Combining Theory and Experiment in Electrocatalysis: Insights into Co-Based Materials Design and Application

電催化理論與實踐相結合: 基於鈷基材料的設計和應用的見解

Student thesis: Doctoral Thesis

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Award date20 Jul 2021


Energy is an essential basis for social development. However, with the increasing energy demands and climate change, significant concerns have been raised over the security of our energy future. Developing sustainable, fossil-free pathways to produce fuels plays a substantial role in reducing carbon dioxide emissions and non-renewable energy consumption. One future goal is to develop electrochemical conversion processes that can convert molecules into higher-value products such as water to hydrogen. In water splitting, the electrocatalyst plays a central role in clean energy conversion, enabling continuous hydrogen generation.

In this thesis, Co-based materials applied in water splitting are introduced:
In chapter 1, water splitting, including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), is a promising approach for large-scale and sustainable hydrogen gas (H2) and oxygen gas (O2) production. However, water splitting kinetics is slow and requires noble-metal catalysts such as Pt, RuO2/IrO2 to operate efficiently. On the other hand, non-noble-metals such as Co-based catalysts associate with low cost, abundance, and comparable noble-metal catalytic performance, good structural stability, making them desirable candidates to replace noble metals for water splitting. In this thesis, the recent advance of Co-based catalysts is reviewed comprehensively and the fundamental knowledge of water electrolysis is focused. Besides, several possible research directions to improve the HER/OER performance are discussed. Finally, the existent challenges and directional perspectives for the development of water-splitting electrocatalysts are outlined.

In chapter 2, low-cost, environmentally friendly, and efficient electrocatalysts are required for large-scale and commercial generation of O2 and H2 by the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Herein, composites composed of Co and carbon nanotubes (Co NPs@CNT) are fabricated by one-step chemical vapor deposition (CVD) using commercial Co nanoparticles and benzene as precursors. In the alkaline medium, Co NPs@CNT exhibits a small overpotential of 380 mV at a current density of 10 mA cm-2, a small Tafel slope of 82.2 mV dec-1, and excellent structural and electrochemical durability. The CNTs provide high conductivity and decrease the adsorption energy of OH* on the surface of Co NPs@CNT. The small energy barrier is responsible for the enhanced OER performance of Co NPs@CNT. In addition to the revelation of the synergistic effects between Co and CNTs, our results provide insights into the development of metal-carbon electrocatalysts, and the simple and effective strategy described here has large commercial potential.

In chapter 3, nanoscale non-noble transition metals are crucial to the development of efficient electrocatalysts for water splitting due to the lower cost compared to Pt-based catalysts, natural abundance, excellent activity, and high utilization per nanoparticle. However, inevitable aggregation during the fabrication and electrochemical processes results in unfavorable oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activities and poor stability and durability. Herein, novel ultrafine metallic Co nanodots 2.2 nm in diameter are embedded in the wall of N-doped carbon nanotubes (N-CNTs) grafted on VN to form the hybrid structure of Co/N-CNT/VN by a simple process. Owing to the abundant active sites on the Co nanodots, high electron/mass transfer ability, and good structural and electrochemical durability, Co/N-CNT/VN has outstanding OER and HER properties such as low overpotentials of 240 mV and 63.4 mV at 10 mV cm-2, respectively. For demonstration, the overall water splitting cell is composed of the bifunctional Co/N-CNT/VN catalyst as both electrodes can be driven by a standard 1.5 V AAA battery. The novel concept and materials can be extended to the preparation of CNTs embedded with other ultrafine metal nanoparticles such as Fe, Co, and Ni for energy applications.

In chapter 4, honeycombed Ni3N-Co3N decorated with carbon speckles (Ni3N-Co3N/C) is prepared on nickel foam as a potent, economical, and durable water-splitting catalyst. The Ni3N-Co3N/C system has excellent properties in the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), such as low overpotentials of 320/175 mV at 100 mA cm-2, small Tafel slopes of 55.2/70.2 mV dec-1, as well as excellent stability for over 7 days. To demonstrate the commercial practicality, an overall water splitting cell composed of the bifunctional Ni3N-Co3N/C catalyst as both the anode and cathode is assembled and can be driven by a standard 1.5 V battery. Based on experimental and theoretical results obtained by in situ Raman scattering, ex situ XPS, and density-functional theory, the precise effects of the active sites and conductivity, roles of Ni3N, Co3N, and C, and reaction mechanism in HER and OER, are investigated and described.

In chapter 5, although many Co-based catalysts have been reported with relative high water splitting activity, there are still some inherent drawbacks like poor conductivity, low number of active sites, and unmatched hydrogen/hydroxide/oxide adsorption, making them not comparable to the benchmark of Pt-based materials for HER and IrO2 based catalysts for OER. Hence, in this thesis, we highlight the efforts devoted to structural design and electronic modulation to improve HER/OER performance: (i) building the specific architectures such as the porous structure, hollow structure, nanorods arrays, core-shell structure to expose a large exposed surface area; (ii) improving the utilization efficiency by decreasing the particles’ size to increase active sites; (iii) combining with conductive supports to accelerate transportation of ions and electrons; (iv) tuning electronic configuration and optimize the thermodynamic hydrogen/hydroxide/oxide adsorption/desorption by constructing hybrid structure, heteroatoms doping, designing alloys. In recent years, continuing breakthroughs have been achieved on devising advanced Co-based catalysts such as single Co atoms dispersed high conductive supports, showing comparable activity to the Pt/C. However, the stability of a single atom catalyst remains a significant challenge owing to the possible aggregation or leaching during the catalytic process. Therefore, it is still a long journey to go before water electrolysis could be commercially applied.