New strategies for the development of nano-engineered catalysts for Fuel Cells and Electrolysers

The continued depletion of strategic resources like fossil fuels, precious metals and other critical raw materials has put the world economy on the brink of a global crisis. Together with this, the continued emission of large amounts of greenhouse gases represents a significant challenge due to the effects of climate change. It is therefore imperative to find alternatives to fossil fuels that are sustainable, keeping in mind a full life cycle analysis. Clean technologies such as fuel cells and water electrolysis will be a fundamental part of the transition to renewable energy. In this context, a key role will be played by molecular hydrogen as “energy vector”. Thanks to its high specific energy density and clean combustion to water, H2 represents a high-quality energy carrier and an ideal candidate to replace fossil fuels. Importantly, H2 can be produced by water electrolysis and can be converted into electricity using Fuel Cells. However, low conversion efficiencies and high capital investment costs, still limit the use of these electrochemical technologies. The search for sustainable, stable, and active electrocatalysts will play a key role in reaching the performance required for these devices. The development and characterisation of such materials is the subject of the research described in this thesis. The first part of this thesis provides an introduction to the field, including a short overview of key electrochemical concepts, and a definition of the two types of devices that are studied, fuel cells and electrolyzers, and their respective anodic and cathodic reactions (Chapter 2). The synthesis and the chemical-physical characterization of all electrocatalysts is reported in Chapter 3. The electrochemical study of these materials in half-cells and their application in complete devices, are described in detail in Chapters 4, 5 and 6. Chapter 4, describes a study of the effect of metal-CeO2 interactions in carbon supported electrocatalysts on alkaline hydrogen oxidation and evolution reactions. A series of transition metal nanoparticle electrocatalysts (Pd, Ir, Ru and Rh) with a metal loading of 10 wt%, were prepared using two supports; carbon and carbon-CeO2 (50:50). Each material is characterized using XRD, XPS, TEM and electrochemical tests, EIS and tafel analysis is performed in order to understand the HER and HOR activities. The presence of CeO2 enhances the activity of Pd, Ir and Rh. Ruthenium has superior activity in term of mass activity, specific activity and i0, both for HER and HOR. The HOR/HER exchange current (i0) of Ru/C has an average value of 106 A gMetal−1. Importantly, EIS and capacitance measurements show that CeO2 promotes catalyst utilization and lowers ionic resistance. Chapter 5 focuses on developing sustainable electrocatalysts for Anion Exchange Membrane Fuel Cells (AEMFC). In this study a high-performance Pd-CeO2/C hydrogen oxidation reaction (HOR) catalyst is integrated into AEMFCs in combination with different Pt and Pt-free cathodes. A H2/O2 AEMFC peak power performance of 2 W cm–2 at 80 °C is obtained when using a Pt/C cathode (2 A cm–2 is achieved at a cell voltage of 0.6 V), which translates to 1 W cm–2 peak power density (0.7 A cm–2 is achieved at 0.6 V) at 60 °C with the switch to a cheap, critical raw material (CRM)-free Fe/C cathode catalyst. In Chapter 6.1, a molecular catalyst for hydrogen evolution was developed and tested in a Polymer Exchange Membrane (PEM) water electrolyser. The dinuclear Ru diazadiene olefin complex, [Ru2(OTf)(μ-H)(Me2dad)(dbcot)2], is shown to be an active catalyst for hydrogen evolution. When supported on high surface area carbon black and at 80 °C, [Ru2(OTf)(μ-H)(Me2dad)(dbcot)2]@C evolves hydrogen at the cathode of a PEM electrolysis cell (400 mA cm−2, 1.9 V). A remarkable turn over frequency (TOF) of 7800 molH2 molcatalyst−1 h−1 is maintained over 7 days of operation. A series of model reactions in homogeneous media and in electrochemical half cells, combined with DFT calculations, are used to rationalize the hydrogen evolution mechanism promoted by [Ru2(OTf)(μ-H)(Me2dad)(dbcot)2]. Lastly, in Chapter 6.2 the development of a non-precious metal cathode catalyst for Anion Exchange Membrane Water Electrolysis (AEMWE) is reported. This study investigates an active HER catalyst synthesized from MoNiO4 nano-rod arrays on nickel foam using high-temperature reductive annealing. Complete characterization of the nanostructure by SEM, HR-TEM and XPS indicates that during synthesis the crystalline MoNiO4 structure of individual rods segregates a surface enriched polycrystalline MoO2 layer rather than a Ni4Mo alloy as reported previously. Mo and Ni electrochemical dissolution was studied by the scanning flow cell technique coupled with inductively coupled plasma mass spectrometry (SFC-ICP-MS). It was found that only Mo undergoes detectable dissolution phenomena, with the MoO2/Ni cathode prepared at 600°C being the most stable. Tests in an AEMWE with a Ni foam anode demonstrate a current density of 0.55 A cm-2 (2 V) at 60 °C and H2 production was stable for more than 300 h (0.5 A cm-2). The synthesis procedure was scaled up to prepare electrodes with an area of 78.5 cm2 that were employed and evaluated in a three-cell AEM electrolyser stack. In conclusion, the research described in this thesis demonstrates how engineering at the nanoscale can be used to improve the electrocatalytic activity and stability of sustainable catalytic materials for both fuel cells and electrolysers. The work conducted here has also gone beyond the study of materials on a lab scale, describing the scale up and application of electrocatalysts in actual devices.