Abrams, Billie

Chorkendorff, Ib

Hansen, Ole

Herbst, Konrad

Pedersen, Thomas

Seger, Brian

Vesborg, Peter Christian Kjærgaard

Journal of The Electrochemical Society

Brian Seger

Konrad Herbst

Thomas Pedersen

Billie Abrams

Peter C. K. Vesborg

Ole Hansen

Ib Chorkendorff

Crossref

Mo3S4Clusters as an Effective H2Evolution Catalyst on Protected Si Photocathodes

This work shows how a molecular Mo3S4 cluster bonded to a photoelectrode surface via a phosphonate ligand can be a highly effective co-catalyst in photocathodic hydrogen evolution systems. Using a TiO2 protected n+p Si photocathode, H2 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 &gt; 635 nm), a saturation current of 20 mA/cm2 is achieved from an electrode containing Mo3S4 dropcasted onto a 100 nm TiO2/7 nm Ti/n+p Si electrode

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Downloaded from orbit.dtu.dk on: Dec 20, 2017Mo3S4 Clusters as an Effective H2 Evolution Catalyst on Protected Si PhotocathodesSeger, Brian; Herbst, Konrad; Pedersen, Thomas; Abrams, Billie; Vesborg, Peter Christian Kjærgaard;Hansen, Ole; Chorkendorff, IbPublished in:Journal of the Electrochemical SocietyLink to article, DOI:10.1149/2.0161412jesPublication date:2014Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Seger, B., Herbst, K., Pedersen, T., Abrams, B., Vesborg, P. C. K., Hansen, O., & Chorkendorff, I. (2014).Mo3S4 Clusters as an Effective H2 Evolution Catalyst on Protected Si Photocathodes. Journal of theElectrochemical Society, 161(12), H722-H724. DOI: 10.1149/2.0161412jesH722 Journal of The Electrochemical Society, 161 (12) H722-H724 (2014)Mo3S4 Clusters as an Effective H2 Evolution Catalyst on ProtectedSi PhotocathodesBrian Seger,a Konrad Herbst,b Thomas Pedersen,c Billie Abrams,b Peter C. K. Vesborg,aOle Hansen,c and Ib Chorkendorffa,∗,zaDepartment of Physics, Technical University of Denmark (DTU), DK–2800 Kongens Lyngby, DenmarkbHaldor Topsøe A/S, DK-2800 Kongens Lyngby, DenmarkcDepartment of Micro- and Nanotechnology, Technical University of Denmark (DTU),DK-2800 Kongens Lyngby, DenmarkThis work shows how a molecular Mo3S4 cluster bonded to a photoelectrode surface via a phosphonate ligand can be a highlyeffective co-catalyst in photocathodic hydrogen evolution systems. Using a TiO2 protected n+p Si photocathode, H2 evolutionoccurs 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 theAM1.5 solar spectrum (λ> 635 nm), a saturation current of 20 mA/cm2 is achieved from an electrode containing Mo3S4 dropcastedonto a 100 nm TiO2/7 nm Ti/n+p Si electrode.© The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative CommonsAttribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in anymedium, provided the original work is properly cited. [DOI: 10.1149/2.0161412jes] All rights reserved.Manuscript submitted June 30, 2014; revised manuscript received August 4, 2014. Published August 12, 2014.It is well established that society cannot indefinitely rely on fossilfuels and must switch to a renewable energy source.1 Solar fuelssuch as hydrocarbons from CO2 reduction and H2 from the watersplitting reaction are highly attractive since these molecular productscan be stored indefinitely2 and used directly in the transportationinfrastructure. Photocatalytic water splitting using a 2-photon devicehas the potential for much higher efficiency than a 1-photon device.3,4In a 2-photon device the optimal bandgaps for the 2 photoabsorbersare approximately 1.7 eV and 1.0 eV.3–5 This allows for the largebandgap material to absorb the blue light leaving the red light to beabsorbed by the small bandgap material. Silicon is very promisingas the small bandgap material since it has a bandgap of 1.1 eV. Amajor problem with Si and other small bandgap materials is that theyhave stability problems in a water splitting environment. Recentlyit was shown that depositing a thin film of TiO2 mitigates stabilityissues.6–8 However, the rectifying properties of this film may restrictthe ability to electrodeposit catalysts onto the protected photoabsorbersurface.9,10 Even though the TiO2 protection