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Reducing landfill waste by 30% and increasing solar energy deployment to 2 GWp (giga-watt peak) are two of the key targets in the Singapore Green Plan 2030. While seemingly mutually-exclusive, increasing solar energy deployment, if not planned and handled properly, will eventually lead to increasing landfill waste. In fact, due to the relatively long useful lifetime (&gt;10 years) of solar cells, it was not until recently that wastes from solar cells starts to be of concerns. It was estimated that by 2030, waste from solar panels alone will reach millions of tons. On the contrary, the current recycling technologies for solar waste is still rudimentary - only precious metals such as silver and copper are economical to be recycled. This presents both environmental and economical problems because the bulk of solar cells which are made of silicon are not recycled. In this work, we recycled Silicon from solar cells into thermoelectrics. Leveraging on our expertise in thermoelectrics, we fine tuned and optimized the carrier concentration using phosphorus, and engineered low thermal conductivity via germanium doping. ZT as high as 0.4 is achieved at 800 K, which is one of the highest in silicon. Our work opens a pathway for recycling semiconductor waste into high performance functional materials for power generation and cooling applications.

MATERIALS FOR HUMANITY (MH 21)

Upcycling of Photovoltaics Silicon into Thermoelectrics by Turning Minority into Majority Carrier

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MH 21 is arranged around 6 themes that focus attention on pressing needs that should be addressed to enhance the well-being of communities and provide a forum for presenting breakthrough discoveries.

Reducing landfill waste by 30% and increasing solar energy deployment to 2 GWp (giga-watt peak) are two of the key targets in the Singapore Green Plan 2030. While seemingly mutually-exclusive, increasing solar energy deployment, if not planned and handled properly, will eventually lead to increasing landfill waste. In fact, due to the relatively long useful lifetime (>10 years) of solar cells, it was not until recently that wastes from solar cells starts to be of concerns. It was estimated that by 2030, waste from solar panels alone will reach millions of tons. On the contrary, the current recycling technologies for solar waste is still rudimentary - only precious metals such as silver and copper are economical to be recycled. This presents both environmental and economical problems because the bulk of solar cells which are made of silicon are not recycled. In this work, we recycled Silicon from solar cells into thermoelectrics. Leveraging on our expertise in thermoelectrics, we fine tuned and optimized the carrier concentration using phosphorus, and engineered low thermal conductivity via germanium doping. ZT as high as 0.4 is achieved at 800 K, which is one of the highest in silicon. Our work opens a pathway for recycling semiconductor waste into high performance functional materials for power generation and cooling applications.

technical paper

Artificial intelligence and machine learning (AI/ML) approaches for material design combined with 3D printing are capable of harnessing the bio-inspired design paradigm, creating de novo structures with extraordinary properties from basic building blocks. To valorize biomass such as food waste, biomass waste from industrial processes, or algae, we use bottom-up processes to convert biomass first into 3D printable materials, reducing need for plastic waste generation, then into advanced, functional materials including architected hierarchical composites, all using a conversion of materials into wave- and sound-based models.

Additive manufacturing of biomass-derived renewable composites with AI accelerated bottom-up design

All-solid-state sodium batteries employing fast ion-conducting sulfides, show great promise for safe, low-cost energy storage systems for large-scale applications. Among the candidate systems, Na3SbS4 (NAS) is particularly promising by being both highly ionic conductive and compatible with humid air while other common sulfide electrolytes suffer a lot from decomposing in ambient environment. The combination of fast Na+ ion migration and low-cost processability significantly benefits the large-scale application and industrialization.
Considering that partial substitution of sulfur by halide ions can induce more mobile sodium vacancies in NAS and potentially improve the interfacial stability towards sodium metal anode, here we explored a low-cost structural modification to NAS by aqueous synthesis to enhance both the rate capability and the cycle life of solid-state batteries based on this optimized solid electrolyte. To this end halide doped NAS electrolytes are synthesized from aqueous solution. Here we focus on the effect of Bromide doping of NAS on its ionic conductivity and kinetic electrochemical stability. It is observed that Br-doped NAS exhibits greatly reduced activation energy. Total ionic conductivity is more than doubled from 0.13 mS/cm to 0.31 mS/cm through this low-cost halide doping approach allowing for higher critical current densities. Stable cycling profiles of the doped NAS vs both sodium-metal anodes and sodium-tin ally anodes evidenced a major enhancement of the electrochemical stability and thus a path towards longer cycle life all-solid state sodium metal batteries.


