Simple Methods for the Preparation of Non-noble Metal Bulk-electrodes for Electrocatalytic Applications

Summary

A facile preparation method of electrodes using the bulk material Fe_4.5Ni_4.5S₈ is presented. This method provides an alternative technique to conventional electrode fabrication and describes prerequisites for unconventional electrode materials including a straightforward electrocatalytic testing method.

Abstract

The rock material pentlandite with the composition Fe_4.5Ni_4.5S₈ was synthesized via high temperature synthesis from the elements. The structure and composition of the material was characterized via powder X-ray diffraction (PXRD), Mössbauer spectroscopy (MB), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and energy dispersive X-ray spectroscopy (EDX). Two preparation methods of pentlandite bulk electrodes are presented. In the first approach a piece of synthetic pentlandite rock is directly contacted via a wire ferrule. The second approach utilizes pentlandite pellets, pressed from finely ground powder, which is immobilized in a Teflon casing. Both electrodes, whilst being prepared by an additive-free method, reveal high durability during electrocatalytic conversions in comparison to common drop-coating methods. We herein showcase the striking performance of such electrodes to accomplish the hydrogen evolution reaction (HER) and present a standardized method to evaluate the electrocatalytic performance by electrochemical and gas chromatographic methods. Furthermore, we report stability tests via potentiostatic methods at an overpotential of 0.6 V to explore the material limitations of the electrodes during electrolysis under industrial relevant conditions.

Introduction

The storage of fluctuating renewable energy sources such as solar and wind energy is of significant social interest due to the gradual fade of fossil fuels and subsequent need of alternative energy sources. In this respect, hydrogen is a promising sustainable candidate for a molecular energy storage solution because of a clean combustion process.¹ Additionally hydrogen could be used as fuel or as starting material for more complex fuels, e.g. methanol. The preferred way for a facile synthesis of hydrogen using carbon neutral resources is the electrochemical reduction of water using sustainable energies.

Currently, platinum and its alloys are known to be the most effective electrocatalysts for the hydrogen evolution reaction (HER) showing low over-potential, a fast reaction rate and operation at high current densities.² However, due to its high price and low natural abundance, alternative non-noble metal catalysts are required. Among the vast amount of alternative non-precious transition metal catalysts,³ especially transition metal dichalcogenides (MX₂; M = Metal; X = S, Se) have been shown to possess high HER activity.^4,5,6,7 In this respect, we recently presented Fe_4.5Ni_4.5S₈ as a highly durable and active 'rock' HER electrocatalyst. This naturally abundant material is stable under acidic conditions and shows a high intrinsic conductivity with a well-defined catalytic active surface.⁸

While numerous materials with high HER activities have been reported, the electrode preparation is often accompanied with multiple problems, e.g. reproducibility and satisfactory stabilities (>24 h). Additionally, since the intrinsic conductivity of transition metal based catalysts in bulk is usually high, electrode preparation requires nano-structured catalysts to allow for an efficient electron transfer. These catalysts are then converted into a catalyst ink containing binders such as Nafion and the catalyst. Afterwards, the ink is drop-coated on an inert electrode surface (e.g. glassy carbon). Whereas being reasonably stable at low current densities an increased contact resistance and mediocre adhesion of the catalyst on the electrode support is commonly observed at high current densities.⁹ Hence, the need for more sufficient preparation methods and electrode materials is evident.

This protocol presents a novel preparation procedure for highly durable and cost efficient electrodes using bulk materials. The prerequisite for such an electrode is a low intrinsic materials resistance. Fe_4.5Ni_4.5S₈ fulfills this criterion and can be obtained from the elements via a simple high-temperature synthesis in sealed silica ampules. The obtained material is characterized with respect to its structure, morphology and composition using powder Xray diffractometry (PXRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and energy dispersive Xray spectroscopy (EDX). The synthesized material is processed to afford two types of bulk electrodes, namely 'rock' and 'pellet' electrodes. The performance of both electrode types is then investigated using standard electrochemical tests and H₂ quantification performed via gas chromatography (GC). A comparison of the performance of both types of electrodes in comparison to commonly used drop-coating experiments is presented.

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Protocol

1. High-temperature Synthesis of Fe _4.5 Ni _4.5 S ₈

NOTE: The herein described procedure for the synthesis of Fe_4.5Ni_4.5S₈ is adopted from the literature.^8,10 The strict application of the reported heating ramps is of high importance to prevent formation of phase impurities and defects of the silica ampule.

1. Mix iron (1.66 g, 29.8 mmol), nickel (1.75 g, 29.8 mmol) and sulfur (1.70 g, 53.1 mmol) thoroughly in a mortar and transfer the mixture to a silica ampule (10 mm diameter).
2. Evacuate the ampule overnight at 10^-2 mbar.
3. Seal the ampule and place it in a tubular furnace.
4. Increase the temperature from room temperature (RT) to 700 °C at 5 °C/min followed by an isothermal step for 3 h.
5. Increase the temperature to 1100 °C within 30 min and keep it isotherm for 10 h.
6. Slowly cool the sample to RT by switching off the furnace. Crack the ampule to collect the solid product. Make sure to separate the Fe_4.5Ni_4.5S₈ completely from silica glass fragments.

