In joint research with Dr. Chang Deuck BAE, a team led by Prof. Hyun Jung SHIN proposed a new catalyst producing hydrogen from water.

In principle, electrolysis of water into oxygen and hydrogen can offer a clean, renewable energy resource. The cost-effective and efficient splitting of water is a critical issue in technologies and economies with energy delivery systems that use hydrogen, enabling zero emission of greenhouse gases. A key challenge in the electrocatalyst design is the performance-cost trade-off of using the platinum-group metals as a cathode for the hydrogen evolution reaction (HER). Two different approaches have constituted the major branches of science in this domain of research. One approach is to use less or optimized amounts of precious elements, such as nanoparticulate Pt on carbon supports. The other approach is to find new alloy compositions based on Pt. In exploring new alloy composition, computational simulations are beneficial in the development of previously unknown single-phase systems. Although more complex surface structures involving platinum have exhibited experimental improvements, efficient electrocatalysts consisting of nonprecious elements have recently been more actively studied. Transition metal chalcogenide–based systems, such as MoS2, provide many possibilities for HER because they have unique anisotropic surface/transport properties and provide surfaces with the desired binding energy/site for H+. However, a material equivalent to platinum in terms of the onset potential, the Tafel slope, and the long-term stability has not yet been developed.

The activation processes on the catalyst surfaces were believed to be the essential mechanisms for HER, and thus, surface energies matter in the development of HER catalysts with high efficacy. This would be true as for the case of monometallic Pt, and seeking platinum-like surfaces seems rational among cost-effective elements. Recent research activities have been heading in this direction accordingly. However, when nonmetallic catalysts are involved, the charge transfer resistance rather than surface reactions themselves would play a significant role and should be considered.

A recent study by Voiry et al. experimentally exhibited that the catalytically inert basal plane of 2H MoS2 can be active by controlling the charge transfer resistance of the system. Beyond identifying platinum-like surfaces, the research team proposes here a novel electrocatalytic concept consisting of two-dimensional (2D) materials in bulk; they refer to this concept as the inorganic bulk layered heterojunction (BLHJ). Our approach is based on a simple fabrication technique for the direct growth of MoS2 on self-supported metallic substrates via sequential gas-phase reactions, which result in the spontaneous formation of dense BLHJ structures via spontaneous sulfidation reactions. These structures contain MoS2 flakes, which are dispersed in the Cu2S matrix (resembling “straw in mud plaster”). The present system not only features distinctive inorganic, dense BLHJ structures, which are difficult to prepare experimentally using any other methods, but also has layered 2D materials as the key anisotropic components that trigger unusual charge transfer processes.

The team selected MoS2 as the representative 2D component with the feasibility of anisotropic transport properties. Incorporating the target layered materials into the bulk chalcogenide host with secure contact interfaces between suitable nanoscale junctions in a controlled manner is difficult. They used the sequential gas-phase surface reaction technique for which the reactants, such as Mo or S, are independently delivered into the substrates, so that only the surface-limited reactions occur. Because the substrate metal used for the self-supporting electrode is simultaneously sulfidizable, this growth mode is expected to incorporate a layered system into the bulk chalcogenide host. In the design of the BLHJ, another important consideration is to select two chalcogenide systems that are thermodynamically immiscible at given temperatures. For example, copper as the substrate material and Mo for sulfidation will lead to the desired immiscible phase separation to form an inorganic BLHJ with MoS2 at relatively lower temperatures (<300°C). In addition to the formation of thermodynamically immiscible systems, it is Earth-abundant and cheap. Moreover, the quality of Cu as the electrochemical electrode has been verified in the field of battery, and thus, the possible side effects could be ruled out.

This work was published in the online version of Science Advances as of March 31^st.