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Electrocatalysts for sustainable hydrogen energy: disordered and heterogeneous nanomaterials
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University.
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With the current global greenhouse gas emissions, our remaining carbon budget is depleted in only 7 years. After that, several biophysical systems are predicted to collapse such as the arctic ice, coral reefs and the permafrost, leading to potentially irreversible consequences. Our emissions are strongly correlated to access of energy and even if we are aware of the planetary emergency today, our emissions still continue to grow. Electrical vehicles have the possibility to reduce the emissions in the transportation sector significantly. However, these vehicles are still expensive and impractical for long-distance or heavy transportation. While political actions and technological development are essential to keep prices down, the driving dis- tance can be increased by replacing the batteries for onboard electricity production. 

In hydrogen fuel cells, electricity is produced by combining hydrogen gas (H2) and oxygen with only water as the by-product and if employed in electrical vehicles, distances of 500 km are enabled with a refueling time in 5 minutes. For other uses than in vehicles, H2 is also promising for large-scale electricity storage and for several industrial processes such as manufacturing CO2-free steel, ammonia and synthetic fuels. However, today most H2 production methods relies on fossil fuels and releases huge amounts of CO2. 

Electrolysis of water is an alternative production method where H2, along with oxygen are produced from water. To split the water, electricity has to be added and if renewable energy sources are used, the method has zero emissions and is considered most promising for a sustainable hydrogen energy economy. The tech- nique is relatively expensive compared to the fossil fuel-based methods and relies on rare noble metals such as platinum as catalysts for decreasing the required energy to split water. For large scale productions, these metals need to be replaced by more sustainable and abundant catalysts to lower the cost and minimize the environmental impacts. 

In this thesis we have investigated such candidates for the water splitting reaction but also to some extent for the oxygen reduction reaction in fuel cells. By combining theory and experiments we hope to aid in the development and facilitate a transition to clean hydrogen energy. We find among other things that i) defects in catalytic materials plays a significant role the performance and efficiency, and that ii) heterogeneity influence the adsorption energies of reaction intermediates and hence the catalytic efficiency and iii) while defects are not often studied for electrocatalytic reactions, these may inspire for novel materials in the future. 

Place, publisher, year, edition, pages
Umeå: Umeå Universitet , 2021. , p. 88
Keywords [en]
Water splitting, Electrochemistry, Nanomaterials, Density functional theory, Hydrogen evolution, MoS2, Fuel cell
National Category
Condensed Matter Physics
Research subject
nanomaterials; Physics; Physical Chemistry
Identifiers
URN: urn:nbn:se:umu:diva-180130ISBN: 978-91-7855-482-9 (electronic)ISBN: 978-91-7855-481-2 (print)OAI: oai:DiVA.org:umu-180130DiVA, id: diva2:1528352
Public defence
2021-03-11, BIO.A.206 – Aula Anatomica, Umeå, 09:00 (English)
Opponent
Supervisors
Available from: 2021-02-18 Created: 2021-02-15 Last updated: 2021-02-16Bibliographically approved
List of papers
1. Stabilizing Active Edge Sites in Semicrystalline Molybdenum Sulfide by Anchorage on Nitrogen-Doped Carbon Nanotubes for Hydrogen Evolution Reaction
Open this publication in new window or tab >>Stabilizing Active Edge Sites in Semicrystalline Molybdenum Sulfide by Anchorage on Nitrogen-Doped Carbon Nanotubes for Hydrogen Evolution Reaction
Show others...
2016 (English)In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 26, no 37, p. 6766-6776Article in journal (Refereed) Published
Abstract [en]

