Prof. Dr. Katharina Krischer
Academic Research Focus
- Dynamics of non-equilibrium systems
- Reaction dynamics of electrochemical systems, such as the CO2 reduction and the hydrogen evolution reaction
- Theoretical studies to address fundamental laws that govern behavior in complex systems (e.g. collective dynamics of coupled oscillators and pattern formation in reaction-diffusion systems)
Fields of Application
Photo-(electro)catalysis
Publications
14.
M J Feil, S Leisibach, M Becherer, K Krischer
Stability of the Au/electrolyte interface during hydrogen evolution: A Cyclic Plasmo-Voltammetry study Journal Article
In: Electrochimica Acta, vol. 513, pp. 145509, 2025, ISSN: 0013-4686.
@article{nokey,
title = {Stability of the Au/electrolyte interface during hydrogen evolution: A Cyclic Plasmo-Voltammetry study},
author = {M J Feil and S Leisibach and M Becherer and K Krischer},
url = {https://www.sciencedirect.com/science/article/pii/S0013468624017456},
doi = {https://doi.org/10.1016/j.electacta.2024.145509},
issn = {0013-4686},
year = {2025},
date = {2025-02-10},
journal = {Electrochimica Acta},
volume = {513},
pages = {145509},
abstract = {Metal-electrolyte interfaces are dynamic entities, the potential and electrolyte dependent mobility of the metal atoms leading to surface restructuring with possible dissolution and degradation. In this work, we investigate the stability of the Au/aqueous electrolyte interface with in situ differential Cyclic Plasmo-Voltammetry (dCPV), augmented by ex situ atomic force microscopy and finite differential time domain simulations. We demonstrate that even the onset of hydrogen evolution is accompanied by pronounced morphological changes of the interface which are by far more prominent than those occurring during Au oxidation and reduction. Furthermore, the stability of the interface heavily depends on pH, the degradation of the electrode being considerably stronger in acidic than in neutral electrolyte. In addition, a clear hydrogen adsorption peak was observed in neutral electrolytes during the cathodic scan, which was more pronounced on a freshly prepared Au electrode than on an aged one. The measured dCPVs in acidic and neutral electrolytes can be explained consistently assuming that (1) adsorbed hydrogen is absorbed into the subsurface region of the Au electrode once HER starts; its subsequent removal as molecular hydrogen causes morphological changes; (2) in the presence of metal cations, adsorbed hydrogen is stabilized through the formation of ternary metal hydrides on the gold surface that stabilize the surface Au-H bonds and hinder further absorption of H into the subsurface region as well as the release of hydrogen into the electrolyte.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Metal-electrolyte interfaces are dynamic entities, the potential and electrolyte dependent mobility of the metal atoms leading to surface restructuring with possible dissolution and degradation. In this work, we investigate the stability of the Au/aqueous electrolyte interface with in situ differential Cyclic Plasmo-Voltammetry (dCPV), augmented by ex situ atomic force microscopy and finite differential time domain simulations. We demonstrate that even the onset of hydrogen evolution is accompanied by pronounced morphological changes of the interface which are by far more prominent than those occurring during Au oxidation and reduction. Furthermore, the stability of the interface heavily depends on pH, the degradation of the electrode being considerably stronger in acidic than in neutral electrolyte. In addition, a clear hydrogen adsorption peak was observed in neutral electrolytes during the cathodic scan, which was more pronounced on a freshly prepared Au electrode than on an aged one. The measured dCPVs in acidic and neutral electrolytes can be explained consistently assuming that (1) adsorbed hydrogen is absorbed into the subsurface region of the Au electrode once HER starts; its subsequent removal as molecular hydrogen causes morphological changes; (2) in the presence of metal cations, adsorbed hydrogen is stabilized through the formation of ternary metal hydrides on the gold surface that stabilize the surface Au-H bonds and hinder further absorption of H into the subsurface region as well as the release of hydrogen into the electrolyte.
13.
L B T De Kam, T L Maier, K Krischer
Electrolyte effects on the alkaline hydrogen evolution reaction: A mean-field approach Journal Article
In: Electrochimica Acta, vol. 497, pp. 144530, 2024, ISSN: 0013-4686.
