EnTV Materials for Efficient Energy Use

Transcription

1 EnTV Materials for Efficient Energy Use Prof. Dr. Rolf Hempelmann Physikalische Chemie der Universität des Saarlandes Chapter 10: Photo-catalytic Water Splitting, Electrochemical CO 2 Reduction content Photo-catalytic water splitting Oxygen Evolution Reaction Hydrogen Evolution vs. Oxygen Reduction Photo-electrochemical water splitting Electrochemical CO 2 reduction dream reactions, Holy Grail of solar energy conversion: Direct storage of solar energy in form of matter 2 1

2 Light2Hydrogen Energy for the future Photocatalytic Splitting of Water to Hydrogen is an international research project, financed by the Federal Ministry of Education and Research within the framework of the program Spitzenforschung & Innovation in den Neuen Ländern with a funding volume of 10 million Euro over five years. 3 In der Natur 4 2

3 Powering the planet: Chemical challenges in solar energy utilization Nathan S. Lewis and Daniel G. Nocera, Powering the planet: Chemical challenges in solar energy utilization, Proc. National Academy of Science 103 (2006) Storage of solar energy in form of matter 6 3

4 Photo-electrochemical cell 7 Photo-catalytic water splitting, relevant processes Akihiko Kudo and Yugo Miseki, Heterogeneous photo-catalyst materials for water splitting, Chem. Soc. Rev., 2009, 38,

5 semiconductor photo-catalysis important: width of the band gap, position of conduction band, valence band: bottom of CB must be more negative than redoxpotential H + / H 2 O; Top of VB must be more positive than redoxpotential von O 2 / H 2 O band gap (in ev) = 1240 /λ (nm) 1,23 V 1100 nm 9 Relation between band structure of semiconductor and redox-potential of water splitting undesired reaction: CdS + 2 h + Cd 2+ + S sulfids are not suited 10 5

6 Photo-corrosion undesired: CdS + 2 h + Cd 2+ + S ZnO + 2h+ Zn 2+ + ½ O 2 11 particle size and grain boundaries /defects 12 6

7 photo-catalytic activity of TiO 2 13 Sacrificial reagents for the observation of HER and OER, respectively 1. OER: Oxygen Evolution Reaction catalysis 2. HER: Hydrogen Evolution Reaction vs. ORR: Oxygen Reduction Reaction desired undesired 14 7

8 why is OER catalysis important? OER is the rate determining step in water-electrolysis, and can hinder the photo-electrochemical production of H 2. IPCE (%) Nanorods, with and without OER Catalyst Wavelength (nm) incident photon to converted electron ratio = Quantenausbeute S116C RuO 2 No Catalyst 0.1M NaOH 100mV applied bias vs Ag/AgCl N 2 Purged 15 why is OER catalysis important? 4H 2 O hν 4OH - CB 4e - 2H 2 + 4OH - e - e- 4h + VB O 2 H 2 O 2 + 2H 2 O n-type metal semiconductor 16 8

9 overall approach Find techniques for OER-catalyst deposition (E-chem, SP) Search for metal compounds that have low onset potential (Ni-Metal, RuO 2 ) this means: reduction of the activation-energy reduction of the overpotential Test the samples for their OER properties and performance (PEC). Long-term stability 17 DEPOSITION METHODS Electrodeposition NiFe, NiCo, etc Spray pyrolysis e.g. RuCl 3 -solution (90 C hotplate) Calcined 350 C Solvent evaporates Solution (e.g.rucl 3 ) Counter Electrode Pt Working Electrode (Substrate) FTOsubstrate Hot plate 90 C/120 C Fe 2 O 3 -layer Reference Electrode Ag/AgCl 18 9

