ordered structure through a modified electrochemical atomic layer deposition (ALD) route, and on studying and manipulating their microstructural and compositional properties. These materials have been integrated into demonstration devices where their characteristics and usefulness are evaluated. The multi-junction nano-materials with a coaxial heterogeneous structure of TiO_2 NTs coated with an ultrathin narrow-band-gap semiconductor have been developed for photocatalysts application. The late-model photocatalysts developed in the project will greatly contribute to the technological improvement of photocatalytic decomposition of water. On the other hand, exploring the proof-of-concept and physical application of the sulphur-carbon/TiO_2 NTs multi-junction nano-materials as the positive electrode material in the rechargeable lithium-sulphur battery also plays a critical role in promoting the full use of renewable energies and the popularity of electric vehicles.
Hydrogen produced by splitting water is receiving significant attention due to the rising global energy demand and growing climate concerns. The photocatalytic decomposition of water converts solar energy into clean hydrogen, and may help mitigate the crisis of fossil fuel depletion. However, the photocatalytic hydrogen production remains challenging to obtain high and stable photoconversion efficiency. By implementing this grant, a highly efficient photoanode based on the coaxial heterogeneous cascade structure of Cu_2Se/CdSe/TiO_2 has been synthesized via the simple room-temperature and low-cost electrochemical deposition method. The microstructure and composition of the Cu_2Se top layer are regulated and controlled by doping Cu with various amounts in different zones of the CdSe/TiO_2 coaxial heterojunction and then using a simple integral annealing process. The maximum stable photocurrent density of the sample with the optimal copper zone and doping concentration has reached up to 28 mA/cm^2, which can be attributed to the success in the uniform dispersion of the three-layer heterogeneous nanojunctions among the anatase nanotube wall from the top to the bottom.
In addition, the formation and characterization of Cuprous oxide/Titanium dioxide (Cu_2O/TiO_2) nanotube-array coaxial heterogeneous structure also have been researched. Such structure is formed by coating nano-particles of Cu_2O onto titanium dioxide nanotube-array walls via multiple-cycle chemical adsorption plus reduction method (MCCAR). The practical deposition technique employs a soaking step to separate the adsorption and reduction processes, thus enhancing the controllability of deposition rate and preventing the clogging of nanotube pores. Cu_2O nano-particle sizes are adjusted by changing the glucose concentration in the reaction solutions. As a result, compact nano-particle film with sufficiently small crystal sizes is uniformly covered on the tube walls, resulting in the formation of coaxial heterogeneous structure. A stable hydrogen production rate of 3.1 mLcm^-2h^-1, which can be targeted for energy application in relation with solar energy driven production of hydrogen from water, has been achieved.
Besides, an epitaxial hetero-structure of CdSe/TiO_2 nanotube arrays as efficient photo-anodes has been obtained. With the help of the similar d spacing with TiO_2, CdSe sensitization layer is epitaxially grown on the tube wall of the TiO_2 nanotubes, resulting in an ideal coherent grain boundary and single crystal growth. The resultant photo-anode produces 30% more photocurrent than those samples without coherent grain boundary. Notably, the especial epitaxial hetero-structure is beneficial to decrease the recombination site and accelerate the separation of photogenerated electron-hole pairs. Furthermore, an ultrathin PEDOT surface layer was developed on the epitaxial hetero-structure of CdSe/TiO_2 nano-tube arrays in which it functions as both a physical passivation barrier and a hole transfer layer. As a result, significantly enhanced photocurrent density and substantially better stability have been achieved.
To improve the photoelectrochemical properties of TiO_2 heterogeneous structure, Cu_2ZnSnS_4 (CZTS) with TiO_2 nanotube arrays also were synthesized. Unlike some chalcogenides such as CdSe and CdS that have strong photo-corrosion weakness, CZTS has very good illumination stability. CZTS/TiO_2 composite can spontaneously form the pn junction, which is helpful to separate photo-generated electron-hole pairs. Considering that its valence band level is higher than the potential of producing oxygen, Ag substitution for Cu has been carried out to reduce the valence band, while replacing Sn with Ge also resulted in further enhancing energy level of the conduction band, which is favourable to electron injection into the conduction band of TiO_2. It has been found that the described Electrochemical ALD method has the considerable advantage for element substitution.
On the other hand, Lithium-sulphur batteries have shown great prospects for the future energy conversion and storage due to its high theoretical specific capacity. However, it has been hindered by rapid capacity decay and low energy efficiency. In this work, the polypyrrole (Ppy)/TiO_2 nanotube with coaxial heterogeneous structure as the substrate of positive electrode has been prepared to improve the electrochemical performance of sulphur electrodes. TiO_2 nanotubes decorated with Ppy provide a highly ordered conductive framework for Li^+ ion diffusion and reaction with sulphur. The suitable architecture also is helpful to trap the produced polysulfides, and as a result attenuates the capacity decay. Furthermore, the thermotreatment temperature in sulphur loading process has been confirmed to be an important impact factor on the overall performance of the resultant battery. The as-designed S/Ppy/TiO_2 nanotube using an elevated heating temperature shows excellent cycling stability with the high discharge capacity of 1150 mAh g^-1 at current rate of 0.1C and 1350 mAh g-1 at 0.05C and has an average coulombic efficiency of 96% after 500 cycles.
In addition, the dispersion for sulphur and restriction for lithium polysulfides have been combined to improve the performance of lithium–sulphur batteries. Sulphur is dispersed on acetylene black particles at first. Results show that sulphur envelops acetylene black particles and is uniformly dispersed in acetylene black–sulphur composite. A layer of deoxidized carbon coating wraps acetylene black–sulphur particles. The carbon coating is the reduction product of carbon precursor. The carbon precursor with carbonyl groups is deoxidized by aluminum powder in acid solution. In comparison with acetylene black–sulphur composite electrode, the carbon–sulphur composite with deoxidized carbon electrode exhibits higher specific capacity of 1020 mAh g^-1 under the current density of 100 mA g^-1 and coulombic efficiencies of 97.9%, which is attributed to the restriction for dissolution of lithium polysulfides through the deoxidized carbon coating.
