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Water splitting as sustainable and eco-friendly energy solution:


To cite this article  use:  Iatsunski, I.,  Coy, E. Water splitting as sustainable and eco-friendly energy solution:Metal oxide and metal carbide co-catalysis alliance. J. Nano Sc. Tech, 4(2016)14-17.

Metal oxide and metal carbide co-catalysis alliance

Igor Iatsunski 

Emerson Coy

The production of hydrogen as energy carrier has been getting increasing amount of attention during the past decade, mainly due to the energetic problems that our society is facing. Hydrogen (H2) is considered to be the most attractive energy carrier of the future, mainly because its combustion is not pollutant.

Hydrogen, when combined with oxygen from the air, releases the chemical energy stored in the H-H bond, generating water vapor as the only by-product of combustion. It can be stored as pressurized gas and as liquid or distributed by pipeline, it is considered that it can replace natural gas in the medium to long term for certain industrial aplications. Since no greenhouse gases are produced during its combustion, hydrogen offers significant potential to reduce CO2 emissions generated during the combustion of fossil precursors. However, hydrogen is not freely found on Earth, which is one of the main reasons which have kept it from being the primary source of energy. Therefore, it has to be produced from different precursors, either by chemical or biochemical processes.

Among the processes for harvesting hydrogen, the one in which hydrogen molecules are extracted from water molecules, the so called water splitting reaction, is the most desirable. Since water is the most plentiful and important element in earth, the efficient and clean extraction of hydrogen from water has driven much excitement in the last decade.

Among the oldest and best known methods for water splitting we find the electrolysis. In essence, in this method an electric current is applied between two electrodes in contact with the water solution, the electric current brings the energy needed to break the O-H-O bounds, while the charge of the electrodes, the anode(+) and cathodes (-), attract the charged molecules of H+ and O respectively. The electrolysis method requires large amount of energy and is industrially uncompetitive.  In practical terms, the hydrogen produced, represent less applicable energy than the one used in order split the molecules, and thus the applicability of this method has been confined to the class rooms of most high schools and universities. Nevertheless, electrolysis still provides some hope, if the electric current, needed for the reaction, is provided by a renewable source, like wind, solar or other.

Additionally, another major drawback to overcome is related to search of highly efficient electrodes, a topic which is also of a great interest for both the industry and academia. As mentioned before, in electrolysis electrodes play two specific roles –  the anode is where the oxygen evolution reaction (OER) takes place and the cathode deals with the hydrogen evolution reaction (HER), thus the whole process is considered as two half-reactions. Sadly, the ideal catalysis for HER is platinum (Pt), which increases the overall cost of the process, since platinum is a precious and not abundant metal, which value has remain comparable, or higher, than gold in the past few years.

In order to solve this issues, nowadays, one of the most interesting topics in energy production is the photoelectrochemical waters splitting, in which the OER is promoted by photoactive interface (metal oxides) and the HER is enabled by a noble metal-free electrodes (carbides or nitrides).

Photoelectrochemical water splitting by metal oxides

Highly efficient hydrogen production depends on the design and synthesis of photocatalysts. The challenge resides on the careful development of strategies for optimal photocatalysts in order to boost their performance. It is here where nanotechnology brings a great deal of ideas and solutions regarding the tailoring and control of the photocatalytic properties of metal oxides towards OEC.

Photoelectrolysis employs? involves? sunlight to directly decompose water into hydrogen and oxygen, and uses semiconductor materials similar to those used in photovoltaic applications. Photoelectrochemical (PEC) cells, offer a promising method for hydrogen production driven directly by solar energy, thus making it affordable and exploitable. PEC cells utilize light energy (photons) to perform a chemical reaction, in this case the splitting of water into hydrogen (H2) and oxygen (O2) gases, specifically on the half reaction devoted to the OEC.

The mechanism follows the same principles as electrolysis: it comprehends an anode and a cathode connected by an external circuit and immersed in an electrolyte (Fig.1). An ideal photocatalyst absorbs light irradiation, thus exciting electrons (e) from the valence band (VB) to the conduction band (CB) while holes (h+) are created in VB. The excited holes migrate to the surface of the catalyst where they will interact with the water. The water then will be oxidized and reduced to oxygen and hydrogen gas by the photoexcited holes and electrons, respectively:

Photoanode: 2H2O + 4h+ → O2 + 4H+

Cathode: 4H+ + 4e → 2H2

figure1

Figure 1. Photoelectrochemical water splitting on TiO2 photoanode.

The energy conversion efficiency of water photoelectrolysis is determined principally by the properties of the materials used for the photoelectrodes. In order to achieve the required efficiency of the photoelectrodes, the following criteria for the material should be met; (a) high stability under sunlight irradiation, (b) efficient charge transport, (c) low conduction band that is higher than the H2/H2O level and a maximum valence band that is lower than the H2O/O2 level, and (d) good absorption capability.

Nanostructured metal oxides (TiO2, ZnO etc.) are of great interest as effective catalysts for a wide range of applications in industry, including chemical conversion, pollutant decomposition under photoirradiation, and clean ener-gy production (i.e., hydrogen generation). The use of metal oxides for PEC water splitting process has proven advantageous due to their semiconducting properties, such as: ability to absorb sunlight, stability in neutral and basic environment, non-toxicity and abundance. Since TiO2 was employed by Fujishima and Honda for the first time in water splitting under UV light irradiation [1], tremendous efforts have been invested in the synthesis and modification of TiO2 based nanomaterials. For example, one-dimensional (1D) TiO2 nanostructures, including nanotubes and nanowires, have been extensively investigated to enhance the efficiency of hydrogen production from water by increasing the surface area as well as reducing carrier diffusion length [2, 3]. However, only the UV irradiation in the solar spectrum can be used by pure TiO2 due to its wide band gap (~3.2 eV for anatase phase), leading to lower conversion efficiency (5%).

