Exploring Hydrogen Incorporation into the Nb4AlC3 MAX Phases: Ab Initio Calculations

MAX Phase (Nb4AlC3) For Electrocatalysis Applications

In search for novel materials to replace noble metal-based electrocatalysts in electrochemical energy conversion and storage devices, special attention is given to a distinct class of materials, MAX phase that combines advantages of ceramic and metallic properties. Herein, Nb4AlC3 MAX phase is prepared by a solid-state mixing reaction and characterized morphologically and structurally by transmission and scanning electron microscopy with energy-dispersive X-ray spectroscopy, nitrogen-sorption, X-ray diffraction analysis, X-ray photoelectron and Raman spectroscopy. Electrochemical performance of Nb4AlC3 in terms of capacitance as well as for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is evaluated in different electrolytes. The specific capacitance Cs of 66.4, 55.0, and 46.0 F g-1 at 5 mV s-1 is determined for acidic, neutral and alkaline medium, respectively. Continuous cycling reveals high capacitance retention in three electrolyte media; moreover, increase of capacitance is observed in acidic and neutral media. The electrochemical impedance spectroscopy showed a low charge transfer resistance of 64.76 Ω cm2 that resulted in better performance for HER in acidic medium (Tafel slope of 60 mV dec-1). In alkaline media, the charge storage value in the double layer is 360 mF cm-2 (0.7 V versus reversible hydrogen electrode) and the best ORR performance of the Nb4AlC3 is achieved in this medium (Tafel slope of 126 mV dec-1).

Effects of TiC, Si, and Al on Combustion Synthesis of Ti3SiC2/TiC/Ti5Si3 Composites

The fabrication of Ti3SiC2 from TiC-containing reactant compacts was investigated by combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS). The initial sample composition was formulated based on (3 - x)Ti + ySi + (2 - x)C + xTiC + zAl, with stoichiometric parameters of x from 0 to 0.7, y = 1.0 and 1.2, and z = 0 and 0.1. For all samples studied, combustion was sufficiently exothermic to sustain the reaction in the SHS manner. Due to the dilution effect of TiC, combustion wave velocity and reaction temperature substantially decreased with TiC content. When compared with the TiC-free sample, the TiC-containing sample facilitated the formation of Ti3SiC2 and the TiC content of x = 0.5 produced the highest yield. Excess Si (y = 1.2) to compensate for the evaporation loss of Si during combustion and the addition of Al (z = 0.1) to promote the phase conversion were effective in improving the evolution of Ti3SiC2. All final products were composed of Ti3SiC2, TiC, and Ti5Si3. For the TiC-containing samples of x = 0.5, the weight fraction of Ti3SiC2 increased from 67 wt.% in the sample without extra Si and Al to 72 wt.% in the Si-rich sample of y = 1.2 and further up to 85 wt.% in the Si-rich/Al-added sample of y = 1.2 and z = 0.1. As-synthesized Ti3SiC2 grains were in a thin plate-like shape with a thickness of 0.5-1.0 μm and length of about 10 μm. Ti3SiC2 platelets were closely stacked into a layered structure.