The basal plane of molybdenum disulfide (MoS2) was recently activated for hydrogen evolution reaction (HER) by creating sulfur (S) vacancies (MoS2-x). However, the HER activity of those S-vacancies depends on the concentration of S-vacancies, imposing a dilemma for either improving activity per site or increasing overall active site density. Herein, we use density functional theory (DFT) calculations and experiments to show that the HER activities of MoS2-x are greatly enhanced by adding cobalt (Co) clusters on the basal plane. Our DFT results show that the highest HER activity is achieved when the Co clusters are anchored on the S-vacancies with the interface of Co-Mo as the preferred active site. Our experiments confirm that the addition of Co enhances the activity per unit active site and increases the electrochemical active surface area. These results demonstrate the basal plane activity of MoS2-x can be enhanced by decorating S-vacancies with transition-metal clusters.

Hydrogen peroxide is a valuable chemical oxidant with a wide range of applications in a variety of industrial processes, especially in water sanitization. Electrochemical synthesis of hydrogen peroxide (H2O2) through a two-electron oxygen reduction reaction (2e-ORR) or a two-electron water oxidation reaction (2e-WOR) has emerged as an appealing process for onsite production of this chemically valuable oxidant. On-site produced H2O2 can be applied for wastewater treatment in remote locations or any applications where H2O2 is needed as an oxidizing agent. This Review studies the theoretical efforts in understanding the challenges in catalysis for electrochemical synthesis of H2O2 as well as providing design principles for more efficient catalyst materials.

Shifting electrochemical oxygen reduction towards 2e^- pathway to hydrogen peroxide (H₂O₂), instead of the traditional 4e^- to water, becomes increasingly important as a green method for H₂O₂ generation. Here, through a flexible control of oxygen reduction pathways on different transition metal single atom coordination in carbon nanotube, we discovered Fe-C-O as an efficient H₂O₂ catalyst, with an unprecedented onset of 0.822 V versus reversible hydrogen electrode in 0.1 M KOH to deliver 0.1 mA cm^-2 H₂O₂ current, and a high H₂O₂ selectivity of above 95% in both alkaline and neutral pH. A wide range tuning of 2e^-/4e^- ORR pathways was achieved via different metal centers or neighboring metalloid coordination. Density functional theory calculations indicate that the Fe-C-O motifs, in a sharp contrast to the well-known Fe-C-N for 4e^-, are responsible for the H₂O₂ pathway. This iron single atom catalyst demonstrated an effective water disinfection as a representative application.

Electrochemical synthesis of H₂O₂ through a selective two-electron (2e^-) oxygen reduction reaction (ORR) is an attractive alternative to the industrial anthraquinone oxidation method, as it allows decentralized H₂O₂ production. Herein, we report that the synergistic interaction between partially oxidized palladium (Pd^δ+) and oxygen-functionalized carbon can promote 2e^- ORR in acidic electrolytes. An electrocatalyst synthesized by solution deposition of amorphous Pd^δ+ clusters (Pd₃^δ+ and Pd₄^δ+) onto mildly oxidized carbon nanotubes (Pd^δ+-OCNT) shows nearly 100% selectivity toward H₂O₂ and a positive shift of ORR onset potential by ~320 mV compared with the OCNT substrate. A high mass activity (1.946 A mg^-1 at 0.45 V) of Pd^δ+-OCNT is achieved. Extended X-ray absorption fine structure characterization and density functional theory calculations suggest that the interaction between Pd clusters and the nearby oxygen-containing functional groups is key for the high selectivity and activity for 2e^- ORR.