@article{nokey,
title = {Electrolyte effects on the alkaline hydrogen evolution reaction: A mean-field approach},
author = {L B T De Kam and T L Maier and K Krischer},
url = {https://www.sciencedirect.com/science/article/pii/S0013468624007709},
doi = {https://doi.org/10.1016/j.electacta.2024.144530},
issn = {0013-4686},
year = {2024},
date = {2024-09-01},
journal = {Electrochimica Acta},
volume = {497},
pages = {144530},
abstract = {This paper introduces the combination of an advanced double-layer model with electrochemical kinetics to explain electrolyte effects on the alkaline hydrogen evolution reaction. It is known from experimental studies that the alkaline hydrogen evolution current shows a strong dependence on the concentration and identity of cations in the electrolyte, but is independent of pH. To explain these effects, we formulate the faradaic current in terms of the electric potential in the double layer, which is calculated using a mean-field model that takes into account the cation and anion sizes as well as the electric dipole moment of water molecules. We propose that the Volmer step consists of two activated processes: a water reduction sub-step, and a sub-step in which OH− is transferred away from the reaction plane through the double layer. Either of these sub-steps may limit the rate. The proposed models for these sub-steps qualitatively explain experimental observations, including cation effects, pH-independence, and the trend reversal between gold and platinum electrodes. We also assess the quantitative accuracy of the water-reduction-limited current model; we suggest that the predicted functional relationship is valid as long as the hydrogen bonding structure of water near the electrode is sufficiently maintained.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
This paper introduces the combination of an advanced double-layer model with electrochemical kinetics to explain electrolyte effects on the alkaline hydrogen evolution reaction. It is known from experimental studies that the alkaline hydrogen evolution current shows a strong dependence on the concentration and identity of cations in the electrolyte, but is independent of pH. To explain these effects, we formulate the faradaic current in terms of the electric potential in the double layer, which is calculated using a mean-field model that takes into account the cation and anion sizes as well as the electric dipole moment of water molecules. We propose that the Volmer step consists of two activated processes: a water reduction sub-step, and a sub-step in which OH− is transferred away from the reaction plane through the double layer. Either of these sub-steps may limit the rate. The proposed models for these sub-steps qualitatively explain experimental observations, including cation effects, pH-independence, and the trend reversal between gold and platinum electrodes. We also assess the quantitative accuracy of the water-reduction-limited current model; we suggest that the predicted functional relationship is valid as long as the hydrogen bonding structure of water near the electrode is sufficiently maintained.
12.
T L Maier, L B T. De Kam, M Golibrzuch, T Angerer, M Becherer, K Krischer
How Metal/Insulator Interfaces Enable an Enhancement of the Hydrogen Evolution Reaction Kinetics Journal Article
In: ChemElectroChem, vol. 11, no. 11, pp. e202400109, 2024, ISSN: 2196-0216.
@article{nokey,
title = {How Metal/Insulator Interfaces Enable an Enhancement of the Hydrogen Evolution Reaction Kinetics},
author = {T L Maier and L B T. De Kam and M Golibrzuch and T Angerer and M Becherer and K Krischer},
url = {https://chemistry-europe.onlinelibrary.wiley.com/doi/abs/10.1002/celc.202400109},
doi = {https://doi.org/10.1002/celc.202400109},
issn = {2196-0216},
year = {2024},
date = {2024-05-16},
journal = {ChemElectroChem},
volume = {11},
number = {11},
pages = {e202400109},
abstract = {Abstract The nanostructuring of electrodes is a common way of increasing electrocatalytic activity. Yet, the fact that the presence of insulating material in nanostructured composites can have a positive effect on efficiency was an unexpected recent finding. The rate enhancement has been linked to different electric fields at the insulator and metal interfaces, facilitating enhanced transport of reaction products into the bulk electrolyte. In this article, we further uncover the origin of the rate enhancement with parameter studies and simulations. We experimentally investigate various parameter dependencies of the alkaline Hydrogen Evolution Reaction (HER) on well-defined nanometer-sized Au arrays embedded in a silicon nitride insulating layer. We find a significant enhancement of the HER for all experimental conditions and opposite activity trends with pH, electrolyte concentration and the cationic species compared to a continuous Au electrode. Using a mean field model, we quantify the electrostatic interfacial pressure above the Au and the insulator patches. Combining the double layer simulations with rate equations, we demonstrate that all parameter variations can be consistently explained by the fact that the double layer structure above the insulator patches is much less rigid than above the metal islands and is independent of the applied potential.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract The nanostructuring of electrodes is a common way of increasing electrocatalytic activity. Yet, the fact that the presence of insulating material in nanostructured composites can have a positive effect on efficiency was an unexpected recent finding. The rate enhancement has been linked to different electric fields at the insulator and metal interfaces, facilitating enhanced transport of reaction products into the bulk electrolyte. In this article, we further uncover the origin of the rate enhancement with parameter studies and simulations. We experimentally investigate various parameter dependencies of the alkaline Hydrogen Evolution Reaction (HER) on well-defined nanometer-sized Au arrays embedded in a silicon nitride insulating layer. We find a significant enhancement of the HER for all experimental conditions and opposite activity trends with pH, electrolyte concentration and the cationic species compared to a continuous Au electrode. Using a mean field model, we quantify the electrostatic interfacial pressure above the Au and the insulator patches. Combining the double layer simulations with rate equations, we demonstrate that all parameter variations can be consistently explained by the fact that the double layer structure above the insulator patches is much less rigid than above the metal islands and is independent of the applied potential.
11.
M J Feil, T L Maier, M Golibrzuch, A C Sterr, M Becherer, K Krischer
Characterization of Different Au/Electrolyte Interfaces via In Situ Differential Cyclic Plasmo-Voltammetry Journal Article
In: The Journal of Physical Chemistry C, vol. 127, no. 40, pp. 20137-20145, 2023, ISSN: 1932-7447.
