Energy Production and Storage from Renewable Sources through Electrocatalysis

The energy transition from fossil fuels to renewables will be a long and difficult process but it is not avoidable since fossil sources depletion, pollution and geopolitical situations are becoming worrisome. There are different challenges for researchers in the field of energy. From renewable energy production data it is clear the remarkable growth of solar electricity technologies, that crystalline silicon photovoltaics (PV) and wind turbines are the workhorses of the first wave of renewable energy deployment around the globe. Anyway these renewable energy are not equally distributed and, for their natural discontinuity, cannot sustain the requirements of our society. In this perspective the energy storage will play a fundamental role in the development of an energy system based on renewable sources. The storage of hydrogen as gas in pressured tanks poses many problems, in particular for transport safety concerns. The alternative is the production on demand of this gas by using storing hydrogen materials. These materials can help developing mobility systems based on hydrogen as fuel, since the reserve of hydrogen would be stored as a solution, for example of sodium borohydride. The interconversion between electricity and hydrogen and biofuels, like bioethanol, could be the keystone of a new sustainable energy economy. The development of two electrochemical devices can serve to this energy system, fuel cells and electrolyzers. Fuel cells are devices that can convert cleanly chemical energy stored in hydrogen or bioalcohols into electrical energy, while electrolyzers use electrical energy to produce molecules, which can be energy vectors such as hydrogen. The electrical energy surplus could be used in another kind of electrolyzers, for example for the reduction of CO2. In these devices it’s possible to obtain different products but the performance of electrocatalysts at the moment are not so good to make this electroreduction particularly appealing due to high energy costs, low activity and stability of the catalysts. The focus on this thesis is to face the energy problems by different points of view: 1) the production of electrical energy from renewable resources with Direct Liquid Fuel Cells (DLFCs), 2) the production of hydrogen from storage materials and from electrolysis, 3) the storage of electrical energy producing useful molecules in electrolyzers. During the three years catalysts have been synthesized and characterized for their morphology and activity, both in electrochemical half-cell and complete cell systems or in properly designed reactors. In Chapter 1 a description of the state of the art of fuel cells and electrolyzers is provided and describes the advantages in replacing the traditional proton exchange membrane electrolytes with anion exchange membranes. In Chapter 2 an introduction of the electrochemical reduction of CO2 is given, focusing on copper and palladium as electrode for this reaction. The discussion of the results will be divided in three main Chapters,(3,4,5). The first one, Chapter 3, is on Direct Liquid Fuel Cells. The electrocatalysts employed are nanostructured Pd at the anode and Fe-Co at the cathode. The performance of microscale direct ethanol fuel cells (DEFCs) was studied with an equipment of a membrane electrode assembly (MEA) of 1 cm2 size with four anion exchange membranes. A fuel concentration of 6 M ethanol+6 M KOH was chosen to test the stability at a constant current density of 1 mA cm−2. The cell ran for 87 days with a potential drop of 3 mV day−1, and the energy delivered was 1.08 Wh. Furthermore, in order to explore functioning in conditions that maximize delivered energy density, a direct formate fuel cell (DFFC) operating at different formate and alkali concentrations has been investigated. The active DFFC at 60 °C with 4 M HCOOK + 4 M KOH as anode fuel and O2fed to the cathode produces a maximum power density of 258 mA cm−2. In Chapter 4, for the production of hydrogen the design and construction of a reactor for sodium borohydride (SBH) hydrolysis and the performance of palladium and rhodium based electrocatalysts in a electrochemical reformer are presented. For SBH hydrolysis a cobalt boride (CoxB) catalyst supported on a commercial Cordierite Honeycomb Monolith (CHM) was used. The electrooxidation performances of Pd/C and Rh/C catalysts were studied at different temperature with different organic molecules as substrates. In particular, Pd/C with formate, exhibits an onset of 200 mV and a specific activity of 2100 A g−1Pd; while Rh/C has an excellent activity for methanol oxidation, showing an onset potential 200 mV lower than Pd/C and a specific activity almost double reaching the value of 2600 A g−1Rh. In addition examples of electrooxidation of biomass-derived alcohols such as EtOH, EG, G and 1,2-P have been studied in an electroreformer containing Rh nanoparticles supported on carbon as the anode electrocatalyst, equipped with an anion exchange membrane and a Pt/C on carbon cloth cathode. The oxidation of alcohols was investigated in electrochemical half-cells at room temperature and at 60–80 °C in alkaline media. The results highlighted the excellent activity of Rh/C in terms of peak current densities (as high as 5700 A gRh-1 for EG at 80 °C) and low onset of potentials. Copper and Palladium based catalysts, studied for the electroreduction of CO2, are reported in Chapter 5. To enhance the activity of these two metals for CO2RR, two different paths were followed: 1) copper surface and morphology were modified to vary the products compositions and enhance activity; 2) palladium was alloyed with a different metal, gold, to improve performance and stability. The surface modification on copper resulted in an enhancement of the surface area of a copper foil and influenced the selectivity of the reduction reaction products. The Au-Pd alloy nanoparticles have shown high activity and selectivity for the electroreduction of CO2, where CO was the only product of the electrocatalysis. In the Chapter 6 there are conclusions and considerations on the whole work during the three years. The importance of the nanostructured catalyst is evident in every application described in this work. These materials showed improved performance and their applications can really represent a turning point in the energy field.