• Energy Research
• Embargo close
• 7. Clean energy close
• Apollo close

Halide perovskites are the leading candidates for next-generation, low-cost optoelectronics with power conversion efficiencies well above 25%. However, operational stability remains a key challenge. Although there is an understanding that the microscale and nanoscale play a consequential role in determining the macroscopic performance and stability, significant gaps remain in the mechanistic understanding of degradation processes at the nanoscale and the mechanisms for stability in cation-alloyed systems. Nanoscale hexagonal phase impurities have been identified as problematic for operational stability, leading to both performance losses and morphological degradation. However, it is still unclear at what stage these phase impurities originate. Understanding this better is critical in order to mitigate the harmful effects of these phase impurities on performance and operational stability. Cation alloying is a commonly used technique in the field to mitigate these hexagonal phase impurities, although not without its challenges. In this thesis study, the nanoscale structural landscape of key halide perovskite compositions is studied. By taking snapshots of the perovskite at different states of the annealing process, the impact of phase impurities on device performance is characterised. Thereon, the mechanism by which composition dictates photostability in FA-rich perovskite absorber layers is studied. It is demonstrated that the composition impacts the degree of octahedral tilt, which is essential to restricting the transition to hexagonal phase impurities. Additionally, it is demonstrated that while a judicious mix of A-site cations can be used to stabilise the photoactive black phase of halide perovskites, it is challenging to achieve this homogeneously over large areas, necessitating a search for alternative or complementary approaches to stabilise perovskite via octahedral tilt. Using scanning electron diffraction (SED) studies, the spacegroup of additive-stabilised-CsPbI3 is demonstrated to be a low symmetry tilted γ-phase. Furthermore, using SED, the nanoscale structural landscape of mixed-phase CsPbI3 absorber layers is studied and it is demonstrated that both narrow-bandgap γ-phase and wide-bandgap δ-phase co-exist at the nanoscale, enabling stable and bright white-light emission. Overall, this thesis provides insights into the role of nanoscale structure in dictating the properties and behaviour of halide perovskites and offers rational guidelines for their optimisation and use in optoelectronic devices. Additionally, it is demonstrated that SED is a powerful tool for studying these materials at the atomic scale, allowing for the detailed characterisation of their structures and properties.

Formate production via both CO2 reduction and cellulose oxidation in a solar-driven process is achieved by a semiartificial biohybrid photocatalyst consisting of immobilized formate dehydrogenase on titanium dioxide (TiO2|FDH) producing up to 1.16±0.04 mmolformate gTiO2-1 in 24 hours. Isotopic labelling experiments with 13C-labelled substrates support the mechanism of stoichiometric formate formation through both redox half-reactions. TiO2|FDH was further immobilized on hollow glass microspheres to perform more practical floating photoreforming allowing vertical solar light illumination with optimal light exposure of the photocatalyst to real sunlight. Enzymatic cellulose depolymerization coupled to the floating photoreforming catalyst generates 0.36±0.04 mmolformate mirr-2 after 24 h. This work thus presents simultaneous solar-driven valorization of waste streams, demonstrates the advantages of biohybrid photocatalysts in photoreforming for the first time and will provide inspiration for the development of future semi-artificial waste-to-chemical conversion strategies.

