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Advanced silicon photoelectrodes for water splitting devices: design, preparation and functional characterization by photo-electrochemistry and high-energy X-ray spectroscopy
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Kemiska institutionen. European Synchrotron Radiation Facility | ESRF - Division of Experiments, Grenoble, France .ORCID-id: 0000-0002-5027-9199
2018 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

For the last century, mankind has been hugely dependent on fossil fuels to meet its energy needs. Harnessing energy from fossil fuels led to the emission of greenhouse gases. Greenhouse gases such as CO2 are a major contributor to global warming. Since the last decade, the global annual average temperature has increased by almost 1 oC, while the annual average temperature of Europe has increased by almost 1.7 oC. It is high time to find an alternative source of energy. Such an energy source must be renewable, sustainable, robust and free of greenhouse gases. Our earth has a non-stop supply of solar energy and water in oceans, harvesting energies from such resources will not only be clean but also inexpensive. Solar fuels such as H2 generated from sunlight and seawater using earth-abundant materials are expected to be a crucial component of a next generation renewable energy mix.

My PhD research was thus focused on the use of solar energy to split water into molecular hydrogen and oxygen, a process that is referred to as ‘artificial photosynthesis’. This can be achieved with the help of semiconductor photocatalysts. As most of the earth crust has a high abundance of silicon (Si), I prepared my semiconductor photoelectrodes using Si. However, Si tends to degrade in an aqueous environment. Thus, my PhD research comprises the synthesis of microstructured Si photoelectrodes and their protection with a TiO2 inter layer followed by functionalization with various earth abundant co-catalysts. The study on the synthesis, morphology and elemental characterization of the photoelectrodes was carried out under the supervision of Prof. Dr. Johannes Messinger at the Chemistry Department of Umeå University. Deep insight on the electronic and atomic structure of the functionalized Si photoelectrodes was obtained by careful experiments at the European Synchrotron Radiation Facility (ESRF) under the supervision of Dr. Pieter Glatzel. I investigated the electronic and geometric structural properties of my photocatalysts using inner shell electron spectroscopy, which is also referred to as ‘X-ray spectroscopy’. Thus, my PhD thesis falls under the broad title of “Artificial Photosynthesis and X-Ray Spectroscopy”.

 With the motivation of developing a bias free photoelectrochemical device for overall water splitting, I first developed cost effective earth abundant photocathodes. The experimental data and detailed analysis of the photocathodes are presented in Paper I. The best photocathode obtained in Paper I (p-Si/TiO2/NiOx) was then coupled with a well-studied FTO/α-Fe2O3 photoanode in parallel-illumination mode. The two most significant information obtained in Paper II were: 1) p-Si/TiO2/NiOx outcompetes Pt as a counter electrode and 2) a space charge region in the pristine hematite can be enhanced using p-Si/TiO2/NiOx as photocathode without bias or using any dopant. The proof of concept device studied in Paper II was further optimized in Paper III by replacing the FTO substrate with the n-Si MW to a obtain n-Si MW/TiO2/α-Fe2O3 photoanode. A record high photocurrent density of 5.6 mA/cm2 was achieved for the undoped hematite photoanode. I also found out that the TiO2 inter layer plays a crucial role in enhancing the overall device performance. The role of TiO2 was thus further studied using valence to core X-ray emission spectroscopy, which opened a new avenue for identifying and investigating the prime components in such devices. Paper I to III discuss the role of TiO2 and of the co-catalysts towards solar water splitting and thus the only material left to study was the Si substrate. For paper IV, a detailed analysis on Si substrate was performed. The electronic structural changes on Si LII, III edge was studied using X-ray Raman spectroscopy. The X-ray spectroscopic studies presented in papers I to III were performed at the ID-26 beamline at ESRF, while the X-ray Raman Spectroscopy presented in Paper IV was performed at the ID-20 beamline at ESRF. The data presented in Paper IV is preliminary and needs to be processed and analyzed further.

Ort, förlag, år, upplaga, sidor
Umeå: Umeå Universitet , 2018. , s. 111
Nyckelord [en]
Photoelectrochemical cell, photoelectrodes, solar-water splitting, artificial photosynthesis, X-ray absorption and emission spectroscopy
Nationell ämneskategori
Materialkemi
Identifikatorer
URN: urn:nbn:se:umu:diva-153400ISBN: 978-91-7601-991-7 (tryckt)OAI: oai:DiVA.org:umu-153400DiVA, id: diva2:1264085
Disputation
2018-12-13, KBE301 (Lilla hörsalen), KBC-huset, Umeå, 10:00 (Engelska)
Opponent
Handledare
Tillgänglig från: 2018-11-22 Skapad: 2018-11-19 Senast uppdaterad: 2018-11-22Bibliografiskt granskad
Delarbeten
1. Photo-electrochemical hydrogen production from neutral phosphate buffer and seawater using micro-structured p-Si photo-electrodes functionalized by solution-based methods
Öppna denna publikation i ny flik eller fönster >>Photo-electrochemical hydrogen production from neutral phosphate buffer and seawater using micro-structured p-Si photo-electrodes functionalized by solution-based methods
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2018 (Engelska)Ingår i: Sustainable Energy and Fuels, ISSN 2398-4902, Vol. 2, nr 10, s. 2215-2223Artikel i tidskrift (Refereegranskat) [Forskning på konstnärlig grund] Published
Abstract [en]

