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Transition Metal-Based Electrocatalysts for Alkaline Water Splitting and CO2 Reduction
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.ORCID iD: 0000-0002-7892-5260
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With excessive usage of fossil fuels and ever-increasing environmental issues, numerous efforts have been devoted to the development of renewable energies for the replacement of traditional fossil fuels to reduce greenhouse gas emission and realize the rapidly growing demand for global energy. Renewable energies, however, often show diurnal and seasonal variations in power output, forming a need for energy storage to meet people’s continuous energy supply. One approach is to use electrolysis and produce a fuel that can be used on demand at a later stage. A full realization of effective electricity-to-fuel conversion, however, is still limited by the large overpotential requirements as well as concerns with the usage of scarce platinum group elements. This thesis presents studies on transition metal-based electrocatalysts for alkaline water splitting and CO2 reduction, which are two technologies to produce a chemical fuel from renewable electricity. Our aim is to develop efficient, inexpensive, and robust electrocatalysts based on earth-abundant elements with high energy conversion efficiencies.

In the first part, we develop and investigate three different electrocatalysts intended for high-performance electrocatalysis of water; NiO nanoflakes (NFs) with tuneable surface morphologies, Fe doped NiO nanosheets (NSs), and self-optimized NiFe layered double hydroxide (LDH) NSs. The self-assembled NiO NFs show drastically different performance for the oxygen evolution reaction (OER). Besides the morphology effect on the catalytic property, the presence of Fe is also functional to improve the catalytic activity for both OER and hydrogen evolution reaction (HER). The NiFe LDH NSs form the most effective system for the overall catalytic performance and is dramatically improved via a dynamic self-optimization, especially for HER, where the overpotential decreases from 206 mV to 59 mV at 10 mA cm-2. In order to get insight into the interfacial reaction processes, a variety of techniques were performed to explore the underlying reasons for the catalytic improvement. Ex-situ X-ray photoelectron spectroscopy, transmission electron microscope and in-situ Raman spectroscopy were utilized to characterize and understand the oxidations states, the crystallinity and the active phases. Electrochemical impedance spectroscopy was applied to investigate the dominating reaction mechanisms during high-performance and stable electrocatalysis.

In the second part, dynamically formed CuInO2 nanoparticles were demonstrated to be high-performance electrocatalysts for CO2 reduction. In-situ Raman spectroscopy was utilized to reveal and understand the formation of CuInO2 nanoparticles based on the Cu2O pre-catalyst onto an interlayer of indium tin oxide under the electrochemical reaction. Density function theory calculation and ex-situ X-ray diffraction further prove the formation of CuInO2 nanoparticles during vigorous catalysis. The findings give important clues on how Cu-based electrocatalysts can be formed into more active materials and can provide inspiration for other Cu-based intermetallic oxides for high-efficiency CO2 reduction.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. , p. 87
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1791
Keywords [en]
Alkaline water splitting, CO2 reduction, Electrocatalyst, In-situ Raman spectroscopy.
National Category
Chemical Sciences Engineering and Technology
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-380575ISBN: 978-91-513-0620-9 (print)OAI: oai:DiVA.org:uu-380575DiVA, id: diva2:1300579
Public defence
2019-05-21, Polhemsalen, 10134, Ångström Laboratory, Lägerhyddsv. 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2019-04-29 Created: 2019-03-29 Last updated: 2019-06-18
List of papers
1. Controlled crystal growth orientation and surface charge effects in self-assembled nickel oxide nanoflakes and their activity for the oxygen evolution reaction
Open this publication in new window or tab >>Controlled crystal growth orientation and surface charge effects in self-assembled nickel oxide nanoflakes and their activity for the oxygen evolution reaction
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2017 (English)In: International journal of hydrogen energy, ISSN 0360-3199, E-ISSN 1879-3487, Vol. 42, no 47, p. 28397-28407Article in journal (Refereed) Published
Abstract [en]

