In-situ formation of highly active electrocatalysts for water splitting

Abstract

Water splitting, to produce hydrogen and oxygen, has attracted considerable interest in the past decades, since it has been regarded as a desirable pathway for converting solar energy into clean and sustainable fuel to meet the rising global energy demand and address the environmental problems. The water splitting overall is a two half-reactions process: the water oxidation half-reaction and the proton reduction half-reaction. The sluggish kinetics of first half-reaction, oxygen-evolution reaction (OER), severely hinder the development of innovative energy storage technologies, including electricity- and solar-driven water splitting and rechargeable metal-air batteries. The second half-reaction, hydrogen-evolution reaction (HER), is also significant in water splitting, since it can be used to convert water into chemical fuel H₂. Therefore, the development of highly active OER and HER electrocatalysts is critically significant. This thesis aims at establishing new design principles for OER and HER electrocatalysts by developing novel approaches to address the main issues in both fields. In chapter 1, we gave a brief introduction to the significance of water splitting, the main issues in water splitting, and the possible approaches to address these issues. In this chapter, we proposed that, to identify the intrinsic OER activity of the first-row transition metal oxyhydroxides, an ideal catalytic model system needs to be constructed. The ideal catalytic model system is expected to address two main issues: Firstly, the electrocatalysts should be in their active form, not the inactive precursors; Secondly, the comparability between different catalysts should be guaranteed. We developed a novel approach, which can be named as non-sacrificial galvanic replacement reactions (NS-GRRs), to study the intrinsic OER activities of 3d transition metal oxyhydroxides. For HER, inspired by the working mechanism of the native hydrogenase, we proposed that it is critical to build-in H⁺ transfer networks for designing effective HER electrocatalysts under neutral condition. By in-situ build-in phosphates in a-MoS_x as H⁺ transfer network, a novel and highly active HER catalyst under neutral condition, a-MoS_x-P_i, was developed. In chapter 2, we demonstrated that the most potent oxidative template Ag[NO₃@Ag₆O₈], with well-defined shape and size, could be fabricated electrochemically on the working electrode. Based on this template, the hollow Ni-, Co- and Fe-based pure, binary and ternary oxyhydroxides were synthesized via GRRs. Their electrocatalytic activities toward water oxidation were studied. This method can guarantee that all the as-synthesized electrocatalysts are of structures identical to that under OER condition. The hollow mesoboxes exhibited exceptional OER catalytic properties in the basic electrolyte, with NiFeOOH mesoboxes showed the best catalytic activity with an onset potential of 185 mV, a small Tafel slope of 34 mV dec^-1, and a great stability. The overpotential required to reach 10 mA cm^-2 for NiFeOOH is only 253 mV, which is among the best compared to the state-of-the-art OER electrocatalysts. Besides, we also examined the effect of the impurity of electrolytes to the observed OER activity. We found that trace amount of Fe impurity did have significant effect on the OER` activities of NiOOH and NiCoOOH. In chapter 3, we developed another oxidative template, Ag₂O₂, in uniform and well-defined octahedral shape and tunable sizes. To the best of our knowledge, this is the first time that Ag₂O₂ with well-defined shape and size could be fabricated. Hollow CoOOH mesoboxss were prepared by employing Ag₂O₂ as the oxidative template via GRR. The as-synthesized hollow CoOOH mesoboxes showed remarkable OER catalytic activities. In chapters 2 and 3, we showed that hollow 3d transition metal oxyhydroxides, which are in their active forms, could be synthesized by structurally well-defined ultra-strong oxidative templates, Ag[NO₃@Ag₆O₈] and Ag₂O₂, via GRRs. However, the ideal OER catalytic model system requires that the comparability of different catalysts should be consistent. Unfortunately, OER electrocatalysts prepared via GRR method cannot meet this requirement. In chapter 4, in order to construct the proposed ideal catalytic model system for OER, we demonstrated that a novel synthetic method, named as non-sacrificial galvanic replacement reactions (NS-GRRs), was successfully developed. Employing the powerful oxidative ability of Au³⁺ containing compounds, Au₂O₃/Au(OH)₃, as the oxidative template, an ultrathin layer of amorphous 3d transition metal oxyhydroxides on gold electrode could be fabricated. We found that the as-synthesized 3d transition metal oxyhydroxides were of identical structure compared to that under OER condition. More importantly, the comparability of different metal based catalysts could be guaranteed. Such catalysts meet all the requirements of the ideal catalytic model system, and were used to study their intrinsic catalytic activities in OER. The intrinsic catalytic activity trend for 3d transition metal oxyhydroxides turned out to be Fe > Co > Mn > Ni, where the specific FeOOH/Au exhibited the highest OER activity and the activity of NiOOH/Au was the lowest one. The FeOOH/Au showed exceptional OER catalytic properties with a small Tafel slope of 24 mV dec^-1 and at overpotential of 300 mV. Its mass activity is as high as 8400 A g^-1, which is among the best compared to the state-of-the-art OER electrocatalysts. The activity of FeOOH/Au underwent dramatical decrease under OER condition, whereas the others were stable. The stability test indicates that the poor stability of FeOOH/Au might be one (or the) reason for the low OER activities of all other reported Fe-based electrocatalysts. The large-scale conversion of solar energy into clean and sustainable hydrogen as a fuel through water splitting requires highly active HER electrocatalysts and benign working conditions, especially the neutral pH. MoS₂ has been shown to be a promising material but currently suffers from low reactivity in neutral media. In chapter 5, a novel bio-inspired HER catalyst named a-MoS_x-P_i was designed by incorporating phosphate anions into molybdenum sulphide, giving rise to an internal proton transfer network for faster proton transfer kinetics and consequentially the improved catalytic activity in neutral condition. This catalyst displayed a striking turnover frequency (TOF) of 0.9 s^-1 with a low overpotential of 245 mV whereas the current density reaches 10 mA cm^-2, about 10 times higher in activity than that of amorphous molybdenum sulfide owing to the presence of phosphate anions. This material represents one of the most efficient catalysts so far under neutral condition and is comparable to the state-of-the-art MoS₂-based HER activity in acid. In chapter 6, a brief summary and future perspectives are given on the basis of the studies carried out in this thesis. Finally, in chapter 7, an appendix about CuO nanoparticles as highly active OER electrocatalysts under near-neutral condition is briefly introduced. We found that when the size of CuO was smaller than 45 nm, the structure of CuO might be tetragonal phase. And we also found that the 6 nm CuO nanoparticles showed excellent OER activities.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology