PhD Dissertation Final Oral Defense (October 2024)

Title of dissertation: Advanced Functional Materials for Electrochemical Hydrogen Generation: From Bulk to Thin Films and Femtosecond Laser Nanostructured High Entropy Alloys

Name of Candidate: Shahbaz Ahmad

Abstract:

The development of non-precious metal-based electrocatalysts has emerged as a critical focus in the scientific community, particularly for enhancing the performance of electrochemical reactions such as the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). This thesis investigates the electrocatalytic properties of multi-principal element medium and high entropy alloys, specifically CoCrNi, CoNiV, and CoNi(Cr/V), utilizing cost-effective transition metals known for their superior mechanical, physical and anticorrosive characteristics. Through systematic synthesis and characterization, we confirmed the single-phase face-centered cubic (FCC) crystal structure of the alloys. Electrochemical experiments revealed remarkable performance, particularly for the CoNi(Cr/V) alloy, which exhibited an operational overpotential of 50 mV at 10 mA/cm² and a Tafel slope of 48 mV/dec, indicating its superior catalytic activity in HER applications.

Further expanding the research, equiatomic alloy thin films of CNioCr, CoNiV, and CoNi(Cr/V) were fabricated using magnetron sputtering and evaluated as bifunctional electrocatalysts for alkaline water splitting. These films were subjected to rigorous electrochemical characterization, which demonstrated that the CoNi(Cr/V) system achieved outstanding performance, with overpotentials remarkably close to those of commercial platinum-based catalysts (Pt/C). The results indicated that these alloys possess excellent corrosion resistance in both acidic and alkaline environments, making them suitable candidates for sustainable energy applications.

In addition, laser structuring was also applied to the CoNi(Cr/V) system, resulting in further enhancement of the electrochemical performance. The laser-structured LS-CoNi(Cr/V) samples demonstrated significant improvements in both HER and OER activities, showcasing lower overpotentials and Tafel slopes compared to their non-structured counterparts. This highlights the dual potential of laser processing in tailoring both the surface characteristics and the electrocatalytic performance of medium entropy alloys.

Furthermore, this thesis also explores the surface engineering of copper (Cu) electrodes through femtosecond (FS) laser processing. This innovative technique facilitated the development of unique surface structures that significantly enhanced the accessibility of active sites for the oxygen evolution reaction (OER). Notably, the LS-Cu-10 sample demonstrated a remarkable improvement in OER activity, achieving a lowest η10 of 345 mV—approximately 70% lower than that of pristine Cu at a current density of 10 mA/cm². The enhanced electrochemical performance can be attributed to the formation of a porous Cu2O layer with a preferred crystalline orientation, coupled with an enlarged electrochemically active surface area. This study underscores the potential of laser processing as an effective method for optimizing the performance of non-precious metal catalysts, further contributing to the advancement of electrocatalytic materials for renewable energy applications.

Overall, the findings of this thesis underscore the potential of multi-principal element alloys and advanced surface structuring methods to develop high-performance, cost-effective electrocatalysts for water splitting applications. The innovative strategies presented herein not only contribute to the fundamental understanding of the electrocatalytic mechanisms involved but also pave the way for future advancements in sustainable hydrogen production. The implications of this research extend beyond the laboratory, offering promising avenues for the practical application of these materials in renewable energy technologies, thus advancing the global transition towards a more sustainable and economically viable energy future.

About the speaker
Name of Candidate: Shahbaz Ahmad; Program: PhD-MSE

Name of supervisor: Dr. Mehmet Egilmez