@article{nokey,
title = {Characterization of Different Au/Electrolyte Interfaces via In Situ Differential Cyclic Plasmo-Voltammetry},
author = {M J Feil and T L Maier and M Golibrzuch and A C Sterr and M Becherer and K Krischer},
url = {https://doi.org/10.1021/acs.jpcc.3c04727},
doi = {10.1021/acs.jpcc.3c04727},
issn = {1932-7447},
year = {2023},
date = {2023-09-28},
journal = {The Journal of Physical Chemistry C},
volume = {127},
number = {40},
pages = {20137-20145},
abstract = {In this article, we describe an improved method that uses in situ plasmonic spectroscopy to reliably track changes of the metal\textendashelectrolyte interface over a large potential window. Utilizing the specific sensitivity of the plasmonic resonance toward changes in the interfacial properties of nanoparticles (NPs), processes such as double-layer charging, surface oxidation/reduction, adsorption and desorption of anions, as well as metal under- and overpotential deposition are resolved. The main contributions to this signal are changes in the charge of the NPs and chemical interface damping due to the adsorbed species. We employ highly homogeneous macroscopic Au nanoarrays with controlled interfaces produced by lift-off nanoimprint lithography (LO-NIL) as the working electrodes for multiparticle differential cyclic plasmo-voltammetry (dCPV). First, plasmonic signals are recorded and compared to known electrochemical processes before the plasmonic signals are used to gain insights beyond those achievable by electrochemical means. These include observation of forced HSO4\textendash and H2PO4\textendash desorption by the onset of the Au oxidation and resolution of the different steps of the monolayer buildup during Cu underpotential deposition on Au.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In this article, we describe an improved method that uses in situ plasmonic spectroscopy to reliably track changes of the metal–electrolyte interface over a large potential window. Utilizing the specific sensitivity of the plasmonic resonance toward changes in the interfacial properties of nanoparticles (NPs), processes such as double-layer charging, surface oxidation/reduction, adsorption and desorption of anions, as well as metal under- and overpotential deposition are resolved. The main contributions to this signal are changes in the charge of the NPs and chemical interface damping due to the adsorbed species. We employ highly homogeneous macroscopic Au nanoarrays with controlled interfaces produced by lift-off nanoimprint lithography (LO-NIL) as the working electrodes for multiparticle differential cyclic plasmo-voltammetry (dCPV). First, plasmonic signals are recorded and compared to known electrochemical processes before the plasmonic signals are used to gain insights beyond those achievable by electrochemical means. These include observation of forced HSO4– and H2PO4– desorption by the onset of the Au oxidation and resolution of the different steps of the monolayer buildup during Cu underpotential deposition on Au.
10.
T L Maier, L B De Kam, M Golibrzuch, T Angerer, M Becherer, K Krischer
How Metal/Insulator Interfaces Enable the Enhancement of the Hydrogen Evolution Reaction Kinetics in Two Ways Journal Article
In: arXiv preprint arXiv:2309.02229, 2023.
@article{nokey,
title = {How Metal/Insulator Interfaces Enable the Enhancement of the Hydrogen Evolution Reaction Kinetics in Two Ways},
author = {T L Maier and L B De Kam and M Golibrzuch and T Angerer and M Becherer and K Krischer},
url = {https://arxiv.org/abs/2309.02229},
doi = {https://doi.org/10.48550/arXiv.2309.02229},
year = {2023},
date = {2023-09-05},
journal = {arXiv preprint arXiv:2309.02229},
abstract = {Laterally nanostructured surfaces give rise to a new dimension of understanding and improving electrochemical reactions. In this study, we present a peculiar mechanism appearing at a metal/insulator interface, which can significantly enhance the Hydrogen Evolution Reaction (HER) from water reduction by altering the local reaction conditions in two ways: facilitated adsorption of hydrogen on the metal catalyst surface and improved transfer of ions through the double layer. The mechanism is uncovered using electrodes consisting of well-defined nanometer-sized metal arrays (Au, Cu, Pt) embedded in an insulator layer (silicon nitride), varying various parameters of both the electrode (size of the metal patches, catalyst material) and the electrolyte (cationic species, cation concentration, pH). In addition, simulations of the electrochemical double layer are carried out, which support the elaborated mechanism. Knowledge of this mechanism will enable new design principles for novel composite electrocatalytic systems.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Laterally nanostructured surfaces give rise to a new dimension of understanding and improving electrochemical reactions. In this study, we present a peculiar mechanism appearing at a metal/insulator interface, which can significantly enhance the Hydrogen Evolution Reaction (HER) from water reduction by altering the local reaction conditions in two ways: facilitated adsorption of hydrogen on the metal catalyst surface and improved transfer of ions through the double layer. The mechanism is uncovered using electrodes consisting of well-defined nanometer-sized metal arrays (Au, Cu, Pt) embedded in an insulator layer (silicon nitride), varying various parameters of both the electrode (size of the metal patches, catalyst material) and the electrolyte (cationic species, cation concentration, pH). In addition, simulations of the electrochemical double layer are carried out, which support the elaborated mechanism. Knowledge of this mechanism will enable new design principles for novel composite electrocatalytic systems.
9.
M Duportal, L M Berger, S A Maier, A Tittl, K Krischer
In: arXiv preprint arXiv:2307.10951, 2023.