Finding an abundant and cost-effective electrocatalyst for the hydrogen evolu-tion reaction (HER) is crucial for a global production of hydrogen from water electrolysis. This work reports an exceptionally large surface area hybrid catalyst electrode comprising semicrystalline molybdenum sulfi de (MoS 2+ x) catalystattached on a substrate based on nitrogen-doped carbon nanotubes (N-CNTs), which are directly grown on carbon fiber paper (CP). It is shown here that nitrogen-doping of the carbon nanotubes improves the anchoring of MoS 2+ xcatalyst compared to undoped carbon nanotubes and concurrently stabilizes a semicrystalline structure of MoS 2+ x with a high exposure of active sites for HER. The well-connected constituents of the hybrid catalyst are shown to facilitate electron transport and as a result of the good attributes, the MoS 2+ x/N-CNT/CPelectrode exhibits an onset potential of −135 mV for HER in 0.5 M H2SO4, a Tafel slope of 36 mV dec −1, and high stability at a current density of −10 mA cm −2.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2016
Keywords
carbon paper, hydrogen evolution reaction, molybdenum disulfide—MoS2, nitrogen doped carbon nanotubes, water splitting catalysts
National Category
Inorganic Chemistry Other Chemical Engineering Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-128753 (URN)10.1002/adfm.201601994 (DOI)000384810300006 ()2-s2.0-84979085766 (Scopus ID)
Available from: 2016-12-14 Created: 2016-12-14 Last updated: 2023-03-23Bibliographically approved
2. Stable Sulfur‐Intercalated 1T′ MoS2 on Graphitic Nanoribbons as Hydrogen Evolution Electrocatalyst
Open this publication in new window or tab >>Stable Sulfur‐Intercalated 1T′ MoS2 on Graphitic Nanoribbons as Hydrogen Evolution Electrocatalyst
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2018 (English)In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 28, no 46, article id 1802744Article in journal (Refereed) Published
Abstract [en]

The metastable 1T′ polymorph of molybdenum disulfide (MoS2) has shown excellent catalytic activity toward the hydrogen evolution reaction (HER) in water‐splitting applications. Its basal plane exhibits high catalytic activity comparable to the edges in 2H MoS2 and noble metal platinum. However, the production and application of this polymorph are limited by its lower energetic stability compared to the semiconducting 2H MoS2 phase. Here, the production of stable intercalated 1T′ MoS2 nanosheets attached on graphitic nanoribbons is reported. The intercalated 1T′ MoS2 exhibits a stoichiometric S:Mo ratio of 2.3 (±0.1):1 with an expanded interlayer distance of 10 Å caused by a sulfur‐rich intercalation agent and is stable at room temperature for several months even after drying. The composition, structure, and catalytic activity toward HER are investigated both experimentally and theoretically. It is concluded that the 1T′ MoS2 phase is stabilized by the intercalated agents, which further improves the basal planes′ catalytic activity toward HER.

Place, publisher, year, edition, pages
WILEY-VCH VERLAG GMBH, 2018
Keywords
DFT calculations, hydrogen evolution reaction, intercalation, MoS2, transition metal chalcogenides
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-154948 (URN)10.1002/adfm.201802744 (DOI)000449887300019 ()2-s2.0-85054188827 (Scopus ID)
Available from: 2019-01-07 Created: 2019-01-07 Last updated: 2023-03-23Bibliographically approved
3. Surface activation of graphene nanoribbons for oxygen reduction reaction by nitrogen doping and defect engineering: An ab initio study
Open this publication in new window or tab >>Surface activation of graphene nanoribbons for oxygen reduction reaction by nitrogen doping and defect engineering: An ab initio study
2018 (English)In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 137, p. 349-357Article in journal (Refereed) Published
Abstract [en]

Introducing heteroatoms and creating structural defects on graphene is a common and rather successful strategy to transform its inert basal plane into an efficient metal-free electrocatalyst for oxygen reduction reaction (ORR). However, the intricate atomic configuration of defective graphenes difficult their optimization as ORR electrocatalysts, where not only a large density of active sites is desirable, but also excellent electrical conductivity is required. Therefore, we used density functional theory to investigate the current-voltage characteristics and the catalytic active sites towards ORR of nitrogen-doped and defective graphene by using 8 zig-zag graphene nanoribbons as model systems. Detailed ORR catalytic activity maps are created for ten different systems showing the distribution of catalytic hot spots generated by each defect. Subsequently, the use of both current-voltage characteristics and catalytic activity maps allow to exclude inefficient systems that exhibit either low electrical conductivity or have adsorption energies far from optimal. Our study highlights the importance of considering not only the interaction energy of reaction intermediates to design electrocatalysts, but also the electrical conductivity of such configurations. We believe that this work is important for future experimental studies by providing insights on the use of graphene as a catalyst towards the ORR reaction. 

