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Water splitting by heterogeneous catalysis
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). (Mats Johnsson)ORCID iD: 0000-0001-8224-1980
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A sustainable solution for meeting the energy demands at our planet is by utilizing wind-, solar-, wave-, thermal-, biomass- and hydroelectric power. These renewable and CO2 emission-free energy sources are highly variable in terms of spatial and temporal availability over the Earth, introducing the need for an appropriate method of storing and carrying energy. Hydrogen has gained significant attention as an energy storage- and carrier media because of the high energy density that is exploited within the ‘power-to-gas’ process chain. A robust way of producing sustainable hydrogen is via electrochemical water splitting.

In this work the search for new heterogeneous catalyst materials with the aim of increasing energy efficiency in water splitting has involved methods of both electrochemical water splitting and chemical water oxidation. Some 21 compounds including metal- oxides, oxofluorides, oxochlorides, hydroxide and metals have been evaluated as catalysts. Two of these were synthesized directly onto conductive backbones by hydrothermal methods. Dedicated electrochemical cells were constructed for appropriate analysis of reactions, with one cell simulating an upscale unit accounting for realistic large scale applications; in this cell gaseous products are quantified by use of mass spectrometry. Parameters such as real time faradaic efficiency, production of H2 and O2 in relation to power input or overpotentials, Tafel slopes, exchange current density and electrochemical active surface area as well as turnover numbers and turnover frequencies have been evaluated.

Solubility, possible side reactions, the role of the oxidation state of catalytically active elements and the nature of the outermost active surface layer of the catalyst are discussed. It was concluded that metal oxides are less efficient than metal based catalysts, both in terms of energy efficiency and in terms of electrode preparation methods intended for long time operation. The most efficient material was Ni-Fe hydroxide electrodeposited onto Ni metal foam as conductive backbone. Among the other catalysts, Co3Sb4O6F6 was of particular interest because the compound incorporate a metalloid (Sb) and redox inert F and yet show pronounced catalytic performance.

In addition, performance of materials in water splitting catalysis has been discussed on the basis of results from electron microscopy, solubility experiments and X-ray diffraction data.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry, Stockholm University , 2017. , 93 p.
Keyword [en]
Faradaic efficiency, electrocatalysis, electrolysis, water oxidation, hydrogen reduction, H2, O2, mass spectrometry
National Category
Inorganic Chemistry
Research subject
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-148181ISBN: 978-91-7797-039-2 (print)ISBN: 978-91-7797-040-8 (electronic)OAI: oai:DiVA.org:su-148181DiVA: diva2:1149996
Public defence
2017-12-06, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2017-11-13 Created: 2017-10-17 Last updated: 2017-11-03Bibliographically approved
List of papers
1. Water splitting catalysis studied by real time Faradaic efficiency obtained by coupled electrolysis and mass spectrometry
Open this publication in new window or tab >>Water splitting catalysis studied by real time Faradaic efficiency obtained by coupled electrolysis and mass spectrometry
2017 (English)In: ChemElectroChem, ISSN 2196-0216Article in journal (Refereed) Epub ahead of print
Abstract [en]

An experimental setup and routine is presented for evaluating potential catalysts for water splitting by means of measuring Faradaic efficiency in real time by coupled potentiometry and mass spectrometry. The aim was to simulate a potential industrial scale setup and generate results such as H2 production versus power input at a certain potential or current density in addition to electrochemical parameters. Three types of electrodes were tested: A) planar metal electrodes; B) metal foam based electrodes; C) porous electrodes with carbon additive. The results verify that the experimental routine yield desired accuracy, sensitivity and a negligible accumulation of gaseous products in the cell; thus the Faradaic efficiency is measured in real time. The metal based electrodes of category A and B proved to be durable with low overpotentials and high gas output to power input, whereas three tested metal oxide electrodes in C revealed (i) potential-dependent deviation in Faradaic efficiency, (ii) phase decomposition and (iii) an optimum operational power range.

National Category
Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-148175 (URN)10.1002/celc.201701086 (DOI)
Available from: 2017-10-17 Created: 2017-10-17 Last updated: 2017-10-18
2. Direct Synthesis of Two Inorganic Catalysts on Carbon Fibres for the Electrocatalytic Oxidation of Water
Open this publication in new window or tab >>Direct Synthesis of Two Inorganic Catalysts on Carbon Fibres for the Electrocatalytic Oxidation of Water
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2017 (English)In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 23, no 3, 568-575 p.Article in journal (Refereed) Published
Abstract [en]

Two electrodes for anodic water oxidation made by direct synthesis of inorganic catalysts onto conductive carbon fibre sheets are evaluated. As catalysts two Co- and Sb-containing phases were tested, that is, Co3Sb4O6F6 and the new compound CoSbO4. The compounds express large differences in their morphology: CoSbO4 grows as thin needles whereas Co3Sb4O6F6 grows as larger facetted crystals. Despite the smaller surface area the latter compound shows a better catalytic performance. When the compound Co3Sb4O6F6 was used it gave a low increase of +0.028 mV h(-1) at an overpotential of eta = 472 mV after 10 h and a stability of +0.48 mV h(-1) at an overpotential of eta = 488 mV after 60 h. The leakages of Co and Sb were negligible and only <0.001 at% Co and approximately 0.02 at% Sb were detected in the electrolyte.

