Engineering of two-dimensional materials for energy storage and conversion

Negative impacts of reliance on fossil fuel on both environment and economy are becoming more apparent and urgent. This calls for a severe paradigm shift and serious actions toward sustainable energy supply chain. Developing clean and green energy especially from non-carbon generating sources is a crucial step in the direction. To this end, renewable energies such as solar and wind energy are perfect solutions. Energy conversion devices such as solar cell and wind turbine can convert these natural energy forms to electricity. Nevertheless, renewable energies are intermittent, which necessitates energy storage devices such as batteries that store the renewable electricity in terms of chemical energy. However, this chemical energy storage is not stable and self-discharge of batteries is inevitable. A more stable energy storage is to store renewable electricity in the form of chemical fuels. For instance, solar-to-hydrogen conversion effectively stores solar energy in stable hydrogen chemical bonds, which are stable for centuries. A key component of these energy conversion and storage device is catalyst, particularly hydrogen and oxygen reaction catalysts. For instance, hydrogen and oxygen evolution catalysts play critical roles in water electrolysis for green hydrogen generation. Oxygen reduction reaction (ORR) catalyst is widely employed in metal-oxygen batteries, a high-capacity energy storage device. An ideal catalyst should have high activity and stability, low cost, and great abundance. The high activity is often achieved by high intrinsic activity of active sites, and high density of these active sites (per unit geometric area), which needs high surface area. High surface area also improves utilization rate of the catalyst materials, which is very crucial for reducing the cost of precious metal catalysts. The high activity also strongly depends on the electrical conductivity of the catalyst. To this end, two-dimensional materials (2DMs) has gained traction for catalysing energy conversion and storage due to their high specific surface area, excellent mechanical properties, great electrical and thermal conductivity. The most alluring feature of 2DMs is their ability to be modified to exhibit different functions. In this thesis, 2DMs are modified to improve their applicability in energy applications, especially for oxygen reduction reaction and solar energy conversion/storage. I will present a showcase of how lithiation activation technique for PdSe2 can balance the stability and performance of the material for ORR catalysis. Moreover, an unprecedented palladium phosphoronitride-based ORR catalyst is produced by a simple annealing of Pd3P2S8. Lastly, I will present the functionalisation of MXene that can tune the work function in a wide range, which could find wide application in optoelectronic devices, e.g., solar cell. Additionally, the functionalisation of a new class of 2DM, MXene, had achieved highly efficient solar light to heat conversion for water desalination.Doctor of Philosoph

Sustainable cold economy via thermal energy storage

The increased demand for cold energy in areas for food distribution and storage, building cooling and domestic refrigeration is a challenging task to meet in the hot tropical climate of Singapore. Without sufficient renewable energy sources and space constraints in our city-state, higher electricity consumption to provide cold energy cannot be sustained. The concept of a sustainable cold economy encompasses recovery of waste cold energy, cold energy storage and higher efficiency in cold generation and better utilization of current resources. Recently, increase in innovative uses for cold energy recovered from cryogenic sources such as re-gasification of liquefied natural gas had seen increase in interest in cold thermal energy storage (CTES). This project’s key objective is in phase change material CTES (PCM CTES) systems and in their heat exchanger components and storage medium. The project covered an extensive review of the methodology for designing of PCM CTES - requirement and methods of assessing PCM for use in CTES and volume sizing methodologies. Following which experiments were conducted using the differential scanning calorimeter (DSC) with various concentrations of aqueous mono ethylene glycol (MEG) solutions to determine their thermal properties. The results from the DSC provided essential information- a range of melting and freezing temperatures and latent heat of fusions values of the various concentration of MEG- which are crucial in a design of CTES system. Graphite powder at micron size was then added to the material to investigate the effects on reduction of supercooling. It was found that 30% volume composition of MEG gave stable result at the desired operation range while 31% volume composition of MEG and 33% volume composition of MEG + 12% weight composition of graphite gave good thermal physical properties at the desired operation range. The enhancement was observed to reduce the supercooling level by around 60% which might be attributed to improvement in nucleation rates. Thermal cyclic tests were conducted on the 2 compositions and results showed that the material requires some initial charge and discharge cycles to reach its full potential and it is thermally stable for at least 50 cycles with no deterioration of the latent heat observed. A CTES system using the investigated material to recover waste cold was proposed with its functional requirement and material selection process.Bachelor of Engineering (Mechanical Engineering

