MXenes as Support for Electrocatalysts towards Oxygen Reduction Reaction in Alkaline Media

Due to increasing concerns about the ever increasing energy demand, the global energy system needs to transition away from fossil fuels, as they have been one of the main causes for climate change and air pollution. Fuel cells offer a good alternative to conventional thermal devices, as they usually use hydrogen and oxygen to produce electricity without greenhouse gases emissions. One of the main drawbacks to fuel cell commercialisation is the use of expensive catalysts, such as platinum, and their long-term stability under operation. Therefore, the design and study of cheap and durable catalysts is required to push the development of fuel cells. MXenes, a new family of two-dimensional transition metal carbides and nitrides materials, have shown some potential as support for ORR catalysts. However, the performance of electrocatalysts is directly related to the available surface where the electrochemical reaction could take place, and MXenes have pretty low specific surface areas compared to conventional supports used in the field of catalysis. This thesis reports the development of a synthesis method focused on improving the final catalyst surface area using MXene (Ti3C2) as support, by intercalating the precursors between the MXene layers before annealing. This is shown to lead to the formation of porous MXene structures, with increased surface area and decomposition products from the precursors on the surface of the MXene layers, which hindered the MXene layer restacking during thermal treatment. The samples where then tested in a three-electrode setup using the RDE method to evaluate the ORR performance of the synthesized materials. It was found that the addition of the precursors improved the overall performance of the materials. The different samples were also characterized to understand the connection between physico-chemical properties and electrochemical properties

Experimental investigation of energy storage properties and thermal conductivity of a novel organic phase change material/MXene as A new class of nanocomposites

Energy storage is a global critical issue and important area of research as most of the renewable sources of energy are intermittent. In this research work, recently emerged inorganic nanomaterial (MXene) is used for the first time with paraffin wax as a phase change material (PCM) to improve its thermo-physical properties. This paper focuses on preparation, characterization, thermal properties and thermal stability of new class of nanocomposites induced with MXene nanoparticles in three different concentrations. Acquired absorbance (UV-Vis) for nanocomposite with loading concentration of 0.3 wt.% of MXene achieved ~39% enhancement in comparison with the pure paraffin wax. Thermal conductivity measurement for nanocomposites in a solid state is performed using a KD2 PRO decagon. The specific heat capacity (cp) of PCM based MXene is improved by introducing MXene. The improvement of cp is found to be 43% with 0.3 wt.% of MXene loaded in PCM. The highest thermal conductivity increment is found to be 16% at 0.3 wt.% concentration of MXene in PCM. Decomposition temperature of this new class of nanocomposite with 0.3 wt.% mass fraction is increased by ~6%. This improvement is beneficial in thermal energy storage and heat transfer applications

Pruning Random Forest with Orthogonal Matching Trees

In this paper we propose a new method to reduce the size of Breiman's Random Forests. Given a Random Forest and a target size, our algorithm builds a linear combination of trees which minimizes the training error. Selected trees, as well as weights of the linear combination are obtained by mean of the Orthogonal Matching Pursuit algorithm. We test our method on many public benchmark datasets both on regression and binary classification and we compare it to other pruning techniques. Experiments show that our technique performs significantly better or equally good on many datasets 1. We also discuss the benefit and shortcoming of learning weights for the pruned forest which lead us to propose to use a non-negative constraint on the OMP weights for better empirical results

Pillared Mo2TiC2 MXene for High-Power and Long-life Lithium and Sodium-ion Batteries

In this work, we apply an amine-assisted silica pillaring method to create the first example of a porous Mo2TiC2 MXene with nanoengineered interlayer distances. The pillared Mo2TiC2 has a surface area of 202 m2 g-1, which is among the highest reported for any MXene, and has a variable gallery height between 0.7 and 3 nm. The expanded interlayer distance leads to significantly enhanced cycling performance for Li-ion storage, with superior capacities, rate capabilities and cycling stabilities in comparison to the non-pillared version. The pillared Mo2TiC2 achieved capacities over 1.7 times greater than multilayered MXene at 20 mA g-1 (≈ 320 mAh g-1) and 2.5 times higher at 1 A g-1 (≈ 150 mAh g-1). The fast-charging properties of pillared Mo2TiC2 are further demonstrated by outstanding stability even at 1 A g-1 (under 8 min charge time), retaining 80% of the initial capacity after 500 cycles. Furthermore, we use a combination of spectroscopic techniques (i.e. XPS, NMR and Raman) to show unambiguously that the charge storage mechanism of this MXene occurs by a conversion reaction through the formation of Li2O. This reaction increases by 2-fold the capacity beyond intercalation, and therefore, its understanding is crucial for further development of this family of materials. In addition, we also investigate for the first time the sodium storage properties of the pillared and non-pillared Mo2TiC2.</p

Pillared Mo2TiC2 MXene for high-power and long-life lithium and sodium-ion batteries

In this work, we apply an amine-assisted silica pillaring method to create the first example of a porous Mo2TiC2 MXene with nanoengineered interlayer distances. The pillared Mo2TiC2 has a surface area of 202 m2 g-1, which is among the highest reported for any MXene, and has a variable gallery height between 0.7 and 3 nm. The expanded interlayer distance leads to significantly enhanced cycling performance for Li-ion storage, with superior capacities, rate capabilities and cycling stabilities in comparison to the non-pillared version. The pillared Mo2TiC2 achieved capacities over 1.7 times greater than multilayered MXene at 20 mA g-1 (≈ 320 mAh g-1) and 2.5 times higher at 1 A g-1 (≈ 150 mAh g-1). The fast-charging properties of pillared Mo2TiC2 are further demonstrated by outstanding stability even at 1 A g-1 (under 8 min charge time), retaining 80% of the initial capacity after 500 cycles. Furthermore, we use a combination of spectroscopic techniques (i.e. XPS, NMR and Raman) to show unambiguously that the charge storage mechanism of this MXene occurs by a conversion reaction through the formation of Li2O. This reaction increases by 2-fold the capacity beyond intercalation, and therefore, its understanding is crucial for further development of this family of materials. In addition, we also investigate for the first time the sodium storage properties of the pillared and non-pillared Mo2TiC2