Electrocatalysis

In order to make clean energy storage and conversion technologies, such as fuel cells, water electrolyser, metal–air batteries, and CO₂ to fuel conversion systems efficient and economically viable, electrocatalysts with significantly enhanced activity and selectivity for desired products are required. Our group focuses on the development of high-performance, stable catalysts for electrochemical reactions of technological interest, such as the oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and CO₂ reduction reaction (CO₂ RR).

Goal of our research is to develop advanced nanoscale catalysts by systemically varying their size, morphology, chemical state and electronic properties. By using advanced operando and in situ techniques, such as X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), nuclear resonant inelastic X-ray scattering (NRIXS), electrochemical atomic force microscopy (EC-AFM), transmission electron microscopy (TEM), differential electrochemical mass spectrometry (DEMS), and electrochemical Fourier-transform infrared (FTIR) spectroscopy, we gain insight into the parameters affecting the activity and selectivity of electrochemical reaction processes.

Particularly in focus are in situ and operando spectroscopic techniques which provide insights into the electronic structure, atomic coordination and lattice vibration dynamics of the electrocatalysts under conditions which are identical or similar to those in an operating electrochemical cell. For example, our group operates an XPS setup which is connected to a quasi in situ electrochemical cell. The elemental composition, chemical state, and surface adsorbate species of the electrocatalysts are determined by XPS immediately after the electrocatalytic reaction, upon transferring the sample in vacuum from the electrochemical cell to the XPS analysis chamber.

Quasi in situ electrochemical - XPS setup.

For determining the chemical state and atomic coordination number of nanoscale electrocatalysts, operando XAFS studies are performed by our group at synchrotron facilities on a regular basis. By acquiring X-ray fluorescence spectra from an actual electrocatalyst while an electrochemical reaction is in progress, reaction-induced changes on the morphology and chemical state of the nanostructures can be monitored as they happen.

Electrochemical EXAFS cell (left) and operando Cu-K edge EXAFS spectra (right) acquired on an O₂-plasma treated Cu-foil during CO₂ electro-reduction .

The effect of mild plasma pretreatments on the morphology and oxidation state of metallic surfaces and the consecutive enhancement of their electrocatalytic performance is also being investigated in our group. For example, in the case of defect-rich copper catalysts, it was shown that their catalytic properties for the electrochemical reduction of CO₂ to C2-C3 products were significantly improved upon O₂ plasma treatment.

Summary of hydrocarbon selectivity of plasma-treated Cu foils for CO₂ electroreduction as a function of the pre-treatment of Cu-foils. [From Mistry et al., Nat. Commun. 7, 12123 (2016)].

Besides the morphology, the chemical state of a nanocatalyst before and, most importantly, during an electrochemical reaction can be also tuned by mild plasma pre-treatments. This can help improve the activity and selectivity of the catalyst, as was demonstrated in the case of a Cu nanocube system for the electrocatalytic reduction of CO₂ to hydrocarbons and alcohols. Treatment of the catalyst’s surface with O₂ plasma prior to the electrocatalytic reaction lead to the stabilization of surface and subsurface oxygen species during the reaction, which results in lower overpotentials and higher ethylene, ethanol, and n-propanol selectivity.

Geometrical current density (left axis) and Faradaic efficiency for C2H4 (right axis) of Cu nanocube electrocatalysts after 1 h of electrochemical reaction at −1.0 V vs RHE in CO₂-saturated 0.1 M KHCO3 for different plasma pre-treatments. [From Gao, Zegkinoglou et al., ACS Nano 11, 4825 (2017)].

Besides XPS and EXAFS, operando nuclear resonant inelastic X-ray scattering (NRIXS) studies can also contribute to the understanding of the factors which determine the performance of electrocatalysts. NRIXS probes the lattice vibrations of Fe-based catalytic materials, providing the phonon density of states as well as values of several thermodynamic properties during the electrochemical reaction. In addition, it can provide new knowledge on temperature-dependent atomic order-disorder transitions, structural phase transitions involving soft phonon modes of NPs, and the physical phenomena underlying electrical and thermal conductivity, heat capacity, vibrational entropy, electron-phonon coupling, Debye temperature etc.

Combined with first principles DFT calculations conducted by collaborating groups, the phonon density of states which is obtained directly from the NRIXS spectra can provide insight into changes in the morphology of the catalyst during the reaction (e.g. alloying or segregation) and give information about the adsorption mechanism of reactants or intermediate products which affect the reaction mechanism. This was, for example, demonstrated on pentlandite (Fe4.5Ni4.5S8) powders used as catalysts for the hydrogen evolution reaction. Operando NRIXS studies, performed at the synchrotron (APS, Argonne) on pentlandite electrodes, revealed a change of the atomic sites which are occupied by H atoms during the reaction as a function of the applied potential due to gradual occupation of the initially existing sulfur vacancies.

Experimental (left) and theoretical (right) Fe-projected vibrational density of states in pentlandite obtained from NRIXS and DFT calculations, respectively, during hydrogen evolution. The location of adsorbed H in the lattice is dependent on the applied potential and the amount of S vacancies. (From Zegkinoglou et al., J. Am. Chem. Soc.139, 14360 (2017)).