Although sustainable hydrogen production from solar energy is a promising route for the future, the cost of the necessary photovoltaic and photoelectrochemical devices as well as a lack of detailed understanding and control of catalyst interfaces in nanomaterials with high catalytic activity are the largest impediments to commercial implementation. Here, we report how a higher catalytic efficiency can be achieved by utilizing an earth-abundant Nickel oxide (NiO) catalyst via an improved control of the crystalline growth orientation and self-assembly. The relationship between the surface charge and the morphology of the nano-catalysts is investigated using a hydrothermal method where the pH is utilized to control both the crystal growth direction and crystallization of Ni(OH)2 and eventually in NiO, where the self-assembly properties of nanoflakes (NFs) into hierarchical flower-like nickel oxide NFs depend on balancing of forces during synthesis. The surface charge ofthe NiO at different pH values was measured with electrophoretic dynamic light scattering (EDLS) and is known to be closely related to that of Ni(OH)2 and is here utilized to control the relative change in the surface charge in the precursor solution. By preparing NiO NFs under variation of the pH conditions of the precursor Ni(OH)2 system, the surface energies of exposed lattice planes of the growing nanostructures can be altered and an enhanced crystal growth orientation in a different direction can be controlled. Specifically, the [111] and [220] growth orientation in cubic NiO can be favored or suppressed with respect to the [200] direction. Benefiting from the large surface area provided by the mesoporous NiO NFs, the catalyst electrode exhibits high activity toward the oxygen evolution reactions in alkaline electrolyte. The NiO nanostructure synthesized at pH 10 displays oxygen evolution reaction (OER) overpotential of 0.29 V and 0.35 V versus the reversible hydrogen electrode (RHE) at 1 mA cm2 and 10 mA cm2 current density, respectively. This is compared to commercial NiO with more than 0.15 V additional overpotential and the same or lower overpotential compared to RuO2 and IrO2 at alkaline conditions. The results show that the OER catalytic activity can be drastically increased by a detailed control of the crystal growth orientation and the self-assembly behavior where the active surface charge around the point of zero charge during synthesis of the metal hydroxides/oxides is introduced as an important design principle for producing efficient electrocatalysts.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Nickel oxide Electrocatalyst, Crystalline growth directio, n Oxygen evolution reaction, Surface charge
National Category
Nano Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-334825 (URN)10.1016/j.ijhydene.2017.09.117 (DOI)000416495200025 ()
Funder
Swedish Energy AgencySwedish Research Council
Available from: 2017-11-28 Created: 2017-11-28 Last updated: 2019-03-29Bibliographically approved
2. In operando Raman spectroscopy of surface phase transformation in iron-doped nickel oxide nanosheets for enhanced overall water splitting
Open this publication in new window or tab >>In operando Raman spectroscopy of surface phase transformation in iron-doped nickel oxide nanosheets for enhanced overall water splitting
(English)In: Nano Energy, ISSN 2211-2855, E-ISSN 2211-3282Article in journal (Refereed) Submitted
National Category
Physical Chemistry Inorganic Chemistry Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-380521 (URN)
Available from: 2019-03-28 Created: 2019-03-28 Last updated: 2019-03-29
3. An electrochemical impedance study of alkaline water splitting using nickel (iron) oxides nanosheets
Open this publication in new window or tab >>An electrochemical impedance study of alkaline water splitting using nickel (iron) oxides nanosheets
(English)In: Article in journal (Refereed) Submitted
National Category
Physical Chemistry Other Chemistry Topics Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-380523 (URN)
Available from: 2019-03-28 Created: 2019-03-28 Last updated: 2019-03-29
4. Direct observation of active catalyst surface phases and the effect of dynamic self-optimization in NiFe-layered double hydroxides for alkaline water splitting
Open this publication in new window or tab >>Direct observation of active catalyst surface phases and the effect of dynamic self-optimization in NiFe-layered double hydroxides for alkaline water splitting
2019 (English)In: Energy & Environmental Science, ISSN 1754-5692, E-ISSN 1754-5706, Vol. 12, no 2, p. 572-581Article in journal (Refereed) Published
Abstract [en]