10 RuO 2 as an OER catalyst Current (ma) Fe 2 O 3 -RuO 2 - electrodeposited Fe 2 O 3 1M NaOH, N 2 Degassed H 2 O/O 2 ~200 mv 660 mv 450 mv Applied Voltage vs Ag/AgCl (V) Best result reached: Improvement of 210 mv, comes closest to the thermodynamic potential of ~200 mv 19 performance of OER catalysts Current (ma) Fe 2 O 3 Fe 2 O 3 -CuNi - electrodeposited Fe 2 O 3 -NiFe - electrodeposited Fe 2 O 3 -Cu - electrodeposited Fe 2 O 3 -Co - electrodeposited Results DEPOSITED METAL CuNi 0.55V NiFe 0.55V Cu 0.53V Co 0.52V ONSET- POTENTIAL Applied Voltage vs Ag/AgCl (V) Onset-potential of the Fe 2 O 3 -reference: 0.64V Performance of the best samples: samples show an improvement compared to hematite, but none of them are as good as the electrodeposited RuO

11 intermediate resume Best improvement reached with E-chem RuO 2 RuO 2 by SP did not show any improvement Some Ni-Metal electrolytes will dissolve Fe 2 O 3 Promising alternative: Ni-compounds if acceptable stability is reachable 21 hydrogen production by photo-electrochemistry hν CB 4e - 4H 2 O and/or O 2 + 2H 2 O 4OH - 2H 2 + 4OH - 4OH - 4h + VB j desired j undesired O 2 + 2H 2 O What is the effect of the oxygen product on the desirable H 2 production reaction?

12 HER on Metals e - H O H H O H 1) + OH - H H k V H k 2 + OH - s k V 2) H H O H k s H 2 OH - OH - Phys. Chem. Chem. Phys., 2001, 3, ORR on metals 4-electron process O O Stable oxide all active metalsites occupied e - k s OO OO k s OH O H OH- k V OH - O H OH- k V OH - O H k s OH k V OH

13 Forms a stable Oxide Forms a metastable Oxide W, Mo are even under cathodic conditions still oxides hydrogen bubbles 26 13

14 alkali-tantalate with NiO co-catalyst 27 NaTaO 3 :La with NiO co-catalyst 28 14

15 Fuel cell design R. Marschall, Ch. Klaysom, A. Mukherji, M. Wark, G. Qing Lu, L. Wang, International Journal of Hydrogen Energy 37 (2012) Photo-reactor

16 PhD thesis of Anja Eberhardt o calibration curve of fuel cell sensor o Test bench for measurement of photo-catalytically formed hydrogen at the semiconductor nanoparticle o photo-catalytic reactions o next steps 31 calibration curve of fuel cell sensor Kalibrierkurve der Brennstoffzelle Kurzschlussstrom [µa] H 2 -Gehalt [ppm] 32 16

17 scheme of apparatus 33 test apparatus 34 17

18 calibration curve of fuel cell sensor ppm 7000 Kalibrierung_ Kurzschlussstrom [µa] ppm 748 ppm 1122 ppm 1496 ppm 1870 ppm Kurzschlussstrom [µa] Zeit [min] H 2 -Gehalt [ppm] 35 Photo-catalytic reactions hardly any hydrogen evolution Lower limit of calibration curve is reached Upon addition of ethanol hydrogen is formed. Ethanol acts as electron donator. Maximal H 2 -evolution: 4.4 ml per hour (11 ml/(g h)) 90 Sol-Gel-NaTaO 3 N 2 -Fluss: 0,1 SLM 1: Einschalten der UV-Lampe 2000 Sol-Gel-NaTaO 3 nach Zugabe EtOH 88 2: Ausschalten der UV-Lampe Kurzschlussstrom [µa] Kurzschlussstrom [µa] N 2 -Fluss: 0,1 SLM Zeit [min] 0 1: Einschalten der UV-Lampe 1 2: Ausschalten der UV-Lampe Zeit [min] 36 18

19 (7,6aTLampe annao3(sg)mit1gewzunähstreinesasserwerden. Photo-catalytic measurement with fuel cell sensor SG-NaTaO 3 with addition of gold 0,7 0,6 SG-NaTaO 3 reines WasserZ0,5 Strom [ma] 0,4 0,3 0,2 0,1 0,0 ugabeelampe an mggoldsäur Zeit [min] 37 Photo-catalytic measurement with fuel cell sensor %Au10 Lampe aus Strom [ma] cw)using SG-NaTaO3/1 wt.-% Au 0 Zugabe EtOH Zeit [min] 38 19