Predicting the work function of 2D MXenes using machine-learning methods

MXenes, which are graphene-like two-dimensional transition metal carbides and nitrides, have tunable compositions and exhibit rich surface chemistry. This compositional flexibility has resulted in exquisitely tunable electronic, optical, and mechanical properties leading to the applications of MXenes in catalysis, electronics, and energy storage. The work function of MXenes is an important fundamental property that dictates the suitability of MXenes for these applications. We present a series of machine learning models to predict the work function of MXenes having generic compositions and containing surfaces terminated by O*, OH*, F*, and bare metal atoms. Our model uses the basic chemical properties of the elements constituting the MXene as features, and is trained on 275 data points from the Computational 2D Materials Database. Using 15 different features of the MXene as inputs, the neural network model predicts the work function of MXenes with a mean absolute error of 0.12 eV on the training data and 0.25 eV on the testing data. Our feature importance analysis indicates that properties of atoms terminating the MXene surface like their electronegativity, most strongly influence the work function. This sensitivity of the work function to the surface termination is also elucidated through experimental measurements on Ti3C2. We introduce reduced-order models comprising of ten-, eight-, and five-features to predict the work function. These reduced-order models exhibit easier transferability to new materials, while exhibiting a marginal increased mean average error. We demonstrate the transferability of these reduced order models to new materials, by predicting the work function of MXenes having surface terminations beyond the original training set, like Br*, Cl*, S*, N*, and NH*. Predicting electronic properties like the work function from the basic chemical properties of elements, paves the way towards rapidly identifying tailored MXenes having a targeted range of properties that are required for a specific application.Ministry of Education (MOE)Nanyang Technological UniversityNational Research Foundation (NRF)National Supercomputing Centre (NSCC) SingaporePublished versionThis work is supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) program and from the Ministry of Education Academic Research Fund Tier 1: RS04/19 and RG 5/22. The computational work for this article was partially performed on resources of the National Supercomputing Centre, Singapore (www.nscc.sg) through Project IDs 12001868, 12002171 and 12002494. P R acknowledges the Visiting Research Student Programme by the India Connect@NTU office. L R acknowledges NTU for a research scholarship

Feasibility study of using porous metal as practical shielding material

This paper presents the feasibility study of using porous metal for electromagnetic shielding purposes. Using the highly porous aluminum (close to 90% porosity) as the material under study, its shielding effectiveness (SE) was studied systematically. A preliminary measurement was carried out to evaluate the SE of porous aluminum sample. This was followed by a full-wave EM modeling of the porous aluminum shielded box. Finally, a shielded enclosure was assembled using porous aluminum panels and the overall SE of this enclosure was measured. With the conventional installation method, the enclosure has been found to provide at least 60 dB shielding effectiveness in the frequency range of 250 kHz to 1 GHz.Published versio

Three-dimensional porous SnO₂@NC framework for excellent energy conversion and storage

SnO2-based materials are deemed to be attractive electrodes for lithium/sodium ion batteries (LIBs and SIBs) and electrocatalytic CO2 reduction reaction (CRR) because of high energy density and large abundance. However, the practical application of the SnO2-based materials is prevented by low electrical conductivity and large volume change. Herein, we construct a three-dimensional (3D) porous network with SnO2 nanoparticles into N-doped carbon (namely P–SnO2@NC) synthesized by freeze drying followed by a pyrolyzation process. In the composite, the 3D hierarchical framework can facilitate the ion penetration and gas diffusion. In addition, the NC network can optimize the conductivity of the material and suppress the electrode material to fall off from the electrode. Therefore, the electrode delivers excellent electrochemical properties with high capacities of 510 mA h g−1 after 1000 cycles for LIBs and 497 mA h g−1 after 500 cycles for SIBs. Furthermore, the electrode shows high selectivity for CRR with a large coulombic efficiency (CE) of 52.7% for HCOOH at 0.6 V.Startup Fund for Natural Science Foundation of Fujian Province (2019J01731 and 2019J01732), Talents of Quanzhou Normal University (H18020), the Young and Middle-Aged Teacher Education Scientific Research Project of Fujian Province (JT180368), National Natural Science Foundation of China (21676222 and U1705252)

Surface group-modified MXene nano-flake doping of monolayer tungsten disulfides

Exciton/trion-involved optoelectronic properties have attracted exponential amount of attention for various applications ranging from optoelectronics, valleytronics to electronics. Herein, we report a new chemical (MXene) doping strategy to modulate the negative trion and neutral exciton for achieving high photoluminescence yield of atomically thin transition metal dichalcogenides, enabled by the regulation of carrier densities to promote electron-bound trion-to-exciton transition via charge transfer from TMDCs to MXene. As a proof of concept, the MXene nano-flake-doped tungsten disulfide is demonstrated to obtain an enhanced PL efficiency of up to ∼five folds, which obviously exceeds the reported efficiency upon electrical and/or plasma doping strategies. The PL enhancement degree can also be modulated by tuning the corresponding surface functional groups of MXene nano-flakes, reflecting that the electron-withdrawing functional groups play a vital role in this charge transfer process. These findings offer promising clues to control the optoelectronic properties of TMDCs and expand the scope of the application of MXene nano-flakes, suggesting a possibility to construct a new heterostructure junction based on MXenes and TMDCs.National Research Foundation (NRF)Published versionThis work is supported by SERC (Grant No. 1426500050) from the Agency for Science, Technology and Research (A*STAR), Singapore National Research Foundation, Competitive Research Program (Grant No. NRF-CRP18-2017-02), Singapore Ministry of Education Tier 2 Program (Grant No. MOE2016-T2- 1-128) and National Natural Science Foundation of China (Grant No. 61704082) and Natural Science Foundation of Jiangsu Province (Grant No. BK20170851). H. Li would like to thank the support from Nanyang Technological University under NAP award (M408050000) and Singapore Ministry of Education Tier 1 program (2018-T1-001-051)