2. Physical Characterization

1. Mount a 10 mm x 5 mm x 3 mm piece of Fe_4.5Ni_4.5S₈ rock on the sample holder and place in the vacuum chamber of the SEM instrument. Record the SEM images at 650X and 6,500X magnification at 20 kV. Simultaneously, use the same sample for EDX analysis at 4.4 kV.
2. For the collection of PXRD data, apply finely ground powder of Fe_4.5Ni_4.5S₈ and mount it on an amorphous silicon wafer using silicon grease. Mount the wafer on the sample holder and collect the data in a continuous scan mode from 10-50° at a scan rate of 0.03° per 5 s using Cu-Kα radiation (λ = 1.5418 Å).
3. For Mössbauer analysis finely ground powder is used and placed in a polyoxymethylene (POM) cup. Record zero-field Mössbauer spectra at 25 °C using a ⁵⁷Co radiation source in a Rh-matrix.
4. For DSC analysis, finely ground powder is placed in a tared α-Al₂O₃ crucible. Perform DSC measurements in the range from RT to 1,000 °C recording the heating and cooling curve at a rate of 10 °C/min. Perform the experiment under a flow of high purity nitrogen.

3. Preparation of 'Rock' Electrodes

1. Solder a copper wire to a wire ferrule.
2. Cut the Fe_4.5Ni_4.5S₈ bulk material into smaller pieces (approx. 5 mm x 5 mm x 5 mm).
3. Place the small piece of Fe_4.5Ni_4.5S₈ in the ferrule in a way that approx. 2 mm of material sticks out of the ferrule.
4. Mantle the ferrule and copper wire with 100 mm of Teflon tubing.
5. Seal the tip of the electrode with two-component epoxide glue and dry the electrode overnight under ambient conditions.
6. Grind off the tip until the shiny surface (metallic finish) of the Fe_4.5Ni_4.5S₈ is exposed. Further polish with fine grade sand paper (20, 14, 3 and 1 µm grit) to obtain a smooth surface.
7. Clean the surface with deionized water and let it dry on air.

4. Preparation of 'Pellet' Electrodes

NOTE: Custom-built Teflon casings with a brass rod were used as contact for 'pellet' electrodes (3 mm diameter).

1. Grind 50 mg of material to obtain a fine powder of the Fe_4.5Ni_4.5S₈ material.
2. Fill the finely ground powder into a compressing tool (3 mm in diameter) and press the material with a maximum weight force of 800 kg/cm².
3. Remove the pellet from the mold using a distance holder.
4. Apply a two-component silver-epoxide glue on the brass rod in the cavity of the Teflon casing. Avoid any pollution of the tip of the Teflon-casing.
5. Place the pellet in the Teflon casing. The flat side of the pellet must stick out ~1 mm.
6. Remove any pollution on the Teflon casing with a paper tissue.
7. Verify the contact between the brass wire and the Fe_4.5Ni_4.5S₈ pellet with a voltmeter to assure proper conductivity.
8. After 12 h of curing the two-component glue at 60 °C, cool down the electrode to ambient temperature.
9. Polish the electrode with sand paper (20, 14, 3 and 1 µm grit) to obtain a shiny flush flat surface within the Teflon case.
10. Clean the surface with deionized water and let it dry under ambient conditions.

5. Electrochemical Testing of Electrodes

NOTE: The experiments were accomplished with a standard three-electrode setup using the Fe_4.5Ni_4.5S₈ electrode as working electrode, Ag/AgCl (sat. KCl or 3 M KCl solution) electrode as reference electrode and Pt wire or Pt-grid as counter electrode. A gas-tight cell equipped with a stirring bar was filled with the electrolyte consisting of 0.5 M H₂SO₄ for all electrochemical experiments. The electrolyte was not exchanged during the electrochemical testing of an electrode. All potentials are referenced to E_RHE (RHE = reversible hydrogen electrode) according to E_RHE = E_Ag/AgCl + X + 0.059 pH with X = 0.197 V (saturated KCl) or X = 0.210 V (3 M KCl), unless noted otherwise.