20 photo-catalytic reactions limiting current of 4.72 ma corresponds to appr ppm H 2 in gas stream ca. 8 ml H 2 per hour (more than 20 ml/(g h)) Sol-Gel-NaTaO 3 mit 1,0 Gew.% NiO N 2 -Fluss: 0,1 SLM 1: Einschalten der UV-Lampe 2: Ausschalten der UV-Lampe Kurzschlussstrom [µa] ,72 ma Zeit [min] 39 photo-catalytic reactions limiting current of 527 µa correponds to appr. 250 ppm H 2 in gas stream ca. 1.5 ml H 2 per hour (0.7 ml/(g h)) 700 nano-tio 2 mit 1 mmol Pt pro mol Katalysator 600 Kurzschlussstrom [µa] µa N 2 -Fluss: 0,1 SLM 1: Einschalten der UV-Lampe 2: Ausschalten der UV-Lampe Zeit [min] 40 20

21 photo-catalytic reactions During platination a lot of hydrogen is formed. A limiting current is not established. Catalyst has been platinized á priori and irradiated for some time. A limiting current is not established kommerzielles TiO 2 nach Zugabe EtOH 1800 kommerzielles TiO 2 mit 1 mmol Pt pro mol Katalysator Kurzschlussstrom [µa] N 2 -Fluss: 0,1 SLM 1, 3, 5, 7: Einschalten der UV-Lampe 2, 4, 6, 8: Ausschalten der UV-Lampe Kurzschlussstrom [µa] N 2 -Fluss: 0,1 SLM 1: Einschalten der UV-Lampe 2: Ausschalten der UV-Lampe Zugabe 2 mg Pt(NO 3 ) 2 Zugabe 2 ml Ethanol Zeit [min] Zeit [min] 41 power to gas 42 21

22 Ox: H 2 O 2 H + + ½ O 2 (0,82V gegen NHE) Red: 2 H e H 2 ( 0,41V gegen NHE) H 2 O H 2 + ½ O 2 ( E=1,23V) 43 Ox: H 2 O 2 H + + ½ O 2 (0,82V gegen NHE) Red 1: 2 H e H 2 ( 0,41V gegen NHE) Red 2: CO H e CH H 2 O ( 0,25V gegen NHE) H 2 O H 2 + ½ O 2 ( E = 1,23V) CO H 2 O CH O 2 ( E = 1,07V) 44 22

23 Ox: H 2 O 2 H + + ½ O 2 (0,82V gegen NHE) Red 1: 2 H e H 2 ( 0,41V gegen NHE) Red 2: CO H e CH H 2 O ( 0,25V gegen NHE) H 2 O H 2 + ½ O 2 ( E = 1,23V) CO H 2 O CH O 2 ( E = 1,07V) 45 Ox: H 2 O 2 H + + ½ O 2 (0,82V gegen NHE) Red: CO H e CH H 2 O ( 0,25V gegen NHE) CO H 2 O CH O 2 ( E = 1,07V) 46 23

24 Ox: H 2 O 2 H + + ½ O 2 (0,82V gegen NHE) Red: CO H e CH H 2 O ( 0,25V gegen NHE) CO H 2 O CH O 2 ( E = 1,07V) - relatively high overpotential of ca. 1V. - little knowledge about parameters which influence distribution of products. - rate-limiting- und key-selectivity-determining steps are discussed controversely in literature

25 Formiat 49 Carbon monoxide 50 25

26 hydrogen 51 methane, ethene, hydrogen 52 26

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28 X. Nie, M. R. Escopi, M. J. Janik, A. Asthagiri: Angew. Chem. 125, 2013, X. Nie, M. R. Escopi, M. J. Janik, A. Asthagiri: Angew. Chem. 125, 2013,

29 X. Nie, M. R. Escopi, M. J. Janik, A. Asthagiri: Angew. Chem. 125, 2013, U PEM-Zelle CO2 GC Gasentnahme Sättigungszelle ph 58 29

30 59 Literatur 60 30