Place, publisher, year, edition, pages
Elsevier, 2018
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-151037 (URN)10.1016/j.carbon.2018.05.050 (DOI)000440661700035 ()2-s2.0-85047971117 (Scopus ID)
Available from: 2018-09-05 Created: 2018-09-05 Last updated: 2022-04-04Bibliographically approved
4. Stainless Steel as A Bi-Functional Electrocatalyst – A Top-Down Approach
Open this publication in new window or tab >>Stainless Steel as A Bi-Functional Electrocatalyst – A Top-Down Approach
2019 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 12, no 13, article id 2128Article in journal (Refereed) Published
Abstract [en]

For a hydrogen economy to be viable, clean and economical hydrogen production methods are vital. Electrolysis of water is a promising hydrogen production technique with zero emissions, but suffer from relatively high production costs. In order to make electrolysis of water sustainable, abundant, and efficient materials has to replace expensive and scarce noble metals as electrocatalysts in the reaction cells. Herein, we study activated stainless steel as a bi-functional electrocatalyst for the full water splitting reaction by taking advantage of nickel and iron suppressed within the bulk. The final electrocatalyst consists of a stainless steel mesh with a modified surface of layered NiFe nanosheets. By using a top down approach, the nanosheets stay well anchored to the surface and maintain an excellent electrical connection to the bulk structure. At ambient temperature, the activated stainless steel electrodes produce 10 mA/cm(2) at a cell voltage of 1.78 V and display an onset for water splitting at 1.68 V in 1M KOH, which is close to benchmarking nanosized catalysts. Furthermore, we use a scalable activation method using no externally added electrocatalyst, which could be a practical and cheap alternative to traditionally catalyst-coated electrodes.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
water splitting, electrolysis, bifunctional, electrocatalysts, hydrogen evolution reaction, oxygen olution reaction, sustainable, stainless steel, nano
National Category
Other Chemical Engineering Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-162337 (URN)10.3390/ma12132128 (DOI)000477043900092 ()31269744 (PubMedID)2-s2.0-85068826298 (Scopus ID)
Available from: 2019-08-16 Created: 2019-08-16 Last updated: 2022-04-04Bibliographically approved
5. Theoretical Analysis of Surface Active Sites in Defective 2H and 1T ' MoS2 Polymorphs for Hydrogen Evolution Reaction: Quantifying the Total Activity of Point Defects
Open this publication in new window or tab >>Theoretical Analysis of Surface Active Sites in Defective 2H and 1T ' MoS2 Polymorphs for Hydrogen Evolution Reaction: Quantifying the Total Activity of Point Defects
2020 (English)In: Advanced Theory and Simulations, E-ISSN 2513-0390, Vol. 3, no 3, article id 1900213Article in journal (Refereed) Published
Abstract [en]

Defect engineering is a common and promising strategy to improve the catalytic activity of layered structures such as MoS2, where in particular the 2H and 1T ' polymorphs have been under intense study for their activity toward the hydrogen evolution reaction. However, the large variety of defects, each with its own distinct and usually unknown effects, complicates the design and optimization of such defective materials. Therefore, it is relevant to characterize in detail the effect of individual defects and to be able to combine these observations to describe more complex materials, such as those seen experimentally. Therefore, nine point defects (antisites defects and vacancies) are theoretically studied on single layer 1T, 1T ', and 2H MoS2 polymorphs, and the variation and spatial distribution in the active sites are identified. It is found that all defective 1T ' monolayers exhibit an increase in the exchange current density of at least 2.3 times when compared to pristine 1T ' MoS2, even if a reduced number of active sites are observed. The results are later used to propose a methodology to study materials containing a mixture of crystal phases, or other alterations that cause inhomogeneous changes in the activity of catalytic sites.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2020
Keywords
density functional theory, hydrogen evolution reaction, molybdenum disulfide, point defects, transition metal chalcogenides
National Category
Condensed Matter Physics Materials Chemistry
Identifiers
urn:nbn:se:umu:diva-168157 (URN)10.1002/adts.201900213 (DOI)000508585100001 ()2-s2.0-85081036235 (Scopus ID)
Available from: 2020-03-18 Created: 2020-03-18 Last updated: 2023-03-24Bibliographically approved
6. Hydrogen Evolution Reaction Activity of Heterogeneous Materials: A Theoretical Model
Open this publication in new window or tab >>Hydrogen Evolution Reaction Activity of Heterogeneous Materials: A Theoretical Model
2020 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 124, no 38, p. 20911-20921Article in journal (Refereed) Published
Abstract [en]

