Professional Education


  • Doctor of Philosophy, Leiden University (2018)
  • Master of Science, Leiden University (2014)
  • Bachelor of Science, Leiden University (2012)

Stanford Advisors


All Publications


  • Base-Accelerated Degradation of Nanosized Platinum Electrocatalysts ACS CATALYSIS Hersbach, T. P., Garcia, A. C., Kroll, T., Sokaras, D., Koper, M. M., Garcia-Esparza, A. T. 2021; 11 (15): 9904-9915
  • Electrochemical Cleaning Stability and Oxygen Reduction Reaction Activity of 1-2 nm Dendrimer-Encapsulated Au Nanoparticles CHEMELECTROCHEM Strasser, J. W., Hersbach, T. P., Liu, J., Lapp, A. S., Frenkel, A. I., Crooks, R. M. 2021; 8 (13): 2545-2555
  • Cathodic corrosion: 21st century insights into a 19th century phenomenon CURRENT OPINION IN ELECTROCHEMISTRY Hersbach, T. P., Koper, M. M. 2021; 26
  • Nanoscale morphological evolution of monocrystalline Pt surfaces during cathodic corrosion PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Arulmozhi, N., Hersbach, T. P., Koper, M. M. 2020; 117 (51): 32267–77

    Abstract

    This paper studies the cathodic corrosion of a spherical single crystal of platinum in an aqueous alkaline electrolyte, to map out the detailed facet dependence of the corrosion structures forming during this still largely unexplored electrochemical phenomenon. We find that anisotropic corrosion of the platinum electrode takes place in different stages. Initially, corrosion etch pits are formed, which reflect the local symmetry of the surface: square pits on (100) facets, triangular pits on (111) facets, and rectangular pits on (110) facets. We hypothesize that these etch pits are formed through a ternary metal hydride corrosion intermediate. In contrast to anodic corrosion, the (111) facet corrodes the fastest, and the (110) facet corrodes the slowest. For cathodic corrosion on the (100) facet and on higher-index surfaces close to the (100) plane, the etch pit destabilizes in a second growth stage, by etching faster in the (111) direction, leading to arms in the etch pit, yielding a concave octagon-shaped pit. In a third growth stage, these arms develop side arms, leading to a structure that strongly resembles a self-similar diffusion-limited growth pattern, with strongly preferred growth directions.

    View details for DOI 10.1073/pnas.2017086117

    View details for Web of Science ID 000601315200017

    View details for PubMedID 33288700

    View details for PubMedCentralID PMC7768681

  • Tailoring the Electrocatalytic Activity and Selectivity of Pt(111) through Cathodic Corrosion ACS CATALYSIS Hersbach, T. P., Ye, C., Garcia, A. C., Koper, M. M. 2020; 10 (24): 15104–13
  • Alkali Metal Cation Effects in Structuring Pt, Rh, and Au Surfaces through Cathodic Corrosion ACS APPLIED MATERIALS & INTERFACES Hersbach, T. P., McCrum, I. T., Anastasiadou, D., Wever, R., Calle-Vallejo, F., Koper, M. M. 2018; 10 (45): 39363–79

    Abstract

    Cathodic corrosion is an electrochemical etching process that alters metallic surfaces by creating nanoparticles and a variety of etching features. Because these features typically have a preferential orientation, cathodic corrosion can be applied to modify and nanostructure electrode surfaces. However, this application of cathodic corrosion is currently limited by an insufficient chemical understanding of its underlying mechanism. This includes the role of alkali metal cations, which are thought to be crucial in both enabling cathodic corrosion and controlling its final facet preference. This work addresses this knowledge gap by exploring the cathodic corrosion of Pt, Rh, and Au in LiOH, NaOH, and KOH through both experimental and theoretical methods. These methods demonstrate that cations are adsorbed during cathodic corrosion and play a major role in controlling the onset potential and final surface morphology in cathodic corrosion. Interestingly, an equally significant role appears to be played by adsorbed hydrogen, based on calculations using literature density functional theory data. Considering the significance of both hydrogen and electrolyte cations, it is hypothesized that cathodic corrosion might proceed via an intermediate ternary metal hydride. This fundamental insight leads to both metal-specific recommendations and more general guidelines for applying cathodic corrosion to structure metallic surfaces.