@article{nokey,
title = {Multi-band metasurface-driven surface-enhanced infrared absorption spectroscopy for improved characterization of in-situ electrochemical reactions},
author = {M Duportal and L M Berger and S A Maier and A Tittl and K Krischer},
url = {https://arxiv.org/abs/2307.10951},
doi = {https://doi.org/10.48550/arXiv.2307.10951},
year = {2023},
date = {2023-07-20},
journal = {arXiv preprint arXiv:2307.10951},
abstract = {Surface-enhanced spectroscopy techniques are the method-of-choice to characterize adsorbed intermediates occurring during electrochemical reactions, which are crucial in realizing a green sustainable future. Characterizing species with low coverages or short lifetimes have so far been limited by low signal enhancement. Recently, metasurface-driven surface-enhanced infrared absorption spectroscopy (SEIRAS) has been pioneered as a promising narrowband technology to study single vibrational modes of electrochemical interfaces during CO oxidation. However, many reactions involve several species or configurations of adsorption that need to be monitored simultaneously requiring reproducible and broadband sensing platforms to provide a clear understanding of the underlying electrochemical processes. Here, we experimentally realize multi-band metasurface-driven SEIRAS for the in-situ study of electrochemical CO2 reduction on a Pt surface. We develop an easily reproducible and spectrally-tunable platinum nano-slot metasurface. Two CO adsorption configurations at 2030 cm-1 and 1840 cm-1 are locally enhanced as a proof of concept that can be extended to more vibrational bands. Our platform provides a 41-fold enhancement in the detection of characteristic absorption signals compared to conventional broadband electrochemically roughened platinum films. A straightforward methodology is outlined starting by baselining our system in CO saturated environment and clearly detecting both configurations of adsorption, in particular the hitherto hardly detectable CO bridge configuration. Then, thanks to the signal enhancement provided by our platform, we find that the CO bridge configuration on platinum does not play a significant role during CO2 reduction in an alkaline environment. We anticipate that our technology will guide researchers in developing similar sensing platforms.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Surface-enhanced spectroscopy techniques are the method-of-choice to characterize adsorbed intermediates occurring during electrochemical reactions, which are crucial in realizing a green sustainable future. Characterizing species with low coverages or short lifetimes have so far been limited by low signal enhancement. Recently, metasurface-driven surface-enhanced infrared absorption spectroscopy (SEIRAS) has been pioneered as a promising narrowband technology to study single vibrational modes of electrochemical interfaces during CO oxidation. However, many reactions involve several species or configurations of adsorption that need to be monitored simultaneously requiring reproducible and broadband sensing platforms to provide a clear understanding of the underlying electrochemical processes. Here, we experimentally realize multi-band metasurface-driven SEIRAS for the in-situ study of electrochemical CO2 reduction on a Pt surface. We develop an easily reproducible and spectrally-tunable platinum nano-slot metasurface. Two CO adsorption configurations at 2030 cm-1 and 1840 cm-1 are locally enhanced as a proof of concept that can be extended to more vibrational bands. Our platform provides a 41-fold enhancement in the detection of characteristic absorption signals compared to conventional broadband electrochemically roughened platinum films. A straightforward methodology is outlined starting by baselining our system in CO saturated environment and clearly detecting both configurations of adsorption, in particular the hitherto hardly detectable CO bridge configuration. Then, thanks to the signal enhancement provided by our platform, we find that the CO bridge configuration on platinum does not play a significant role during CO2 reduction in an alkaline environment. We anticipate that our technology will guide researchers in developing similar sensing platforms.
8.
L M Berger, M Duportal, L D S Menezes, E Cortés, S A Maier, A Tittl, K Krischer
Improved In Situ Characterization of Electrochemical Interfaces Using Metasurface-Driven Surface-Enhanced IR Absorption Spectroscopy Journal Article
In: Advanced Functional Materials, vol. 33, iss. 25, pp. 2300411, 2023, ISSN: 1616-301X.