Keyword
antimony, cobalt, electrochemistry, inorganic catalysts, water oxidation
National Category
Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-141394 (URN)10.1002/chem.201603085 (DOI)000393625600014 ()27862443 (PubMedID)
Available from: 2017-04-26 Created: 2017-04-26 Last updated: 2017-10-18Bibliographically approved
3. A transition metal oxofluoride offering advantages in electrocatalysis and potential use in applications
Open this publication in new window or tab >>A transition metal oxofluoride offering advantages in electrocatalysis and potential use in applications
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2016 (English)In: Faraday discussions (Online), ISSN 1359-6640, E-ISSN 1364-5498, Vol. 188, 481-498 p.Article in journal (Refereed) Published
Abstract [en]

The recently described solid solution ( Co,Ni,Mn)(3)Sb4O6F6 has proved stable and efficient as a catalyst for electrocatalytic water oxidation. The end component Co3Sb4O6F6 was found to be most efficient, maintaining a current density of j = 10 mA cm(-2) at an overpotential of 443 mV with good capability. At this current density, O-2 and H-2 were produced in the ratio 1 : 2 without loss of faradaic current against a Pt-cathode. A morphological change in the crystallite surface was observed after 0.5 h, however, even after 64.5 h, the overall shape and size of the small crystallites were unaffected and the electrolyte contained only 0.02 at% Co. It was also possible to conclude from in situ EXAFS measurements that the coordination around Co did not change. The oxofluorides express both hydrophilic and hydrophobic surface sites, incorporate a flexible metalloid element and offer the possibility of a mechanism that differs from other inorganic catalytic pathways previously described.

National Category
Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-133258 (URN)10.1039/c5fd00169b (DOI)000380099200027 ()27064139 (PubMedID)
Available from: 2016-09-06 Created: 2016-09-05 Last updated: 2017-10-18Bibliographically approved
4. Application of novel catalysts: general discussion
Open this publication in new window or tab >>Application of novel catalysts: general discussion
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2016 (English)In: Faraday discussions (Online), ISSN 1359-6640, E-ISSN 1364-5498, Vol. 188, 399-426 p.Article in journal (Refereed) Published
National Category
Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-148179 (URN)10.1039/C6FD90018F (DOI)
Available from: 2017-10-17 Created: 2017-10-17 Last updated: 2017-10-18Bibliographically approved
5. An Oxofluoride Catalyst Comprised of Transition Metals and a Metalloid for Application in Water Oxidation
Open this publication in new window or tab >>An Oxofluoride Catalyst Comprised of Transition Metals and a Metalloid for Application in Water Oxidation
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2015 (English)In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 21, no 37, 12991-12995 p.Article in journal (Refereed) Published
Abstract [en]

The application of the recently discovered oxofluoride solid solution (CoxNi1-x)(3)Sb4O6F6 as a catalyst for water oxidation is demonstrated. The phase exhibits a cubic arrangement of the active metal that forms oxo bridges to the metalloid with possible catalytic participation. The Co3Sb4O6F6 compound proved to be capable of catalyzing 2H(2)OO(2)+4H(+)+4e(-) at 0.33V electrochemical and 0.39V chemical overpotential with a TOF of 4.410(-3), whereas Ni3Sb4O6F6 needs a higher overpotential. Relatively large crystal cubes (0.3-0.5mm) are easily synthesized and readily handled as they demonstrate both chemical resistance to wear after repeated insitu tests under experimental conditions, and have a mechanical hardness of 270V0.1 using Vickers indentation. The combined properties of this compound offer a potential technical advantage for incorporation to a catalytic interface in future sustainable fuel production.

Keyword
cobalt, nickel, oxofluorides, solid solutions, water oxidation catalysts
National Category
Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-136190 (URN)10.1002/chem.201501452 (DOI)000360583000022 ()26219925 (PubMedID)
Available from: 2016-11-30 Created: 2016-11-30 Last updated: 2017-10-18Bibliographically approved
6. Cobalt selenium oxohalides: catalysts for water oxidation
Open this publication in new window or tab >>Cobalt selenium oxohalides: catalysts for water oxidation
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2014 (English)In: Dalton Transactions, ISSN 1477-9226, E-ISSN 1477-9234, Vol. 43, no 10, 3984-33989 p.Article in journal (Refereed) Published
Abstract [en]

Two new oxohalides Co4Se3O9Cl2 and Co3Se4O10Cl2 have been synthesized by solid state reactions. They crystallize in the orthorhombic space group Pnma and the monoclinic space group C2/m respectively. The crystal structure of the two compounds are made up of similar building blocks; Co4Se3O9Cl2 is made up of [CoO4Cl2], [CoO5Cl] and [SeO3] polyhedra and Co3Se4O10Cl2 is made up of [CoO4Cl2] and [SeO3] polyhedra. As several Co-containing compounds have proved to be good catalysts for water oxidation, the activities of the two new compounds were compared with the previously found oxohalide Co5Se4O12Cl2 in reference to CoO and CoCl2. The one electron oxidant Ru(bpy)33+ was used as oxidizing species in a phosphate buffer and it was found that the activities of the oxohalide species were in between CoO and CoCl2. The roles of Cl and PO43− ions are discussed.

National Category
Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-148173 (URN)10.1039/C3DT53452A (DOI)
Available from: 2017-10-17 Created: 2017-10-17 Last updated: 2017-10-18Bibliographically approved

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