Solar fuels such as H2 generated from sunlight and seawater using earth-abundant materials are expected to be a crucial component of a next generation renewable energy mix. We herein report a systematic analysis of the photo-electrochemical performance of TiO2 coated, microstructured p-Si photoelectrodes (p-Si/TiO2) that were functionalized with CoOx and NiOx for H2 generation. These photocathodes were synthesized from commercial p-Si wafers employing wet chemical methods. In neutral phosphate buffer and standard 1 sun illumination, the p-Si/TiO2/NiOx photoelectrode showed a photocurrent density of 1.48 mA cm2 at zero bias (0 VRHE), which was three times and 15 times better than the photocurrent densities of p-Si/TiO2/CoOx and p-Si/TiO2, respectively. No decline in activity was observed over a five hour test period, yielding a Faradaic efficiency of 96% for H2 production. Based on the electrochemical characterizations and the high energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) and emission spectroscopy measurements performed at the Ti Ka1 fluorescence line, the superior performance of the p-Si/TiO2/ NiOx photoelectrode was attributed to improved charge transfer properties induced by the NiOx coating on the protective TiO2 layer, in combination with a higher catalytic activity of NiOx for H2-evolution. Moreover, we report here an excellent photo-electrochemical performance of p-Si/TiO2/NiOx photoelectrode in corrosive artificial seawater (pH 8.4) with an unprecedented photocurrent density of 10 mA cm2 at an applied potential of 0.7 VRHE, and of 20 mA cm2 at 0.9 VRHE. The applied bias photon-to-current conversion efficiency (ABPE) at 0.7 VRHE and 10 mA cm2 was found to be 5.1%

Nyckelord
solar water splitting, artificial photosynthesis, X-ray Spectroscopy
Nationell ämneskategori
Materialkemi
Identifikatorer
urn:nbn:se:umu:diva-153381 (URN)10.1039/c8se00291f (DOI)000447950800007 ()
Forskningsfinansiär
Wallenbergstiftelserna, KAW 2011.0055
Tillgänglig från: 2018-11-19 Skapad: 2018-11-19 Senast uppdaterad: 2018-11-20Bibliografiskt granskad
2. A microstructured p-Si photocathode outcompetes Pt as a counter electrode to hematite in photoelectrochemical water-splitting
Öppna denna publikation i ny flik eller fönster >>A microstructured p-Si photocathode outcompetes Pt as a counter electrode to hematite in photoelectrochemical water-splitting
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2019 (Engelska)Ingår i: Dalton Transactions, ISSN 1477-9226, E-ISSN 1477-9234, Vol. 48, nr 4, s. 1166-1170Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Herein, we communicate about an Earth-abundant semiconductor photocathode (p-Si/TiO2/NiOx) as an alternative for the rare and expensive Pt as a counter electrode for overall photoelectrochemical water splitting. The proposed photoelectrochemical (PEC) water-splitting device mimics the "Z"-scheme observed in natural photosynthesis by combining two photoelectrodes in a parallelillumination mode. A nearly 60% increase in the photocurrent density (Jph) for pristine α-Fe2Oand a 77% increase in the applied bias photocurrent efficiency (ABPE) were achieved by replacing the conventionally used Pt cathode with an efficient, cost effective p-Si/TiO2/NiOx photocathode under parallel illumination. The resulting photocurrent density of 1.26 mA cm−2 at 1.23VRHE represents a new record performance for hydrothermally grown pristine α-Fe2O3 nanorod photoanodes in combination with a photocathode, which opens the prospect for further improvement by doping α-Fe2O3 or by its decoration with co-catalysts. Electrochemical impedance spectroscopy measurements suggest that this significant performance increase is due to the enhancement of the space-charge field in α-Fe2O3. 

Ort, förlag, år, upplaga, sidor
Royal Society of Chemistry, 2019
Nationell ämneskategori
Materialkemi
Identifikatorer
urn:nbn:se:umu:diva-153415 (URN)10.1039/c8dt03653e (DOI)000459625900002 ()30534760 (PubMedID)
Tillgänglig från: 2018-11-20 Skapad: 2018-11-20 Senast uppdaterad: 2019-03-27Bibliografiskt granskad
3. Valence to core X-ray spectroscopic insight on the performance enhancing TiO2 interlayer of functionalized Si photoelectrodes for bias-free solar water splitting
Öppna denna publikation i ny flik eller fönster >>Valence to core X-ray spectroscopic insight on the performance enhancing TiO2 interlayer of functionalized Si photoelectrodes for bias-free solar water splitting
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(Engelska)Manuskript (preprint) (Övrigt vetenskapligt)
Nationell ämneskategori
Materialkemi
Identifikatorer
urn:nbn:se:umu:diva-153416 (URN)
Tillgänglig från: 2018-11-20 Skapad: 2018-11-20 Senast uppdaterad: 2018-11-20
4. Si L-edge X-ray Raman spectroscopy insight on Si/TiO2 interaction in photoelectrodes
Öppna denna publikation i ny flik eller fönster >>Si L-edge X-ray Raman spectroscopy insight on Si/TiO2 interaction in photoelectrodes
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(Engelska)Manuskript (preprint) (Övrigt vetenskapligt)
Nationell ämneskategori
Materialkemi
Identifikatorer
urn:nbn:se:umu:diva-153417 (URN)
Tillgänglig från: 2018-11-20 Skapad: 2018-11-20 Senast uppdaterad: 2018-11-20

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