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In-situ activated hydrogen evolution from pH-neutral electrolytes
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The goal of this work was to better understand how molybdate and trivalent cations can be used as additives to pH neutral electrolytes to activate the Hydrogen Evolution Reaction (HER). Special emphasis was laid on the chlorate process and therefore also to some of the other effects that the additives may have in that particular process.

Cathode films formed from the molybdate and trivalent cations have been investigated with electrochemical and surface analytical methods such as polarization curves, potential sweep, Electrochemical Impedance Spectroscopy (EIS), current efficiency measurements, Scanning Electron Microscope (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), X-Ray Fluorescence (XRF) and Inductively Coupled Plasma (ICP) analysis.

Trivalent cations and molybdate both activate the HER, although in different ways. Ligand water bound to the trivalent cations replaces water as reactant in the HER. Since the ligand water has a lower pKa than free water, it is more easily electrochemically deprotonated than free water and thus catalyzes the HER. Sodium molybdate, on the other hand, is electrochemically reduced on the cathode and form films which catalyze the HER (on cathode materials with poor activity for HER). Molybdate forms films of molybdenum oxides on the electrode surface, while trivalent cation additions form hydroxide films. There is a risk for both types of films that their ohmic resistance increases and the activity of the HER decreases during their growth. Lab-scale experiments show that for films formed from molybdate, these negative effects become less pronounced when the molybdate concentration is reduced.

Both types of films can also increase the selectivity of the HER by hindering unwanted side reactions, but none of them as efficiently as the toxic additive Cr(VI) used today in the chlorate process. Trivalent cations are not soluble in chlorate electrolyte and thus not suitable for the chlorate process, whereas molybdate, over a wide pH range, can activate the HER on catalytically poor cathode materials such as titanium.

Abstract [sv]

Målsättningen med detta doktorsarbete har varit att bättre förstå hur trivalenta katjoner och molybdat lösta i elektrolyten kan effektivisera elektrokemisk vätgasproduktion. Tillämpningen av dessa tillsatser i kloratprocessen och eventuella sidoeffekter har undersökts.

De filmer som bildas på katoden av tillsatserna har undersökts med både elektrokemiska och fysikaliska ytanalysmetoder: polarisationskurvor, potentialsvep, elektrokemisk impedansspektroskopi (EIS), strömutbytesmätningar, svepelektronmikroskopi (SEM), energidispersiv röntgenspektroskopi (EDS), röntgenfotoelektronspektroskopi (XPS), röntgenfluorensens (XRF) och induktivt kopplat plasma (ICP).

Både trivalenta katjoner och molybdat kan aktivera elektrokemisk vätgasutveckling, men på olika sätt. Vatten bundet till trivalenta katjoner ersätter fritt vatten som reaktant vid vätgasutveckling. Eftersom vatten bundet till trivalenta katjoner har lägre pKa-värde, går det lättare att producera vätgas från dessa komplex än från fritt vatten. Natriummolybdat däremot reduceras på katoden och bildar filmer som kan katalysera vätgasutvecklingen på substratmaterial som har låg katalytisk aktivitet för reaktionen. Molybdat bildar molybdenoxider på ytan medan trivalenta katjoner bildar metallhydroxider. Båda typerna av film riskerar att bilda filmer som är resistiva och deaktiverar vätgasutvecklingen. Laboratorieexperiment visar att problemen minskar med minskad molybdathalt.

Båda filmerna kan öka selektiviteten för vätgasutveckling genom att hindra sidoreaktioner. Filmerna är dock inte lika effektiva som de filmer som bildas från den ohälsosamma tillsatsen Cr(VI), vilken används i kloratprocessen idag. Trivalenta katjoner är inte lösliga i kloratelektrolyt och är därför inte en lämplig tillsats i kloratprocessen. Molybdat har god löslighet och kan aktivera vätgasutveckling i ett stort pH‑intervall på titan och andra substratmaterial som själva har betydlig sämre aktivitet för vätgasutveckling.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , 45 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2012:26
Keyword [en]
molybdate, trivalent cations, electrolysis, hypochlorite reduction, films, electrolysis, chlorate process
Keyword [sv]
molybdat, trivalenta katjoner, elektrolys, hypokloritreduktion, filmer, kloratprocessen
National Category
Engineering and Technology
Research subject
SRA - Energy
Identifiers
URN: urn:nbn:se:kth:diva-95369ISBN: 978-91-7501-391-6 (print)OAI: oai:DiVA.org:kth-95369DiVA: diva2:527964
Public defence
2012-06-15, E3, Lindstedtsvägen 3, entréplan, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
c6839
Funder
StandUp
Note

