Tuning Surface Structure Of Water Splitting Catalysts For Improved Properties

The development of human society has been blocked by energy crisis and environmental pollution. Seeking for clean and sustainable energy is likely to be an essential approach to cope with these problems fundamentally. Hydrogen, a new generation of green sustainable energy carrier, has been widely regarded as a promising alternative to the gradually exhausted fossil fuels that meet most of the global energy demand nowadays. The water-splitting technology is one of the “Holy Grails” to achieve the sustainable production of hydrogen. Therefore, it is a key issue to develop and design efficient catalysts for water splitting. To date, noble-metal-based catalysts, such as Pt, Ru or Ir oxides, exhibit the most active catalytic activities for both half-reactions, but suffer from high cost and scarcity, which severely impede their wide application. As a result, a current hot spot is to develop cheap but efficient catalysts based on high earth-abundance elements for water splitting. Since the catalytic reactions mainly take place on the surface of catalysts, the catalytic performance is closely related to the surface structure of catalysts. In this work, we focus on the study of the surface structure of water splitting catalysts. Several highly active noble-metal-free catalysts have been successfully designed and developed for water splitting. And what’s more, the relationship between these catalytic materials’ surface structure and their catalytic performance is also investigated.The main contents of this thesis include the following aspects:1. Co₉S₈ nanoparticles（NPs） have been identified as noble-metal-free electrocatalysts toward hydrogen evolution reaction（HER） under neutral media（p H 7）. Co₉S₈@C material is successfully prepared through a carbon-arming strategy to improve the stability and activity of Co₉S₈ NPs over a wider p H range. The resulting Co₉S₈@C material shows high electrocatalytic HER performance, and exhibits nearly 100% Faradaic yield during HER under all-p H（0¹4） conditions. This work provides a new impetus to rational design and development of low-cost and high-performance noble-metal-free water splitting catalysts.2. Based on above research about Co₉S₈, we further find that Co₉S₈ is a metallic metal sulfide with high conductivity. This prompts us to synthesize Co₉S₈ material with hierarchical nanostructures for more surface catalytic sites and better catalytic performance. We prepare metallic spongy-like Co₉S₈ nanosheets in situ grown on 3D flexible carbon cloth（CC） by a facile one-pot solvothermal method. The introduction of a tiny amount of Zn2+ ions（Zn: Co mol ratio of 0.5-1:100） in the synthesis system can reduce the thickness, improve the crystallinity, and optimize the surface structure of Co₉S₈ nanosheets, without Zn-doping. The as-obtained material(synthesized in the presence of Zn²⁺ ions) affords a current density of 10 m A/cm2 at a low overpotential of 175 m V, has great catalytic durability as long as 100 h, and gives nearly 100% Faradaic yield in neutral media（p H 7）. This work gives a new idea for the development of electrocatalyst for HER with high efficiency and high stability under neutral medium.3. We successfully synthesize stable, {（?）10} high-index faceted Ni₃S₂ nanosheet arrays in situ grown on nickel foam（NF） via a facile one-pot hydrothermal method. The resulting material, dubbed Ni₃S₂/NF, can act as a highly active, ultrastable, binder-free bifunctional electrocatalysts for HER and OER. Ni₃S₂/NF is found to give ～100% Faradaic yield toward both HER and OER and to show remarkable catalytic stability（for >200 h）. In addition, {001} low-index faceted Ni₃S₂ nanosheet arrays are obtained by changing the reation time. At last, we identify the fact that the electrocatalytic activity toward HER and OER of the {（?）10}-exposed Ni₃S₂/NF is higher than that of the {001}-exposed Ni₃S₂ nanosheet arrays, and reveal that Ni₃S₂/NF’s superior catalytic performance towards HER and OER is mainly ascribed to the synergistic effect between its nanosheet array architecture and exposed {（?）10} high-index facet. This work is selected as the front cover of J. Am. Chem. Soc. and highlighted by J. Am. Chem. Soc. with a title of “Water Splitting Gets a Boost from Mineral-Based Catalyst”.4. A novel “self-template” route is designed and developed to synthesize successfully nanoporous Sr-rich SrTiO₃（denoted SrTiO₃-1）. The porous structure endows the material with a high surface area, resulting in higher density of surface reactive sites. Besides, a slight substitution of Ti⁴⁺ with excess Sr²⁺ is present on Sr-rich surface of SrTiO₃-1, which can effectively inhibit the formation of Ti³⁺. The experimental results have shown that SrTiO₃-1 exhibits a remarkable photocatalytic activity, which is three times higher than that of non-porous SrTiO₃. What’s more, there is no loss of photocatalytic activity over 60 h cycle test. It is worth mentioning that the synthetic strategy can be extended to the preparation of other porous nanostructured functional materials.