Over the past decade, lead-halide perovskites (LHPs) have demonstrated significant potential in terms of their performance across a wide range of optoelectronic devices, including solar cells, photodetectors and light-emitting diodes. However, the toxicity of lead and instability issue of LHPs are still concerns for their widespread implementation. These successes, but also the challenges of LHPs have motivated great efforts across multiple disciplines to search for lead-free and stable alternatives that can have similar optoelectronic properties to LHPs, namely ‘perovskite-inspired materials (PIMs)’. With the deeper understanding of defect tolerance displayed in LHPs, a large number of PIMs have been identified until now. Among all the identified PIMs, ternary chalcogenides or ABZ2 materials, are believed to be one of the most promising alternatives so far, owing to their simple fabrication protocols, strong absorption and high stability in air. Particularly, AgBiS2 solar cells have demonstrated the highest efficiency (9.17%) among all bismuth-based solar cells. Nevertheless, studies into ternary chalcogenides are mostly limited to AgBiS2 photovoltaics, and the investigations into other potential ABZ2 materials or broader applications are rare so far. Therefore, this thesis will aim to investigate the optoelectronic properties of another promising while rarely investigated ABZ2 material – NaBiS2, and also the potential of AgBiS2 as near-infrared (NIR) photodetectors. In the first project of this thesis, NaBiS2 nanocrystals (NCs) have been shown to exhibit extremely strong absorption, along with a comparatively sharp absorption onset. However, optical-pump-terahertz-probe (OPTP) measurements indicated that most free charge-carriers in NaBiS2 NCs will be localised within a few picoseconds. These localised charge-carriers only exhibited low mobility of around 0.03 cm2 V-1 s-1 and could not transport effectively even though they might be rather long-lived in NaBiS2 and unaffected by intentionally-introduced defects. With help from density functional theory (DFT) calculations, all of these unusual characteristics in NaBiS2 have been shown to closely associate with intrinsic cation disorder, which was also observed in AgBiS2. Although post-annealing is effective for improving cation inhomogeneity and enhancing absorption in AgBiS2, its effect on NaBiS2 was found to be rather minor, which also indicated that the charge-carrier localisation process in NaBiS2 could not be significantly mitigated after annealing. Based on the fundamental insights acquired in the first project, the possibility of further improving charge-carrier transport in NaBiS2 NCs through ligand exchange treatment was investigated in my second project. Using a variety of correlated spectroscopic characterisation techniques, I found that NaBiS2 NCs treated by inorganic iodide ligands had enhanced sum mobility and surface photovoltage (SPV) signals, which implies an improvement in the macroscopic charge-carrier transport. However, the ultrafast localisation process was still observed in these iodide-treated NaBiS2 NCs, suggesting that their cation disorder was not greatly changed. At the same time, the defect capture rates were also found to be lower in the iodide-treated NaBiS2 NCs based on my two proposed models for describing charge-carrier dynamics. As a result, solar cells based on these iodide-treated NaBiS2 NCs could exhibit a peak external quantum efficiency (EQE) value over 50%, along with a power conversion efficiency exceeding 0.7%. Although this is an order of magnitude larger than previous reports, I found ion migration to be a limiting factor for NaBiS2 devices from temperature-dependent transient current measurements, where a low activation energy of only 88 meV was extracted. In my third project, AgBiS2 photodetectors were fabricated and characterised in depth. Aside from the broadband photo-response across from ultra-violet (UV) to near-infrared (NIR) region, AgBiS2 photodetectors have demonstrated an extremely high cut-off frequency (f-3dB) on MHz order, indicating their great potential in applications requiring fast device response such as optical communications. The mechanism behind this fast response was studied, and a relatively long drift length compared to the AgBiS2 film thickness is believed to be the key reason. Similar to NaBiS2 devices, ion migration was also found easy in AgBiS2 devices with an activation energy of 124 meV, which could lead to their increasing noise currents with time. Importantly, these noise currents could be also effectively suppressed when optimising the AgBiS2 film thickness, in which a balance between large shunt resistant and cumulative quantity of defects should be reached. Finally, owing to the small bandgap of AgBiS2 NCs (~1.2 eV), AgBiS2 photodetectors could effectively monitor the heartbeat rates by probing the transmission change of blood vessels illuminated by NIR light, which has been widely used in the medical field owing to its deeper penetration in tissues. These three projects not only uncovered several remarkable optoelectronic characteristics of ABZ2 materials, but also investigated possible methods to further alter these characteristics. Although ABZ2 materials have shown great potential as light harvesters, it can be seen that both cation disorder (or charge-carrier localisation) and ion migration are still limiting the performance. More studies on the root causes of both phenomena, and how to effectively suppress their effects on the materials, would be hence crucial in the future work. With more understandings on this material class, we could expect more efficient, stable, and cleaner optoelectronic devices to be realised in the future.