QC 20120530

Available from: 2012-05-30 Created: 2012-05-23 Last updated: 2013-04-18Bibliographically approved
List of papers
1. Cathodic reactions on an iron RDE in the presence of Y(III)
Open this publication in new window or tab >>Cathodic reactions on an iron RDE in the presence of Y(III)
2008 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 155, no 10, E136-E142 p.Article in journal (Refereed) Published
Abstract [en]

During electrolysis of a solution containing Y(III) ions, a hydrous Y(OH)(3) film forms in the alkaline layer close to a hydrogen-evolving cathode. The film hinders the reduction of dissolved oxygen and activates the reduction of water, hydrogen evolution. The ability to hinder certain reactions while catalyzing hydrogen evolution may be useful in electrolysis applications. In this work the electrochemical properties of an in situ formed yttrium-hydroxide film were studied on an iron rotating disk electrode (RDE) in 0.5 M NaCl with addition of YCl3, NaClO, and of NaNO3. It was found that the film also hinders the reduction of protons, hypochlorite ions, and nitrate ions. At low concentration of Y(III) or at high current density, when the hydrogen evolution was vigorous, no activation of hydrogen evolution was observed. Under these conditions the film still hindered the reduction of ions. The reactant in the catalyzed hydrogen evolution reaction is most likely water molecules within the hydrous film. Nitrate ions were easily reduced on an iron cathode when no Y(III) ions were present in the solution. When studying effects of yttrium addition to a chloride solution the use of YCl3, rather than Y(NO3)(3), as Y(III) source is recommended.

Keyword
Concentration (process), Current density, Disks (machine components), Dissolution, Dissolved oxygen, Electrochemical properties, Electrochemistry, Electrolysis, Hydrogen, Ions, Nitrates, Nonmetals, Oxygen, Reduction, Rotating disks, Sodium chloride, Yttrium, Applications., Cathodic reactions, Chloride solutions, High current densities, Hydrogen evolution, Hydrogen evolution reaction, In-situ, Low concentrations, Nitrate ions, NO activation, Reduction of protons, Rotating disk electrode, Water molecules, Yttrium addition
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-8150 (URN)10.1149/1.2958299 (DOI)000258976500042 ()2-s2.0-51849102885 (Scopus ID)
Note
20100901. Uppdaterad från submitted till published (20100901).Available from: 2008-03-27 Created: 2008-03-27 Last updated: 2017-12-14Bibliographically approved
2. Rare earth metal salts as potential alternatives to Cr(VI) in the chlorate process
Open this publication in new window or tab >>Rare earth metal salts as potential alternatives to Cr(VI) in the chlorate process
2010 (English)In: Journal of Applied Electrochemistry, ISSN 0021-891X, E-ISSN 1572-8838, Vol. 40, no 8, 1529-1536 p.Article in journal (Refereed) Published
Abstract [en]

Chromate is today added to industrial chlorate electrolyte, where it forms a thin cathode film of chromium hydroxide that hinders unwanted reduction of hypochlorite and chlorate. The aim of this study was to investigate rare earth metal (REM) ions as an environmentally friendly alternative to the toxic chromate addition. Potential sweeps and iR-corrected polarisation curves were recorded using rotating disc electrodes of iron and gold. Addition of Y(III), La(III) or Sm(III) to 5 M NaCl at 70 A degrees C suppressed hypochlorite reduction. Activation of hydrogen evolution by REM ion addition to 0.5 M NaCl was more significant at 25 A degrees C than at 50 and 70 A degrees C. Increasing the chloride concentration to 5 M had a detrimental effect on this activation. The major problem in replacing chromate with REM salts is the poor solubility of REM ions at normal chlorate process conditions, and therefore REM salts are not a realistic alternative to chromate addition.

Keyword
Rare earth metals, Chlorate production, Yttrium, Hydrogen evolution, Hypochlorite, In situ electrodeposition
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-27266 (URN)10.1007/s10800-010-0136-4 (DOI)000279033200010 ()2-s2.0-77956907518 (Scopus ID)
Note

QC 20101216

Available from: 2010-12-16 Created: 2010-12-09 Last updated: 2017-12-11Bibliographically approved
3. In-situ activation of hydrogen evolution in pH-neutral electrolytes by additions of multivalent cations
Open this publication in new window or tab >>In-situ activation of hydrogen evolution in pH-neutral electrolytes by additions of multivalent cations
2012 (English)In: International journal of hydrogen energy, ISSN 0360-3199, E-ISSN 1879-3487, Vol. 37, no 12, 9496-9503 p.Article in journal (Refereed) Published
Abstract [en]

Activation of the hydrogen evolution reaction (HER) in close to pH-neutral electrolytes can be achieved by addition of trivalent cations. This activation has been investigated using steady state polarization, electrochemical impedance spectroscopy (EIS) and chemical analysis of cathode films for yttrium. Several multivalent cations were included in this study, such as Al(III), Mg(II), Y(III), Sm(III), La(III) and Sc(III). In general the more acidic the metal ions the larger is the activation. Metal hydroxide films formed in the alkaline diffusion layer at the cathode surface can have a negative impact on the magnitude of this activation, and therefore complicate the interpretation of the results. The activation corresponds to a transport of metal ion complexes to the electrode surface and the reduction of bound ligand water to form hydrogen.

