High Intensity Ultrasound Processing of AlNi (50 wt.% Ni) Particles for Electrocatalytic Water Splitting

The present thesis is focused on the processing of metal alloy particles with ultrasound of high intensity (HIUS) for potential application in electrocatalytic water splitting process for hydrogen generation. During ultrasonic treatment of metal particles the changes in bulk (crystallite size, microstrain) and surface (composition, morphology) properties were monitored in order to unravel the fundamental aspects of acoustic cavitation and their effect on sonicated matter as well as to explain the enhancement of electrocatalytic performance of the initially inactive metal alloy catalysts. Through the appropriate choice of sonication medium, concentration of sonicated particles suspension, and duration of the ultrasonic treatment it became possible to provide insights into the phenomenon of cavitation and associated physical (energy transfer, thermal impact, solid state atomic diffusion) and chemical (phase transformations, red-ox reaction) processes. As a result, by adjusting the ultrasound treatment conditions an AlNi based electrocatalyst with significantly improved properties (reduced overpotential, higher current output) toward hydrogen evolution reaction (HER) was fabricated. A novel method for quantitative evaluation of energy transfer between collapsing cavitation bubbles and sonicated matter was developed. The method is based on analysis of crystallographic material parameters using powder X-ray diffraction technique. Upon monitoring of the crystallite sizes of Al3Ni and Al3Ni2 intermetallic phases present in the alloy using the Scherrer and Williamson–Hall methods, it was revealed that a temperature gradient that propagates in sonicated metal particles, triggers atomic diffusion and leads to an increase in crystallites’ sizes and reduction of microstrain in the system. The method proposed here for the evaluation of the impact of cavitation on solids was applied for the estimation of the average minimum temperature (T ̅_particle^min) up to which the particle can be heated. The cavitation induced temperature gradient strongly depends on physical properties of the sonication medium such as vapor pressure and viscosity and increases in the row ethylene glycol < ethanol < water < decane. Furthermore, based on the obtained data it was estimated that the energy transfer from collapsing cavitation bubble to sonicated particle is ~ 17 % more efficient in decane than in ethylene glycol. Simultaneously with solid state atomic diffusion in metal bulk, thermal, mechanical, and chemical impact of cavitation bubbles on the metal surface triggers the phase transformation reactions in a nm-thick interfacial layer. In collaboration with Prof. Dr. Juergen Senker from the department of Inorganic Chemistry III at the University of Bayreuth using 27Al solid state NMR it has been demonstrated that even though the formation of the Al3Ni2 phase on the surface of AlNi alloys is kinetically restricted, collapsing cavitation bubbles heat the surface above 1124 K, triggering the near-surface transformation of the Al3Ni phase into Al3Ni2. Furthermore, in collaboration with Prof. Muthupandian Ashokkumar from the School of Chemistry at the University of Melbourne and having performed the X-ray photoelectron spectroscopy (XPS) studies, it was found that use of a sonication medium such as ethanol or decane promotes the reduction processes on the surface of the treated alloy and, thus, affects the atomic ratio and chemical composition in metal alloys. Gradient changes in phase composition and crystal size that HIUS produced in reductive media (ethanol/decane) lead to significant enhancement of electrocatalytic properties of AlNi alloys. After performing electrochemical test measurements (linear sweep voltammetry (LSV)) it was found that HIUS enables near-surface structuring of AlNi alloy particles toward electrocatalytic HER with significantly improved electrocatalytic properties such as reduced overpotential (η) and increased exchange current density (i0). In particular, it has been shown that HIUS treatment in ethanol results in almost 146-fold increase in i0 as compared to untreated alloy particles, placing sonochemical processing of metals/metal alloys is among the most promising methods for creation of an electrocatalytically active interface for hydrogen evolution. The experimentally determined electrocatalytic activity of the Al3Ni2 intermetallic phase was confirmed by means of density functional theory (DFT) calculations which were performed by Prof. Dr. Stephan Kuemmel from the department of Theoretical Physics at the University of Bayreuth. DFT calculations proved the concept proposed here of beneficial structuring of a catalytically active (Al3Ni2) phase with preferential orientation of the crystal planes (100) in the ultrasonically treated alloys for optimum hydrogen adsorption. The obtained fundamental knowledge was successfully applied on development of materials with significantly enhanced electrocatalytic properties. Thus, with respect to formation of an electrocatalytically active interface, ultrasound treatment satisfies several requirements which are essential for the catalyst to be efficient. First, US treatment leads to overall structuring of HER active phases, namely their growth and exposure on the catalyst surface due to accelerated solid state atomic diffusion caused by the created temperature gradient. Second, through the appropriate choice of the sonication medium it is possible to controllably avoid the formation of high surface area for prevention of hydrogen bubble trapping and associated increased electrolyte ohmic resistance. Additionally, sonication activates the catalyst surface, which is required for the achievement of the necessary hydrogen coverage. These steps may drastically reduce the applied overpotential for the HER process. Overall, seemingly highly undesirable for industrial engineering applications the cavitation phenomenon (acoustically induced) has a great number of positive impacts in the area of catalytic materials formation. In other words, the method of ultrasound treatment is a perfect example of turning initially disadvantageous cavitation effects into highly beneficial ones. Thus, US treatment can be simply considered as a unique “one-pot” surface modification method which opens new prospective for inexpensive earth abundant metals such as aluminum and nickel to be used for fabrication of robust and highly efficient alternatives to platinum as electrocatalyst toward hydrogen evolution.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2015

Author dc:creator

• Cherepanov, Pavel

Contributors dc:contributor

• Andreeva-Bäumler, Daria

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/2521/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:2521