Solar Water Splitting: A Step Towards Carbon-Free Energy and Environment
Md. Golam Kibria
Department of Electrical and Computer Engineering, McGill University. Montreal, Quebec, Canada.
Email Address: email@example.com
Research Highlights: Received 12 December 2011; received in revised form 24 June 2012; online published 16 July 2012
1. Global Energy Crisis
Our planet is heading towards an energy famine. According to the U.S. Department of Energy, the worldwide primary energy consumption in 2008 was ~17 terrawatts (TW, one TW equals 1012 watts, or 1012 joules per second), which is expected to double by 2050 and triple by 2100 . Needless to say, this is because of the growing world population, which was 6.2 billion in 2006, 7.0 billion in 2011 and is projected to be 9.4 billion by 2050 . Therefore, by 2050, an additional 17 TW energy supply is required for the extra 2.4 billion people, considering the current living standard of world population and with the best conservation of energy. Considering Bangladesh as an example, only 50% of Bangladeshis have access to electricity at present with a per capita generation of ~252 KWH . According to the Power Sector Master Plan 2010 , the power demand in 2030 will be nearly 34,000 MW. With the current generation of around 5,200 MW , Bangladesh will require an additional ~29,000 MW power in 2030. Therefore, it is imperative to explore alternate sources of energy to cope up with this energy crisis.
2. Alternative Energy Sources
There are different types of energy sources currently available; however, not all of them can follow our current and future energy demand curve. For example, biomass offers a limited energy supply because of the low efficiency of photosynthesis . Alternatively, nuclear energy plant is assumed to be a solution to this energy crisis. However, the deployment of expensive nuclear plant is difficult to build fast enough to cope up with the energy demand. Also nuclear power plant requires widespread public acceptance, since they can be a point of catastrophic disaster as seen by the world recently in Fukushima, Japan . Wind energy is a poor choice because of its low energy density and intermittency . While the fossil energy resources (i.e., gas, oil, coal, etc.) can satiate the global energy appetite, the atmospheric CO2 released from burning this fossil fuel significantly increases greenhouse gas (GHG), which can perturb the ecosystem of the planet. Therefore, it is vital to find alternative carbon-neutral energy sources that can mitigate the current and future global energy appetite.
3. Solar Energy
Solar energy is the ultimate solution offering carbon-free energy. About 1.2× 105 TW power from sunlight falls on the Earth’s surface in 1 hr, which is higher than the global energy consumed by all human activities in 1 year . Approximately 600 TW power from sunlight strikes the Earth’s terrestrial surfaces that can be used as practical sites for solar energy harvesting . Covering 0.16% of the land on Earth with 10% efficient solar conversion systems would provide 20 TW of solar power, which is nearly twice the world’s fossil fuel consumption rate . Clearly, solar energy is the largest renewable carbon-free resource amongst all other renewable energy options. However, the major challenges in implementing large scale solar energy system are the low efficiency and the high manufacturing cost of solar panels. Furthermore, for penetrating the global energy market, new technologies need to be developed to store solar energy, which can be released in a form that one desires, whether it is electrical, chemical or mechanical.
4. Importance of Solar Energy Storage
Because of the diurnal nature and intermittency arising from variable atmospheric conditions, it is imperative to find a solution to store and supply solar energy for 24 hours a day, 7 days a week. Solar energy storage offers the following significant advantages: (1) an efficient storage mechanism allows synchronization between solar power supply and demand curves, (2) a cost effective solar storage system will offer highly distributed off-grid power supply system, (3) because of its highly distributed and decentralized nature, solar energy storage offers geopolitical stability . Therefore, it is vital to innovate and design new materials and techniques to store solar energy in cost effective ways.
5. Current Techniques for Solar Energy Storage
The current methods of storing solar energy are too expensive and inefficient for large scale applications. There are mainly four forms of energy storage technologies : (1) potential energy (pumped-hydroelectric, compressed-air, electric charge in super/ultra capacitors), (2) kinetic energy (flywheels), (3) thermal energy (concentrated solar thermal, geothermal), and (4) chemical energy (in the form of batteries or fuels). The energy storage system based on potential energy, kinetic energy and thermal energy experiences high cost of deployment, short time storage, and low energy density to be implemented for large scale applications. The electrochemical energy storage in the form of batteries or capacitors is only limited to small scale and short term applications because of their low energy density and short life time.
On the other hand, energy storage in the form of chemical fuels, such as hydrogen (H2) gas or hydrocarbons offers sustainable energy for small/large scale, and short/long term applications. Although H2 has the highest energy density by mass (143 MJ Kg), it suffers from low volumetric energy densities . On the other hand, hydrocarbon fuels offer the optimum volumetric energy density, and therefore can be used as a renewable energy carrier for modern society.
6. Necessity for Solar Water Splitting
At present, hydrogen is mainly produced from fossil fuels and therefore the production process emits CO2. However, H2 needs to be produced from abundant natural resources, such as water and sunlight. Splitting water into H2 and O2 using sunlight allows storing solar energy in the form of chemical energy and makes solar energy 24 hours a day, 7 days a week. This process produces hydrogen: a clean, storable and renewable source of energy. The released H2 from water splitting can be used in fuel-cells to produce electricity or can be directly combusted in an engine, wherein the reaction byproduct is nothing but water. This process will make solar energy highly distributed, from small to large scale applications. Therefore, research on solar water splitting is an urgent need. However, the current technologies for solar water splitting in a cost effective and efficient way are still at their infancy.