    View details for DOI 10.1021/acsami.8b13883

    View details for Web of Science ID 000451100500078

    View details for PubMedID 30351902

  • Hydrogen adsorption on nano-structured platinum electrodes FARADAY DISCUSSIONS Diaz-Morales, O., Hersbach, T. P., Badan, C., Garcia, A. C., Koper, M. M. 2018; 210: 301–15

    Abstract

    The "hydrogen region" of platinum is a powerful tool to structurally characterize nanostructured platinum electrodes. In recent years, the understanding of this hydrogen region has improved considerably: on Pt(111) sites, there is indeed only hydrogen adsorption, while on step sites, the hydrogen region involves the replacement of adsorbed hydrogen by adsorbed hydroxyl which interacts with co-adsorbed cations. However, the hydrogen region features an enigmatic and less well-understood "third hydrogen peak", which develops on oxidatively roughened platinum electrodes as well as on platinum electrodes with a high (110) step density that have been subjected to a high concentration of hydrogen. In this paper, we present evidence that the peak involves surface-adsorbed hydrogen (instead of subsurface hydrogen) on a locally "reconstructed" (110)-type surface site. This site is unstable when the hydrogen is oxidatively removed. The cation sensitivity of the third hydrogen peak appears different from other step-related peaks, suggesting that the chemistry involved may still be subtly different from the other features in the hydrogen region.

    View details for DOI 10.1039/c8fd00062j

    View details for Web of Science ID 000449765000009

    View details for PubMedID 29987308

  • Local structure and composition of PtRh nanoparticles produced through cathodic corrosion PHYSICAL CHEMISTRY CHEMICAL PHYSICS Hersbach, T. P., Kortlever, R., Lehtimaki, M., Krtil, P., Koper, M. M. 2017; 19 (16): 10301–8

    Abstract

    Alloy nanoparticles fulfill an important role in catalysis. As such, producing them in a simple and clean way is much desired. A promising alloy nanoparticle production method is cathodic corrosion, which generates particles by applying an AC voltage to an alloy electrode. However, this harsh AC potential program might affect the final elemental distribution of the nanoparticles. In this work, we address this issue by characterizing the time that is required to create 1 μmol of Rh, Pt12Rh88, Pt55Rh45 and Pt nanoparticles under various applied potentials. The corrosion time measurements are complemented by structural characterization through transmission electron microscopy, X-ray diffraction and X-ray absorption spectroscopy. The corrosion times indicate that platinum and rhodium corrode at different rates and that the cathodic corrosion rates of the alloys are dominated by platinum. In addition, the structure-sensitive techniques reveal that the elemental distributions of the created alloy nanoparticles indeed exhibit small degrees of elemental segregation. These results indicate that the atomic alloy structure is not always preserved during cathodic corrosion.

    View details for DOI 10.1039/c7cp01059a

    View details for Web of Science ID 000400117700011

    View details for PubMedID 28393941

  • Anisotropic etching of platinum electrodes at the onset of cathodic corrosion NATURE COMMUNICATIONS Hersbach, T. P., Yanson, A. I., Koper, M. M. 2016; 7: 12653

    Abstract

    Cathodic corrosion is a process that etches metal electrodes under cathodic polarization. This process is presumed to occur through anionic metallic reaction intermediates, but the exact nature of these intermediates and the onset potential of their formation is unknown. Here we determine the onset potential of cathodic corrosion on platinum electrodes. Electrodes are characterized electrochemically before and after cathodic polarization in 10 M sodium hydroxide, revealing that changes in the electrode surface start at an electrode potential of -1.3 V versus the normal hydrogen electrode. The value of this onset potential rules out previous hypotheses regarding the nature of cathodic corrosion. Scanning electron microscopy shows the formation of well-defined etch pits with a specific orientation, which match the voltammetric data and indicate a remarkable anisotropy in the cathodic etching process, favouring the creation of (100) sites. Such anisotropy is hypothesized to be due to surface charge-induced adsorption of electrolyte cations.

