Mo₃S₄Clusters as an Effective H₂Evolution Catalyst on Protected Si Photocathodes

Abstract

This work shows how a molecular Mo 3 S 4 cluster bonded to a photoelectrode surface via a phosphonate ligand can be a highly effective co-catalyst in photocathodic hydrogen evolution systems. Using a TiO 2 protected n + p Si photocathode, H 2 evolution occurs with an onset of +0.33 V vs. RHE in an acid solution with this precious metal-free system. Using just the red part of the AM1.5 solar spectrum (λ > 635 nm), a saturation current of 20 mA/cm 2 is achieved from an electrode containing Mo 3 S 4 dropcasted onto a 100 nm TiO 2 /7 nm Ti/n + p Si electrode. It is well established that society cannot indefinitely rely on fossil fuels and must switch to a renewable energy source.1 Solar fuels such as hydrocarbons from CO 2 reduction and H 2 from the water splitting reaction are highly attractive since these molecular products can be stored indefinitely 2 and used directly in the transportation infrastructure. Photocatalytic water splitting using a 2-photon device has the potential for much higher efficiency than a 1-photon device. 3,4In a 2-photon device the optimal bandgaps for the 2 photoabsorbers are approximately 1.7 eV and 1.0 eV. [3][4][5] This allows for the large bandgap material to absorb the blue light leaving the red light to be absorbed by the small bandgap material. Silicon is very promising as the small bandgap material since it has a bandgap of 1.1 eV. A major problem with Si and other small bandgap materials is that they have stability problems in a water splitting environment. Recently it was shown that depositing a thin film of TiO 2 mitigates stability issues.6-8 However, the rectifying properties of this film may restrict the ability to electrodeposit catalysts onto the protected photoabsorber surface.9,10 Even though the TiO 2 protection layer can be detrimental to certain electrodeposition procedures, it may also open up a new avenue to help improve the stability of molecular catalysts.Previous work showed molecular clusters such as Mo 3 S 4 incomplete cubane clusters to be quite effective as H 2 evolution catalysts, 11,12 however the materials are also known to be water soluble. This solubility makes it quite difficult to keep these catalysts adsorbed on an electrode surface in a working environment. A previous approach to resolve this issue involved attaching hydrophobic ligands to the clusters. 13,14 Unfortunately this approach decreased catalytic activity and stability in air. One of the great difficulties in depositing these catalysts was the limited freedom in interacting with the Si surface due to its extreme propensity to oxidize to SiO 2 . However with the emergence of TiO 2 as a protection layer, there now is a much larger freedom to design molecular catalysts that can interact with the electrode.In the present work we investigated attaching a (hydrophilic) molecular linker to help bind Mo 3 S 4 clusters to the TiO 2 surface. Previous work showed that phosphonate groups are quite effective at binding to TiO 2 in photocatalytic H 2 evolving systems.15 Thus a N-(phosphonomethyl)iminodia...

Artificial photosynthesis: opportunities and challenges of molecular catalysts

“…The performance of the Mo 3 S 4 -cluster-modified p-Si photocathode was dramatically enhanced by introducing a 100 nm protective coating of TiO 2 to the p-Si photocathode and using a covalent grafting method to immobilize Mo 3 S 4 clusters as the surface HER catalyst. 530 The Reisner group has also developed several p-Si photocathodes with the modification of Ni-based HECs. [531][532][533] It is worth mentioning that one of those p-Si photocathodes is integrated with the [NiFeSe]-hydrogenase to form an enzyme-material hybrid device, 533 which is also known as a semiartificial photosynthetic system.…”

Recent Development in Hydrogen Evolution Reaction Catalysts and Their Practical Implementation

“…28 Changing ligands on the cubanes since resulted in increased stability against dissolution 29 and recently also increased stability against oxidation by O 2 . 30 Recently, the Jaramillo group reported good results using Mo 4 S 13 clusters exhibiting one of the highest per site activity for any molybdenum sulfide material. 31 The Hu group took another approach in which they cyclically reduced and oxidized a molybdenum sulfide precursor to electrodeposit a highly porous and sufficiently conductive network of molybdenum sulfide.…”

Electrocatalytic properties of {Mo3S4}-based complexes with regard to the hydrogen evolution reaction and application to PEM water electrolysis

“…They can be used to develop strategies for catalyst's immobilization onto surfaces, or for tuning physical-chemical properties of the {Mo 3 S 4 } core according to the redox potential of the semi-reaction of interest. 54 Herein, we report on {Mo 3 S 4 }based hybrid electrocatalyst using alkyl b-dikenotate (noted acac-R) as bidentate ligands. The HER electrocatalytic performances of this hybrid catalyst have been investigated in both homogeneous and heterogenous phase, with the goal to design the first {Mo 3 S 4 }-based PEM (proton exchange membrane) cathode for water electrolysis.…”