@article{nokey,
title = {Improved In Situ Characterization of Electrochemical Interfaces Using Metasurface-Driven Surface-Enhanced IR Absorption Spectroscopy},
author = {L M Berger and M Duportal and L D S Menezes and E Cort\'{e}s and S A Maier and A Tittl and K Krischer},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.202300411},
doi = {https://doi.org/10.1002/adfm.202300411},
issn = {1616-301X},
year = {2023},
date = {2023-03-20},
urldate = {2023-03-20},
journal = {Advanced Functional Materials},
volume = {33},
issue = {25},
pages = {2300411},
abstract = {Abstract Electrocatalysis plays a crucial role in realizing the transition toward a zero-carbon future, driving research directions from green hydrogen generation to carbon dioxide reduction. Surface-enhanced infrared absorption spectroscopy (SEIRAS) is a suitable method for investigating electrocatalytic processes because it can monitor with chemical specificity the mechanisms of the reactions. However, it remains difficult to detect many relevant aspects of electrochemical reactions such as short-lived intermediates. Herein, an integrated nanophotonic-electrochemical SEIRAS platform is developed and experimentally realized for the in situ investigation of molecular signal traces emerging during electrochemical experiments. A platinum nano-slot metasurface featuring strongly enhanced electromagnetic near fields is implemented and spectrally targets the weak vibrational mode of the adsorbed carbon monoxide at ≈2033 cm−1. The metasurface-driven resonances can be tuned over a broad range in the mid-infrared spectrum and provide high molecular sensitivity. Compared to conventional unstructured platinum films, this nanophotonic-electrochemical platform delivers a 27-fold improvement of the experimentally detected characteristic absorption signals, enabling the detection of new species with weak signals, fast conversions, or low surface concentrations. By providing a deeper understanding of catalytic reactions, the nanophotonic-electrochemical platform is anticipated to open exciting perspectives for electrochemical SEIRAS, surface-enhanced Raman spectroscopy, and other fields of chemistry such as photoelectrocatalysis.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract Electrocatalysis plays a crucial role in realizing the transition toward a zero-carbon future, driving research directions from green hydrogen generation to carbon dioxide reduction. Surface-enhanced infrared absorption spectroscopy (SEIRAS) is a suitable method for investigating electrocatalytic processes because it can monitor with chemical specificity the mechanisms of the reactions. However, it remains difficult to detect many relevant aspects of electrochemical reactions such as short-lived intermediates. Herein, an integrated nanophotonic-electrochemical SEIRAS platform is developed and experimentally realized for the in situ investigation of molecular signal traces emerging during electrochemical experiments. A platinum nano-slot metasurface featuring strongly enhanced electromagnetic near fields is implemented and spectrally targets the weak vibrational mode of the adsorbed carbon monoxide at ≈2033 cm−1. The metasurface-driven resonances can be tuned over a broad range in the mid-infrared spectrum and provide high molecular sensitivity. Compared to conventional unstructured platinum films, this nanophotonic-electrochemical platform delivers a 27-fold improvement of the experimentally detected characteristic absorption signals, enabling the detection of new species with weak signals, fast conversions, or low surface concentrations. By providing a deeper understanding of catalytic reactions, the nanophotonic-electrochemical platform is anticipated to open exciting perspectives for electrochemical SEIRAS, surface-enhanced Raman spectroscopy, and other fields of chemistry such as photoelectrocatalysis.
7.
M P S Rodrigues, A H B Dourado, A G Sampaio De Oliveira-Filho, A P De Lima Batista, M Feil, K Krischer, S I Córdoba De Torresi
Gold–Rhodium Nanoflowers for the Plasmon-Enhanced CO2 Electroreduction Reaction upon Visible Light Journal Article
In: ACS Catalysis, pp. 267-279, 2022.
@article{nokey,
title = {Gold\textendashRhodium Nanoflowers for the Plasmon-Enhanced CO2 Electroreduction Reaction upon Visible Light},
author = {M P S Rodrigues and A H B Dourado and A G Sampaio De Oliveira-Filho and A P De Lima Batista and M Feil and K Krischer and S I C\'{o}rdoba De Torresi},
url = {https://doi.org/10.1021/acscatal.2c04207},
doi = {10.1021/acscatal.2c04207},
year = {2022},
date = {2022-12-15},
journal = {ACS Catalysis},
pages = {267-279},
abstract = {Bimetallic nanostructures combining catalytic and plasmonic properties are a class of materials that might possess improved efficiency and/or selectivity in electrocatalytic reactions. In this paper, we described the application of gold\textendashrhodium core\textendashshell nanoflowers (Au@Rh NFs) as a model system for the electrochemical CO2 reduction reaction. The nanoparticles consist of a gold nucleus surrounded by rhodium branches, combining Au localized surface plasmon resonance (LSPR) in the visible range of the spectrum and Rh catalytic properties. The influence of LSPR excitation on the catalytic properties was evaluated for different excitation wavelengths and various Au@Rh NF metallic ratios. Our catalysts showed enhanced activity upon LSPR excitation, demonstrating that LSPR excitation may lead to improved performance even with a low content of metallic NFs (2% Au + Rh in Carbon Vulcan). Electrochemical impedance spectroscopy (EIS) experiments performed under LSPR excitation suggest that the superior activity under illumination is related to lower energetic barriers that facilitate the desorption of adsorbed species compared to dark conditions.L},
keywords = {},
pubstate = {published},
tppubtype = {article}
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Bimetallic nanostructures combining catalytic and plasmonic properties are a class of materials that might possess improved efficiency and/or selectivity in electrocatalytic reactions. In this paper, we described the application of gold–rhodium core–shell nanoflowers (Au@Rh NFs) as a model system for the electrochemical CO2 reduction reaction. The nanoparticles consist of a gold nucleus surrounded by rhodium branches, combining Au localized surface plasmon resonance (LSPR) in the visible range of the spectrum and Rh catalytic properties. The influence of LSPR excitation on the catalytic properties was evaluated for different excitation wavelengths and various Au@Rh NF metallic ratios. Our catalysts showed enhanced activity upon LSPR excitation, demonstrating that LSPR excitation may lead to improved performance even with a low content of metallic NFs (2% Au + Rh in Carbon Vulcan). Electrochemical impedance spectroscopy (EIS) experiments performed under LSPR excitation suggest that the superior activity under illumination is related to lower energetic barriers that facilitate the desorption of adsorbed species compared to dark conditions.L
6.
