Solar-Driven Electrochemical Green Fuel Production from CO2 and Water Using Ti3C2Tx MXene-Supported CuZn and NiCo Catalysts

Summary

This protocol guides the fabrication and electrochemical analysis of MXene-supported CuZn and NiCo bi-metallic electrocatalysts for green fuel production from carbon dioxide and water using solar energy.

Abstract

This protocol describes the synthesis and activity mapping of bi-metallic, MXene-supported cathodes for carbon dioxide reduction (CO₂R) and anodes for water splitting utilizing solar energy in alkaline media. A reverse current pulse electrodeposition technique was used to control the nanostructure, grain refinement, and alloy composition of the fabricated electrodes. Both anodes and cathodes are noble metal-free and supported on two-dimensional (2D) titanium carbide (Ti₃C₂T_x) MXene, which reduces contact resistance and facilitates the charge transfer from substrate to reactants via catalysts. Copper zinc (CuZn) supported on Ti₃C₂T_x MXene are used as cathodes for methanol production in thermocatalytic CO₂ conversion, and here we have demonstrated their performance in electrochemical CO₂ reduction (CO₂R) for the first time. Nickel cobalt (NiCo) supported on Ti₃C₂T_x MXene are similarly fabricated via pulse electrodeposition and tested as anodes for water electrolysis driven by a solar cell under simplified laboratory conditions. The activity also describes the structural characterization of metallic thin films. Detailed setup for the integration of the state-of-the-art perovskite silicon tandem solar cell with electrochemical cell, which in turn feeds in-line gas chromatography, is demonstrated for both CO₂R and water electrolysis (WE). A setup detailing the water electrolysis at commercial conditions of highly caustic alkaline solutions (30% KOH), high temperatures (60 °C), and in a zero-gap cell is demonstrated on the fabricated anode paired with a Pt/C spray-coated cathode.

Introduction

Replacing fossil fuels with zero-emission alternatives is critical to decarbonizing the energy sector and increasing the use of renewable energy^1,2,3. Conversion of carbon dioxide (CO₂) into carbon monoxide (CO), methane (CH₄), and other carbonaceous fuels is becoming an important route to prevent further CO₂ emissions and create a circular carbon economy⁴. Similarly, replacing fossil fuels with high energy density hydrogen is projected to accelerate the energy transition from fossil-based to zero-emission fuels^5,6,7,8. The energy generation system can be further made cost-effective, and eco-friendly by using direct sunlight^9,10,11,12. The green energy transition can generate positive socio-economic outcomes, particularly by enhancing social impacts and thereby increasing the social capital associated with zero-emission fuel generation¹³.

An extensive body of research has been dedicated to solar-driven electrochemical reduction of CO₂ and hydrogen generation^14,15. Copper-based materials have shown excellent activity (>70% faradaic efficiency towards C₂ product) towards CO₂ reduction^16,17,18,19. Bimetallic systems consisting of copper lead to an increase in Faradaic Efficiencies (FE) and overall carbon dioxide conversion rate. The main challenges are high overpotentials, lower faradaic efficiencies, and selectivity of a single product²⁰. Integrating photovoltaic (PV) systems with electrochemical reactions requires an overlap between the electrocatalyst's faradaic activity and the solar cell's maximum power point to achieve sustainable fuel production rates^21,22. Ni and Co bimetallic systems have been extensively reported as high functional anodes for alkaline water electrolysis due to their <600 mV overpotential at currents >100 mA/cm^2 23,24. Rapid degradation of the catalyst at industrially relevant currents limits their viability as commercial anodes. The electrolyte-induced short circuiting by corrosion of the interface between the metal layer and substrate further makes water electrolysis sluggish^25,26.

MXenes, a growing family of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, have emerged as promising materials for electrochemical CO₂R reaction due to their unique structural and electronic properties^27,28. First discovered in 2011, MXenes are mostly derived from bulk-layered M_n+1AX_n phases (where n = 1- 4) which consists of n+1 layers of one or more transition metals (M, typically from groups 4-6), interleaved with X layers, carbon (C), nitrogen (N) or both (CN), and an A group element (usually from groups 13-16). Their compositions can be tuned using different transition metals of groups 4 to 6, X sublattice (C/N), and T_x represents surface terminations (e.g., -O, -OH, -F, -Cl)^27,29,30. In CO₂R, MXenes are being actively investigated as co-catalysts or supports due to their tunable surface chemistry, catalytically active sites, high specific surface area, and metal-like electrical conductivity.

