Solar-Driven Electrochemical Green Fuel Production from CO₂ and Water Using Ti₃C₂T_x MXene-Supported CuZn and NiCo Catalysts

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.

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.

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-spacing, and peak shifts due to alloy formation. Scanning electron microscopy (SEM) is used to show the microscopic structure and topology of the deposited material. SEM images show the shape, size, and conformality of the film.

X-ray diffraction (XRD) pattern and scanning electron microscope (SEM) images obtained for Ti3AlC2 MAX phase, and Ti3C2Tx MXene are shown in Figure 1. The diffraction pattern of Ti3AlC2 MAX exhibited characteristic (002) peaks for the M3AlC2 structure, and after the selective etching and delamination, the (002) peaks shifted to lower angles, suggesting the synthesis of Ti3C2Tx MXene (Figure 1A). SEM image of the Ti3AlC2 MAX phase reveals its layered morphology (Figure 1B). After selective etching of Ti3AlC2 MAX and delamination of multilayered Ti3C2Tx MXene, the delaminated Ti3C2Tx MXene deposited on an alumina anode exhibits a flake-like morphology consisting of single to a few layers, as shown in the SEM image (Figure 1C).

XRD and scanning electron microscopic images of CuZn@Ti3C2Tx MXene@CFP and NiCo@Ti3C2Tx MXene@Ni-Foam are shown in Figure 2A,B. Figure 3 shows the PV cell, H-cell, solar-driven gas evolution, and zero-gap cell assembly with electrolyte flow. Figure 4 shows (A) CV in CO2 reduction and water electrolysis, (B) EIS in H-cell for cathode in CO2 reduction and anode in water electrolysis, and (C) CV on anode for a full cell. Supplementary Figure 1 shows EIS @OCP in a zero-gap cell, Supplementary Figure 2 shows gas chromatograms on TCD and FID for CO2 reduction, and Supplementary Figure 3 shows gas chromatograms for water electrolysis. Supplementary File 1 shows the oxygen evolution reaction mechanism on NiCo anodes in an alkaline medium.

Figure 1: X-ray diffraction (XRD) pattern and scanning electron microscope (SEM) images for Ti3AlC2 MAX phase, and Ti3C2Tx MXene. (A) XRD pattern showing characteristic peaks for Ti3C2Tₓ (indicated by stars) and Ti3AlC2 MAX phase (indicated by diamonds); (B) SEM image of Ti3AlC2 MAX phase; (C) SEM image of delaminated Ti3C2Tₓ MXene. Please click here to view a larger version of this figure.

Figure 2: XRD and scanning electron microscopic images of CuZn@Ti3C2Tx MXene@CFP and NiCo@Ti3C2Tx MXene@Ni-Foam. XRD (A) and SEM images of the fabricated bi-metallic electrocatalyst films, CuZn@Ti3C2Tx MXene@CFP cathode for CO2 reduction (B,C), and NiCo@Ti3C2Tx MXene@Ni-F anode for water electrolysis (D,E). Please click here to view a larger version of this figure.

Figure 3: PV cell, H-cell, solar-driven gas evolution, and zero-gap cell assembly with electrolyte flow. (A) State-of-the-art perovskite silicon tandem PV assembly used to drive the electrochemical reduction of CO2 and water at zero applied current, (B) H-cell used to reduce CO2 and electrolyze water with a membrane to stop product oxidation, (C) gas bubbles observed on cathode when connected to the terminals of PV cell showing the reaction taking place, and (D) zero-gap cell assembly used to perform water electrolysis at industrial parameters (30% KOH, 1 A/cm2, 60 °C). Please click here to view a larger version of this figure.

Figure 4: Electrochemical performance of NiCo@MXene@Ni-foam anode for CO2 reduction and water electrolysis. (A) CV obtained in 1 M KOH in a H-cell for CO2 reduction (CO2R) (red trace), and OER (blue trace) using H-cell assembly, (B) EIS at open circuit potential for anode and cathode in CO2R and water electrolysis, (C) CV and (D) chrono amperograms observed at different applied cell potentials (1.6 V to 2.6 V on NiCo@MXene@Ni-Foam anode in 30% KOH, 60 °C, and electrolyte flow rate of 50 mL/min in a zero-gap cell (mimicking commercial AEM-WE). Please click here to view a larger version of this figure.

Supplementary Figure 1: EIS recorded at 0V on NiCo@MXene@Ni-Foam anodes in a zero-gap cell. The total cell resistance is <1ohm, showing high conductivity. Please click here to download this figure.

Supplementary Figure 2: Gas chromatograms at TCD 1, 2, and FID detectors operating at a method specially created for the detection and identification of permanent gases in CO2 reduction (A) FID, (B) TCD1, and (C) TCD2. The peaks for CH4, C2H4, C2H6, and H2 are detected by comparing them to a calibration curve computed using standard concentrations for these cases. Please click here to download this figure.

Supplementary Figure 3: Gas chromatograms at TCD 1 and FID detectors operating at a method specially created for the detection and identification of hydrogen and oxygen (A) TCD, and (B) FID. The peaks for H2 and O2 are detected by comparing them to a calibration curve computed using standard concentrations for these gases. It can be seen that there is no signal on FID as water splitting is expected. Please click here to download this figure.

Supplementary File 1: Oxygen evolution reaction mechanism on NiCo anodes in an alkaline medium. Please click here to download this file.

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 combination of catalysts (anodes and cathodes) that function at the peak performance of the PV cell used¹⁰. H-cell is used to separate anodic and cathodic products and to prevent re-oxidation of cathodic products at the anode. The alkaline exchange membrane should be placed carefully between the two halves of the cell, which will lead to better transport of ions and avoiding gas crossover.

