Hybrid solar cells powering electrochemical reduction reactions

Introduction Photovoltaic (technology has evolved rapidly in the past few decades and now encompasses a large variety of materials and device structures. A key aspect to be considered in any PV technology is the operational durability under real outdoor conditions, as well as the sustainability of materials/components and the facile integration with energy storage systems. In the last five years, perovskite solar cells (PSCs) and dye-sensitized solar cells (DSSCs) containing water-based electrolytes have been considered as of the possible breakthroughs towards hybrid photovoltaic concepts large-scale diffusion. If opportunely developed and optimized, these solar cells system (especially DSSCs) can be truly considered as zero-impact photovoltaic devices fabricated with non-toxic components. These intriguing concepts have to face with the Paris climate agreement, where the European member states engaged to mitigate global warming and to play a leading role in the fight against climate change. Technologies allowing the transition to a low-emission society are still not available on a large-scale level and significant research and development efforts are crucially needed. The enormous increase of photovoltaic capacity worldwide shows that a consolidated alternative to fossil energy carriers already exists for electricity production. However, storing efficiently and reliably surplus electric energy remains one of today top challenges. Storage processes converting electricity and solar energy into chemical energy would be highly desirable. For the transport and heating sector, fossil fuels are an unmatched energy source, coming along with a huge, existing infrastructure. Also chemical industry, supplying a variety of indispensable bulk chemicals for everyday life (e.g. hydrogen peroxide and ammonia), is completely dependent on fossil-based raw materials such as crude oil. Generating alternative fuels and chemical raw materials from renewable energy sources represents a game changer and one of today biggest challenges. The goal of this work is to provide a sustainable alternative to the fossil-based, energy-intensive production of fuels and base chemicals. The needed energy will be provided by sunlight, the raw materials will be molecules abundantly available in the atmosphere, such as carbon dioxide, oxygen and nitrogen. Results and Discussion The first part of this work concerns the conversion of carbon dioxide into hydrocarbons using solar energy, i.e. an attractive strategy for storing such a renewable source of energy into the form of chemical energy (a fuel). This can be achieved in a system coupling a photovoltaic cell to an electrochemical cell for CO2 reduction. To be beneficial and applicable, such a system should use low-cost and easily processable photovoltaic cells and display minimal energy losses associated with the catalysts at the anode and cathode and with the electrolyzer device. We have considered all of these parameters altogether to set up an integrated system for CO2 reduction to hydrocarbons. By using the same original and efficient Cu-based catalysts at both electrodes of the electrolyzer, and by minimizing all possible energy losses associated with the electrolyzer device, we have achieved CO2 reduction to ethylene and ethane with a 21% energy efficiency. Coupled with a state-of-the-art, low-cost perovskite photovoltaic minimodule, this system reaches a 2.3% solar-to-hydrocarbon efficiency, setting a benchmark for an inexpensive all–earth-abundant photovoltaic–electrochemical cell system. The second part of the work describes our preliminary approaches to face electrochemical N2 reduction as a sustainable and renewable energy-based technology to replace the well-established Haber-Bosch process for NH3 production, one of the top-3 chemicals produced worldwide, at the base of fertilizers synthesis and a viable green energy carrier, since it is easier transportable and safer than H2. Li3N is the only stable nitride of alkali metals and its formation is thermodynamically favored also in ambient conditions. Li-mediated N2 reduction was demonstrated in 1994 and it has recently shownpromising results, both in continuous systems with Li+ ions and a proton donor in the same cell, and in discontinuous processes involving Li3N formation and subsequent protonolysis into NH3. In the first setup, E-NRR carried out in aqueous or ethanol environments has been proposed and a recent study demonstrated a Faradaic efficiency (FE) of around 30%, far greater than that of standard E-NRR. In the second approach, cell design is similar to that of Li-air batteries and shows an intriguing possibility, i.e. to avoid the main competitive reaction (hydrogen reduction reaction); in this way, it is possible to reach Faradaic efficiency values higher than 60%.