Ammonia has been responsible for feeding population growth in the 20th century through synthetic fertilizer, and is poised to become the preferred energy storage medium for a society powered by renewable electricity in the 21st century. However, conventional brown ammonia production through the Haber-Bosch process is optimized for utilization of centralized and steady energy supply from fossil-fuels. When shifting to distributed and intermittent energy supply through wind and solar energy, a re-optimization is required for a low-capital and flexible green ammonia production processes. This thesis re-designs and Haber-Bosch process by targeting the integration of reaction and separation in a single process vessel at low pressures, thereby achieving the simplification and down-scaling of the high pressure recycle loop of the Haber-Bosch process. Materials are developed for this purpose, the feasibility of integration is demonstrated, and mathematical modeling is utilized for assessing the application of the single-vessel process to a range of renewable energy sources in comparison to competing ammonia production processes. Herein, a catalyst with low-temperature (< 350°C) and high-conversion (i.e. near equilibrium) activity is developed using ruthenium nanoparticles as the active metal supported on ceria and promoted with cesium to mitigate hydrogen and ammonia inhibition, respectively. This catalyst is compared to commercial iron-based catalyst from the perspective of the final application. Concurrently, a high-temperature (> 300°C) manganese chloride absorbent is developed that resists decomposition and is stable when supported on silica. These catalyst and absorbent are integrated in a layered reactor configuration to demonstrate the feasibility of the integrated process by exceeding single-pass reaction equilibrium. Mathematical modelling of ammonia production processes illustrates that at small-scales (< 1 t day-1) the single-vessel process is optimal compared to the Haber-Bosch process due to its modular design. In addition, it can achieve simpler ramping because the Haber-Bosch process is constrained by heat-integration in the recycle loop and the potential for runaway reaction. For final application, the pairing of ammonia production processes with examples of intermittent solar and wind sources demonstrates that the flexibility of the production process is essential when considering non-ideal sources of energy with a long-term (e.g. seasonal) oscillations. Flexible ammonia production also expands the economic usage of ammonia as an energy storage vector from the seasonal to the weekly time-scale, with advantage compared to batteries or hydrogen. The work of this thesis provides a framework for advancing the electrification of the chemical industry given the novel constrains of intermittent and distributed renewable energy. A systems level approach is applied from the ground up, starting from material design and progressing to optimized process design and application.

The aggravating global problems of energy crisis, rising atmospheric greenhouse gas concentrations and accumulation of persistent waste have attracted the attention of scientists, policy-makers and global organisations to come up with effective and expeditious solutions to address these challenges. In this context, the development of sustainable technologies driven by renewable energy sources for the production of clean fuels and commodity chemicals from diverse waste feedstocks is an appealing approach towards creating a circular economy. Over the years, semiconductor photocatalysts based on TiO₂, CdS, carbon-nitrides (CNx) and carbon dots (CDs) have been widely used for the photocatalytic reforming (PC reforming) of pre-treated waste substrates to organic products, accompanied with clean hydrogen (H₂) generation. However, these conventional solar-driven processes suffer from major drawbacks such as low production rates, poor product selectivity, CO₂ release, challenging process and catalyst optimisation, and harsh waste pre-treatment conditions, which limit their commercial applicability. These challenges are tackled in this thesis with the introduction of new and efficient photoelectrochemical (PEC) and chemoenzymatic processes for reforming a diverse range of waste feedstocks to sustainable fuels. Solar-driven PEC reforming based on halide perovskite light-absorber is first developed as an attractive alternative to PC reforming. The