Earth-abundant transition metal-based compounds are of high interest as catalysts for sustainable hydrogen fuel generation. The realization of effective electrolysis of water, however, is still limited by the requirement of a high sustainable driving potential above thermodynamic requirements. Here, we report dynamically self-optimized (DSO) NiFe layered double hydroxide (LDH) nanosheets with promising bi-functional performance. Compared with pristine NiFe LDH, DSO NiFe LDH exhibits much lower overpotential for the hydrogen evolution reaction (HER), even outperforming platinum. Under 1 M KOH aqueous electrolyte, the bi-functional DSO catalysts show an overpotential of 184 and -59 mV without iR compensation for oxygen evolution reaction (OER) and HER at 10 mA cm(-2). The material system operates at 1.48 V and 1.29 V to reach 10 and 1 mA cm(-2) in two-electrode measurements, corresponding to 83% and 95% electricity-to-fuel conversion efficiency with respect to the lower heating value of hydrogen. The material is seen to dynamically reform the active phase of the surface layer during HER and OER, where the pristine and activated catalysts are analyzed with ex situ XPS, SAED and EELS as well as with in situ Raman spectro-electrochemistry. The results show transformation into different active interfacial species during OER and HER, revealing a synergistic interplay between iron and nickel in facilitating water electrolysis.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY, 2019
National Category
Other Chemical Engineering
Identifiers
urn:nbn:se:uu:diva-379268 (URN)10.1039/c8ee03282c (DOI)000459741700005 ()
Funder
Swedish Energy AgencySwedish Research Council, VR-2016-03713Swedish Research Council Formas, 2016-00908Knut and Alice Wallenberg Foundation
Available from: 2019-03-18 Created: 2019-03-18 Last updated: 2019-03-29Bibliographically approved
5. Unravelling in-situ formation of highly active mixed metal oxide CuInO2 nanoparticles during CO2 electroreduction
Open this publication in new window or tab >>Unravelling in-situ formation of highly active mixed metal oxide CuInO2 nanoparticles during CO2 electroreduction
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2018 (English)In: Nano Energy, ISSN 2211-2855, E-ISSN 2211-3282, Vol. 49, p. 40-50Article in journal (Refereed) Published
Abstract [en]

Technologies and catalysts for converting carbon dioxide (CO2) to immobile products are of high interest to minimize greenhouse effects. Copper(I) is a promising catalytic active state of copper but hampered by the inherent instability in comparison to copper(II) or copper(0). Here, we report a stabilization of the catalytic active state of copper(I) by the formation of a mixed metal oxide CuInO2 nanoparticle during the CO2 electroreduction. Our result shows the incorporation of nanoporous Sn:In2O3 interlayer to Cu2O pre-catalyst system lead to the formation of CuInO2 nanoparticles with remarkably higher activity for CO2 electroreduction at lower overpotential in comparison to the conventional Cu nanoparticles derived from sole Cu2O. Operando Raman spectroelectrochemistry is employed to in-situ monitor the process of nanoparticles formation during the electrocatalytic process. The experimental data are collaborated with DFT calculations to provide insight into the electro-formation of the type of Cu-based mixed metal oxide catalyst during the CO2 electroreduction, where a formation mechanism via copper ion diffusion across the substrate is suggested.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2018
Keywords
Cuprous oxide, Copper indium oxide, CO2 electroreduction, Operando Raman spectroelectrochemistry, Density functional theory
National Category
Physical Chemistry Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-358275 (URN)10.1016/j.nanoen.2018.04.013 (DOI)000434829500006 ()
Funder
Stiftelsen Olle Engkvist ByggmästareGöran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of TechnologyJ. Gust. Richert stiftelseSwedish National Infrastructure for Computing (SNIC), 2017-1-57Swedish National Infrastructure for Computing (SNIC), 2016-10-23
Available from: 2018-08-30 Created: 2018-08-30 Last updated: 2019-03-29Bibliographically approved

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