Self-activation of copper electrodes during CO electro-oxidation in alkaline electrolyte

Abstract

The development of low-temperature fuel cells for clean energy production is an appealing alternative to fossil-fuel technologies. CO is a key intermediate in the electro-oxidation of energy carrying fuels and, due to its strong interaction with state-of-the-art Pt electrodes, it is known to act as a poison. Here we demonstrate the ability of Earth-abundant Cu to electro-oxidize CO efficiently in alkaline media, reaching high current densities of ≥0.35 mA cm−2 on single-crystal Cu(111) model catalysts. Strong and continuous surface structural changes are observed under reaction conditions. Supported by first-principles microkinetic modelling, we show that the concomitant presence of high-energy undercoordinated Cu structures at the surface is a prerequisite for the high activity. Similar CO-induced self-activation has been reported for gas–surface reactions at coinage metals, demonstrating the strong parallels between heterogeneous thermal catalysis and heterogeneous electrocatalysis.

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Fig. 1: CO electro-oxidation on Cu(111) with an onset prior to copper bulk oxidation.
Fig. 2: In situ EC-IRRAS during CO electro-oxidation on Cu(111) model catalysts.
Fig. 3: First-principles microkinetic modelling of the catalytic activity.
Fig. 4: Direct EC-STM visualization of Cu(111) under CO oxidation reaction conditions.
Fig. 5: Schematic representation of the self-activation of Cu(111) during CO electro-oxidation.

Data availability

All the data supporting the findings of this study are available within the paper and its Supplementary Information files. The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Code availability

The scripts used for the modified microkinetic simulations using the open source CatMAP code are given separately from the Supplementary Information in the following GitHub repository: https://github.com/mieand/COelectroCATMAPfiles.git.

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Acknowledgements

A.A. is a recipient of a doctorate (DOC) Fellowship of the Austrian Academy of Sciences at the Institute of Physical Chemistry. E.-M.W. and J.K.-L. acknowledge funding by the Austrian Science Foundation (FWF) through grant I-4114. N.G.H. and K.R. acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany´s Excellence Strategy – EXC 2089/1 – 390776260. Additionally, we thank B. Kindler for assistance with preparation of the Cu single crystals.

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Contributions

J.K.-L and K.R. supervised and coordinated the project. A.A and E.-M.W. designed and conducted the experiments. A.A. carried out the cyclic voltammetry and in situ EC-STM experiments. E.-M.W. conducted the in situ EC-IRRAS measurements. A.A. and N.B. did the RDE measurements. N.G.H. performed the DFT calculations and M.A. performed the microkinetic modelling. A.A., M.A., E.-M.W., K.R. and J. K.-L. contributed to the manuscript writing. All authors discussed and revised the manuscript.

Corresponding author

Correspondence to Julia Kunze-Liebhäuser.

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Extended data

Extended Data Fig. 1 Current transients show high electrocatalytic reversibility.

Potential steps to -0.63VSHE (black) and -0.45VSHE (grey), showing the high reversibility of CO oxidation on Cu(111). The reference potential was set to -0.85VSHE. Insets: current densities versus step number.

Extended Data Fig. 2 Control EC-STM studies in CO-free electrolyte.

EC-STM at -0.45 VSHE in Ar-saturated 0.1 M NaOH, where the surface is imaged over a long period of time (> 1.5 hours). In comparison to the structural changes during the CO bulk oxidation at the exact same potential, the images in the CO-free 0.1 M NaOH indicate unidirectional Cu transport upon Cu(111) reconstruction. Although a high degree of step edge roughening due to OH adsorption-induced reconstruction can be observed, there is only little formation of nanoscale Cu adatom islands. A much higher stability of the surface morphology after reconstruction is observed. No additional growth of adatom islands and nanostructures is seen upon scanning of up to an hour, which means there is no apparent surface diffusion. a, 100 x 100 nm2 (Itip = 1 nA) and bd, 50 x 50 nm2 (Itip = 4 nA). Etip = -0.5 VSHE.

Extended Data Fig. 3 Electronic structure analysis of OH and CO on Cu adatoms at Cu(111).

Atom and momentum projected PDOS of a, OH and b, CO adsorbed on top of a Cu adatom (labelled as Cuad) located in the fcc site of Cu(111). All results are from SCCS implicit solvation calculations, which allow an absolute alignment of the energy scale with the vacuum level and the simulation of appropriate molecular reference states, namely CO(aq) and charged OH-(aq). The Fermi level (EF) and d-band centres are indicated by dashed and dotted lines. c, The charge-density difference plot for OH-(aq) adsorption indicates partially remaining negative charge on the Cuad–OH complex. The shown isosurface corresponds to 0.0135e3 (red) and -0.0135e3 (blue), the purple region indicates the implicit solvation cavity. d, The changes in the 3d PDOS of adsorbed Cuad and of surface Cu(111) show the significant difference of CO adsorption as compared to OH.

Supplementary information

Supplementary Information

Supplementary Figs. 1–11, Table 1, Notes 1–10, input files, and references 1–25.

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Auer, A., Andersen, M., Wernig, E. et al. Self-activation of copper electrodes during CO electro-oxidation in alkaline electrolyte. Nat Catal 3, 797–803 (2020). https://doi.org/10.1038/s41929-020-00505-w

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