Data underlying the PhD dissertation: Electrochemical CO2 Reduction to Multicarbon Products on MoS2 Catalysts

by: Eszter Madai

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                                 Experimental techniques:
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Molecular Dynamics Simulations
All molecular dynamics (MD) simulations were conducted using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) package on the DelftBlue supercomputer. The interactions within MoS2 were modeled using the REBOMOS potential developed by Jiang and colleagues. Water molecules were represented using the SPC/E model, and their rigid structure was maintained through the SHAKE algorithm. A transferable ion force field developed by Loche et al. was used to model KCl and NaCl. The simulation box was constructed using the Packmol software. 

Electrode Preparation
MoS2 nanopowder (99% trace metal basis, Sigma Aldrich) was mixed with Carbon Black (CB, Fischer Scientific) and poly(vinyl alcohol) (PVA) in a weight ratio of 90:5:5, respectively. The components were combined in a 5 mL vial and diluted with 2 mL of deionized (DI) water. The resulting ink was sonicated in a water bath for 10 minutes to ensure homogenization.

Glassy carbon substrates (1×1 cm, Sigma Aldrich) were cleaned with DI water, followed by sonication in isopropyl alcohol for 5 minutes, then dried with compressed air. After sonication, 25 µL of the ink was dropcasted onto each substrate, which were then left to air dry.

To prepare the electrodes, a copper tape was attached to the back of each substrate and covered with insulating tape, exposing only a 10 mm diameter circular area. The intercalation of sodium (Na⁺) and potassium (K⁺) ions into the MoS2 was then performed to enhance its conductivity.

Intercalation of alkali metal ions into MoS2 is known to induce a phase transition from the semiconducting 2H-phase to the metallic 1T-phase. This 1T-phase is metastable and can revert to the 2H-phase upon heating or over time. Specifically, Li⁺ and Na⁺ ions, due to their smaller size, can reversibly intercalate and somewhat stabilize the 1T-phase. In contrast, the larger K⁺ ions cause greater lattice distortion and less reversibility. The 1T-phase may transform back to the 2H-phase at temperatures as low as 95°C. To prevent this phase relaxation, all sample preparation and characterization were performed at or below room temperature.

Electrochemical Techniques
Electrochemical measurements were performed using a BioLogic VMP-3e Multichannel Potentiostat controlled with EC-Lab software. Each electrode was immersed in 40 mL of either 1 mM KHCO3 (for K⁺ intercalation) or 1 mM NaHCO3 (for Na⁺ intercalation) in an electrochemical H-cell. The working electrode was placed in the cathode compartment along with a saturated calomel electrode (SCE), while a platinum mesh served as the anode.

The setup was enclosed in a Faraday cage to minimize noise. Electrochemical impedance spectroscopy (EIS) was first conducted over a frequency range of 1.0 MHz to 100 mHz. Then, chronoamperometry (CA) was performed for one hour at a predetermined intercalation potential for each ion type. After intercalation, the electrode was rinsed with DI water to remove any surface-adsorbed layers, followed by a second EIS measurement to assess conductivity changes.

Scanning Electron Microscopy
Scanning Electron Microscopy (SEM) was carried out using a JEOL IT800SHL instrument equipped with a field emission gun. The system operated at 10.00 kV with a beam current of 42.20 nA. Energy Dispersive Spectroscopy (EDS) was performed using an Oxford Instruments Maxim 100 detector.

X-ray Diffraction (XRD)
XRD patterns were recorded using a Bruker D8 Advance diffractometer in Bragg-Brentano geometry, equipped with a Lynxeye position-sensitive detector and Cu Kα radiation. The system was operated at 45 kV and 40 mA. The divergence slit used was V12 with a scatter screen height of 5 mm. Detector settings were LL 0.11 and W 0.14.

For VS-MoS2 and K⁺-intercalated VS-MoS2, measurements were conducted from 5° to 80° with a step size of 0.03° and a counting time of 2 seconds per step. For Na⁺-intercalated VS-MoS2, a step size of 0.06° was used.

Raman Spectroscopy
Raman spectra were collected using a WiTec Alpha300R Raman Imaging Microscope with a 532 nm laser set at 1 mW to prevent sample damage. A 63X/0.9 water-dipping objective was used, with an integration time of 5 seconds and 10 accumulations per spectrum. A 1800 g/mm grating ensured spectral resolution better than 1 cm⁻¹.

