Materials for electrocatalysis
The world’s transition to a more sustainable future requires innovative solutions for energy conversion and storage. Strategies to develop stable, active and durable catalytic materials are an essential part of this development. The continued and concerted effort of different groups at the MPIE to enhance our understanding of the fundamental processes governing the stability, activity and degradation of, e.g., electrolysers in realistic environments and under operando conditions, opens new routes towards designing efficient and long-lived catalysts.
Three main directions were pursued within the last years: (i) exploiting metal dealloying to enhance catalytic activity, (ii) understanding the role of dopants, impurities and local inhomogeneities on catalytic activity and (iii) the study of oxides and oxidised compounds, which exhibit an enhanced stability in the conventionally harsh environments developing during catalytic reactions. In all of these, particular attention is paid to resolve the atomistic structure, as the gained insights have proven highly beneficial in explaining catalytic performance.
Our work demonstrated that electrochemical dealloying can greatly improve the catalytic activity of AgAu nanoparticles for the hydrogen evolution reaction . By combining electrochemistry with identical location electron microscopy analyses and linking them to the electrocatalytic properties of the obtained nanocatalysts, they established guidelines for the selection of dealloying parameters to reach highly porous and active materials beyond the previous “trial and error” attempts. Using AgAu alloy nanoparticles and the hydrogen evolution reaction as a model system, the influence of cyclic voltammetry parameters on the catalytic activity upon electrochemical dealloying was investigated. Increasing cycle numbers initially result in a decreased Ag content and a sharp improvement in catalyst activity. Additional dealloying increased the nanoparticle porosity, while marginally altering their composition, due to surface motion of atoms. Since this is accompanied by particle aggregation, further cycling results in a decrease of catalytic activity. This transition between porosity formation and particle aggregation marks the optimum for nanocatalyst post-production. The gained insights will allow speeding up the development of new materials by electrochemical dealloying as an easy-to-control post-processing route to tune properties of existing nanoparticles, instead of having to alter usually delicate synthesis routes as a whole.
Chemical synthesis is a conventional route by which catalytic materials are produced and/or designed. Wet-chemical methods are however, more complex than expected and often reported. Careful analysis of samples we studied in the context of electrocatalysis, revealed the presence of impurities integrated within the structure during synthesis. The presence of traces of such spurious elements may enhance or detrimentally affect the properties of materials and thus, affect their performance. Identification and quantification of such impurities is however extremely challenging. To better tackle these issues, (scanning) transmission electron microscopy ((S)TEM), atom probe tomography (APT) and X-ray photoelectron spectroscopy (XPS) were combined to carefully analyse the structural features and chemical composition of MoS2 sheets . The obtained results showed that impurity elements from the precursor were incorporated into the nanosheets during synthesis, pointing towards the necessity to carefully control the synthesis environment to avoid contamination. These findings also suggest a strategy to optimize materials performance, which we now exploit for targeted materials design. One example is the intentional synthesis of Pd nanoparticles with different levels of B doping . Analysis by STEM and APT, complemented by density functional theory calculations and cyclic voltammogram measurements, enable us to assess changes in the catalytic activity of the nanoparticles towards the hydrogen evolution and/or hydrogen oxidation reactions (HER, HOR) due to the presence of impurities, and their effect on the grain boundary cohesion and hence, material service lifetime.
Another example focuses on the role of Sn in enhancing the efficiency of hematite (a-Fe2O3) for water splitting . Looking both at advantages and limitation of Sn-doping of hematite thin films photoanodes, the study reveals that Sn ions at the surface eliminate surface states, preventing hole trapping and thus, enhancing the catalytic activity towards the oxygen evolution reaction (OER) on the (0001) hematite surface. An aberration corrected STEM image, the corresponding elemental distribution obtained by electron energy loss spectroscopy (EELS) and APT of a sample where Sn is located only within the top few nanometre of the hematite film is given in Fig. 1. On the other hand, the homogeneous distribution of Sn throughout the thickness of a hematite film impairs crystallization, due to grain boundary segregation of Sn, leading to inferior performance.
Next to high chemical activity towards a targeted catalytic reaction, the development of a sustainable energy economy requires longevity of the used catalytic materials. The latter is a particular obstacle, given the harsh electrochemical environments developing during reactions. Oxides show an enhanced stability in such conditions and may therefore, provide blue-prints for achieving long-term stability of catalytic materials. With this in mind, several of our studies focused on different aspects of the oxides using Ir and Ru, aiming to understand the connection between structure, stability and catalytic activity. These studies reveal the role of lattice oxygen atoms in rutile IrO2 with respect to stability, surface hydrogenation and amorphization, and catalytic activity through a direct visualisation of the electrochemically active body of the catalyst [5, 6]. They also show that the local microstructure controls the growth rate of individual oxide grains and how crystalline defects act as diffusion pathways during oxidation in different IrxRuyO2 . Other studies specifically target the effect different electrolytes have on the degradation, as exemplified by the example of the dissolution behaviour of the BiVO4 photoanode . Ongoing studies deal with FexMn1-xO2 as electrocatalyst. With the help of Raman spectroscopy and TEM, the effect of the crystal structure as well as the morphology (nanosheets, nanocones and nanotubes) on the (photo)electrochemical performance are explored. Focus is placed on the stability of the nanostructures upon electrochemical cycling, which we will explore by identical location TEM experiments (IL-TEM).
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- Zhang, S.; Ahmet, I.; Kim, S.-H.; Kasian, O.; Mingers, A.M.; Schnell, P.; Kölbach, M.; Lim, J.; Fischer, A.; Mayrhofer, K.J.J., Cherevko, S.; Gault, B.; van de Krol, R.; Scheu, C.: ACS Appl. Energy Mater 3 (2020), 9523.