Low cost aqueous synthesis and doping of a high performance sodium solid electrolyte

Rechargeable lithium−oxygen batteries (LOBs) have attracted increasing attention due to their high energy density but highly rely on the development of efficient oxygen catalysts for reversible Li2O2 deposition/decomposition. Herein, highly porous carbon nanocubes with a specific surface area up to 1600 m2 g−1 are synthesized and utilized to tightly anchor Ru nanoparticles for using as the oxygen-cathode catalyst in LOBs, achieving a low charge/discharge potential gap of only 0.75 V, a high total discharge capacity of 17,632 mA h g−1, and a superb cycling performance of 550 cycles at 1000 mA g−1. Comprehensive ex situ and operando characterizations unravel that the outstanding LOB performance is ascribed to the highly porous catalyst structure embedding rich active sites that synergistically function in reducing overpotentials, suppressing parasitic reactions, accommodating reaction products, and promoting mass and charge transportation.

Ru-embedded Highly Porous Carbon Nanocubes Derived from Metal Organic Frameworks for Catalyzing Reversible Li2O2 Formation

Co-doped molybdenum disulfide nanosheet (denoted Co-MoS2) was designed as a cathode catalyst in a lithium-sulfur battery, and was prepared by a hydrothermal method. It was demonstrated that cobalt doping induces the transformation of MoS2 from the 2H phase to the metallic 1T phase. The consequents increase the proportion of 1T phase MoS2 (1T-MoS2) with high catalytic activity while improving the stability of the catalytic site. The introduction of Co atoms also provides abundant electrons to the basal plane of the MoS2 matrix, thus improving the conductivity of semiconductor MoS2. Consequently, a sulfur cathode with Co-MoS2 contributes to a high rate performance (943 mAh g−1 at a high rate of 2 C relative to 1220 mAh g−1 at 0.3 C) as well as long cyclic stability (an ultralow capacity fading rate of 0.029% per cycle over 1000 cycles at the high current rate of 0.2 C).

Co-doped Molybdenum Disulfide Facilitating Polysulfides Transformation in Advanced Lithium-sulfur Batteries

Hybrid capacitive deionization (HCDI) is emerging as an energy-efficient alternative for brackish water desalination with low expenditure cost and favourable ion removal effectiveness by adopting battery-type electrodes in the traditional capacitive deionization (CDI) system. Herein, an unscalable electrospinning method is introduced to contrive cathode material for HCDI system. Co3O4 and nitrogen-doped carbon nanotube decoration are formed successfully and uniformly distributed with the hollow structure of free-standing carbon nanofibers (Co3O4@CNF@CNT). The conductive “bridges” provided by the CNF matrix significantly shorten the diffusion length of Na+ and promote the electrical conductivity of the Co3O4 nanoparticles. Moreover, benefiting from the incorporation of nitrogen-doped CNTs as the “medium”, the contact between electrolyte and electrode material is improved and the electrical conductivity is further enhanced. The as-prepared Co3O4@CNF@CNT cathode shows excellent pseudocapacitive performance of 395 F g-1 at a scan rate of 1 mV s-1 and superior rate performance of 279 F g-1 at 100 mV s-1. The HCDI system delivers an outstanding salt adsorption capacity (SAC) of 58.6 mg g-1 and a highest salt adsorption rate (SAR) of 12.27 mg g-1 min-1 with a potential difference of 1.4 V, indicating the great potential of applying Co3O4@CNF@CNT in the practical HCDI system.

Hierarchical tubular Co3O4/CNT structures for Efficient Hybrid Capacitive Deionization

Li-O2 battery is one of the most desired energy storages form due to its extraordinary high energy density and the readily available oxygen source from atmosphere. However, significant challenges still line up ahead from both the anode and cathode sides before the technology can be finally deployed to benefit human life. Herein, a two-birds-one-stone tactic is implemented to fabricate high-performance and flexible Li-O2 batteries utilizing only one structural backbone composed of graphene oxide (GO) aerogel embedding hollow NiCo2O4 (NCO) microspheres for both the anode and cathode. While the composite Li anode fabricated through lithium infusion exhibits an ultrahigh capacity of 3398.4 mAh g-1 and significantly inhibited dendrite growth and volume expansion, the freestanding oxygen cathode demonstrates great capability in promoting energy efficiency by catalyzing reversible Li2O2 formation. When coupled together, the resultant Li-O2 batteries manifest a superb long-term cycling stability with highly reversible discharge/charge for more than 400 cycles, apart from the remarkable mechanical deformability demonstrated by flexible pouch cells surpassing 400 hours of steady operation. By employing a full spectrum of ex-situ and operando spectroscopic and microscopic tools, mechanistics into the remarkable Li-O2 performance is unraveled.