It is shown that electrodeposition can be used to deposit metallic catalysts depending on the desired reaction and structure. A pulsed deposition offers more control over the microscopical structure and alloy composition compared to constant deposition^40,41. On applying a negative pulse, the positively charged metal ions are dragged to the cathode, where they are discharged and deposited as metal particles. A null pulse repopulates the electron diffusion layer, thus controlling the grain size and texture. A reverse pulse refines the grain and optimizes the alloy composition^42,43,44. The method used in this activity is a combination of forward (-ve amplitude pulse), null (zero amplitude pulse), and reverse (+ve amplitude pulse) repeated 1000 times to deposit a thin conformal catalyst layer, as shown in Figure 1. CuZn@Ti₃C₂T_x MXene@CFP shows a nano-sheet structure and a conformal deposition on CFP. NiCo@Ti₃C₂T_x MXene@Ni-Foam shows the compact film deposited on the nickel foam. It is important to optimize the metal ratio in the precursor for a desired alloy composition. From XRD graphs (Figure 2A), the phase obtained is α-brass with copper content around 64% and Zn about 36%⁴⁵. The NiCo bi-metallic system is deposited in the FCC phase with metallic Ni, Co, and Ni-Co alloy with a 1:1 ratio⁴⁶. The electrochemical potential of metal ions plays a crucial role in the deposition process, thus requiring attention^47,48.

Electrochemical reduction of CO₂ is done in an H-cell, which is gas-tight and in-line with GC. To analyze the electrochemical performance, a CV at a 50 mV/s scan rate is recorded in 1 M KOH in a 3-electrode setup. A 3-electrode setup includes a reference electrode, which monitors working electrode potential to compare the activity and overpotential to state-of-the-art materials for CO₂ to fuels. An EIS is used to determine the charge transfer resistance and total IR losses associated with the cell, electrolyte, substrate, and catalysts. The PV cell illuminated with 1 sun light is connected with the H-cell and left to run for 2 h. The gas evolved from the cell is fed to the GC to analyze the products formed every 10 min. A method that uses a thermal conductivity detector and flame ionization detector to detect the CO₂ products is used in loops to identify and quantify the gases generated. A calibration curve generated by using standard concentrations of permanent gases (O₂, N₂, CO, CH₄, C₂H₄, C₂H_6, etc) is used to calculate the faradaic efficiency of products obtained in the CO₂R^49,50.

A CV and EIS were recorded in H-cell with NiCo@Ti₃C₂T_x MXene@Ni-Foam as anode, Pt/C as cathode, and Hg/HgO as reference electrode, and 1 M KOH as electrolyte at scan rate of 50 mV/s and open circuit potential, respectively, to assess the electrochemical activity of the cathode. To test the solar-driven water electrolysis performance, illuminated PV cell terminals were connected to H-cell electrodes (anode and cathode), and a chronopotentiometric curve was recorded at zero applied current. The cell was gas-tight and connected to a GC to quantify the hydrogen produced every 10 min. The GC method used TCD in single loops to quantify the hydrogen produced by comparing the peak area and flow rate of gas (H₂, O₂, N₂ (carrier gas)) from the cell with a calibration curve generated using a standard concentration of hydrogen. It is important to take care while measuring the area under the hydrogen peak for consistent results.

A zero-gap cell is used to limit the IR losses prevalent in an H-cell. Standard industrial conditions of electrolyte concentration (30% KOH), current densities (1 A/cm²), and temperatures (60 °C) were used to show hydrogen production at commercial demands. Care must be taken while assembling a zero-gap cell to avoid any leak of electrolyte or gas. Using screws to keep in place the end plate, place the anode on the flow channel and secure it in place by using a gasket, now layer AEM (moist and activated in 1 M KOH for 24 h), and secure with a gasket, then add the counter electrode and second end plate and tighten the screws to stop any leaking. Connecting the inlet of the zero gap cell with the outlet of the peristaltic pumps and the outlet of the cell to the electrolyte reservoir (30% KOH) that is held at 60 °C. The pump inlet is connected to the electrolyte reservoir. An applied current of 10 kA/m² is used to study the performance of the cell and the durability of catalysts. Cell voltage observed gives insights into the performance, and degradation/change of cell voltage over time gives insights into the durability of the cell system. A good heat exchanger is needed to maintain the desired temperature of the electrolyte flow. Ensuring no leak from the cell or any connecting tubing is vital to avoid performance loss and risk mitigation.

The method of deposition has practical application in fabricating thin metal films for catalyst-coated substrate electrolysers. The deposition method can be used with small adjustments to coat a surface with poly-metal alloys of choice for other electrocatalytic applications. The water electrolysis in a zero-gap cell under industrial conditions can be used to generate hydrogen in higher quantities by scaling up.

Troubleshooting

It is important to take necessary precautions regarding safety and precise control of reaction parameters. However, due to asymmetric personal and instrumental errors, the experiment may fail. Ensure that connections from the electrochemical cell and electrodes are secure and free from rust for the PV connection test; on illumination, potential/current should be generated to drive the reaction. For electrodeposition, the counter and working electrodes should face each other, with the reference electrode placed near the working electrode without blocking its front side. The electrodeposition method is applicable to all first-row transition metals and can be diversified by varying the current ON pulses and adjusting pH. Normally, only metallic films are obtained via this method, but oxide films can be achieved by applying oxidative potential. The process is limited by the nature of the metals and the current amplitude; unoptimized current amplitude and metal concentration can lead to asymmetrical and coarse deposition, which hampers the reaction considerably. The extent of hydrogen generation or CO2 products depends on the electrode area, intrinsic activity of the electrode, nature of the substrate, potential limit of the instrument, and gas transport from the electrode to the outside.