M P S Rodrigues, A H B Dourado, K Krischer, S I C Torresi
Gold–rhodium nanoflowers for the plasmon enhanced ethanol electrooxidation under visible light for tuning the activity and selectivity Journal Article
In: Electrochimica Acta, vol. 420, pp. 140439, 2022, ISSN: 0013-4686.
@article{nokey,
title = {Gold\textendashrhodium nanoflowers for the plasmon enhanced ethanol electrooxidation under visible light for tuning the activity and selectivity},
author = {M P S Rodrigues and A H B Dourado and K Krischer and S I C Torresi},
url = {https://www.sciencedirect.com/science/article/pii/S0013468622006016},
doi = {https://doi.org/10.1016/j.electacta.2022.140439},
issn = {0013-4686},
year = {2022},
date = {2022-05-12},
journal = {Electrochimica Acta},
volume = {420},
pages = {140439},
abstract = {Direct ethanol fuel cells (DEFCs) are a promising power source, but the low selectivity to ethanol complete oxidation is still challenging. The localized surface plasmon resonance (LSPR) excitation has been reported to accelerate and drive several chemical reactions, including the ethanol oxidation reaction (EOR), coming as a strategy to improve catalysts performance. Nonetheless, metallic nanoparticles (NPs) that present the LSPR excitation in the visible range are known for leading to the incomplete oxidation of ethanol. Thus, we report here the application of gold-rhodium nanoflowers (Au@Rh NFs) towards the plasmon-enhanced EOR. These hybrid materials consist of a Au spherical nucleus covered by Rh branches shell, combining plasmonic and catalytic properties. Firstly, the Au@Rh NFs metallic ratio was investigated in dark conditions to obtain an optimal catalyst. Experiments were also performed under light irradiation. Our data demonstrated an improvement of 352% in current density and 36% in selectivity to complete ethanol oxidation under 533 nm laser incidence. Moreover, the current density showed a linear increase with the laser power density, indicating a photochemical effect and thus enhancement due to the LSPR properties.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Direct ethanol fuel cells (DEFCs) are a promising power source, but the low selectivity to ethanol complete oxidation is still challenging. The localized surface plasmon resonance (LSPR) excitation has been reported to accelerate and drive several chemical reactions, including the ethanol oxidation reaction (EOR), coming as a strategy to improve catalysts performance. Nonetheless, metallic nanoparticles (NPs) that present the LSPR excitation in the visible range are known for leading to the incomplete oxidation of ethanol. Thus, we report here the application of gold-rhodium nanoflowers (Au@Rh NFs) towards the plasmon-enhanced EOR. These hybrid materials consist of a Au spherical nucleus covered by Rh branches shell, combining plasmonic and catalytic properties. Firstly, the Au@Rh NFs metallic ratio was investigated in dark conditions to obtain an optimal catalyst. Experiments were also performed under light irradiation. Our data demonstrated an improvement of 352% in current density and 36% in selectivity to complete ethanol oxidation under 533 nm laser incidence. Moreover, the current density showed a linear increase with the laser power density, indicating a photochemical effect and thus enhancement due to the LSPR properties.
5.
M Golibrzuch, T L Maier, M J Feil, K Krischer, M Becherer
Tuning the feature size of nanoimprinting stamps: A method to enhance the flexibility of nanoimprint lithography Journal Article
In: Journal of Applied Physics, vol. 131, no. 12, pp. 124301, 2022.
@article{nokey,
title = {Tuning the feature size of nanoimprinting stamps: A method to enhance the flexibility of nanoimprint lithography},
author = {M Golibrzuch and T L Maier and M J Feil and K Krischer and M Becherer},
url = {https://aip.scitation.org/doi/abs/10.1063/5.0079282},
doi = {10.1063/5.0079282},
year = {2022},
date = {2022-03-09},
journal = {Journal of Applied Physics},
volume = {131},
number = {12},
pages = {124301},
abstract = {In the field of nanoimprinting lithography, fabricating large-area imprinting stamps is often the most time- and resource-consuming step. Specifically in research, it is often not reasonable to produce a new imprinting stamp for each new experimental configuration. Therefore, the lack of flexibility in feature sizes makes prototyping and tailoring the feature sizes according to their application challenging. To overcome these restrictions, we developed an imprinting stamp reproduction and tuning method which enables the size of the features of existing imprinting stamps to be tuned within nanometer precision. For replication, we first fabricate a chromium nanoisland array on silicon dioxide using the to-be tuned imprinting stamp. Then, the silicon dioxide is anisotropically etched in a reactive ion etching process with chromium as a hard mask. The formed replica of the imprinting stamp is subsequently tuned in an isotropic etching step with hydrofluoric acid. The method enables us to tune the size of the features of our nanoimprinting stamps within nanometer precision without influencing their shape with a yield above 96%. The tuned stamps are then used to fabricate metal nanoisland arrays with the respective tuned sizes. To evaluate the influence of the feature sizes, we exemplarily study the plasmonic resonance of gold nanoisland arrays fabricated using stamps with different feature diameters. Here, we see a good agreement between measured and simulated plasmonic resonance wavelengths of the samples. Hence, with the tuning method, we can tailor specific size-dependent properties of our nanoisland arrays according to individual experiments and applications.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In the field of nanoimprinting lithography, fabricating large-area imprinting stamps is often the most time- and resource-consuming step. Specifically in research, it is often not reasonable to produce a new imprinting stamp for each new experimental configuration. Therefore, the lack of flexibility in feature sizes makes prototyping and tailoring the feature sizes according to their application challenging. To overcome these restrictions, we developed an imprinting stamp reproduction and tuning method which enables the size of the features of existing imprinting stamps to be tuned within nanometer precision. For replication, we first fabricate a chromium nanoisland array on silicon dioxide using the to-be tuned imprinting stamp. Then, the silicon dioxide is anisotropically etched in a reactive ion etching process with chromium as a hard mask. The formed replica of the imprinting stamp is subsequently tuned in an isotropic etching step with hydrofluoric acid. The method enables us to tune the size of the features of our nanoimprinting stamps within nanometer precision without influencing their shape with a yield above 96%. The tuned stamps are then used to fabricate metal nanoisland arrays with the respective tuned sizes. To evaluate the influence of the feature sizes, we exemplarily study the plasmonic resonance of gold nanoisland arrays fabricated using stamps with different feature diameters. Here, we see a good agreement between measured and simulated plasmonic resonance wavelengths of the samples. Hence, with the tuning method, we can tailor specific size-dependent properties of our nanoisland arrays according to individual experiments and applications.