High‐Performance Li–O2 Batteries Based on All‐Graphene Backbone

Silver bismuth iodide has been explored extensively for its potential as absorbers in perovskite inspired solar cells, due to its less toxic environment friendly nature. However, the conventional spin coating methods employed for synthesis of thin films make them dubious when it comes to commercial applications. These methods can be tedious and can facilitate fabrication of one to a few devices in a single run, making it unfitting for commercial applications, though they may be useful for lab scale device fabrication intended for research purposes. Here we propose a modified evaporation method which can be employed for large scale fabrication of silver bismuth iodide thin films. The method involves sequential evaporation of metallic layers of silver and bismuth followed by exposure to sublimated iodine vapours to obtain highly uniform pin hole free silver bismuth iodide thin films. Following this method, with multiple optimization steps, we were able to synthesise Ag3BiI7 thin films with a band gap of around 1.86eV. The photo voltaic devices fabricated, using the hence developed films as absorbers yielded an efficiency of 0.68% for devices with active area of around 36mm2. Thus, we believe that this work would help develop commercially feasible absorbers for perovskite inspired solar cells and provide a new dimension for the associated research for large area devices.

Silver Bismuth Iodide for Photovoltaic Applications Using Modified Evaporation Method

Surface passivation has been proven as an effective strategy to improve power conversion efficiency and stability of perovskite solar cells. However, the rationale of choosing appropriate passivator is still not clear yet, especially the binding interaction of different molecules with perovskite beneath. Here, we elucidated the binding interaction of two judiciously chosen molecules, fluorinated phosphonic acid and its corresponding phosphonate salt, and monitor the extent of interaction between these molecules and perovskite interface. Higher photoluminescence in perovskite film treated with phosphonate salt was observed which possesses stronger binding interaction with perovskite. The effect on photovoltaic performance and its stability were also examined. The corresponded device shows enhancement in conversion efficiency from 18.27% to 19.44%. Furthermore, water contact angle of treated perovskite film exceeding 110.9° was demonstrated which allows super hydrophobic nature, enabling excellent long term stability of devices by retaining over 90% of their initial efficiency after 90 days.

Toward Efficient and Stable Perovskite Photovoltaics with Fluorinated Phosphonate Salt Surface Passivation

Understanding the fundamental properties of buried interfaces in perovskite photovoltaics is of paramount importance to the enhancement of device efficiency and stability. Nevertheless, accessing buried interfaces poses a sizeable challenge because of their non-exposed feature. In this presentation, the buried mysteries in full device stacks will be unveiled by a series of dedicated techniques including the lift-off strategy, the in-situ mapping spectroscopy, and the cross-sectional high-resolution microscopy. By establishing the microstructure–property relations, the basic losses at the contact interfaces are systematically presented, which are induced by both the sub-microscale extended imperfections and lead-halide inhomogeneities. Furthermore, a brand new mechanism for the most popular ammonium-halide post-treatment is explored and called molecule-assisted microstructural reconstruction. The losses can be considerably mitigated by the use of the passivation-molecule-assisted microstructural reconstruction, which unlocks the full potential for improving device performance.

Buried interfaces in halide perovskite photovoltaics

The CsPbI2Br perovskite solar cells based on carbon electrode (CE) are considered to be low-cost and stable devices. Nevertheless, the insufficient contact and energy level mismatch in the interface of CsPbI2Br/CE hinder the hole extraction and limit the enhancement of cell efficiency. Herein, a carbon black (CB) interlayer was introduced between the CsPbI2Br and CE. [1] The hole extraction was facilitated due to the larger contact area and suitable energy band alignment in the CsPbI2Br/CB interface. Moreover, the carbon based CsPbI2Br solar cells maintained over 90% of original efficiency after constant 85 oC heating for 2000 hours, showing excellent thermal stability of carbon based CsPbI2Br solar cells. Further investigation indicated the diffusion of CB nanoparticles from CE or CB layer to CsPbI2Br film after a certain period of time. We disclosed the formation of a CB-CsPbI2Br bulk heterojunction structure due to the carbon diffusion, which resulted in an efficiency enhancement. As a result, a high efficiency of 13.13% was achieved for carbon based CsPbI2Br solar cells. This work indicates that the diffusion of CB nanoparticles in CB-contained perovskite solar cells is universal and inevitable, although this kind of diffusion results in the enhancement of the cell efficiency.

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Upcycling of Photovoltaics Silicon into Thermoelectrics by Turning Minority into Majority Carrier

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