Amorphous alloy : a potential electrocatalyst in water splitting

• Main Author: Cai, Weizheng
• Other Authors: Liu Bin
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Bioengineering
• Online Access: https://hdl.handle.net/10356/143590
• Institution: Nanyang Technological University

Catalytic water splitting to produce hydrogen gas driven by renewable electricity is regarded as one of the most promising methods for the sustainable energy conversion, but its efficiency is greatly restricted due to the high overpotential required for the anodic water oxidation half reaction. To accelerate the sluggish four-electron transfer kinetics for the oxygen evolution reaction (OER), it has attracted numerous attention on developing earth-abundant materials as efficient electrocatalysts. Amorphous catalysts are reported to have better activities of water oxidation than their crystalline counterparts, but little is known about the underlying origin, which retards the development of high-performance amorphous oxygen evolution reaction (OER) catalysts. In the first work, to gain an in-depth understanding of the structure-performance relationship for OER on amorphous alloy catalysts, we present a room temperature wet-chemical technique to prepare an amorphous nickel-iron alloy catalyst for water oxidation. And crystalline nickel-iron alloy catalysts without changing composition can be obtained via thermal annealing of amorphous catalyst in N2 atmosphere. Through theoretical analysis including cyclic voltammetry and electrochemical impedance spectroscopy studies, and following by experimental methods of isotope (18O) labeling studies and in situ X-ray absorption spectroscopy (XAS) on both amorphous and crystalline nickel-iron alloy catalysts, it was demonstrated that the amorphous nickel-iron alloy catalyst could be electrochemically activated to expose active sites by applying a positive anodic potential because of the short-range ordering of the amorphous structure. This process greatly increased the number of active sites and thus significantly improved the water oxidation activity. Additionally, the amorphous nickel-iron alloy catalyst with Ni to Fe atomic ratio of 3:1 could reach a water oxidization current density of 10 mA/cm2 at an overpotential of 265 mV (iR corrected) on a glassy carbon electrode, which is 100 mV smaller as compared to the crystalline counterpart. Additionally, by coating the as-prepared amorphous nickel-iron alloy catalyst onto a nickel foam, long term OER stability in 1 M KOH at 80 oC and 500 mA/cm2 could be achieved, rendering the amorphous nickel-iron alloy catalyst practically feasible for high performance water electrolysis. To gain an in-depth understanding of the electronic-performance relationship for OER on amorphous alloy catalysts, we develop the amorphous multimetal alloy catalyst with different electronic structures toward oxygen evolution reaction. In the second work, we provide a room temperature solution technique to synthesis the homogeneously dispersed, amorphous multimetal alloy catalysts. Based on analysis of HAADF-STEM, XPS valence band, electrochemical simulation, and using methanol as a probing molecule to measure binding energy, we success to modulate the 3d electronic structure of amorphous multimetal alloy and thus tune the adsorption energies for oxygenated intermediates. It is demonstrated that near optimal adsorption energy is achieved for oxygenated intermediates by adding high-valence metal (molybdenum) resulting in significantly improved water oxidation activity. Additionally, the amorphous nickel-iron-molybdenum alloy catalyst could reach a water oxidization current density of 10 mA/cm2 at an overpotential of 220 mV on a glassy carbon electrode, which is the best OER catalyst as compared to recently reported works. Additionally, by coating the as-prepared amorphous nickel-iron-molybdenum alloy catalyst onto nickel foam as anode coupled with a crystalline nickel boron catalyst as the hydrogen-evolving cathode, long term OER stability at 80 oC in 6 M KOH and 500 mA/cm2 could be achieved with input voltage around 1.48 V (1.63 V in 1 M KOH at room temperature), making the amorphous nickel-iron-molybdenum alloy catalyst practically feasible for high performance water electrolysis. In this thesis, we revealed the mechanism of amorphous catalyst outperforming much better OER performance than the crystalline counterpart using both in situ and ex situ measurements, demonstrating that the amorphous nickel iron alloy catalyst could be electrochemically activated to expose active sites under an anodic potential due to the short-range ordering structure. And we further successfully designed the OER catalyst, which showed the highest activity over the world, based on the deep comprehension of electronic-performance relationship for OER on the amorphous alloy.