The essence of green carbon science is carbon balance and recycling. The underlying chemistry for carbon balance or carbon neutrality is the balance of oxidation and reduction, two opposite and unified reactions. Carbon-based energy molecules are composed of three elements: carbon, hydrogen and oxygen. On the basis of the ternary elemental system, a number of ternary molecular systems can be formed in molecular transformations. The contribution that water can make to the chemistry of oxidation and reduction reactions defines the central role of water in green carbon science. 

In addition to the dissociation of water to form hydrogen and oxygen molecules, green carbon science explores different approaches to using water either as a direct source of hydrogen in reactions or as an activated medium. In catalytic approaches, the mechanistic understanding associated with the coupling of oxygen atoms is critical, and is determined by both extrinsic factors and intrinsic factors. The extrinsic factors refer to those geometrical and electronic factors that help facilitate the approach of oxygen atoms, which are well interpreted by the d-band theory, while the intrinsic factors refer to those geometrical and electronic factors that directly affect the coupling of oxygen atoms, which can be attributed to the p-band theory proposed by the authors. 

In terms of engineering approaches, the understanding of the role of supercritical water and microdroplets, and the efforts to apply them to practical conversion processes, reveals that the interactions and changes in electronic configuration between oxygen atoms occur under specific conditions. Based on this understanding, it is possible to foresee good prospects for the development of new conversion processes. 

Beyond water oxidation, recent research hotspots focus on the synergy or coupling of multiple factors in finding more effective ways to directly use water as a source of hydrogen.