Molecular hydrogen (H
2) is a clean and sustainable energy carrier that has the
potential to meet the ever-growing global energy demands at no environmental cost.
Water splitting is one of the most promising ways to produce hydrogen but it is a
thermodynamically uphill reaction and thus requires external energies such as
electricity to initiate the reaction. Efficient water splitting requires highly active, earth-abundant
and robust catalysts to promote the two half reactions, namely the hydrogen
evolution and oxygen evolution reactions (HER and OER). Cobalt phosphides have
been shown to be active toward HER, especially in acidic medium, and OER in alkaline
medium. However, they show much inferior HER activity in alkaline medium due to
the extra water dissociation step. Thus it...[
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Molecular hydrogen (H
2) is a clean and sustainable energy carrier that has the
potential to meet the ever-growing global energy demands at no environmental cost.
Water splitting is one of the most promising ways to produce hydrogen but it is a
thermodynamically uphill reaction and thus requires external energies such as
electricity to initiate the reaction. Efficient water splitting requires highly active, earth-abundant
and robust catalysts to promote the two half reactions, namely the hydrogen
evolution and oxygen evolution reactions (HER and OER). Cobalt phosphides have
been shown to be active toward HER, especially in acidic medium, and OER in alkaline
medium. However, they show much inferior HER activity in alkaline medium due to
the extra water dissociation step. Thus it is still a challenge to improve the catalytic
activity of cobalt based phosphides for both HER and OER in alkaline solution. My
thesis is focused on the nanohybridization of metal with phosphide for enhancing water
splitting activity in alkaline medium and the mechanism of their performance.
This thesis contains 5 chapters. Chapter 1 mainly introduces electorcatalytic water
splitting, the development of cobalt based phosphides as electrocatalysts,
nanohybridization and the objectives of my thesis. Chapter 2 introduces the techniques
used in my experiments. Major findings of my study are presented and discussed in
chapter 3 and 4. And the conclusions and outlooks are shown in chapter 5.
In chapter 3, to increase the electrocatalytic HER activity of cobalt phosphide in
alkaline solution and figure out the mechanism, I synthesized 3D porous Co-P/Co
nanohybrids nanosheets array grown on carbon cloth. This catalyst showed efficient
HER performance in alkaline medium. It showed a low overpotential (128 mV at 10
mA cm
-2), low Tafel slope (58 mV dec
-1), and prominent electrochemical stability.
Moreover, it exhibited superior activity and stability than Co-P/CC. Most importantly,
we found that cobalt phosphide undergoes a phase change to Co(OH)
2 during HER
process and the hybridized cobalt facilitated this change. This in situ generated
Co(OH)
2 could facilitate water dissociation, which contributed to the good performance.
In chapter 4, to further improve the OER activity of cobalt phosphide, two
strategies were adopted, doping hetero transition metal and nanohybridizing with metal.
Using similar synthesis methods described in chapter 3, I prepared Ni doped Co-P/Co
nanohybrids (Ni-Co-P/NiCo) grown on carbon cloth. With the optimizing doping
amount, Ni/Co ratio of 1/10, this catalyst showed an excellent OER performance with
low overpotential (mV at 10 mA cm
-2), low Tafel slope (mV dec
-1) and excellent
electrochemical stability. It exhibited much better activity than Co-P/Co, which proved
the importance of Ni doping. In addition, it showed similar performance to Ni-Co-P but
superior stability, due to the good electric conductivity resulted from hybridized metal
species.
Chapter 5 gives the summary of my thesis and some outlook about the
nanohybridization of cobalt based phosphides in water splitting field.
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