Recent theoretical studies predict that decorating MXenes with bimetallic or single atoms can substantially enhance CO₂R performance by modulating their electronic structure. These design strategies shift the d-band center, modulate the intermediate adsorption energies, and lower the Gibbs free energy barriers of rate-determining steps^31,32,33. For instance, Mo₂ZC₂ MXenes (Z = Ti, V, etc.) have been shown to strengthen -HOCH₂O adsorption and weaken -OCH₂O binding in electrochemical CO₂RR, thereby reducing the limiting potential for CH₄ production due to an upshift in the d-band center of Mo atoms. In another study, Cu-doped Ti₃C₂T_x MXene achieved a Faradaic efficiency of 58.1% toward HCOO^- production by introducing polarized sites that facilitate intermediate adsorption and electron transfer. These few theoretical studies guide the potential of exploring MXene-based bimetallic catalysts in tailoring CO₂RR pathways.

Here, we have synthesized CuZn@Ti₃C₂T_x MXene cathodes and NiCo@Ti₃C₂T_x MXene anodes for CO₂R and water electrolysis by simple one-pot electrodeposition. The optimized procedure can be used to deposit a range of bi-metallic or polymetallic systems in concentration ranges from 5 to 100 mM each with pH control. Each set of desired systems would demand tuning the pH and current density according to their electrochemical potentials, deposition potential, reaction conditions, etc. Although the deposition can be carried out on a two-electrode setup, better control over the deposition voltage through reference electrodes is advised. The method can deposit large structures as well as fine particles by controlling reaction parameters like current density, ON-OFF time ratio, and pH. The refined and optimized grain structure tuned by reverse current pulses shows high Faradaic efficiency (56% for hydrocarbons) for CO₂R with very low catalyst degradation. Water electrolysis is demonstrated on NiCo@Ti₃C₂T_x MXene in an H-cell for laboratory conditions with 98% FE and in a zero-gap cell for industrial conditions. A demonstration of in-line product determination by gas chromatography is also featured. The overall system integration and its operation have been closely monitored using the safe-by-design protocols established in our lab³⁴.

Protocol

The reagents and the equipment used in this study are listed in the Table of Materials.

1. Synthesis of Ti₃C₂T_x MXene

1. Wash 1 g of optimized Ti₃AlC₂ MAX phase using 9 M hydrochloric acid (HCl) for 18 h to remove intermetallic impurities.
2. Prepare the etchant solution by mixing 12 M HCl, deionized (DI) water, and 50 wt% hydrofluoric acid (HF) in a 6:3:1 volume ratio.
   NOTE: Hydrofluoric acid (HF) is highly toxic and corrosive. Perform all steps involving HF in a certified fume hood while wearing appropriate personal protective equipment (PPE), including gloves, lab coat, and face shield.
3. Add the HCl-washed MAX powder to the etchant solution and stir at 400 rpm for 24 h at 35 °C.
4. Wash the etched Ti₃C₂T_x MXene with DI water by repeated centrifugation at 3234 x g for 4-5 cycles (~200 mL each), each cycle of 10 min, until the supernatant reaches a neutral pH of approximately 6.
5. Delaminate the MXene by adding the washed sediment to a lithium chloride (LiCl) solution (50 mL per gram of etched powder) and stir at 400 rpm for 1 h at 65 °C under argon gas flow.
6. Wash the mixture by centrifugation at 3234 x g for 5 min, 10 min, 15 min, and 20 min sequentially at room temperature.
7. Vortex mix the final suspension for 30 min at room temperature, then centrifuge at 2380 x g for 30 min to obtain single-to-few-layered Ti₃C₂T_x MXene flakes.

2. Preparation of precursors

1. Disperse 300 mg of Ti₃C₂T_x MXene in 100 mL of DI water. Sonicate for 5 min to ensure stable dispersion.
2. Prepare a 30 mM copper citrate solution and a 10 mM zinc oxalate solution in DI water. Ensure to make all solutions in a hood with proper PPE, avoiding spillage or direct contact with skin.
3. Prepare 50 mM solutions of nickel nitrate and cobalt nitrate in DI water.
4. Prepare a dispersion of 5 mg of Pt/C in 50 mL of DI water and sonicate for 10 min at room temperature.

