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 H
2. 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[NO
3@Ag
6O
8],
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, Ag
2O
2, in uniform and well-defined
octahedral shape and tunable sizes. To the best of our knowledge, this is the first time that
Ag
2O
2 with well-defined shape and size could be fabricated. Hollow CoOOH mesoboxss
were prepared by employing Ag
2O
2 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[NO
3@Ag
6O
8] and Ag
2O
2, 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 Au
3+ containing compounds, Au
2O
3/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. MoS
2 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 MoS
2-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.
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