Hydrogen produced by splitting water is receiving significant attention due to the rising global energy demand and growing climate concerns. The photocatalytic decomposition of water converts solar energy into clean hydrogen, and may help mitigate the crisis of fossil fuel depletion. However, the photocatalytic hydrogen production remains challenging to obtain high and stable photoconversion efficiency. By implementing this grant, a highly efficient photoanode based on the coaxial heterogeneous cascade structure of Cu_2Se/CdSe/TiO_2 has been synthesized via the simple room-temperature and low-cost electrochemical deposition method. The microstructure and composition of the Cu_2Se top layer are regulated and controlled by doping Cu with various amounts in different zones of the CdSe/TiO_2 coaxial heterojunction and then using a simple integral annealing process. The maximum stable photocurrent density of the sample with the optimal copper zone and doping concentration has reached up to 28 mA/cm^2, which can be attributed to the success in the uniform dispersion of the three-layer heterogeneous nanojunctions among the anatase nanotube wall from the top to the bottom.
In addition, the formation and characterization of Cuprous oxide/Titanium dioxide (Cu_2O/TiO_2) nanotube-array coaxial heterogeneous structure also have been researched. Such structure is formed by coating nano-particles of Cu_2O onto titanium dioxide nanotube-array walls via multiple-cycle chemical adsorption plus reduction method (MCCAR). The practical deposition technique employs a soaking step to separate the adsorption and reduction processes, thus enhancing the controllability of deposition rate and preventing the clogging of nanotube pores. Cu_2O nano-particle sizes are adjusted by changing the glucose concentration in the reaction solutions. As a result, compact nano-particle film with sufficiently small crystal sizes is uniformly covered on the tube walls, resulting in the formation of coaxial heterogeneous structure. A stable hydrogen production rate of 3.1 mLcm^-2h^-1, which can be targeted for energy application in relation with solar energy driven production of hydrogen from water, has been achieved.
Besides, an epitaxial hetero-structure of CdSe/TiO_2 nanotube arrays as efficient photo-anodes has been obtained. With the help of the similar d spacing with TiO_2, CdSe sensitization layer is epitaxially grown on the tube wall of the TiO_2 nanotubes, resulting in an ideal coherent grain boundary and single crystal growth. The resultant photo-anode produces 30% more photocurrent than those samples without coherent grain boundary. Notably, the especial epitaxial hetero-structure is beneficial to decrease the recombination site and accelerate the separation of photogenerated electron-hole pairs. Furthermore, an ultrathin PEDOT surface layer was developed on the epitaxial hetero-structure of CdSe/TiO_2 nano-tube arrays in which it functions as both a physical passivation barrier and a hole transfer layer. As a result, significantly enhanced photocurrent density and substantially better stability have been achieved.
To improve the photoelectrochemical properties of TiO_2 heterogeneous structure, Cu_2ZnSnS_4 (CZTS) with TiO_2 nanotube arrays also were synthesized. Unlike some chalcogenides such as CdSe and CdS that have strong photo-corrosion weakness, CZTS has very good illumination stability. CZTS/TiO_2 composite can spontaneously form the pn junction, which is helpful to separate photo-generated electron-hole pairs. Considering that its valence band level is higher than the potential of producing oxygen, Ag substitution for Cu has been carried out to reduce the valence band, while replacing Sn with Ge also resulted in further enhancing energy level of the conduction band, which is favourable to electron injection into the conduction band of TiO_2. It has been found that the described Electrochemical ALD method has the considerable advantage for element substitution.
On the other hand, Lithium-sulphur batteries have shown great prospects for the future energy conversion and storage due to its high theoretical specific capacity. However, it has been hindered by rapid capacity decay and low energy efficiency. In this work, the polypyrrole (Ppy)/TiO_2 nanotube with coaxial heterogeneous structure as the substrate of positive electrode has been prepared to improve the electrochemical performance of sulphur electrodes. TiO_2 nanotubes decorated with Ppy provide a highly ordered conductive framework for Li^+ ion diffusion and reaction with sulphur. The suitable architecture also is helpful to trap the produced polysulfides, and as a result attenuates the capacity decay. Furthermore, the thermotreatment temperature in sulphur loading process has been confirmed to be an important impact factor on the overall performance of the resultant battery. The as-designed S/Ppy/TiO_2 nanotube using an elevated heating temperature shows excellent cycling stability with the high discharge capacity of 1150 mAh g^-1 at current rate of 0.1C and 1350 mAh g-1 at 0.05C and has an average coulombic efficiency of 96% after 500 cycles.
In addition, the dispersion for sulphur and restriction for lithium polysulfides have been combined to improve the performance of lithium–sulphur batteries. Sulphur is dispersed on acetylene black particles at first. Results show that sulphur envelops acetylene black particles and is uniformly dispersed in acetylene black–sulphur composite. A layer of deoxidized carbon coating wraps acetylene black–sulphur particles. The carbon coating is the reduction product of carbon precursor. The carbon precursor with carbonyl groups is deoxidized by aluminum powder in acid solution. In comparison with acetylene black–sulphur composite electrode, the carbon–sulphur composite with deoxidized carbon electrode exhibits higher specific capacity of 1020 mAh g^-1 under the current density of 100 mA g^-1 and coulombic efficiencies of 97.9%, which is attributed to the restriction for dissolution of lithium polysulfides through the deoxidized carbon coating.