    View details for DOI 10.1038/ncomms12653

    View details for Web of Science ID 000382456300001

    View details for PubMedID 27554398

    View details for PubMedCentralID PMC4999510

  • Anisotropic etching of rhodium and gold as the onset of nanoparticle formation by cathodic corrosion FARADAY DISCUSSIONS Hersbach, T. P., Mints, V. A., Calle-Vallejo, F., Yanson, A. I., Koper, M. M. 2016; 193: 207–22

    Abstract

    Cathodic corrosion is a phenomenon in which negatively polarized metal electrodes are degraded by cathodic etching and nanoparticle formation. Though these changes are dramatic and sometimes even visible by eye, the exact mechanisms underlying cathodic corrosion are still unclear. This work aims to improve the understanding of cathodic corrosion by studying its onset on rhodium and gold electrodes, which are subjected to various constant cathodic potentials in 10 M NaOH. After this polarization, the electrodes are studied using cyclic voltammetry and scanning electron microscopy, allowing a corrosion onset potential of -1.3 V vs. NHE for rhodium and -1.6 V vs. NHE for gold to be defined. The mildness of the potentials on both metals suggests that cathodic corrosion is less extreme and more ubiquitous than expected. Furthermore, we are able to observe well-defined rectangular etch pits on rhodium. Combined with rhodium cyclic voltammetry, this indicates a strong preference for forming (100) sites during corrosion. In contrast, a (111) preference is indicated on gold by voltammetry and the presence of well-oriented quasi-octahedral nanoparticles. This different etching behavior is suggested to be caused by preferential adsorption of sodium ions to surface defects, as is confirmed by density functional theory calculations.

    View details for DOI 10.1039/c6fd00078a

    View details for Web of Science ID 000390786000012

    View details for PubMedID 27722596

  • Enhancement Effect of Noble Metals on Manganese Oxide for the Oxygen Evolution Reaction JOURNAL OF PHYSICAL CHEMISTRY LETTERS Seitz, L. C., Hersbach, T. J., Nordlund, D., Jaramillo, T. F. 2015; 6 (20): 4178-4183

    Abstract

    Developing improved catalysts for the oxygen evolution reaction (OER) is key to the advancement of a number of renewable energy technologies, including solar fuels production and metal air batteries. In this study, we employ electrochemical methods and synchrotron techniques to systematically investigate interactions between metal oxides and noble metals that lead to enhanced OER catalysis for water oxidation. In particular, we synthesize porous MnOx films together with nanoparticles of Au, Pd, Pt, or Ag and observe significant improvement in activity for the combined catalysts. Soft X-ray absorption spectroscopy (XAS) shows that increased activity correlates with increased Mn oxidation states to 4+ under OER conditions compared to bare MnOx, which exhibits minimal OER current and remains in a 3+ oxidation state. Thickness studies of bare MnOx films and of MnOx films deposited on Au nanoparticles reveal trends suggesting that the enhancement in activity arises from interfacial sites between Au and MnOx.

    View details for DOI 10.1021/acs.jpclett.5b01928

    View details for Web of Science ID 000363083900031

  • Electro-Oxidation of Glycerol on Platinum Modified by Adatoms: Activity and Selectivity Effects TOPICS IN CATALYSIS Kwon, Y., Hersbach, T. P., Koper, M. M. 2014; 57 (14-16): 1272–76
  • Electrochemical and Spectroelectrochemical Characterization of an Iridium-Based Molecular Catalyst for Water Splitting: Turnover Frequencies, Stability, and Electrolyte Effects JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Diaz-Morales, O., Hersbach, T. P., Hetterscheid, D. H., Reek, J. H., Koper, M. M. 2014; 136 (29): 10432–39

    Abstract

    We present a systematic electrochemical and spectroelectrochemical study of the catalytic activity for water oxidation of an iridium-N-dimethylimidazolin-2-ylidene (Ir-NHC-Me2) complex adsorbed on a polycrystalline gold electrode. The work aims to understand the effect of the electrolyte properties (anions and acidity) on the activity of the molecular catalyst and check its stability toward decomposition. Our results show that the iridium complex displays a very strong dependence on the electrolyte properties such that large enhancements in catalytic activity may be obtained by adequately choosing pH and anions in the electrolyte. The stability of the adsorbed compound was investigated in situ by Surface Enhanced Raman Spectroscopy and Online Electrochemical Mass Spectrometry showing that the catalyst exhibits good stability under anodic conditions, with no observable evidence for the decomposition to iridium oxide.

    View details for DOI 10.1021/ja504460w

    View details for Web of Science ID 000339471900040

    View details for PubMedID 24977640