Molecular hydrogen (H₂) 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...[ Read more ]

Molecular hydrogen (H₂) 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.