Design and Preparation of Novel Electrocatalysts for Oxygen Evolution Reaction

Abstract

The generation of hydrogen by electrochemical water splitting is a potential route to store energy from intermittent renewable energy sources such as solar or wind. However, the development of efficient and cost effective water splitting technologies is limited by the sluggish kinetics of the anodic reaction, oxygen evolution reaction (OER; 4OH- → O2 + 2H2O + 4ein alkali or 2H2O → O2 + 4H+ + 4ein acid), which can be slow even when state-of-the-art noble metal catalysts such as iridium oxide (IrO2) and ruthenium oxide (RuO2) are applied. This research aims to develop economical, endurable and efficient OER catalysts operated at room temperature.The first strategy investigated in this thesis (Chapter 4) is to prepare porous Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) perovskites via a novel in-situ tetraethoxysilane (TEOS)-templating method. The TEOS formed amorphous silica nanoparticles in a homogenous gel with the BSCF precursor and, when the silica was removed from the calcined BSCF, this template left 3–10 nm wide mesopores in the perovskite. The highest surface area BSCF exhibited a mass-normalized OER activity (35.2 A g-1 @ η= 0.4 V) 5.3 times greater than the nonporous BSCF prepared by conventional synthesis. The mass-normalized activities of the porous BSCFs reported here are comparable with the most active noble metal oxide catalysts.The second study (Chapter 5) aims to develop an evaporation-induced precipitation (EIP) process to synthesise amorphous basic nickel carbonate particles for OER at a temperature as low as 60 °C. The amorphous structure can be tuned by simply adjusting the H2O/Ni ratio in the precursor mixture. The basic nickel carbonate catalysts, which are featured by highly amorphous and hierarchical structures, achieve a mass activity of 51.1 A g−1 at a low overpotential of 0.35 V and a small Tafel slope of 60 mV dec−1, comparing favourably to state-of-the-art RuO2 catalysts. No activity loss was observed during a chronoamperometry test during 10 000 s, indicating outstanding stability under harsh OER conditions. By comparing its performance to conventional crystalline β-Ni(OH)2 and NiOx synthesized by a similar system, we experimentally demonstrate that high OER activities can be achieved with amorphous phases. These results highlighted amorphous catalysts as competitive candidates against crystalline catalysts for water oxidation.The third study (Chapter 6) aims to fabricate boron-doped Ni/Fe nano-chains using a magnetic-field assisted chemical reduction method. Importantly, the synthesis can be performed via a one-step process at room temperature. Boron effectively reduced the magnetic moment of the product, resulting in a high specific surface area of 73.4 m2 g-1. The B-doped Fe/Ni nano-chain also exhibited highly amorphous structure and uniform elemental distribution that are favourable for OER catalysis, leading to a nearly two-magnitude (91.4 times) activity increase from pure FCC Ni nano-chain. The current density of B-doped Fe/Ni nano-chain catalysts compared favourably to the state-of-the-art OER catalysts, BSCF perovskite and Ru(IV) oxide, identifying amorphous metal borides as promising OER catalysts.The fourth study (Chapter 7) aims to prepare a series of amorphous metal borides as OER catalysts by a chemical reduction method. The amorphous borides catalysts are featured by their large specific surface areas and electron-enriched transition-metal sites, and compared favourably against the corresponding crystalline metal oxides, namely spinel, LDH and perovskite, which have been reported as promising structures for OER. More importantly, three amorphous samples, Co1Fe2-AMOR, Co3Fe1-AMOR and Ni3Fe1-AMOR exhibited outstanding OER activities, which transcended that of the state-of-the-art RuO2 catalysts. This indicates that efficient OER catalysts can be developed by using chemical reduction method. The homogeneous and strictly controlled elemental distributions of the amorphous borides were justified by further research into the synthesis of quaternary samples (Ba-Sr-Co-Fe and La-Sr-Co-Fe).