In this study, we present a new comprehensive methodology to quantify the catalytic activity of heterogeneous materials for the hydrogen evolution reaction (HER) using ab initio simulations. The model is composed of two parts. First, the equilibrium hydrogen coverage is obtained by an iterative evaluation of the hydrogen adsorption free energies (ΔGH) using density functional theory calculations. Afterward, the ΔGH are used in a microkinetic model to provide detailed characterizations of the entire HER considering all three elementary steps, i.e., the discharge, atom + ion, and combination reactions, without any prior assumptions of rate-determining steps. The microkinetic model takes the equilibrium and potential-dependent characteristics into account, and thus both exchange current densities and Tafel slopes are evaluated. The model is tested on several systems, from polycrystalline metals to heterogeneous molybdenum disulfide (MoS2), and by comparing to experimental data, we verify that our model accurately predicts their experimental exchange current densities and Tafel slopes. Finally, we present an extended volcano plot that correlates the electrical current densities of each elementary reaction step to the coverage-dependent ΔGH.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2020
National Category
Physical Chemistry Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-176146 (URN)10.1021/acs.jpcc.0c05243 (DOI)000575823600029 ()2-s2.0-85095916115 (Scopus ID)
Funder
Swedish Research Council, 2017-04862Swedish Research Council, 2018-03937Swedish Energy Agency, 45419-1Olle Engkvists stiftelse, 186-0637
Available from: 2020-10-22 Created: 2020-10-22 Last updated: 2023-03-24Bibliographically approved
7. Magnetically collected platinum/nickel alloy nanoparticles – insight into low noble metal content catalysts for hydrogen evolution reaction
Open this publication in new window or tab >>Magnetically collected platinum/nickel alloy nanoparticles – insight into low noble metal content catalysts for hydrogen evolution reaction
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-180128 (URN)
Available from: 2021-02-15 Created: 2021-02-15 Last updated: 2021-02-16
8. Solar-driven water splitting at 13.8 % solar-to-hydrogen efficiency by an earth-abundant PV-electrolyzer
Open this publication in new window or tab >>Solar-driven water splitting at 13.8 % solar-to-hydrogen efficiency by an earth-abundant PV-electrolyzer
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2021 (English)In: ACS Sustainable Chemistry and Engineering, E-ISSN 2168-0485, Vol. 9, no 42, p. 14070-14078Article in journal (Refereed) Published
Abstract [en]

We present the synthesis and characterization of an efficient and low cost solar-driven electrolyzer consisting of Earth-abundant materials. The trimetallic NiFeMo electrocatalyst takes the shape of nanometer-sized flakes anchored to a fully carbon-based current collector comprising a nitrogen-doped carbon nanotube network, which in turn is grown on a carbon fiber paper support. This catalyst electrode contains solely Earth-abundant materials, and the carbon fiber support renders it effective despite a low metal content. Notably, a bifunctional catalyst–electrode pair exhibits a low total overpotential of 450 mV to drive a full water-splitting reaction at a current density of 10 mA cm–2 and a measured hydrogen Faradaic efficiency of ∼100%. We combine the catalyst–electrode pair with solution-processed perovskite solar cells to form a lightweight solar-driven water-splitting device with a high peak solar-to-fuel conversion efficiency of 13.8%.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2021
Keywords
Solar-driven electrolysis, Earth-abundant materials, Nanostructured catalyst, Perovskite solar cells, Cost analysis
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-180129 (URN)10.1021/acssuschemeng.1c03565 (DOI)000711203000009 ()2-s2.0-85118127026 (Scopus ID)
Note

Originally included in thesis in manuscript form.

Available from: 2021-02-15 Created: 2021-02-15 Last updated: 2023-09-05Bibliographically approved

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