layer can be detrimentalto certain electrodeposition procedures, it may also open up a newavenue to help improve the stability of molecular catalysts.Previous work showed molecular clusters such as Mo3S4 incom-plete cubane clusters to be quite effective as H2 evolution catalysts,11,12however the materials are also known to be water soluble. This sol-ubility makes it quite difficult to keep these catalysts adsorbed onan electrode surface in a working environment. A previous approachto resolve this issue involved attaching hydrophobic ligands to theclusters.13,14 Unfortunately this approach decreased catalytic activ-ity and stability in air. One of the great difficulties in depositingthese catalysts was the limited freedom in interacting with the Si sur-face due to its extreme propensity to oxidize to SiO2. However withthe emergence of TiO2 as a protection layer, there now is a muchlarger freedom to design molecular catalysts that can interact with theelectrode.In the present work we investigated attaching a (hydrophilic)molecular linker to help bind Mo3S4 clusters to the TiO2 surface.Previous work showed that phosphonate groups are quite effectiveat binding to TiO2 in photocatalytic H2 evolving systems.15 Thusa N-(phosphonomethyl)iminodiacetate (Hpmida)-Mo3S4 cluster wassynthesized and used as a catalyst. Rather than dropcasting the cat-alyst onto the electrodes, we soaked the electrodes in a solution ofthe Mo3S4-Hpmida overnight. We then thoroughly washed off theelectrode with ultrapure water (18 M-cm) to ensure that only boundMo3S4-Hpmida remained on the 100 nm TiO2/7 nm Ti /n+p Si elec-trodes. This was found to give the same performance as dropcasting,∗Electrochemical Society Active Member.zE-mail: ibchork@dtu.dkthus indicating an adsorbed monolayer is all that is needed to max-imize catalytic efficiency. A photoirradiated p-n junction was usedrather than simply a degenerately doped Si to show that potentialphoton absorption from Mo3S4 clusters is not a significant issue forblocking light to the Si nor in the Mo3S4 catalysis of protons to H2.Figure 1 shows a cyclic voltammogram (CV) of red light irradi-ated (λ > 635 nm) 100 nm TiO2/7 nm Ti/n+p Si photocathodes witheither Mo3S4-Hpmida or Pt nanoparticles as catalysts. The electrolytewas 1.0 M HClO4. This material has an onset potential (i.e. at currentdensity of 1 mA/cm2) of +0.32(4) V vs. Reversible Hydrogen Elec-trode (RHE), which is essentially the same as the +0.33(0) V vs. RHEachieved earlier with a MoSx catalyst. However, unlike in our previ-ous work using Mo3S4 clusters with a hydrophobic ligand,13,14 theMo3S4-Hpmida clusters were relatively stable in air, thus no specialprecautions were necessary to use these materials. It was found thatleaving the catalysts in atmospheric conditions for 2 years did resultin a slight decrease (∼30 mV) in activity. See Supporting Informationfor details.Since the Mo3S4-Hpmida clusters are water soluble, there is therisk that they dissolve into solution during operation. In previouswork, there was substantial current loss after less than 10 scans(corresponding to approximately 5 minutes). To test for durability, aFigure 1. Cyclic voltammograms of either Pt or Mo3S4-PMIDA depositedon 100 nm TiO2 / 7 nm Ti/n+p Si electrodes for photocathodic H2 evolution.These catalysts were deposited via adsorption from a solution containing thecatalyst.) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 192.38.67.112Downloaded on 2014-09-04 to IP Journal of The Electrochemical Society, 161 (12) H722-H724 (2014) H723Figure 2. A) Cyclic voltammograms of photoirradiated Mo3S4-Hpmida de-posited on 100 nm TiO2 / 7 nm Ti/n+p Si electrodes initially and after 1 hour.B) Chronoamperometry test of the photoelectrodes used in part A. These cat-alysts were deposited via adsorption from a solution containing the catalyst.The electrode was held at +0.2 V vs. RHE during the test.chronoamperometry test was run. Figure 2A shows a CV before and af-ter a 1 hour chronoamperometry test was ran at a potential of +0.20 Vvs. RHE. Figure 2B shows the corresponding 1 hour