4.
M Salman, C Bick, K Krischer
Bifurcations of Clusters and Collective Oscillations in Networks of Bistable Units Journal Article
In: arXiv preprint arXiv:2110.15004, 2021.
@article{nokey,
title = {Bifurcations of Clusters and Collective Oscillations in Networks of Bistable Units},
author = {M Salman and C Bick and K Krischer},
year = {2021},
date = {2021-10-28},
journal = {arXiv preprint arXiv:2110.15004},
abstract = {We investigate dynamics and bifurcations in a mathematical model that captures electrochemical experiments on arrays of microelectrodes. In isolation, each individual microelectrode is described by a one-dimensional unit with a bistable current-potential response. When an array of such electrodes is coupled by controlling the total electric current, the common electric potential of all electrodes oscillates in some interval of the current. These coupling-induced collective oscillations of bistable one-dimensional units are captured by the model. Moreover, any equilibrium is contained in a cluster subspace, where the electrodes take at most three distinct states. We systematically analyze the dynamics and bifurcations of the model equations: We consider the dynamics on cluster subspaces of successively increasing dimension and analyze the bifurcations occurring therein. Most importantly, the system exhibits an equivariant transcritical bifurcation of limit cycles. From this bifurcation, several limit cycles branch, one of which is stable for arbitrarily many bistable units.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We investigate dynamics and bifurcations in a mathematical model that captures electrochemical experiments on arrays of microelectrodes. In isolation, each individual microelectrode is described by a one-dimensional unit with a bistable current-potential response. When an array of such electrodes is coupled by controlling the total electric current, the common electric potential of all electrodes oscillates in some interval of the current. These coupling-induced collective oscillations of bistable one-dimensional units are captured by the model. Moreover, any equilibrium is contained in a cluster subspace, where the electrodes take at most three distinct states. We systematically analyze the dynamics and bifurcations of the model equations: We consider the dynamics on cluster subspaces of successively increasing dimension and analyze the bifurcations occurring therein. Most importantly, the system exhibits an equivariant transcritical bifurcation of limit cycles. From this bifurcation, several limit cycles branch, one of which is stable for arbitrarily many bistable units.
3.
M Salman, C Bick, K Krischer
Collective oscillations of globally coupled bistable, nonresonant components Journal Article
In: Physical Review Research, vol. 2, no. 4, pp. 043125, 2020.
@article{nokey,
title = {Collective oscillations of globally coupled bistable, nonresonant components},
author = {M Salman and C Bick and K Krischer},
url = {https://link.aps.org/doi/10.1103/PhysRevResearch.2.043125},
doi = {10.1103/PhysRevResearch.2.043125},
year = {2020},
date = {2020-10-23},
journal = {Physical Review Research},
volume = {2},
number = {4},
pages = {043125},
abstract = {Bistable microelectrodes with an S-shaped current-voltage characteristic have recently been shown to oscillate under current control, when connected in parallel. In other systems with equivalently coupled bistable components, such oscillatory instabilities have not been reported. In this paper, we derive a general criterion for when an ensemble of coupled bistable components may become oscillatorily unstable. Using a general model, we perform a stability analysis of the ensemble equilibria, in which the components always group in three or fewer clusters. Based thereon, we give a necessary condition for the occurrence of collective oscillations. Moreover, we demonstrate that stable oscillations may persist for an arbitrarily large number of components, even though, as we show, any equilibrium with two or more components on the middle, autocatalytic branch is unstable.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Bistable microelectrodes with an S-shaped current-voltage characteristic have recently been shown to oscillate under current control, when connected in parallel. In other systems with equivalently coupled bistable components, such oscillatory instabilities have not been reported. In this paper, we derive a general criterion for when an ensemble of coupled bistable components may become oscillatorily unstable. Using a general model, we perform a stability analysis of the ensemble equilibria, in which the components always group in three or fewer clusters. Based thereon, we give a necessary condition for the occurrence of collective oscillations. Moreover, we demonstrate that stable oscillations may persist for an arbitrarily large number of components, even though, as we show, any equilibrium with two or more components on the middle, autocatalytic branch is unstable.
