Our world has witnessed the devastating effects of fossil fuels extraction; the primary energy source for modern society. Transitioning to sustainable energy sources requires exploring alternative fuel production methods, focusing on utilizing captured anthropogenic CO2. Electrochemical reduction of CO2 (CO2RR) using renewable electricity presents an attractive option for storing energy in chemical bonds. While many electrocatalysts efficiently reduce CO2 to CO, only a few, such as copper, can produce high energy-dense molecules like ethanol or methanol with selectivities exceeding 60%. Molecular catalysts (MCs) hold promise as alternatives to metallic electrocatalysts, particularly when immobilized on the surface of gas diffusion electrodes (GDEs), where state-of-the-art CO2RR takes place. However, their efficiency and scalability must be addressed for industrial applications. Our work focuses on evaluating the impact of engineering the micro-environment at the interface where CO2RR occurs, for the purposes of optimizing the performance of MCs systems.

Specifically, the electrode active-site interaction for the CO2-to-ethanol pathway was investigated using a nickel iron-tetraphenylporphyrin (Ni-FeTPP) electrocatalyst. We explored the challenges of translating a molecular catalyst system from an unconventional 3D electrode design to a GDE-based electrolyzer. The difficulty lies in replicating the strong electronic coupling between Ni and FeTPP necessary for enabling the CO2-to-ethanol pathway; which led to two methods of coupling Ni to FeTPP: drop-casting and spray-coating. Although our system did not achieve CO2 reduction to ethanol, it provided significant insights into the impact of subtle micro-environment changes. These changes uncovered possible unintended interactions, suggesting the formation of nickel hydride species that could greatly influence the CO2-to-ethanol pathway catalyzed by Ni-FeTPP. Moreover, the competing behavior between CO2 and CO to enhance the CO2-to-methanol pathway was adressed by using cobalt phthalocyanine supported on multi-walled carbon nanotubes (CoPc/MWCNT). We engineered the local environment where molecular catalysts interact by introducing silver nanoparticles and PTFE particles to increase local CO availability near the active sites and promote further CO reduction. This approach resulted in a remarkable 4-fold and 17-fold increase in the Faradaic efficiency of methanol, from the addition of silver nanoparticles and PTFE particles, respectively. Our results demonstrate the CO2-to-methanol pathway is enhanced through the modification of the catalyst surface microenvironment of the GDE. Our research contributes to a deeper understanding of the prospect for improving the challenging electrochemical reduction of CO2 beyond CO.

Light-emitting electrochemical cells (LECs) form a cheap and easily produced alternative to (organic) light-emitting diodes ((O)LEDs) due to their simple device structure and the ability to form an in-situ pi- n junction. Usage of quantum dots (QDs) as a luminophore in LECs is expected to result in devices with improved emission properties and stability compared to polymer-based LECs. LECs that use QDs as their only luminophore have, however, not been operated successfully without the presence of additional charge carrier injection layers and/or the inclusion of polyvinylcarbazole (PVK) as host (and light-emitting) polymer, yet. In this thesis, I show that three-layered, purely QD-based LECs can be fabricated and operated fruitfully. A crucial step to achieving this was carrying out a ligand exchange (LE) on the QDs, replacing the long aliphatic ligands that passivate the QD surface with short BF4- ions and thereby improving the conductivity of the QD film. LECs were then produced using the ligand exchanged QDs, both with and without additional charge carrier injection layers. The three resulting devices were confirmed to operate as LECs and show light emission at positive bias. The current density and electroluminescence (EL) intensity increase as the applied bias is increased for all three the LECs. The three types of LECs were compared on their electrical response and emission. Turn-on voltages and stability windows were also determined for the three types of devices. Improvement of LEC performance is suggested to be achieved by enhancing the photoluminescent quantum yield (PLQY) of the QDs, excluding side reactions that might take place in the LEC under operation or optimizing the current device structure.

Due to their simple structure and the possibility for in situ doping, light emitting electrochemical cells (LECs) are shaping up to be a cheap and easily produced alternative to LEDs. To date, little research has been done to combine the luminescent properties of colloidal quantum dots (QDs) with the LEC concept. In this thesis, steps are taken towards developing a QD-based LEC. The LECs were produced based on the semiconducting polymer MEH-PPV, ZnO and CdSe/CdS/ZnS quantum dots. Furthermore, drift-diffusion simulations were performed to discover general trends in LECs. MEH-PPV devices performed as expected. ZnO QDs were found to be unsuitable for LECs due to their high intrinsic conductivity and low luminescence efficiency, while CdSe/CdS/ZnS QDs were identified as promising candidates. Results from the drift-diffusion simulations suggested that unlike in LEDs, the type of electrode material has little effect on device operation. Charge carrier mobility was identified as the limiting parameter for luminescence intensity. It is thus suggested that improving carrier mobility in QD films is the most important step towards QD-based LECs.

Quantum dots (QDs) are nano-crystal semiconductors (1-100 nm) in which charge carriers (electrons and holes) are confined in all 3 dimensions by potential barriers that cause them to behave differently from conventional bulk semiconductors. QD research in the past decade has progressed rapidly, allowing
a deeper understanding of the physics behind the functioning and the effective synthesis of such materials. The wide spread opto-electronic applications of such QD semiconductors in LEDs, lasers, electrochemistry and solar cells with the potential to outperform traditional bulk semiconductors has fuelled inspired research in this field.
Quantum dot solar cells (QDSC) are solution-based third generation solar cells that possess the potential to overcome the Shockley-Queisser limit using multiple exciton generation (MEG). The large Bohr radius, wide bandgap tunability and large light absorption coefficients of PbS QDs have made them the most common material used in the absorber layer of such solar cells and shall also be the material used in this thesis. PbS QDs are used in conjunction with an n-type metal oxide to form a heterojunction that enhances charge separation at the interface. ZnO and TiO2 are common metal oxides used for this purpose while different synthesis methods of the same metal has been observed to show different results. While different research groups have used different metal oxide layers, there has been no systematic study on the interaction of the metal oxide layer synthesized by different methods with the absorber layer. Such a study could help improve understanding of the heterojunction interface and shall
be briefly looked into in this thesis. Another area of improvement in device performance is the depletion region across the heterojunction. Varying the doping of the n-type material affects the depletion width which has been explored in the past by adding impurity atoms to the metal oxide. However, the
emergence of an alternate method to dope ZnO electrochemically has triggered an interesting novel pathway to integrate doped materials into solar cells. In this thesis, we have successfully fabricated PbS QD solar cells for the first time in TU Delft at the Synthesis lab of the Applied Sciences faculty with
power conversion efficiencies exceeding 5%. Additionally, a systematic study on the absorber layer and the metal oxide layer was also carried out to optimize the device performance and set protocols for any future work. Finally, electrochemical doping of the PbS absorber layer was attempted to develop
a precise doping mechanism for QD films.