Angel Angelov Topalov

Dissolution of Platinum: Limits for the Deployment of Electrochemical Energy Conversion?

Platinum stability: Dissolution of Pt, which is one major degradation mechanism in, for example, hydrogen/air fuel cells, was monitored under potentiodynamic and potentiostatic conditions. The highly sensitive and time-resolving dissolution monitoring enables the distinction between anodic and cathodic dissolution processes during potential transient and chronoamperometric experiments, and the precise quantification of the amount of dissolved Pt.

Toward Highly Stable Electrocatalysts via Nanoparticle Pore Confinement

The durability of electrode materials is a limiting parameter for many electrochemical energy conversion systems. In particular, electrocatalysts for the essential oxygen reduction reaction (ORR) present some of the most challenging instability issues shortening their practical lifetime. Here, we report a mesostructured graphitic carbon support, Hollow Graphitic Spheres (HGS) with a specific surface area exceeding 1000 m(2) g(-1) and precisely controlled pore structure, that was specifically developed to overcome the long-term catalyst degradation, while still sustaining high activity. The synthetic pathway leads to platinum nanoparticles of approximately 3 to 4 nm size encapsulated in the HGS pore structure that are stable at 850 °C and, more importantly, during simulated accelerated electrochemical aging. Moreover, the high stability of the cathode electrocatalyst is also retained in a fully assembled polymer electrolyte membrane fuel cell (PEMFC). Identical location scanning and scanning transmission electron microscopy (IL-SEM and IL-STEM) conclusively proved that during electrochemical cycling the encapsulation significantly suppresses detachment and agglomeration of Pt nanoparticles, two of the major degradation mechanisms in fuel cell catalysts of this particle size. Thus, beyond providing an improved electrocatalyst, this study describes the blueprint for targeted improvement of fuel cell catalysts by design of the carbon support.

Design criteria for stable Pt/C fuel cell catalysts.

Summary Platinum and Pt alloy nanoparticles supported on carbon are the state of the art electrocatalysts in proton exchange membrane fuel cells. To develop a better understanding on how material design can influence the degradation processes on the nanoscale, three specific Pt/C catalysts with different structural characteristics were investigated in depth: a conventional Pt/Vulcan catalyst with a particle size of 3–4 nm and two Pt@HGS catalysts with different particle size, 1–2 nm and 3–4 nm. Specifically, Pt@HGS corresponds to platinum nanoparticles incorporated and confined within the pore structure of the nanostructured carbon support, i.e., hollow graphitic spheres (HGS). All three materials are characterized by the same platinum loading, so that the differences in their performance can be correlated to the structural characteristics of each material. The comparison of the activity and stability behavior of the three catalysts, as obtained from thin film rotating disk electrode measurements and identical location electron microscopy, is also extended to commercial materials and used as a basis for a discussion of general fuel cell catalyst design principles. Namely, the effects of particle size, inter-particle distance, certain support characteristics and thermal treatment on the catalyst performance and in particular the catalyst stability are evaluated. Based on our results, a set of design criteria for more stable and active Pt/C and Pt-alloy/C materials is suggested.

Towards a comprehensive understanding of platinum dissolution in acidic media

Platinum is one of the most important electrode materials for continuous electrochemical energy conversion due to its high activity and stability. The resistance of this scarce material towards dissolution is however limited under the harsh operational conditions that can occur in fuel cells or other energy conversion devices. In order to improve the understanding of dissolution of platinum, we therefore investigate this issue with an electrochemical flow cell system connected to an inductively coupled plasma mass spectrometer (ICP-MS) capable of online quantification of even small traces of dissolved elements in solution. The electrochemical data combined with the downstream analytics are used to evaluate the influence of various operational parameters on the dissolution processes in acidic electrolytes at room temperature. Platinum dissolution is a transient process, occurring during both positive- and negative-going sweeps over potentials of ca. 1.1 VRHE and depending strongly on the structure and chemistry of the formed oxide. The amount of anodically dissolved platinum is thereby strongly related to the number of low-coordinated surface sites, whereas cathodic dissolution depends on the amount of oxide formed and the timescale. Thus, a tentative mechanism for Pt dissolution is suggested based on a place exchange of oxygen atoms from surface to sub-surface positions.