3. Fabrication of electrodes

1. Clean the nickel foam using acetone and then sonicate in DI water for 5 min.
2. Activate the carbon fiber paper (CFP) by soaking it in 1 M nitric acid (HNO₃) for 20 min. Activate the CFP in a hood and use gloves and proper PPE.
3. Immerse the cleaned nickel foam (5 cm × 5 cm) in MXene solution for 5 min. Dry at room temperature under vacuum. Label this sample as MXene-Ni-Foam.
4. Spray-coat MXene solution onto activated CFP. Label the sample as MXene-CFP. Load 50 mL of Mxene solution in a spray gun. At a distance of 5 cm, spray the ink on the activated CFP, covering the whole surface of the 2 cm x 2 cm CFP.
5. In a glass electrochemical cell, add the Cu and Zn precursor solutions. Use an Ag/AgCl reference electrode, a platinum counter electrode, and MXene-CFP as the working electrode, and connect the electrodes to their respective connection on the potentiostat.
   NOTE: For example, the MXene-CFP electrode will be connected to the working electrode connection, and likewise. The electrolyte bath consisted of 50 mL of 30 mM Cu citrate solution and 10 mM Zn oxalate in DI water.
6. Apply the following pulsed current deposition sequence: -10 mA/cm² for 1 s, 0 mA/cm² (null pulse) for 0.5 s, +10 mA/cm² for 0.5 s.
7. Repeat for 1000 cycles to deposit CuZn. Label the resulting electrode as CuZn@Ti₃C₂T_x-MXene-CFP.
8. In another cell, add Ni and Co precursor solutions. Use the same electrode configuration but replace the working electrode with MXene-Ni-Foam. Ensure the electrodes in the electrochemical cell are connected properly with the respective connections on the potentiostat. The electrolytic bath consists of 50 mL of 50 mM Ni nitrate and Co nitrate in DI water.
9. Apply the same pulsed deposition cycle (as in step 3.6) for 1000 sets to obtain NiCo@Ti₃C₂T_x-MXene-Ni-Foam.
10. Fill 50 mL of Pt/C in the spray gun. From a distance of 5 cm, spray-coat Pt/C onto Ni mesh and dry in a vacuum oven at 60 °C to prepare the reference cathode for water electrolysis.

4. Structural characterization

1. Cut each electrode into 0.5 cm × 1 cm pieces.
2. Perform X-ray diffraction (XRD) and scanning electron microscopy (SEM) for phase and morphology analysis.
   NOTE: The size of electrode pieces should be similar to what fits the XRD and SEM instrument sample preparation guidelines.

5. Electrochemical CO₂ reduction

1. Assemble an H-cell with an alkaline exchange membrane separating the two chambers.
2. Use Ni-Foam as anode and CuZn@Ti₃C₂T_x-MXene-CFP (2 cm × 2 cm) as cathode. Use 1 M KOH as an electrolyte in both chambers. Check thoroughly for any electrolyte leaks at the membrane junction. Tighten the junction for any leaks.
   NOTE: KOH is highly corrosive; wear proper PPE, put all solutions in a fume hood, and avoid contact with skin. Keep all solutions on flat surfaces away from personal intrusion and electrical plugs.
3. Insert an Hg/HgO reference electrode in the cathode compartment. Seal the system to be gas-tight. Add a tubing for the CO₂ inlet and one for the gas outlet in the cathodic chamber.
4. Purge CO₂ into the cathodic chamber at 30 mL/min for 15 min to saturate the electrolyte.
5. Illuminate the photovoltaic (PV) cell with 1-sun intensity and connect it to the cell (positive to anode, negative to cathode).
6. Record cyclic voltammetry (CV) from 0 V to -2.5 V at 50 mV/s and EIS (100 kHz to 0.1 Hz) at open circuit potential.
7. Perform a 0 A chrono potentiometric measurement for 2 h, recording current periodically using a multimeter. The recorded current can be used to calculate the faradaic efficiency of products.
8. Connect the cathodic chamber outlet to a gas chromatograph (GC) for in-line sampling every 10 min. Program the GC for the detection and quantification of permanent gases. Use a packed column, such as the Molecular sieve, to identify the gases.
   NOTE: The temperature of the oven is set at ramping with an initial temperature of 150 °C with a hold time of 2 min and further ramped to 200 °C with a hold time of 1 min to allow proper separation and elution of the gases from the mixture.

6. Electrochemical water electrolysis (OER)

1. Repeat the H-cell setup with NiCo@Ti₃C₂T_x-MXene-Ni-Foam as anode and Pt/C@Ni mesh as cathode. Insert an Hg/HgO electrode as a reference electrode in the anodic chamber with the working electrode.
2. Fill both chambers with 1 M KOH electrolyte.
3. Record CV from 0 V to 1.2 V at 50 mV/s and EIS (10 kHz to 0.1 Hz, 10 mV amplitude) at open circuit potential. On the autolab potentiostat, use the OCP (open circuit potential) determination function to record OCP.
4. Illuminate the PV cell with 1-sun light using a solar simulator placed 6 cm away, or luminosity of 1 sun on the cell.
5. Connect PV terminals to electrodes and record a 0 A chrono potentiometric curve. Monitor and log current for efficiency calculations.
6. Connect the cathode outlet to GC and analyze hydrogen production every 10 min using a thermal conductivity detector (TCD) with nitrogen as carrier gas.