chronoamper-ometry test. The variations during the 1 hour chronoamperometry testmay be related to mass transfer issues near the catalyst. The CV af-ter 1 hour shows negligible change in catalytic activity. Compared toprevious work with water soluble, air stable catalysts,11 the Hpmidafunctional group greatly stabilized the catalysts. Further long termtesting at more reductive potentials show that some cathodic stabilityissues remain. Since previous results show that TiO2/Ti is stable,6we attribute these losses to catalyst detachment/deactivation. How-ever, re-deposition of the catalyst allowed for a completely regain inperformance (See Supporting Information).Previous research has shown that Mo3S4 entities can easily un-dergo degradation.14,16 To investigate if this could potentially be anissue, the Mo3S4-Hpmida catalyst/100 nm TiO2/7 nm Ti/n+p Si wasphotoirradiated and cycled to highly oxidative potentials as shown inFigure 3. The scan before and after the oxidation cycle show no dif-ference indicating that anodic corrosion does not occur on the photoelectrode. This inability for the 100 nm TiO2/7 nm Ti/n+p Si to cor-rode the Mo3S4 was actually expected due to the TiO2 layer forminga depletion layer, which blocks electron transfer.9 Previous researchhas shown that when using moderately doped TiO2, anodic electrontransfer cannot occur at potentials much more anodic H+/H2 redoxpotential.9 In Figure 3 the 5 nm Pt/n+p Si shows anodic current dueFigure 3. Cyclic voltammograms of Mo3S4-PMIDA deposited on 100 nmTiO2 / 7 nm Ti/n+p Si electrodes. These CV’s show that the photoelectrodecan go to corrosive potentials without corroding the catalyst due to TiO2 actingas a blocking layer. These catalysts were deposited via dropcasting.to H2 oxidation, however Mo3S4 cannot oxidize H2, thus no current isseen for the Mo3S4-Hpmida/100 nm TiO2/7 nm Ti/n+p Si sample.16It should be noted that different Si wafers were used in Figure 1and Figure 3. Each Si wafer has a slightly different photovoltage,thus accounting for the small difference in onset potentials betweenFigure 1 and Figure 3. In Figure 1 and Figure 3 the electrode with Ptand the electrode with Mo3S4-Hpmida came from the same wafer sotheir photovoltages should be equivalent.While Figure 3 shows that catalyst oxidation is highly unlikely dueto blocking action of the TiO2 layer, there are other potential degrada-tion mechanisms which the catalyst may suffer. In a commercial de-vice, catalysts may degrade by poisoning via electrolyte impurity, me-chanical instability, oxidation from oxygen crossover, or many othermechanisms. Thus regeneration/redeposition of the catalyst may bean important process that needs to be taken into consideration. Thesemolecular Mo3S4-Hpimda clusters have the advantage over the typicalnanoparticle catalysts in that they are soluble in water, and will adsorbto the TiO2 surface of the composite photocathode. Thus deposition(and redeposition) is as simple as soaking/wash-coating the photo-cathode in a solution of catalyst. Nanoparticles must contend withdispersion issues, thus adding an extra step for optimization. Whileelectrodeposition or photoelectrodeposition also typically uses watersoluble precursors, an electrical bias needs to be applied, thus addinganother step to this procedure as well. While optimization of bothnanoparticles and electrodeposited particles is possible, the simplicityof having an aqueous solution of a catalyst adsorb to the semiconduc-tor surface gives molecular catalysts such as cubane clusters a distinctadvantage of simplicity.In summary, this work demonstrates that a Mo3S4-Hpmida/100nm TiO2/7 nm Ti/n+p Si composite photocathode can produce anonset of +0.32(4) V vs. RHE for H2 evolution. Importantly, wedemonstrate that anchoring the cubane catalyst to the electrode viaphosphonate-TiO2 gives a very significant improvement in the stabil-ity of this efficient, molecular H2 evolution catalyst, although roomfor improvement remains. Future work will focus on analyzing otherpotential deactivation mechanisms and designing the photocathodicsystem to