2.
T L Maier, M Golibrzuch, S Mendisch, W Schindler, M Becherer, K Krischer
Lateral silicon oxide/gold interfaces enhance the rate of electrochemical hydrogen evolution reaction in alkaline media Journal Article
In: Journal of Chemical Physics, vol. 152, iss. 15, 2020, ISSN: 0021-9606.
@article{,
title = {Lateral silicon oxide/gold interfaces enhance the rate of electrochemical hydrogen evolution reaction in alkaline media},
author = {T L Maier and M Golibrzuch and S Mendisch and W Schindler and M Becherer and K Krischer},
url = {\<Go to ISI\>://WOS:000529243500002},
doi = {10.1063/5.0003295},
issn = {0021-9606},
year = {2020},
date = {2020-04-21},
urldate = {2020-04-21},
journal = {Journal of Chemical Physics},
volume = {152},
issue = {15},
abstract = {The production of solar hydrogen with a silicon based water splitting device is a promising future technology, and silicon-based metal-insulator-semiconductor (MIS) electrodes have been proposed as suitable architectures for efficient photocathodes based on the electronic properties of the MIS structures and the catalytic properties of the metals. In this paper, we demonstrate that the interfaces between the metal and oxide of laterally patterned MIS electrodes may strongly enhance the catalytic activity of the electrode compared to bulk metal surfaces. The employed electrodes consist of well-defined, large-area arrays of gold structures of various mesoscopic sizes embedded in a silicon oxide support on silicon. We demonstrate that the activity of these electrodes for hydrogen evolution reaction (HER) increases with an increase in gold/silicon oxide boundary length in both acidic and alkaline media, although the enhancement of the HER rate in alkaline electrolytes is considerably larger than in acidic electrolytes. Electrodes with the largest interfacial length of gold/silicon oxide exhibited a 10-times larger HER rate in alkaline electrolytes than those with the smallest interfacial length. The data suggest that at the metal/silicon oxide boundaries, alkaline HER is enhanced through a bifunctional mechanism, which we tentatively relate to the laterally structured electrode geometry and to positive charges present in silicon oxide: Both properties change locally the interfacial electric field at the gold/silicon oxide boundary, which, in turn, facilitates a faster transport of hydroxide ions away from the electrode/electrolyte interface in alkaline solution. This mechanism boosts the alkaline HER activity of p-type silicon based photoelectrodes close to their HER activity in acidic electrolytes.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The production of solar hydrogen with a silicon based water splitting device is a promising future technology, and silicon-based metal-insulator-semiconductor (MIS) electrodes have been proposed as suitable architectures for efficient photocathodes based on the electronic properties of the MIS structures and the catalytic properties of the metals. In this paper, we demonstrate that the interfaces between the metal and oxide of laterally patterned MIS electrodes may strongly enhance the catalytic activity of the electrode compared to bulk metal surfaces. The employed electrodes consist of well-defined, large-area arrays of gold structures of various mesoscopic sizes embedded in a silicon oxide support on silicon. We demonstrate that the activity of these electrodes for hydrogen evolution reaction (HER) increases with an increase in gold/silicon oxide boundary length in both acidic and alkaline media, although the enhancement of the HER rate in alkaline electrolytes is considerably larger than in acidic electrolytes. Electrodes with the largest interfacial length of gold/silicon oxide exhibited a 10-times larger HER rate in alkaline electrolytes than those with the smallest interfacial length. The data suggest that at the metal/silicon oxide boundaries, alkaline HER is enhanced through a bifunctional mechanism, which we tentatively relate to the laterally structured electrode geometry and to positive charges present in silicon oxide: Both properties change locally the interfacial electric field at the gold/silicon oxide boundary, which, in turn, facilitates a faster transport of hydroxide ions away from the electrode/electrolyte interface in alkaline solution. This mechanism boosts the alkaline HER activity of p-type silicon based photoelectrodes close to their HER activity in acidic electrolytes.
1.
J A Nogueira, K Krischer, H Varela
Coupled Dynamics of Anode and Cathode in Proton-Exchange Membrane Fuel Cells Journal Article
In: Chemphyschem, vol. 20, no. 22, pp. 3081-3088, 2019, ISSN: 1439-4235.
@article{,
title = {Coupled Dynamics of Anode and Cathode in Proton-Exchange Membrane Fuel Cells},
author = {J A Nogueira and K Krischer and H Varela},
url = {\<Go to ISI\>://WOS:000478934200001},
doi = {10.1002/cphc.201900531},
issn = {1439-4235},
year = {2019},
date = {2019-07-19},
journal = {Chemphyschem},
volume = {20},
number = {22},
pages = {3081-3088},
keywords = {},
pubstate = {published},
tppubtype = {article}
}