To enhance visible light absorption and to improve charge transport, various strategies have been explored for the modification of  TiO2, such as chemical functionalization [4], coating, doping [5], and defect-engineering, forming heterojunction with other materials [4, 6], and introducing plasmonic effect by coating/ alloying noble metal nanoparticles [7, 8].

Despite the huge quantity of publi-cations for the last years, the key challenges to advance PEC innovation toward the market concern pro-gress in materials science and engineering. It is very important to develop photo-electrode materials and their processing technologies with high-efficiency (performance) and corrosion-resistance (longevity) characteristics, paving the path toward smart system integration and engineering. Since no “ideal” photo-electrode material commercially exists for water splitting, tailored materials have to be engineered and much space for fundamental and applied research is still available.

Noble metal-free hydrogen evolution catalysts (carbides)

While photocatalytic surfaces are ex-tensively studied from the OEC point of view, non-noble metal counter electrodes are also extensively studied. Since Pt shows the highest efficiency and electrocatalytic properties, most of the materials are benchmarked against platinum.

The first approach used was the implementation of the others from so called platinum group metals (PGM) which include Pt, Ru, Rh, Ir and Pd, however, as in the case of Pt, these elements are expensive and found in rather low abudancy in nature.

Nevertheless, in 1973 R.B. Levy and M. Boudart discovered that tungsten carbide (WC) possessed some platinum-like catalytic properties, specifically its similarity in electron density (d-band) to platinum. This discovery meant an order of magnitude reduction in the overall price of the electrodes and opened a whole new approach to the noble metal-free electrodes. Metal carbides are well known for their application as protective coatings and extraordinary mechanical and tribological properties.

They are generally refractory materials with high operation temperature and metallic properties such as electrical conductivity. This properties result in a general advantage for the physical properties of the electrode, since platinum and other pure metals are rather soft and highly plastic, making them not highly resilient to mechanical stress. To date the best results are the ones provided by WC and MoC electrodes.

Nanotechnology has quickly attempt to optimize the performance of this electrodes. Since in essence, catalysis is a surface effect, the nano engineering of porous, 2D or 1D nanostructures of MoC and WC has proven advantageous and has allowed for achieving rather competitive hydrogen production to date. However, although a great advancement has been achieved by Mo and W carbides, the availability of these two metals is just slightly higher than that of platinum and far from the ideal market materials, such as Ni, Co and Fe (Fig. 2), which are plentiful and overall economic to mine.

figure-2

Figure 2.  Photoelectrochemical water splitting on Pt electrode and element availability in earth crust. Image repurposed from [9] © Royal Society of Chemistry.

Conclusion

Photoelectrochemical water splitting is one the most promising solutions to the energy crisis nowadays. The careful selection and tailoring of surface effects of both anode and cathode electrodes has increased the perspectives of this technology and  has showed that nanotechnological solutions and applications are still pushing forward the boundaries of knowledge and narrowing the gap between application and  industry.

Much research is needed and this field is still attracting much investment and development of energy agencies. The spread of these technologies is considered a key strategy of the European Union and the global market [10]. According to the U.S  department of Energy[11], the high cost of hydrogen production, low availability of the hydrogen production systems, and the challenge of providing safe production and delivery systems are all early penetration barriers for wide spread of technology usage.  Additionally to this, hydrogen delivery and production options need to be determined and assessed as part of system demonstrations for every potential production technology.

Massive production and posterior transport of hydrogen is not a safe or feasible option. Validation of integrated systems is required to optimize component development, which demands basic research and prototype testing. Meanwhile at the European level, the Fuel Cells and Hydrogen Joint Undertaking (FCH)[12], a joint initiative between industry and academy supported by the European Commission, has establish platinum group metals (PGM) as one of the specific challenges to be addressed due to their critical availability. This couples with the European Union initiative that promotes the decarbonisation of the economy, regarding combustion and energy production, with a goal of substantially reduce the market by the year 2030 [13] and the ongoing funding of the Horizon2020 programme.

References

[1] Fujishima, A. & Honda, K., Nature, 238, 3 7(1972).

[2] In Sun Cho et al., Nano Lett.,  15 (9), 5709–5715 (2015).

[3] Rahman,  M.A., et al., Energy Environ. Sci., 8, 3363-3373 (2015).

[4] Wang, R.,  et al., Nanoscale, 7, 11082-11092(2015).

[5] Wang et al., J. Mater. Chem. A,   2, 17820-17827 (2014).

[6] Gu, Q., et al., Adv. Mater. Interfaces, 3, 1500631 (2016).

[7] Yang , Y.,  et al., Int. J. of Hydrogen Production, 39, 7664-7671 (2014).

[8] Melvin, A.A., et al., Nanoscale, 2015, 7, 13477-13488 (2015).

[9] Zou, X.,  et al.,  Chem. Soc. Rev., 44, 5148-5180 (2015).

[10] http://www.marketsandmarkets.com/Press Releases/hydrogen.asp

[11] http://energy.gov/ (Office of Energy Efficiency and Renewable Energy)

[12] http://www.fch.europa.eu/  (Fuel Cells and Hydrogen)

[13] Communication From The Commission To The European Parliament, The Council, The European Economic And Social Committee, The Committee Of The Regions And The European Investment Bank State Of The Energy Union 2015

______________________

Igor Iatsunski Ph.D.

NanoBioMedical Centre, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland  E-mail: igoyat@amu.edu.pl

Emerson Coy Ph.D.

NanoBioMedical Centre, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland  E-mail: coyeme@amu.edu.pl

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