Advanced nano-array catalysts for electrochemical energy conversion reactions

Abstract

The development of next-generation energy technologies, such as water-alkali electrolysers and rechargeable metal-air batteries, provides a highly desirable path forward sustainable energy by converting electricity derived from renewable sources in the form of chemical energy, thus allowing for reducing the dependence on conventional fossil fuels as well as the greenhouse gases emissions thereof. Typically, such promising energy systems require precious metal based catalysts (e.g. Pt, RuO₂ and IrO₂) to improve the sluggish kinetics of related reaction processes, such as hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, their high cost, low abundance, and poor stability greatly restrict the large-scale practical applications. Therefore, it is of paramount importance to explore earth-abundant, efficient, and robust alternatives. To this end, this thesis aims to design and synthesize a series of self-supported nanoarrays as advanced electrocatalysts for electrochemical energy conversion reactions. The first aspect of this thesis is to identify the pH-dependence of the HER mechanism for a rational design of functional nanocomposite with interfacial synergy. A Ru/MoS₂ hybrid, consisting of Ru nanoparticles modified defect-rich MoS₂ nanosheets arrays with strong interfacial interaction vertically aligned on carbon paper, was assembled via a simple we-chemical method. By a comprehensive comparison of the HER behaviours in alkaline and acidic media, along with HER mechanism analysis based on Tafel slopes, a pH-dependent synergy on Ru/MoS₂ interface was demonstrated, whereby Ru facilitates water dissociation and nearby MoS₂ accelerates hydrogen recombination into molecular hydrogen in alkaline electrolyte. Due to the synergistic effect, the resulting Ru/MoS₂ electrode exhibited superior alkaline HER activity (10 mA cm⁻² at −13 mV in 1.0 M KOH) to commercial 20 wt.% Pt/C catalyst and almost all Ru-based and MoS₂-based electrocatalysts. The second aspect of this thesis focuses on enhancing the conductivity and stability of NiFe-based materials as highly efficient and durable electrocatalysts towards the OER. Well-defined single-crystalline Fe-doped Ni(OH)₂ nanoflake arrays were grown on nickel foam (Fe-Ni(OH)₂/NF) by a facile hydrothermal reaction. It is discovered that the as-fabricated Fe- Ni(OH)₂/NF featured unique physiochemical properties, namely, strong electronic interaction between Fe³⁺ and Ni²⁺ species, good electrical conductivity, and robust structure, in favour of improved OER activity and long-term operation. As a result, it achieved remarkable OER performance in 1.0 M KOH, even outperforming benchmark IrO₂ catalyst. Furthermore, self activation during prolonged test, which reflected by a dramatically reduced overpotential from 267 mV to 235 mV to afford 10 mA cm⁻² after 75 h cycling, was correlated with increased active oxyhydroxide species, making it very attractive for practical application. The third aspect of this thesis aims to develop effective bifunctional electrocatalysts for both the HER and OER in order to realize overall water splitting. NiFe-based oxides are known as one of the best OER electrocatalysts, yet their HER activity is usually unsatisfactory. Combining component manipulation with nanostructure engineering, sulfur-incorporated NiFe₂O₄ nanosheets composed of ultra-small nanoparticles (~2 nm) were deposited on nickel foam through a simple thiourea-assisted electrodeposition. Benefitting from the homogeneous sulphur doping and hierarchical structure, the as-obtained S-NiFe₂O₄/NF electrode showed fascinating OER and HER activities for overall water splitting under alkaline and neutral conditions. Besides, according to previous theoretical studies, NiO is proposed to be a bifunctional promotor for RuO₂ towards alkaline water electrolysis, because potential-induced interfacial synergy between NiO and RuO₂ could maximize the rate of both the OER and HER. Specifically, NiO-derived NiOOH can reinforce the oxygen binding energy for enhanced OER at the anode, and NiO is able to promote water dissociation for improved HER on RuO₂-derived Ru at the cathode. The proof-of-concept studies were carried out by preparing porous nanosheet arrays consisting of strongly coupled NiO and RuO₂ nanoparticles grown on nickel foam. As expected, the as-designed RuO₂/NiO/NF electrode displayed significantly higher OER and HER performance with a cell voltage of 1.5 V to deliver 10 mA cm⁻² in 1.0 M KOH, while the PtC/NF||IrO₂/NF couple needed 1.56 V. Moreover, the self-consistent interpretation of the OER and HER Tafel slopes supported the interfacial bifunctional synergy. These strategies might be extended to a wide range of electrocatalytic systems, thereby opening a new dimension for constructing higher-performance electrocatalysts.