PEC systems consist of a perovskite|Pt photocathode for clean H₂ production and a Cu-Pd alloy anode for reforming diverse waste streams, including pre-treated cellulosic biomass, polyethylene terephthalate (PET) plastics, and industrial by-product glycerol into industrially-relevant, value-added chemicals (gluconic acid, glycolic acid and glyceric acid) without any externally applied bias or voltage. Additionally, the single light-absorber PEC systems can also convert the airborne waste stream and greenhouse gas CO₂ to diverse products with the simultaneous reforming of PET plastics with no applied voltage. The perovskite-based photocathode enables the integration of different CO₂ reduction catalysts such as a molecular cobalt porphyrin, a Cu-In alloy and formate dehydrogenase enzyme, which produce CO, syngas and formate, respectively. The versatile PEC systems, which can be assembled in either a ‘two-compartment’ or standalone ‘artificial leaf’ configurations achieve 60‒90% oxidation product selectivity (with no over-oxidation) and >100 µmol cm‾² h‾¹ product formation rates, corresponding to 10²‒10⁴ times higher activity than conventional PC reforming systems. In addition to developing PEC platforms, this thesis also explores avenues for circumventing the harsh alkaline pre-treatment strategies (pH >13, 60‒80 ºC) adopted for photoreforming waste substrates. For this purpose, a chemoenzymatic pathway is introduced whereby PET and polycaprolactone plastics were deconstructed using functional enzymes under benign conditions (pH 6‒8, 37‒65 ºC), followed by PC reforming using Pt loaded TiO₂ (TiO₂|Pt) or Ni₂P loaded carbon-nitride (CNx|Ni₂P) photocatalysts. The chemoenzymatic reforming process demonstrates versatility in upcycling polyester films and nanoplastics for H₂ production at high yields reaching ∼10³‒10⁴ µmol gsub‾¹ and activities at >500 µmol gcat‾¹ h‾¹. The utilisation of enzyme pre-treated plastics also allowed the coupling of plastic reforming with photocatalytic CO₂-to-syngas conversion using a phosphonated cobalt bis(terpyridine) co-catalyst immobilised on TiO₂ (TiO₂|CotpyP). Finally, moving beyond solar-driven systems, a bio-electrocatalytic flow process is demonstrated for the conversion of microbe pre-treated food waste to ethylene (an important feedstock in the chemical industry) on graphitic carbon electrodes via succinic acid as the central intermediate. In conclusion, with its focus on improving efficiencies, achieving selective product formation, building versatile platforms, diversifying substrate and product scope, and reducing carbon footprint and economic strain, this thesis aims to bring sustainable waste-to-fuel technologies a step closer to commercial implementation.

This research provides new techno-economic insights into integrating flexible combined-cycle gas turbines with post-combustion carbon capture and storage (CCGT-CCS) for low-carbon power systems. This study developed a versatile unit-commitment optimisation model of CCGT-CCS. This research highlights the model’s adaptability, accommodating diverse techno-economic configurations, feed gases (e.g., biomethane or fossil natural gas), carbon capture rates, and policy instruments. This generalisation empowers seamless application in various policy and market contexts, making the model a potent tool for researchers and policymakers. While the case study focuses on the UK, the findings are relevant for most low-carbon power systems with variable renewable supplies. Analysing the UK’s net-zero scenarios from 2030 to 2050, the economic viability of flexible CCGT-CCS was highlighted. Intertemporal flexibility proves highly valuable with greater electricity price volatility, with a total ROI range of 81–246 %, surpassing the CCGT-CCS plant’s ROI (7–64 %). A flexible solvent storage solution should be seen in the context of the overall system ‘flexibility’ requirements of a low-carbon power system. On a cost basis, solvent storage represents just a fraction of the capital costs of more “mainstream” energy storage technologies, such as lithium-ion batteries or hydro-pumped storage, while CCGT-CCS offers firm power. Overall, while seen as a rather technical solution, if abated fossil fuel generation is to be part of a future low-carbon power system, having this flexibility adds economic benefits not just to operators but also improves overall system security and complements high shares of variable renewables on the grid.

