Enhancing the oxygen evolution reaction activity of CuCo based hydroxides with V2CTx MXene

The oxygen evolution reaction (OER) is a key reaction in the production of green hydrogen by water electrolysis. In alkaline media, the current state of the art catalysts used for the OER are based on non-noble metal oxides. However, despite their huge potential as OER catalysts, these materials exhibit various disadvantages including lack of stability and conductivity that hinder the wide-spread utilization of these materials in alkaline electrolyzer devices. This study highlights the innovative chemical functionalization of a mixed copper cobalt hydroxide with the V2CTx MXene to enhance the OER efficiency, addressing the need for effective electrocatalytic interfaces for sustainable hydrogen production. The herein synthesized CuCo@V2CTx electrocatalysts demonstrate remarkable activity, outperforming the pure CuCo catalysts for the OER and moreover show increased efficiency after 12 hours of continuous operation. This strategic integration improved the water oxidation performance of the pure oxide material by improving the composite's hydrophilicity, charge transfer properties and ability to hinder Cu leaching. The materials were characterized using an array of materials characterization techniques to help decipher both structure of the composite materials after synthesis and to elucidate the reasoning for the OER enhancement for the composites. This work demonstrates the significant potential of TMO-based nanomaterials combined with V2CTx for advanced innovative electrocatalytic interfaces in energy conversion applications.


Graphene and graphene composite synthesis
To prepare a typical graphene dispersion, 180 mg of graphite was dispersed in 180 ml of isopropyl alcohol (IPA) (starting concentration of graphite in IPA: 1 mg ml -1 ) using a 1.3 hour sonication process at 60 % amplitude, with a pulsation pattern of 6 seconds on and 2 seconds off to avoid damage to the processor and reduce solvent heating, and, thus, evaporation.The beaker was connected to a cooling system that allowed for cold water (under 5 °C) to flow around the dispersion during sonication.The resulting dispersion was centrifuged for 90 minutes at 500 rpm.After centrifugation, the upper 45 ml of the dispersion was retained for use.The final concentration of graphene nanosheets in IPA were 0.5 mg ml −1 .
CuCo@graphene (CCG) was synthesized through a urea-assisted hydrothermal method.In a typical synthesis process 5 mmol urea, 1 mmol Cu(CH 3 COO) 2 • H 2 O and 2 mmol Co(CH 3 COO) 2 • 4 H 2 O were added to a graphene suspension, and it was stirred for 30 minutes to fully dissolve all the compounds.The solution was transferred into a Teflon-lined stainless-steel autoclave and kept at 120 °C for 6 h for hydrothermal treatment.After cooling down to room temperature, the precipitate was collected by centrifugation at 5000 rpm for 10 min, and then repeatedly washed with deionized water (3 x) and ethanol (3 x).The sediment was dried overnight at 60 °C for 10 h.The materials were labelled CCG50 and CCG25 for 50 % and 25 % graphene content, respectively.

Material characterizations
The phase analysis of the samples was performed at room temperature (RT) by powder X-ray diffraction (XRD) utilizing a Bruker D8 ADVANCE powder diffractometer with Cu-K  radiation of 40 kV and a beam current of 40 mA ((Cu-K 1 ) = 0.1541 nm and (Cu-K 2 ) = 0.1544 nm).Diffraction patterns were collected between 3.5° and 80° applying a step size of 0.015°.The morphologies of the as-prepared samples were observed using a Karl Zeiss MERLIN scanning electron microscope (SEM) using a 0.1-30 keV field emission gun.X-ray photoelectron spectroscopy (XPS) measurements were conducted using a JEOL JPS-9030 setup with a base pressure of 2E-9 mbar.The powders were evenly distributed on carbon tape for the measurements.A nonmonochromated Al source with 300 W power was used for excitation and a hemispherical analyzer with pass energy of 50 eV (surveys) and 20 eV (narrow scans) was used to detect the emitted photoelectrons.The analyzer binding energy scale was calibrated by measuring sputter cleaned gold and copper foils just before the measurements and setting the Au4f_7/2 peak to 84.00 eV and the Cu2p_3/2 peak to 932.62 eV.Since the samples exhibited charging, the C-C component of the carbon tape was set to 285.0 eV for comparison.
CasaXPS was used to fit the spectra, employing Tougaard (Cu2p) and Shirley (all other core levels) backgrounds and Voigt-functions.X-ray Absorption Near Edge Structure (XANES) spectra was acquired using Scanning Transmission X-Ray Microscopy (STXM) at the BESSY-II electron storage ring.The measurements were performed at the MAXYMUS end station.The x-ray beam was focused with a zone plate and an order selective aperture on the transmissive sample.The samples were measured ex-situ under vacuum (~10 -6 mbar).Scanning Transmission Electron Microscopy (STEM) was performed using a FEI Titan 80-300 Thermo Fisher Scientific, fitted with a Schottky field emission gun set to an operating voltage of 300 kV.Elemental composition was determined by Energy Dispersive X-Ray Spectroscopy (EDS) using a Bruker XFlash 6-30 EDS detector.Au grids TEM grids were used to allow for the true Cu signal of the materials to be detected.Contact angle measurements were carried using an Ossila Contact Angle Goniometer, in which a deionised water droplet (5 µl) was used to characterize the wettability of the catalysts.The Inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements were carried out with a iCAP 7400 DV from ThermoFisher in axial measurement mode.

Figure S1 .
Figure S1.SEM images of pure Co and pure Cu.

Figure S2 .
Figure S2.EDS map of V 2 CT x .

Figure
Figure Transmission image A. at maximum absorption of V-L 3 edge at 517 eV of pristine V 2 CT X , and B. of CC50.NOTE: The XANES spectra in Figure 3C in the main paper is an averaged spectra over these images.Lower optical density represents the background and correspondingly increasing contrast indicates the thickness of the sample.

Figure S9 .
Figure S9.CV of the CC composite materials highlighting the A1 redox peak.

Figure S10 .
Figure S10.Stability test of the CC50 for 24 hours.

Table S1 .
Fitted values of the charge transfer resistance during OER.

Table S2 .
Literature comparison of Cu/Co/V based materials.