7. Zero-gap electrolyzer assembly

1. Wash with water and prepare a zero-gap cell. Prepare clean polyvinyl propylene tubing and other accessories, like push and pull values that fit the tubing to create junctions between various parts of the alkaline water electrolysis assembly.
2. Stack the following in sequence: Cell anode plate, NiCo@Ti₃C₂T_x-MXene-Ni-Foam anode, gasket (same or slightly thicker (0.1 mm) than the thickness of anode), alkaline exchange membrane, gasket, Pt/C cathode, and finally cathode cell plate. Use positioning rods for alignment if available, or make sure to keep the layers firm and stationary on a table.
3. Assemble the electrolyzer with proper alignment of all layers and end plates. Secure tightly using screws.
4. Connect the cell to peristaltic pumps circulating 30% KOH at 30 mL/min. The flow rate of electrolytes can be adjusted, and depending on the gas evolution and temperature, the electrolytes can be increased for better gas removal and activity.
5. Maintain the electrolyte reservoir at 60 °C using an oil bath and monitor with a temperature probe. Ensure not touching the cell or reservoir without thermal gloves.
   NOTE: 30% KOH is highly caustic and can cause burns. Perform all experiments using proper personal protective equipment. Store extra electrolytes in a fume hood and keep the electrolyte reservoir on a flat surface that is well supported and balanced to avoid spills.

8. Calculation of Faradaic efficiency

1. Use the following equation:
   
   Where  Q_prod is the charge used to form the product (e.g., hydrogen or CO₂R product), and Q_total is the total charge passed.
   NOTE: Dispose of all HF- and HCl-containing waste following institutional hazardous waste protocols. Consult Material Safety Data Sheets (MSDS) and environmental health and safety (EHS) personnel for disposal guidance.

Results

The X-ray diffraction technique is used to analyse the solid crystal structure of the metal films. Cut appropriately sized (fits the sample stub of the XRD machine) film samples. Load the samples in the machine and scan a range of 2Θ from 10°to 80°. The XRD graph obtained shows peak signals for the crystal planes present in the material. Use the International Centre for Diffraction Data (ICDD) reference pattern to identify and further analyse the crystal structure, like main peaks, d-spac...

Discussion

This study presents the protocol for the synthesis of bi-metallic electrodes for solar-driven redox reactions for fuel generation. Decarbonizing the energy sector heavily depends on CO₂ conversion0^35,36,37 and use of zero-emission fuels like hydrogen generated from water^38,39. Solar-driven electrochemical transformation of carbon dioxide and water needs a...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Autolab 302	Metrohm UK	PGSTAT302N	It assess the electrochemical activity of the system (CO2R and WE)
Cobalt nitrate hexahydrate	Sigma aldrich	98 10026-22-9	precursor for water electrolysis anode
Copper nitrate trihydrate	Sigma aldrich	10031-43-3.	precursor for CO2R cathode
Light source	SOION tech	xenon lamp	Light source to illuminate the solar cell
Nickel nitrate hexahydrate	Sigma aldrich	13478-00-7	precursor for water electrolysis anode
Online gas chromatography	Agilent	8890	connected to the electrochemical cell and loops a sample , ~10microlitre gas every 10 minutes or time described in the command for identification and quantification.
Peristaltic pumps	SOION tech	BT100-3J	to flow electrolyte in zero-gap cell.
Potassium hydroxide	Sigma aldrich	1310-58-3	Electrolyte for CO2R and water electrolysis.
Solar cell (photovoltaic cell)	Fuel cell store		Silicon tandem cell that can supply upto 4 volts of potential and currents of ~300mA
Sustainion membrane	Dioxide materials	X37-50 Grade RT	anion exchange membrane used for ion transport from cathode to anode and stop gas and product crossovers.
Ti3C2Tx MXene	Anasori Lab, Purdue University
Vionic	Metrohm UK	3500001080	It assess the electrochemical activity of the system (CO2R and WE) at higher currents. It has higher current and potential windows.
Zinc citrate	Sigma aldrich	480762	precursor for CO2R cathode