Carbon-Based Electrocatalysts for the Water Splitting and Oxygen Reduction Reactions

Abstract

The development of high-efficiency and sustainable techniques to address the current energy shortage and climate change is extremely urgent. Electrocatalytic water splitting and the oxygen reduction reaction (ORR) are two promising processes for sustainable energy conversion and storage. Carbon-based electrocatalysts with their abundant sources and unique physicochemical properties have been widely investigated as an important catalyst class in the field of electrocatalysis. In this doctoral thesis, three types of carbon-based catalysts, i.e. carbon nanotube (CNT)-encapsulated metal nanoparticles, graphene-coordinated single-atom catalysts (SACs) and graphene-supported single-site molecules are synthesized as three model systems for the investigation of water splitting and ORR. In addition, specific emphasis is also placed on the understanding of formation mechanisms, real active sites, structure-performance correlations, and catalytic reaction pathways.
The first research target presented in the third chapter is a delicate architecture for overall water splitting with NiFe alloys encapsulated into CNTs coupled with graphene nanosheets. The catalyst was synthesized through a g-C3N4 assisted “reduction–nucleation–growth” formation process. This facile and versatile synthetic route was then extended to other transition metal elements, such as Ni, Co, Fe, and their mixed alloys. A wide range of characterization methods combined with density functional theory (DFT) calculations uncovers the synergetic effects between CNTs and metal alloys during the catalytic process of water splitting. This work has also revealed the role of N dopants in the water splitting reaction through combined experiments and theoretical calculations. Their excellent electrocatalytic performance and straightforward synthesis render the CNT-encapsulated architectures quite competitive and promising for large-scale water splitting applications.
Following a similar synthetic protocol as presented in the third chapter, a graphene-coordinated SAC class was further developed via applying glucose as a bifunctional additive in the fourth chapter. A systematic study on the formation mechanism demonstrates the key role of glucose in the dispersion of single metal atoms onto the graphene lattice and its catalytic function in converting g-C3N4 into graphene sheets. The resulting single-site Ni, Co and Fe SACs show excellent ORR performance. Meanwhile, the dual-site NiFe SAC exhibits a dramatic enhancement in the oxygen evolution reaction (OER) performance and undergoes a structure evolution process during OER. Further in situ XAS studies in combination with DFT calculations indicate that the OER follows a dual-site reaction mechanism over the newly formed Ni-O-Fe bonds during OER, which accounts for the enhanced OER performance. This study provides a novel synthesis strategy for the rational design of carbon-based SACs and sheds light on the understanding of the catalytic mechanism of carbon-based SACs toward OER. Based on the understanding of the performance-structure relationship of SAC in the fourth chapter, a coordination-dependent mechanism for OER was further proposed in the fifth chapter. A series of single-site SACs were prepared through a soft-landing strategy by anchoring MPcs on graphene sheets (i.e. MPc-GO (M=Ni, Co, Fe)). The as-prepared SACs show not only outstanding ORR performance but also excellent OER activity. The combination of in situ XAS monitoring and DFT calculations show that the high valence of Fe3+ centers with a lower number of d electrons exhibit the strongest bonding affinity toward O2 molecules, thus endowing them with the highest ORR performance among the series. In contrast, the coordination environment of NiPc-GO and CoPc-GO provides a wider selection of alternative N/C-site OER pathways, resulting in their higher OER activity. This work provides a perspective of understanding the coordination-performance relationship of SACs and offers an alternative and new option to design highly efficient SACs for OER.