Solar-driven water electrolysis : new multijunction solar cells and electrolysis materials

Abstract

The unpredictability and intermittency associated with solar energy render it unsuccessful to fully replace fossil fuels. Expansion of the deployment of renewable energy will also depend on appropriate energy storage methods and water electrolysis arises as one of the most promising of such technologies. However, solar-driven water electrolysis currently relies on noble metal and metal oxide electrocatalysts and bipolar plates, perfluorinated polyelectrolytes, and III-V multijunction solar cells that have high economic and environmental burdens. Developing and implementing zero-carbon and cost-effective solutions is thus fundamental to reduce the global dependence on fossil fuels and achieve carbon-neutrality. This work focuses on testing proton-exchange membranes based on fluorinated polymers, preparing membrane-electrode assemblies by printing methodologies, and combining a water electrolysis cell with perovskite-based multijunction solar cells to enable high solar-to-hydrogen conversion. The research is part of the eSCALED project that aims at creating new materials and devices to eventually combine into an artificial leaf, mimicking natural photosynthesis. Firstly, in chapter 2, to ensure reproducible and comparable results with literature work when using state-of-the-art materials, a water electrolysis setup and cell were built and optimized. The optimization process involved the study of the cell clamping torque, cell temperature, flow field’s material, porous transport layers (PTLs) materials and sample storage environment. After the improvement of the resistive losses, reaction kinetics, and overall stability, the proton exchange membrane water electrolysis reached current densities as high as 1 A cm−2 below 1.70 V. These results established the benchmark for the new electrolysis materials and preparation methodology investigated in later chapters. In chapter 3, a monolithic two-terminal multijunction solar cell that combined a wide-bandgap perovskite (PVK) semiconductor with a narrow-bandgap crystalline silicon (c-Si) was connected to the water electrolysis cell for solar-driven water electrolysis operation under 1-Sun equivalent light intensity. Two-terminal multijunction solar cells provide an open-circuit voltage (Voc) beyond the standard cell potential for water electrolysis while also increasing the power conversion efficiency (PCE) above the limit of series-connected single-junction cells. The PVK-Si tandem solar cell attains a Voc above 1.75 V, enough to conduct water electrolysis, while reaching high current densities that enable a solar-to-hydrogen efficiency (STH) of 21.5% when using state-of-the-art catalysts and membranes. This STH value is presently the highest reported value for a system operating without sunlight concentration. The system also represents the first example of a two-terminal PVK-Si multijunction solar cell coupled to a flow electrochemical cell operating in normal sunlight. In chapter 4, the PVK-Si tandem solar cell is replaced by an all-perovskite tandem solar cell that provides a higher Voc of almost 2 V, widening the operating voltage range at the expense of some current density. The slightly increased Voc is beneficial in case less efficient, but cost-effective electrocatalysts are used as the overpotential for water electrolysis rises. Solar-driven water electrolysis conducted with the all-perovskite tandem solar cell reached a STH close to 19% while using a comparatively inexpensive semiconductor and state-of-the-art catalysts and membranes. Additionally, this chapter describes the optimization of a narrow-bandgap perovskite solar cell to potentially increase the PCE of the all-perovskite tandem solar cell and STH of the coupled system. The use of bulk additives and top surface treatments of the perovskite layer enabled a maximum PCE of 18.6%, a 3.1% increase over the previous procedure. Such optimization combined with further improvements on the wide-bandgap sub-cell might elevate the PCE of the all-perovskite tandems above 26% and STH to almost 20%. In the next chapter, the water electrolysis performance and hydrogen permeability of different proton exchange membranes (PEMs) made of sulfonated derivatives of polypentafluorostyrene and poly(arylene thioether)s are compared to Nafion as standard membrane. These membranes were developed by other researchers in eSCALED. The ionic transport properties of the new PEMs are mostly better than Nafion, however, Nafion still outperformed them in terms of energy and faradaic efficiency in water electrolysis. This was mainly attributed to the larger water uptake and swelling ratios of the new membranes that increased mass transfer losses at the electrodes and allowed more hydrogen crossover. Hence, the design of new ionomers for PEMs should combine high ion transport properties and low water uptakes to avoid excessive gas permeation and energy losses. Titanium is widely used to manufacture the porous transport layers and bipolar plates, however, it results in high capital cost, which is higher than the one associated with the noble metal catalysts, decreasing the economic viability of water electrolysis. The growing field of printed electronics may provide a suitable answer to attain high throughput and low-cost manufacturing of electrodes. Graphite-based electrodes printed directly on Nafion with diverse patterns are studied in chapter 6. The patterns allowed proton transport across the membrane that resulted in successful water electrolysis, but also revealed several shortcomings in terms of resistive losses, graphite oxidation, reproducibility and overall performance. Additional improvements on the ink formulation and testing other patterns may enhance the efficiency of these electrodes, whilst using minimal amount of materials. In the last chapter, a life cycle assessment (LCA) of a solar-driven water electrolysis device that integrates the new materials (PEM and molecular catalysts developed in eSCALED project) is conducted. In this LCA, the eSCALED device is further compared with a device employing state-of-the-art materials. The study considers all the environmental impacts from raw material extraction (cradle) to manufacture (gate) of the devices to identify the most environmentally critical processes and materials. Overall, the environmental impact and energy demand of the eSCALED device were larger than of the state-of-the-art device. The low efficiency of the molecular catalysts in particular, prevents operation of the electrochemical cell at high current densities, resulting in larger material and energy consumption in the coupled system (photovoltaics and electrolysis components). Finally, the identification of the most environmentally impactful processes led to a better understanding of the environmental burden of the devices and where to improve them in the future.