Anion-Redox Mechanism of MoO(S2)2(2,2'-bipyridine) for Electrocatalytic Hydrogen Production.

Redox processes of molybdenum-sulfide (Mo-S) compounds are important in the function of materials for various applications from electrocatalysts for the hydrogen evolution reaction (HER) to cathode materials for batteries. Our group has recently described a series of Mo-S molecular HER catalysts based on a MoO(S2)2L2 structural motif. Herein, reductive pathways of MoO(S2)2bpy (Mo-bpy) (bpy = 2,2'-bipyridine) are presented from both experimental and theoretical studies. We tracked chemical reduction of Mo-bpy with UV-vis spectroscopy using sodium napthalenide (NaNpth) as the reducing agent and found that Mo-bpy undergoes anionic persulfide reduction to form the tetragonal Mo(VI) complex [MoOS3]2-. We also identified silver mercury amalgam as an inert working electrode (WE) for spectroectrochemical (SEC) studies. UV-vis spectra in the presence of trifluoroacetic acid with an applied potential confirmed that Mo-bpy maintains its structure during catalytic cycling. Finally, theoretical catalytic reaction pathways were explored, revealing that Mo=O may function as a proton relay. This finding together with the observed anion reduction as the redox center is of broad interest for amorphous Mo-S (a-MoSx) electrocatalytic materials and anion-redox chalcogel battery materials.

[MoO(S2)2L]1- (L = picolinate or pyrimidine-2-carboxylate) Complexes as MoSx-Inspired Electrocatalysts for Hydrogen Production in Aqueous Solution.

Crystalline and amorphous molybdenum sulfide (Mo-S) catalysts are leaders as earth-abundant materials for electrocatalytic hydrogen production. The development of a molecular motif inspired by the Mo-S catalytic materials and their active sites is of interest, as molecular species possess a great degree of tunable electronic properties. Furthermore, these molecular mimics may be important for providing mechanistic insights toward the hydrogen evolution reaction (HER) with Mo-S electrocatalysts. Herein is presented two water-soluble Mo-S complexes based around the [MoO(S2)2L2]1- motif. We present 1H NMR spectra that reveal (NEt4)[MoO(S2)2picolinate] (Mo-pic) is stable in a d6-DMSO solution after heating at 100 °C, in air, revealing unprecedented thermal and aerobic stability of the homogeneous electrocatalyst. Both Mo-pic and (NEt4)[MoO(S2)2pyrimidine-2-carboxylate] (Mo-pym) are shown to be homogeneous electrocatalysts for the HER. The TOF of 27-34 s-1 and 42-48 s-1 for Mo-pic and Mo-pym and onset potentials of 240 mV and 175 mV for Mo-pic and Mo-pym, respectively, reveal these complexes as promising electrocatalysts for the HER.

Tunable Molecular MoS2 Edge-Site Mimics for Catalytic Hydrogen Production.

Molybdenum sulfides represent state-of-the-art, non-platinum electrocatalysts for the hydrogen evolution reaction (HER). According to the Sabatier principle, the hydrogen binding strength to the edge active sites should be neither too strong nor too weak. Therefore, it is of interest to develop a molecular motif that mimics the catalytic sites structurally and possesses tunable electronic properties that influence the hydrogen binding strength. Furthermore, molecular mimics will be important for providing mechanistic insight toward the HER with molybdenum sulfide catalysts. In this work, a modular method to tune the catalytic properties of the S-S bond in MoO(S2)2L2 complexes is described. We studied the homogeneous electrocatalytic hydrogen production performance metrics of three catalysts with different bipyridine substitutions. By varying the electron-donating abilities, we present the first demonstration of using the ligand to tune the catalytic properties of the S-S bond in molecular MoS2 edge-site mimics. This work can shed light on the relationship between the structure and electrocatalytic activity of molecular MoS2 catalysts and thus is of broad importance from catalytic hydrogen production to biological enzyme functions.

A double-acceptor as a superior organic dye design for p-type DSSCs: high photocurrents and the observed light soaking effect.

Herein, we report three novel single donor double acceptor dyes, BH2, 4, and 6, for use in p-type dye sensitized solar cells (DSSCs). BH4 yields one of the highest photocurrents, 7.4 mA cm(-2), to date. The high performance is achieved via a shorter synthetic route and no exotic materials or cell-building techniques. We suggest a structural principle when building dyes whereby one adopts a double acceptor/single anchor when a triphenylamine moiety is incorporated into a dye for p-type DSSCs. This strategy increases the molar extinction coefficient while simultaneously reducing the number of synthetic steps. The molar extinction coefficients (99 980 M(-1) cm(-1)) reported herein are among the highest reported. Finally, we report the first-ever-observed light soaking effect in p-type DSSCs.