Dissolution of noble metals during oxygen evolution in acidic media

The electrochemical production of hydrogen and hydrocarbons is considered to play a decisive role in the conversion and storage of excess amounts of renewable energy. The electrocatalysis of the oxygen evolution reaction (OER), however, faces significant challenges for practical implementation of electrolyzers. In this work, a comparative study on the activity and stability of oxidized polycrystalline noble metals during the OER is presented. All studied metals exhibit transient and steady‐state dissolution. Transient dissolution takes place during oxide formation and reduction. Steady‐state dissolution depends on the OER mechanism on each surface: On metals such as Ru and Au, for which oxygen from the oxide participates in the OER, the Tafel slope is low and the dissolution rate is high. In contrast, on metals for which oxygen evolves directly from adsorbed water, such as Pt and presumably Pd, the Tafel slopes are high and the dissolution rates are low. This should be considered in the design of optimal OER catalysts.

Near-surface ion distribution and buffer effects during electrochemical reactions

The near-surface ion distribution at the solid-liquid interface during the Hydrogen Oxidation Reaction (HOR)/Hydrogen Evolution Reaction (HER) on a rotating platinum disc electrode is demonstrated in this work. The relation between reaction rate, mass transport and the resulting surface pH-value is used to theoretically predict cyclic voltammetry behaviour using only thermodynamic and diffusion data obtained from the literature, which were confirmed by experimental measurements. The effect of buffer addition on the current signal, the surface pH and the ion distribution is quantitatively described by analytical solutions and the fragility of the surface pH during reactions that form or consume H(+) in near-neutral unbuffered solutions or poorly buffered media is highlighted. While the ideal conditions utilized in this fundamental study cannot be directly applied to real scenarios, they do provide a basic understanding of the surface pH concept for more complex heterogeneous reactions.

Rational design of the electrode morphology for oxygen evolution – enhancing the performance for catalytic water oxidation

The fundamental understanding of the electrode/electrolyte interface is of pivotal importance for the efficient electrochemical conversion and storage of electrical energy. However, the reasons for the low rate of electrocatalytic oxygen evolution and issues of long-term material stability, which are central constraints for attaining desirable efficiency for sustainable technologies like water electrolysis or electrochemical CO2 reduction, are still not completely resolved. While a lot of attention has been directed towards the search for new materials with unique (electro)catalytic properties, experimental results accumulated during the last four decades and prediction from models suggest that RuO2 possesses superior activity for oxygen evolution under acidic conditions. Considering that RuO2 is a material of choice, we show that tailoring the surface morphology on the meso- and macroscale has great potential for the improvement of the efficiency of this gas evolving reaction. Advanced analytical tools have been utilized for the combined investigation of both activity and stability. Namely, the potential dependent frequencies of gas-bubble evolution, an indicator for the activity of the electrode, were acquired by scanning electrochemical microscopy (SECM), while the dissolution of RuO2 was monitored using a micro electrochemical scanning flow cell combined with an inductively coupled plasma mass spectrometer (SFC-ICP-MS). The obtained fundamental insights will aid improving the design and thus performance of electrode materials for water oxidation.

Temperature-Dependent Dissolution of Polycrystalline Platinum in Sulfuric Acid Electrolyte

Commercial proton exchange membrane (PEM) fuel cells, various types of water electrolyzers and recently proposed unified, regenerative fuel cells are usually operated at elevated temperatures. Higher-operation temperatures bring several advantages: (a) increase of the rate of slow oxygen reactions, (b) improved mass transport, and (c) minimization of the electrolyte (ionic conductor) resistance. However, at the same time, it is expected that degradation processes will be accelerated at such temperatures. In the current work, electrochemistry and in situ mass spectrometry are utilized to investigate how increased temperature affects the rate of (electro)chemical dissolution of platinum. The steady state dissolution rate during potentiostatic polarization decreases to a value below the detection limit after several minutes at all temperatures—dissolution thus remains a transient process controlled by oxide formation kinetics as reported previously for room temperature. Deconvolution of anodic and cathodic dissolution branches in potentiodynamic experiments reveals that the increase in temperature results in higher amounts of platinum being dissolved during oxide formation, while dissolution during oxide reduction decays with increasing temperature. In contrast to most literature reports, the total amount of dissolved platinum during 1 potential cycle is found to decrease with increasing temperature.

Development and integration of a LabVIEW-based modular architecture for automated execution of electrochemical catalyst testing

This paper describes a system for performing electrochemical catalyst testing where all hardware components are controlled simultaneously using a single LabVIEW-based software application. The software that we developed can be operated in both manual mode for exploratory investigations and automatic mode for routine measurements, by using predefined execution procedures. The latter enables the execution of high-throughput or combinatorial investigations, which decrease substantially the time and cost for catalyst testing. The software was constructed using a modular architecture which simplifies the modification or extension of the system, depending on future needs. The system was tested by performing stability tests of commercial fuel cell electrocatalysts, and the advantages of the developed system are discussed.