1. Preliminary steps
   1. Connect all three electrodes with the wires of the potentiostat.
   2. Add 25 mL of electrolyte (0.5 M H₂SO₄) into the electrochemical cell and adjust the electrodes to ensure that the electrodes are fully immersed into solution. Subsequently, switch on the potentiostat.
   3. Switch on the magnetic stirring.
2. Electrochemical cleaning of the electrode surface
   1. Perform a cyclic voltammetry (CV) experiment to obtain fast overview on the electrochemical processes that can be observed.
   2. Set the potential range from 0.2 to -0.2 V with a scan rate of 100 mV/s (non-catalytic potential area). Further, set the number of cycles to 20.
   3. Start the cycling process and wait until the last cycle is finished. If at least the last 3 to 4 obtained cycles coincide, the electrochemical electrode cleaning is completed. In case of divergence add more cycles until stable curves are obtained.
3. Measurement of the catalytic performance – linear sweep voltammetry
   1. Before starting the experiment determine the iR compensation value for the electrochemical setup.
   2. Select the program for linear sweep voltammetry (LSV) experiments and set the potential range from 0.2 to -0.6 V and the scan rate to 5 mV/s, including the iR drop into the experiment. Start the experiment.
   3. Repeat the linear sweep experiments to ensure reproducibility. In case of non-reproducible results start over from step 5.2.
4. Stability measurement & quantification
   1. Perform a controlled potential coulometry experiment (CPC).
   2. Set the potential to -0.6 V with an experiment time of at least 20 h (72,000 s).
   3. Simultaneously collect gas samples with a gas tight syringe from the headspace of the sealed cell through a septum for every hour for at least 4 h of the experiment. Inject the samples into a GC instrument for quantification and determine the amount of hydrogen produced using a calibration curve recorded on this instrument.
5. Estimation of the electrochemical surface area (ESCA)
   NOTE: Do not stir the electrolyte solution during this experiment.
   1. Determine the iR compensation to measure the resistance of the solution.
   2. Select a potential range between 0.1 and 0 V in the cyclic voltammetry experiment and set the scan rate to 10 mV s^-1. Use the iR drop correction. Set the number of cycles for the experiment to 5.
   3. Repeat steps 5.4.1) to 5.4.2) for scan rates of 20, 30, 40, 50 and 60 mV s^-1.
   4. From the obtained CV curves pick the fifth cycle for further interpretation.
   5. Determine the charging current density differences (Δj = j_a j_c) and plot these values as a function of the scan rate. The linear slope is equivalent to twice of the double-layer capacitance C_dl, which is proportional to the electrochemical surface area (ECSA).
6. Electrochemical impedance spectroscopy (EIS)
   1. Record electrochemical impedance spectra in the frequency range from 50 kHz to 1 Hz at the corresponding open-circuit potential and an overpotential of 0.3 V.
   2. Plot the Nyquist plot from the received data to determine the charge transfer resistance.

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Results

The successful synthesis of Fe_4.5Ni_4.5S₈ possessing the Pentlandite structure is confirmed by powder X-ray diffraction experiments due to the prominent (111), (311), (222), (331) and (511) reflections being present (Figure 1a). A proper temperature control during the reaction, however, is the key to obtain phase pure materials. Notably, mono-sulfide solid solutions (mss), a common impurity of pentlandite materials

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Discussion

The synthesis of Fe_4.5Ni_4.5S₈ was performed in a vacuum-sealed ampule to prevent oxidation of the material during synthesis. During the synthesis, temperature control is the key to obtain a pure product. The first, very slow heating step thereby prevents superheating of the sulfur, which might cause cracking of the ampule due to high sulfur pressure. Even more crucial is the prevention of phase impurities like mono-sulfide solid solutions (mss) by slow heating of the sample. The subsequen...

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Materials

Name	Company	Catalog Number	Comments
Iron, powder	Sigma-Aldrich, http://www.sigmaaldrich.com	12310-500G-R
Nickel, powder	Sigma-Aldrich, http://www.sigmaaldrich.com	203904-25G	H: 351-372-317-412; P: 281-273-308-313-302+352
Sulfur, powder	Sigma-Aldrich, http://www.sigmaaldrich.com	13803-1KG-R	H: 315
Silver Epoxy Glue EC 151 L	Polytec PT, http://www.polytec-pt.de/de/	161010-1	-
Two Component Epoxy Glue Uhu Plus Endfest	Uhu, http://www.uhu.com	-	H: 315-319-317-411;  P: 101-102-261-272-280-302+352-333+313-362-363-305+351+338-337+313
Sulfuric Acid >95%	VWR, https://ru.vwr.com	231-639-5	H: 290-314; S: (1/2)-26-30-45
PTFE Tube	-	-	Prepare 8 cm long peaces
Iron Sleeves	-	-	Connect to the copper wire
Copper Wire	-	-	-
Lapping Film 3µm, 215.9 mm x 279 mm	3M, http://3mpro.3mdeutschland.de	60-0700-0232-8	Polish with a small amount of water
Lapping Film 1µm, 215.9 mm x 279 mm	3M, http://3mpro.3mdeutschland.de	60-0700-0266-6	Polish with a small amount of water
Sand Paper 20 µm, SiC	-	-	-
Sand Paper 14 µm, SiC	-	-	-
Dremel Model 225	Dremel, https://www.dremeleurope.com	2615022565	Use grinding pulley wheel for cutting
Hand Made Pellet Press	Hand Made	-	-
Stirring Plate	-	-	-
GAMRY Reference 600	GAMRY Instruments, https://www.gamry.com	-	-
Gero Furnace 30-3,000 °C	http://www.carbolite-gero.de	-	-
Quartz glass ampule	Hand Made	-	-
Vacuum pump	-	-	-
Hydraulic press	-	-	-