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 Fe4.5Ni4.5S8 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 Fe4.5Ni4.5S8 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 Fe4.5Ni4.5S8 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. Fe4.5Ni4.5S8 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.
Protocol
Representative Results
Discussion
The synthesis of Fe4.5Ni4.5S8 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…
Divulgazioni
The authors have nothing to disclose.
Acknowledgements
We thank B. Konkena und W. Schuhmann for valuable scientific discussions. Financial support by the Fonds of the Chemical Industry (Liebig grant to U.-P.A.) and the Deutsche Forschungsgemeinschaft (Emmy Noether grant to U.-P.A., AP242/2-1).
Materials
| 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 x 279 mm | 3M, http://3mpro.3mdeutschland.de | 60-0700-0232-8 | Polish with a small amount of water |
| Lapping Film 1µm, 215.9 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 30-3000°C | http://www.carbolite-gero.de | – | – |
| Quartz glass ampule | Hand Made | – | – |
| Vacuum pump | – | – | – |
| Hydraulic press | – | – | – |
Riferimenti
- May, M. M., Lewerenz, H. -. J., Lackner, D., Dimroth, F., Hannappel, T. Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure. Nat Comm. 6, 8286 (2015).
- Sheng, W., et al. Correlating hydrogen oxidation and evolution activity on platinum at different pH with measured hydrogen binding energy. Nat Comm. 6, 5848 (2015).
- Li, X., Hao, X., Abudula, A., Guan, G. Nanostructured catalysts for electrochemical water splitting: Current state and prospects. J. Mater. Chem. A. 4 (31), 11973-12000 (2016).
- Merki, D., Hu, X. Recent developments of molybdenum and tungsten sulfides as hydrogen evolution catalysts. Energy Environ. Sci. 4 (10), 3878 (2011).
- Kibsgaard, J., Chen, Z., Reinecke, B. N., Jaramillo, T. F. Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis. Nat Mater. 11 (11), 963-969 (2012).
- Kong, D., Cha, J. J., Wang, H., Lee, H. R., Cui, Y. First-row transition metal dichalcogenide catalysts for hydrogen evolution reaction. Energy Environ. Sci. 6 (12), 3553 (2013).
- Voiry, D., et al. Enhanced catalytic activity in strained chemically exfoliated WS(2) nanosheets for hydrogen evolution. Nat Mater. 12 (9), 850-855 (2013).
- Konkena, B., et al. Pentlandite rocks as sustainable and stable efficient electrocatalysts for hydrogen generation. Nat Comm. 7, 12269 (2016).
- Jeon, H. S., et al. Simple Chemical Solution Deposition of Co₃O₄ Thin Film Electrocatalyst for Oxygen Evolution Reaction. ACS Appl Mater Interfaces. 7 (44), 24550-24555 (2015).
- Xia, F., Pring, A., Brugger, J. Understanding the mechanism and kinetics of pentlandite oxidation in extractive pyrometallurgy of nickel. Mine Eng. 27-28, 11-19 (2012).
- Drebushchak, V. A., Kravchenko, T. A., Pavlyuchenko, V. S. Synthesis of pure pentlandite in bulk. J Crystal Growth. 193 (4), 728-731 (1998).
- Knop, O., Huang, C. -. H., Reid, K., Carlow, J. S., Woodhams, F. Chalkogenides of the transition elements. X. X-ray, neutron, Mössbauer, and magnetic studies of pentlandite and the π phases π(Fe, Co, Ni, S), Co8MS8, and Fe4Ni4MS8 (M = Ru, Rh, Pd). J Solid State Chem. 16 (1-2), 97-116 (1976).
- Kullerud, G. Thermal stability of pentlandite. The Canadian Mineralogist. 7 (3), 353-366 (1963).
- Siracusano, S., et al. An electrochemical study of a PEM stack for water electrolysis. Int J Hydrogen Energy. 37 (2), 1939-1946 (2012).