Rambutan‐like hollow carbon spheres decorated with vacancy‐rich nickel oxide for energy conversion and storage

Transition metal oxides hold great promise for lithium‐ion batteries (LIBs) and electrocatalytic water splitting because of their high abundance and high energy density. However, designing and fabrication of efficient, stable, high power density electrode materials are challenging. Herein, we report rambutan‐like hollow carbon spheres formed by carbon nanosheet decorated with nickel oxide (NiO) rich in metal vacancies (denoted as h ‐NiO/C) as a bifunctional electrode material for LIBs and electrocatalytic oxygen evolution reaction (OER). When being used as the anode of LIBs, the h ‐NiO/C electrode shows a large initial capacity of 885 mA h g−1, a robust stability with a high capacity of 817 mA h g−1 after 400 cycles, and great rate capability with a high reversible capacity of 523 mA h g−1 at 10 A g−1 after 600 cycles. Moreover, working as an OER electrocatalyst, the h ‐NiO/C electrode shows a small overpotential of 260 mV at 10 mA cm−2, a Tafel slope of 37.6 mV dec−1 along with good stability. Our work offers a cost‐effective method for the fabrication of efficient electrode for LIBs and OER.MOE (Min. of Education, S’pore)Published versio

Vertical silver@silver chloride core-shell nanowire array for carbon dioxide electroreduction

Core-shell nanowires with conductive metal cores and active catalyst shells hold great promise for making a catalyst electrode with high total activity. Indeed, core-shell nanowire arrays have been demonstrated as the catalyst in various electrochemical reactions, yet their catalytic roles are underexplored for carbon dioxide (CO2) electroreduction. Herein, we report a reduced silver@silver chloride (Ag@AgClx) core-shell nanowire array for electrocatalytic CO2 reduction. The Ag@AgClx core-shell array electrode shows high total activity including low onset potential and high current density. The strong electronic coupling between AgClx and the Ag core results in the record-high specific CO2RR activity to date.MOE (Min. of Education, S’pore)Accepted versio

An empirical approach to develop near-field limit for radiated-emission compliance check

Based on measurements from a near-field scanner and far-field measurements obtained in a semi-anechoic chamber, a statistical relationship is established between a magnetic field in the near field and an electric field in the far field. The relationship makes it possible to transform a radiated-emission regulatory limit from the far-field to the near-field zone. The transformed near-field limit can allow efficient prediction of radiated-emission compliance for high-speed printed circuit boards. The presented results demonstrate the feasibility of the proposed method for a quick radiated-emission precompliance check without heavy equipment investment.Accepted versio

Functionalized MXene enabled sustainable water harvesting and desalination

Solar irradiation is a promising resource for sustainable energy conversion and storage in many technological fields. Interfacial solar steam generator is emerging as a sustainable system for water harvesting under natural solar irradiance. However, natural sunlight is usually insufficient for conventional interfacial (2D absorber) solar steam generators to achieve high solar steam efficiency (>85%). Herein, a functionalized MXene-polymeric membrane-based solar steam generator in a heat localization structure is reported as an efficient water harvesting and desalination system. The functionalized MXene flakes are homogeneously dispersed and encapsulated in polymeric networks of cellulose acetate in the crosslinking process, where water is automatically pumped through the absorber in the embedded nano/micro channels. As such, the fabricated solar steam generator shows a net evaporation rate (with respect to that in the dark condition) of 1.47 kg m−2 h−1 with 92.1% efficiency under 1 sun illumination. Moreover, the salt rejection rate for high salinity seawater (original Na+ concentration is 18 000 mg L−1) reaches up to 97.5%, generating drinkable water that meets the WHO standard. The new photothermal membrane absorber is cost-effective, scalable, washable, and stable under harsh conditions; holding great promise for practical solar desalination applications.Ministry of Education (MOE)Nanyang Technological UniversityThis work was supported by Nanyang Technological University under NAP Award (No. M408050000) and Singapore Ministry of Education Tier 1 program (2018-T1-001-051)