7. Solar Water Splitting
Splitting water into hydrogen and oxygen with sunlight is one of the ‘holy grails’ of chemistry. Since the pioneering work by Fujishima and Honda , considerable researches have been focused over the last 40 years on developing a stable and efficient photocatalyst material for solar water splitting , with limited success. A photocatalyst material is required to absorb solar photons and subsequently use the photon energy to break the chemical bonds of water. A photocatalyst material needs to satisfy three requirements to be able to split water: (i) the band gap has to be higher than 1.23 eV (as discussed later), (ii) suitable band-edge potentials with respect to water oxidation and reduction potential (as discussed later), and (iii) stable against photocorrosion . Although a number of metal oxides satisfy all of the above mentioned requirements, they are not, however, suitable for practical applications. This is because metal oxides usually possess a large bandgap (> 3 eV), and hence can harvest only the UV portion of solar spectrum. Since, only ~4% of solar spectrum lies in the UV region, the solar to hydrogen conversion efficiency of the metal oxides remains impractical. Therefore, tremendous effort has been made in the last decade to harvest visible photons, as visible light accounts for ~46% of solar irradiation .
There are mainly two approaches for water splitting: photocatalytic and photoelectrochemical (PEC) water splitting. In photoelectrochemical water splitting, the photogenerated carriers (electron or holes) in the semiconductor photocatalyst (i.e., TiO2) are channeled through a metallic conductor to an electrode, which is modified by catalysts (Pt) (Fig. 1 A). In this case, some external bias is usually required for efficient carrier separation and to overcome the resistance between the electrodes in the solution. In photocatalytic water splitting, the catalysts (Pt, in this case) are integrated onto the semiconductor (TiO2) and the photogenerated carriers are directly captured by the catalyst (Fig. 1 B) .
Figure 1: (A) Photoelectrochemical water splitting on TiO2 photoanode, (B) Photocatalytic water splitting on TiO2 particle .
In both approaches the water splitting reaction converts solar energy into chemical energy with a positive change in Gibbs free energy (i.e., uphill reaction) .
H2O → O2 + H2, ΔG=+237.178 KJ/mol
Therefore, water splitting reaction can store 237.178 KJ/mol at 25 °C and 1 bar. This reaction is regarded as artificial photosynthesis, as it resembles the natural process of photosynthesis by which green plants store solar energy.
8. Main Processes Involved in Solar Water Splitting
The water splitting reaction generally involves three main processes [13, 15] as shown in Figure 2 . (1) The first step is band gap absorption of photons and generation of electron–hole pairs. Thermodynamically, when the energy of incident light is larger than the band gap energy, electrons and holes are generated in the conduction and valence bands, respectively. (2) The second step consists of charge separation and migration of photogenerated carriers. The physical size of the photocatalyst determines the activity of the photocatalyst. If the size is small, the photogenerated carriers will have to travel a small distance to reach the surface and hence there will be less probability of carrier recombination. Therefore, development of high quality nanoscale material is of great interest.
Figure 2: (Three main processes involved in photocatalytic water splitting. Here, the host nanowire photocatalyst is used to capture solar photons. The nanowire is decorated with some metal nanoparticles as H2/O2 evolution catalyst to enhance the redox reaction .
(3) The final step involves the reduction and oxidation (redox) of water on the photocatalyst surface via the photogenerated electrons and holes, respectively. It is a common practice to incorporate some H2 and O2 evolution catalyst on the host photocatalyst (nanowire in this case) to enhance the extraction of electrons/holes and reduce the activation energy barrier for gas evolution, as shown in Figure 2.
For water splitting, first, the O-H bonds of two water molecules need to be broken with the simultaneous formation of one O=O double bond as follows .
H2O ↔ O2 +4e- + 4H+, Eanodic=1.23 V – 0.059 (pH) V (NHE) …………….(1)
Since this reaction requires a high oxidizing potential, 1.23 V vs. NHE (normal hydrogen electrode) (pH=0), the top level of valence band has to be more positive than this potential, so that the photogenerated holes have enough energy to oxidize water. This reaction releases four protons (H+) and four electrons (e?), which need to be combined to form two H2 molecules, as follows :
4H+ + 4e- ↔ 2H2, Ecathodic = 0 V – 0.059 (pH) V (NHE) ………….(2)
Therefore, the conduction band of the semiconductor has to be more negative than water reduction potential (0 V vs. NHE (pH=0)). Thus, water molecules are oxidized by the holes to form O2 and reduced by the electrons to form H2 for overall water splitting. Therefore, the theoretical minimum band gap for water splitting is 1.23 eV that corresponds to light wavelength of about 1000 nm. A photocatalytic water splitting reaction over a semiconductor material is schematically shown in Figure 3 .
Figure 3: Schematic diagram of band edge requirements for water splitting reaction .
