The overall goal of this procedure, is to demonstrate a simple and straight forward preparation in electro-chemical testing method for electrodes, based on a conductive solid state catalyst as an alternative to a conventional drop coating preparation for the hydrogen evolution reaction. This method can help to answer key questions in the field of electrode materials preparation. The main advantage of this technique is that it provides an easy route towards highly robust, high performance electrodes without the need of any artificial nanostructuring.
We first had the idea for this method when we synthesized bipedalite, enhanced stability and reproducibility issues preparing electrodes using conventional drop coating methods. Visual demonstration of this technique is critical as the process parameters can strong influence the performance and stability of the catalyst and electrode. Demonstrating this procedure, will be Mathias Smialkowski, a student from our lab.
To begin this procedure, mix iron, nickel and sulfur thoroughly with a mortar and pestle. Transfer the mixture to a 10 millimeter diameter silica ampoule. Then, evacuate the ampoule overnight at 10 to the minus two milli bar.
After sealing the ampoule, place it in a tubular furnace. Increase the temperature of the furnace from room temperature to 700 degree celsius at five degree celsius per minute, followed by an isothermal step for three hours. Next, increase the temperature of the furnace to 1, 100 degree celsius within 30 minutes, followed by an isotherm step for 10 hours.
Then, slowly cool the sample to room temperature, by switching off the furnace. After removing the ampoule from the furnace, crack it to collect the solid product. Grind the pentlandite bulk material to obtain a fine powder.
Transfer approximately 50 milligrams of the powder into a three millimeter diameter compressing tool and press the material with the maximum weight force. Then, remove the pellet from the mold, using a distance holder. Following this, apply a two component silver epoxide glue on the brass rod, in the cavity of a Teflon casing.
Place the pellet in the Teflon casing, so that the flat side of the pellet sticks out approximately one millimeter. Remove any debris on the Teflon casing with a paper tissue. Next, verify the contact between the brass wire and the pentlandite pellet with the voltmeter, to ensure proper conductivity.
After 12 hours of curing the two component glue at 60 degree celsius, cool the electrode to room temperature. Polish the cooled electrode with a series of fine grade sandpaper, to obtain a shiny flush flat surface within the Teflon casing. Then, clean the surface with deionized water and allow it to dry under ambient conditions.
Connect all three electrodes with the wires of a potentiostat. Next, add 25 milliliters of electrolyte to the electrochemical cell and adjust the electrodes to ensure that they are fully immersed in the solution. Then, purge the solution with Argon for 30 minutes.
After purging, switch on the potentiostat and the magnetic stirring. Perform a cyclic voltammetry experiment, by first setting the potential range from 0.2 to minus 0.2 volts, the scan rate to 100 millivolts per second and the number of cycles to 20. Next, start the cycling process, waiting until the last cycle is finished.
If the last three to four obtained cycles coincide, the electrochemical electrode cleaning is completed. In case of divergence, add more cycles until stable curves are obtained. Before starting the linear sweep voltammetry experiment, determine the internal resistance compensation value for the electrochemical set up.
Following this, select the program for linear sweep voltammetry experiments, and set the potential range from 0.2 to minus 0.6 volts, and the scan rate to five millivolts per second, including the internal resistance drop into the experiment. Then, start the experiment. Repeat the linear sweep experiments, to ensure reproducibility.
Now, perform a controlled potential coulometry experiment, by first setting the potential to minus 6.6 volts with an experiment time of at least, 20 hours. Simultaneously collect gas samples, with the gas tight syringe from the head space of the sealed cell, to receptum for every hour for at least four hours of the experiment. Inject the samples into a JC instrument for quantification and determine the amount of hydrogen produced, using a calibration curve, recorded on the instrument.
Select a potential range between 0.1 and zero volts in the cyclic voltammetry experiment and set the scan rate to 10 millivolts per second. Use the internal resistance drop correction as demonstrated, and set the number of cycles for the experiment to five. Repeat the previous steps, for scan braids of 20, 30, 40, 50 and 60 millivolts per second.
From the obtained cyclic voltammetry curves, pick the fifth cycle for further interpretation. Determine the charging current density differences and plot these values as a function of the scan braid. The synthesis of pentlandite possessing a pentlandite structure, was confirmed by powder XRD.
Monosulfite solid solution impurities have been observed with faster heating, with a slower heating rate of five degree celsius per minute, these impurities can be reduced or eliminated. Mossbauer spectroscopy also confirmed the pentlandite structure and revealed two iron sites. DSC revealed two phase transitions, confirming the absence of undesirable phases.
SEM images of pentlandite polished rock and pellet electrodes are depicted here. From EDX analysis, the ratio between iron, nickel and sulfur is consistent with the observation of a pure pentlandite phase in the XRD. The linear sweep voltammegrams of both electrodes are shown here.
An exfoliated drop coated electrode, did not show improved electro catalytic performance. The amount of hydrogen produced depending on the time of electrolysis using a rock electrode, is comparable to a pellet electrode. Charging current density as a function of scan rate, shows marginal differences in the electrochemical surface area and number of active sites, which confirms that the electrodes have similar performance.
A nyquist plot of a rock electrode, reveals a low charge transfer resistance, which is consistent with the material's high intrinsic conductivity, pellet electrodes exhibit similar behavior. Once mastered, this technique can be utilized for multiple materials within a very short time frame, if performed properly. While attempting this procedure, it is important to remember that the material of interest needs to be conductive.
Following this procedure, will give you standard protocol that allows you to evaluate electro catalytic materials and properly compare your results. After watching this video, you should have a good understanding on how to prepare stable electrodes from bulk catalysts without the use of pioneers and how to test them properly with respect to the electro catalytic performance.