eliminate them.ExperimentalFor anchoring on hydroxylated TiO2 surfaces, the Mo3S4 clusterwas derivatized with N-(phosphonomethyl)iminodiacetate (Hpmida)ligands. A suspension of the free acid H4pmida in water was depro-tonated by 3 molar equivalents of NaHCO3 leading to dissolutionof the ligand. Dropwise addition of this mixture into an aqueous) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 192.38.67.112Downloaded on 2014-09-04 to IP H724 Journal of The Electrochemical Society, 161 (12) H722-H724 (2014)MoMoMoSSSSNOO COCONNOOOONa5POOOHFigure 4. Plot of the Na5[Mo3S4(Hpmida)3] catalyst structure used in thiswork as an H2 evolution catalyst.solution of [Mo3S4(H2O)9]Cl4 led to a light green solution.17 Fromthis solution the cluster Na5[Mo3S4(Hpmida)3] was isolated usingslow crystallization after addition of 0.5 vol-eq. of ethanol to theaqueous mixture.The formation of the new cluster is shown in the reaction schemebelow, and simplified formula plots are shown in Figure 4.H4pmida + 3 NaHCO3 → Na3(Hpmida) + 3 H2O + 3 CO2 [1]3 Na3(Hpmida) + [Mo3S4(H2O)9]Cl4→ Na5[Mo3S4(Hpmida)3] + 4 NaCl [2]The electrodes were made and tested in a manner similar toour previous work.9 The exact details are given in the supportinginformation.AcknowledgmentWe gratefully acknowledge the Danish Ministry of Science forfunding the CAtalysis for Sustainable Energy (CASE) initiative, theDanish National Research Foundation for funding The Center forIndividual Nanoparticle Functionality-CINF (DNRF54).References1. P. V. Kamat, Journal of Physical Chemistry C, 111, 2834 (2007).2. N. S. Lewis and D. G. Nocera, Proceedings of the National Academy of Sciences ofthe United States of America, 103, 15729 (2006).3. S. Haussener, C. Xiang, J. M. Spurgeon, S. Ardo, N. S. Lewis, and A. Z. Weber,Energy & Environmental Science, 5, 9922 (2012).4. M. F. Weber and M. J. Dignam, International Journal of Hydrogen Energy, 11, 225(1986).5. L. C. Seitz, Z. Chen, A. J. Forman, B. A. Pinaud, J. D. Benck, and T. F. Jaramillo,ChemSusChem, 7, 1372 (2014).6. B. Seger, D. S. Tilley, T. Pedersen, P. Vesborg, O. Hansen, M. Gra¨tzel, andI. Chorkendorff, RSC Advances (2013).7. A. Paracchino, N. Mathews, T. Hisatomi, M. Stefik, S. D. Tilley, and M. Gratzel,Energy & Environmental Science, 5, 8673 (2012).8. Y. W. Chen, J. D. Prange, S. Duehnen, Y. Park, M. Gunji, C. E. D. Chidsey, andP. C. McIntyre, Nature Materials, 10, 539 (2011).9. B. Seger, T. Pedersen, A. B. Laursen, P. C. K. Vesborg, O. Hansen, andI. Chorkendorff, Journal of the American Chemical Society, 135, 1057 (2013).10. C. G. Morales-Guio, S. D. Tilley, H. Vrubel, M. Gratzel, and X. Hu, Nat Commun, 5(2014).11. T. F. Jaramillo, J. Bonde, J. D. Zhang, B. L. Ooi, K. Andersson, J. Ulstrup, andI. Chorkendorff, Journal of Physical Chemistry C, 112, 17492 (2008).12. J. Kibsgaard, T. F. Jaramillo, and F. Besenbacher, Nat Chem, 6, 248 (2014).13. Y. Hou, B. L. Abrams, P. C. K. Vesborg, M. E. Bjorketun, K. Herbst, L. Bech,B. Seger, T. Pedersen, O. Hansen, J. Rossmeisl, S. Dahl, J. K. Norskov, andI. Chorkendorff, Journal of Photonics for Energy, 2 (2012).14. Y. Hou, B. L. Abrams, P. C. K. Vesborg, M. E. Bjorketun, K. Herbst, L. Bech,A. M. Setti, C. D. Damsgaard, T. Pedersen, O. Hansen, J. Rossmeisl, S. Dahl,J. K. Norskov, and I. Chorkendorff, Nature Materials, 10, 434 (2011).15. E. Reisner, D. J. Powell, C. Cavazza, J. C. Fontecilla-Camps, and F. A. Armstrong,Journal of the American Chemical Society, 131, 18457 (2009).16. J. Bonde, P. G. Moses, T. F. Jaramillo, J. K. Norskov, and I. Chorkendorff, FaradayDiscussions, 140, 219 (2008).17. T. Shibahara, M. Yamasaki, G. Sakane, K. Minami, T. Yabuki, and A. Ichimura,Inorganic Chemistry, 31, 640 (1992).) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 192.38.67.112Downloaded on 2014-09-04 to IP 

Mo3S4 Clusters as an Effective H2 Evolution Catalyst on Protected Si Photocathodes

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Mo<sub>3</sub>S<sub>4</sub> Clusters as an Effective H<sub>2</sub> Evolution Catalyst on Protected Si Photocathodes

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