For practical applications, water splitting has to be achieved under visible light (> 400 nm) using an Earth abundant, stable and efficient photocatalyst material. Researches on visible light responsive photocatalyst are limited as there are very few stable materials that satisfy the thermodynamic and kinetic potential for overall water splitting under visible light [17-18]. Therefore, different band engineering methods have been developed to transform ultraviolet (UV) light (<400 nm) active materials into visible-light active photocatalysts [19-21]. While band engineering improves the visible light activity of these photocatalyst materials to some extent, there are still very few reliable and efficient photocatalyst for overall water splitting under visible light. Therefore, it is vital to explore new visible light responsive photocatalysts that are stable and efficient for overall water splitting.
On the other hand, recent advances in nanotechnology possess immense potential for efficient harvesting of solar energy. For example, one dimensional (1D) nanostructures, such as nanowire, nanotubes or nanobelts can be made using two approaches, namely top-down and bottom-up. Because of their high crystallinity and surface-to-volume ratio, these 1D nanostructures are capable of efficient light absorption and carrier separation . Hence, a boost in solar to hydrogen conversion efficiency is expected. Figure 4 shows a schematic of large scale H2 production system via solar water splitting, as depicted by Domen et al. . According to their calculations, about 10,000 solar water splitting power plant (25 km2 each) would be required to supply one-third of the projected energy needs of human society in 2050.
Figure 4: Large scale H2 production via solar water splitting .
Although H2 can be synthesized via water splitting for subsequent use in fuel cell or in an engine, it is not a convenient means to store solar energy. H2 is highly volatile and therefore requires high pressure or cryogenic container to maintain a temperature of -253 °C. The handling of potentially explosive H2 requires special conditions: high pressure, minimum diffusion or leakage and extensive safety precautions. Additionally, the volumetric energy density of liquid H2 is about one-third of that of gasoline. Further, the infrastructure needed for H2 fuel would be expensive, thus limiting its potential use. In order to overcome these issues, George A. Olah (Nobel Prize winner in Chemistry in 1994) proposed a “methanol economy”, wherein methanol can be used as a means of energy storage instead of H2 and/or fossil fuels . Methanol production for methanol economy can be synthesized by solar water splitting in the presence of CO2, as discussed below.
9. Solar Water Splitting for CO2 Reduction
Solar water splitting not only has the potential to mitigate the energy crisis but also has the capability to reduce CO2 in the environment. The four electrons and the four holes released from water oxidation reaction (Eq-1) can be combined with CO2 to produce liquid alcohol or hydrocarbon fuel (CO, CH4 or CH3OH) , as follows :
CO2 + 6e- + 6H+ → CH3OH + H2O ………………. (3)
Liquid fuel (i.e., CH3OH) can also be produced by catalytic hydrogenation of CO2 and H2, wherein H2 is obtained from solar water splitting, as follows :
CO2 + 3H2 → CH3OH + H2O …….. ……….. (4)
The produced Methanol can be used directly in a “Direct Methanol Fuel Cell”. The required CO2 for the above reactions can be captured from fossil fuel burning power plants or other industries. Considering the diminishing fossil fuel resources and therefore CO2, the CO2 in air can also be used for the above reactions. Since the concentration of CO2 in air is very low (0.037%) , new technologies need to be developed to efficiently and economically capture CO2 from air.
Figure 5: Schematic diagram of solar water splitting for energy production and environment purification.
Therefore, environment purification (i.e., CO2 reduction) and energy production (i.e., liquid fuel) can be achieved simultaneously, as shown schematically in Figure 5. Because of their higher volumetric energy densities, hydrocarbon fuels can alleviate hydrogen storage issues. Further, hydrocarbon fuels can be used in existing gasoline infrastructure with limited modifications . Hence, building expensive liquid H2 infrastructure can be avoided. Therefore, there is an immense potential in solar water splitting both for carbon-free energy and environment. Research and development in this area are under advancement.
10. Current Technological Challenges
The major technological challenges for the large scale deployment of solar water splitting either for H2 generation or for CO2 reduction may be summarized as follows: (1) development of stable, highly efficient and visible light active photocatalyst from Earth abundant materials for water oxidation and reduction; (2) development of stable and efficient catalyst for CO2 reduction; (3) development of a technology to separate simultaneously produced H2 and O2 from solar water splitting; (4) development of new materials to store large amounts of H2 in a small volume at low pressure, near-ambient temperature, and with high efficiency during energy release cycle. Success in addressing these technological challenges will enable a carbon-free clean planet wherein there will be no energy crisis, no geopolitical instability and therefore a peaceful humanity.
Solar energy is the ultimate solution to mitigate the current and future energy crises. However, for widespread market penetration, solar energy has to be harnessed more efficiently and stored for large scale long-term applications. Solar water splitting allows producing H2 gas, a clean, renewable and carbon-free energy carrier. Solar water splitting can also be used for CO2 reduction from environment with simultaneous production of hydrocarbon fuel. However, many significant challenges need to be addressed for translating this technology from R&D laboratory to the commercial world. How soon this technology will reach marketplace depends on how soon breakthroughs are made in discovery research.
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