Metal-halide perovskites are materials at the forefront of the next generation of optoelectronic materials. Of particular interest is their remarkable power conversion efficiencies when incorporated into thin film solar cells. The properties of next-generation semiconductors such as perovskites are dominated by microscopic variations in their structure, composition and photophysics. Perovskites show extraordinary levels of disorder and this has considerable implications for their function. Gaining a microscopic understanding into how the optoelectronic quality of perovskite thin films and their interfaces with contact layers affects their performance is crucial to enabling solar cells with sufficient performance and stability to commercialise. In this thesis, I detail the development of a multi-modal microscopy toolkit to probe the optoelectronic quality of perovskite thin films and devices and spatially correlate these measurements with microscopic chemistry and structural information. In the first experimental chapter, I detail the capabilities of a hyperspectral, wide-field optical microscope, capable of measuring spatially resolved photoluminescence, reflectance and transmittance spectra with diffraction resolution. With a variety of perovskite thin film samples, I show that thin-film morphology and surface passivation play a huge role in photoluminescence intensity, spectrum and stability. The second experimental chapter applies calibration tools to the hyperspectral microscope, enabling the extraction of device relevant metrics such as the quasi-Fermi level splitting and Urbach Energy microscopically. We spatially correlate these measurements with nanoprobe X-ray diffraction and fluorescence to probe structure and chemistry. Applying this multimodal toolkit to state-of-the-art alloyed perovskites, we find that nanoscale variations in chemical composition dominate the optoelectronic properties of these perovskite films and form energetic funnels that carriers fall down and away from trap states. This study helps to explain the remarkable defect tolerance of these materials. The final experimental chapter augments the optical microscopy setup to measure voltage dependent photoluminescence maps. Voltage dependent photoluminescence allows the extraction of pseudo current-voltage curves of the devices, enabling the recombination and charge transport losses of perovskite solar cells to be mapped microscopically. I show that microscopic performance heterogeneity has a large impact on both macroscopic performance and stability. By mapping the same areas before devices before and after ageing, the microscopic effects of degradation on charge extraction can be imaged. Taken together, the results here show the important microscopic influences on performance from thin films to complete devices and the powerful multi-modal methodologies developed are widely applicable to a wide array of disordered semiconductors.

The construction industry in Europe is in transition. In the last decade, challenges related to inefficiencies in the sector, the shortage of skilled labour, and environmental concerns initiated a shift towards off-site manufacturing. In Hungary, the first examples of prefabricated residential buildings have just appeared after a 30-year-long break. At the same time, in post-socialist countries, the general attitude towards modern methods of construction is rather complex. While the Western examples of modular constructions are admired, local examples of prefabricated and standardised homes from the socialist era are neglected or criticised for their uniformity and inability to change. This thesis examines the social limits of standardisation in the Hungarian context, specifically focusing on how we can ensure that in the future, mass-manufactured buildings will be sustainable and retain their social respectability, technical qualities and economic value for a long time. It is found that standardisation does not necessarily limit creativity and can be socially sustainable, provided that it does not result in uniform constructions. Findings rely on an extensive review of the literature and real-life architectural examples, statistical results from two online surveys on preconceptions about mass-manufactured buildings, and space syntactical investigations of preferred home layouts. The findings of the project include showing that young Hungarian adults associate mass produced buildings with the loss of diversity, but they find these buildings environmentally friendly, fast to produce, progressive and fashionable. In addition, it is shown that it is possible to use small graph matching and density-based clustering to find the most suitable layouts for socially-conscious mass manufacturing. The practical outcomes of this project include an exemplar dwelling that showcases good design, a framework for discussing standardised buildings, and a Plug-in that can evaluate any new apartments created in Autodesk Revit based on the developed guidelines.