Place, publisher, year, edition, pages
Elsevier, 2012
National Category
Engineering and Technology
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-95356 (URN)10.1016/j.ijhydene.2012.03.061 (DOI)000305106300006 ()2-s2.0-84861201363 (Scopus ID)
Projects
C6839
Funder
StandUp
Note

QC 20120530

Available from: 2012-05-23 Created: 2012-05-23 Last updated: 2017-12-07Bibliographically approved
4. In-situ Activated Hydrogen Evolution by Molybdate Addition to Neutral and Alkaline Electrolytes
Open this publication in new window or tab >>In-situ Activated Hydrogen Evolution by Molybdate Addition to Neutral and Alkaline Electrolytes
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Activation of the hydrogen evolution reaction (HER) by in-situ addition of Mo(VI) to the electrolyte has been studied in alkaline and pH neutral electrolytes, the latter with the chlorate process in focus. Catalytic molybdenum containing films formed on the cathodes during polarization were investigated using scanning electron microscopy (SEM), energy-dispersive X‑ray analysis (EDS), X-ray photoelectron spectroscopy (XPS), and X‑ray fluorescence (XRF). In-situ addition of Mo(VI) activates the HER on titanium in both alkaline and neutral electrolytes and makes the reaction kinetics independent of the substrate material. Films formed in neutral electrolyte consisted of molybdenum oxides and contained more molybdenum than those formed in alkaline solution. Films formed in neutral electrolyte in the presence of phosphate buffer activated the HER, but were too thin to be detected by EDS. Since molybdenum oxides are generally not stable in strongly alkaline electrolyte, films formed in alkaline electrolyte were thinner and probably co-deposited with iron. A cast iron‑molybdenum alloy was also investigated with respect to activity for HER. When polished in the same way as iron, the alloy displayed a similar activity for HER as pure iron.

Keyword
molybdate, molybdenum dioxide, electrodeposition, electrolysis
National Category
Engineering and Technology
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-95363 (URN)
Projects
c6839
Funder
StandUp
Note

QS 2012

Available from: 2012-05-23 Created: 2012-05-23 Last updated: 2013-07-02Bibliographically approved
5. On the Suppression of Cathodic Hypochlorite Reduction by ElectrolyteAdditions of Molybdate and Chromate Ions
Open this publication in new window or tab >>On the Suppression of Cathodic Hypochlorite Reduction by ElectrolyteAdditions of Molybdate and Chromate Ions
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The goal of this study was to gain a better understanding of the feasibility of replacing Cr(VI) in the chlorate process by Mo(VI), focusing on the cathode reaction selectivity for hydrogen evolution on steel and titanium in a hypochlorite containing electrolyte.

To evaluate the ability of Cr(VI) and Mo(VI) additions to hinder hypochlorite reduction, potential sweep experiments on rotating disc electrodes and cathodic current efficiency (CE) measurements on stationary electrodes were performed. Formed electrode films were investigated with scanning electron microscopy and energy-dispersive X-ray spectroscopy. Cathodic hypochlorite reduction is hindered by the Mo-containing films formed on the cathode surface after Mo(VI) addition to the electrolyte, but much less efficient compared to Cr(VI) addition. Very low levels of Cr(VI), in the mM range, can efficiently suppress hypochlorite reduction on polished titanium and steel. Phosphate does not negatively influence the CE in the presence of Cr(VI) or Mo(VI) and the Mo-containing cathode films become thinner if the electrolyte during the film build-up also contains phosphate. For a RuO2-TiO2 anode polarized in electrolyte with 40 mM Mo(VI), the anode potential increased and on the electrode surface, increased molybdenum levels were detected. In an earlier study 40 mM Mo(VI) gave increased by-product oxygen.

National Category
Engineering and Technology
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-95367 (URN)
Projects
c6839
Funder
StandUp
Note
QS 2012Available from: 2012-05-23 Created: 2012-05-23 Last updated: 2012-05-30Bibliographically approved

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