To monitor structural changes due to intercalation, in situ Raman measurements were conducted at the exact same location on the sample surface before and after the electrochemical process. The sample was immersed in electrolyte in a beaker directly mounted on the Raman stage. The pristine spectrum was recorded first, after which the objective was retracted and the intercalation was carried out. The objective was then repositioned to the same coordinates (with a spatial resolution of ~25 nm) and focus depth to acquire the post-intercalation spectrum. This ensured that any spectral differences were due to structural changes rather than variations in sample position or thickness.

X-ray Photoelectron Spectroscopy (XPS)
XPS measurements were performed using a Physical Electronics (PHI) 5400 system with a non-monochromatic Al Kα X-ray source (200 W, 13.5 kV). Spectra were recorded with a step size of 0.1 eV and a dwell time of 1.25 seconds, using a spherical capacitor analyzer at a pass energy of 71.55 eV.

Data analysis was carried out using MultiPak software. Peaks were normalized and fitted using the iterated-Shirley background method.



Product Analysis
Product analysis was conducted using a two-compartment, three-electrode electrochemical H-cell (Pine Research), connected to a BioLogic SP-50e potentiostat. The anode compartment contained a platinum wire serving as the counter electrode. The cathode compartment included the working electrode, a micro Ag/AgCl reference electrode (Mengel Engineering), and a glass tube for continuous CO₂ purging into the electrolyte.

The working electrode was prepared by drop-casting MoS2 onto a 15 mm × 15 mm glassy carbon substrate. The electrode was masked to expose a 10 mm diameter circular area and connected to the potentiostat using copper tape.

Both compartments were filled with an electrolyte composed of 70% deionized water and 30% respective ionic liquid (Iolitec). Before the electrochemical measurements, the electrolyte was purged with CO₂ gas for one hour at a flow rate of 10 mL per minute. A constant potential was then applied for one hour, during which CO₂ was continuously bubbled into the system at 10 mL per hour to maintain a steady supply for the CO₂ reduction reaction.

Gas samples were collected every 10 minutes from the cathodic compartment using a 10 mL Hamilton gas-tight syringe and analyzed via gas chromatography (CompactGC 4.0, Interscience). The gas chromatograph was equipped with three detectors: two thermal conductivity detectors (TCDs) using Carboxen101 columns, and one flame ionization detector (FID). The first TCD channel used helium (99.999%, Linde) as the carrier gas to detect O₂, N₂, CO, and CO₂. The second TCD used argon (99.999%, Linde) to detect H₂. The FID was used to detect hydrocarbon species.

Before measurements, the GC system was baked out and calibrated with certified gas standards (Linde), including H₂, CH₄, C₂H₆, C₃H₈, C₄H₁₀, C₅H₁₂, C₅H₁₀, C₆H₁₄, C₆H₁₂, and C₇H₁₄.

For calculating Faradaic efficiencies (FEs), chromatographic peak areas were processed using Chromeleon software and converted to parts per million (ppm) based on the GC calibration curves. Faradaic efficiencies were then calculated following the methodology described by Asadi and co-workers, using the formula:

FE (%) = (moles of product × n × F) / (I × t) × 100

Where:
n is the number of electrons transferred per molecule of product
F is the Faraday constant
I is the applied current
t is the total electrolysis time


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                               Data files
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XPS data files:
- first column: binding energy (eV)
- second column: intensity (c/s)

Raman data files:
- first column: Raman shift (cm^(-1))
- second column: Intensity (a.u.)

XRD data files:
- first column: 2$\Theta$ ($\degree$)
- second column: Intensity (c/s)

Gibbs free-energy profiles from Molecular Dynamics simulations:
- first column: x-axis coordinate (A)
- second column: U (kcal/mol)

EIS data files:
- first column: Re(Z) (Ohm)
- second column: -Im(Z) (Ohm)

LSV data files:
- first column: Voltage (V vs RHE)
- second column: Current density (mA/cm^(2))

Faradaic efficiency data files:
- first column: Voltage (V vs RHE)
- second column: Faradaic Efficiency (%) [Calculation described above]
