- Open Access
The production of renewable energy in the form of methane using electrolytic hydrogen generation
© Hashimoto et al.; licensee Springer. 2014
Received: 4 March 2014
Accepted: 8 July 2014
Published: 12 August 2014
An extrapolation of the world energy consumption from 1990 to 2010 indicates a complete exhaustion of the world reserves of oil, natural gas, uranium, and coal by 2043, 2047, 2051, and 2055, respectively. For the survival of all people in the whole world, intermittent and fluctuating electricity generated from renewable energy should be supplied in the form of usable fuel. We have been working on research and development of global CO2 recycling for the use of renewable energy produced via electrolytic hydrogen generation in the form of methane. We created energy-saving cathodes for H2 production, anodes for O2 evolution without chlorine formation in seawater electrolysis, and catalysts for methanation of CO2 by the reaction with H2, and built pilot plants on an industrial scale. The development of new anode and cathode for alkali water electrolysis, the improvement of the anode for seawater electrolysis and the catalysts for CO2 methanation, as well as industrial applications are in progress.
The future of energy
The usage of renewable energy
In 2010, renewable electric power, excluding hydroelectric power, reached only about 1.4% of the energy consumed in the whole world. For a long-term sustained survival of human beings over the globe, we have to substitute 100% of this energy with renewable energy. There are sufficient renewable energy sources on our planet. For example, in 2010, the total energy consumed worldwide was 538.66 × 1018 J year-1. The corresponding amount of energy can be generated in the form of electricity by photovoltaic cells with an energy conversion efficiency of 15% in a desert area supplying a solar intensity of 1,000 Wm-2 when they were operated 8 h per day and employed only 2.11% of the area of the main deserts on the Earth. This estimate is rather encouraging because it indicates that we can survive on our planet.
The technologies of converting different types of renewable energy sources to electric power have already been developed. The inherent disadvantage of renewable energy sources in general is their instability with fluctuation with time. Thus, when electricity generated by renewable energy sources is directly integrated in the existing electricity supply network system, an undesirable frequency fluctuation of the supplied alternating current (AC) is inevitable. In modern industry, an AC electric power supply with a frequency fluctuation margin of ± 0.2 Hz is claimed to be unacceptable . In Japan, the frequency fluctuation margin of AC electricity supplied to the customer is set at ±0.2 Hz by all the electric power supply companies excluding Hokkaido Electric Power Company, which sets the margin at ±0.3 Hz . According to an estimate made by Hokkaido Electric Power Company , the frequency error margin of ±0.3 Hz cannot be held when electricity generated by wind exceeds 250 MW (4.8% of the maximum total power in the network). As such, it is difficult to integrate electricity generated by renewable energy sources with its inherently fluctuating nature exceeding about 5% of the maximum instantaneous power output in the existing electricity supply grid. Therefore, the electric energy generated by intermittent and fluctuating renewable sources should be converted into some form of fuel.
Global carbon dioxide recycling
As early as the 1970s, we had been dreaming to supply hydrogen to all people in the whole world, generating hydrogen by seawater electrolysis using the renewable energy origin electricity. However, we understood that there exists no infrastructure for a mass transportation of hydrogen and no combustion technology for hydrogen. Thus, for establishing renewable-energy-fuelled communities worldwide, we need to convert the energy generated by renewable energy sources to some form of fuel material for which the combustion technology as well as the mass transportation infrastructures have been established.
Energy necessary for all people in the whole world is converted to electricity from solar energy in deserts.
The electricity is used for H2 production by seawater electrolysis in plants installed along the coasts near the deserts.
H2 is converted to CH4 by the reaction with CO2 in the plants next to the electrolyzers.
Using the available transportation infrastructure, the generated CH4 is sent to the areas of consumption.
The only difference from the current practice is that CO2 should be captured from the chimney and be sent to the place where the electricity is available from renewable energy. If this is realized, we can use solar energy forever, using CO2 as the feedstock without emitting it into the atmosphere.
An industrial technology for the conversion of renewable energy sources to electricity is available, and the infrastructures for CH4 transportation and combustion technologies have been developed. Capturing of CO2 gas from the chimney has also been proven to be industrially viable. The properties of liquefied CO2 are comparable to those of liquefied petroleum gas (LPG), and therefore the transportation of the liquefied CO2 is possible. Thus, the realization of the global CO2 recycling can be performed through the establishment of the technologies to yield H2 by seawater electrolysis and to produce CH4 by the reaction of H2 with CO2.
Key materials for global carbon dioxide recycling
The key materials toward the realization of a global CO2 recycling are the cathode and anode for seawater electrolysis and a catalyst for the conversion of H2 to CH4 through the reaction with CO2.
The rate-determining reaction and Tafel slope at 90°C
Tafel slope, (∂E/∂logi)/mV decade−1
Electrode shown in Figure6
Ni, Fe, Ni-Fe
These cathodes with mechanistically highest activity made of Ni-based alloys or Co-based alloys  can be used not only for direct seawater electrolysis but also for high-temperature alkali water electrolysis.
Anode materials for seawater electrolysis
The direct seawater electrolysis for the generation of hydrogen is certainly a clean option of acquiring an energy source, but there are numerous technical problems to be solved. In terms of energy consumption at present, this direct seawater electrolysis cannot compete with the high-temperature alkali water electrolysis undertaken for desalinated seawater. Therefore, at the moment, for a large-scale industrial plant, the hot alkali water electrolysis is preferred against direct seawater electrolysis. For the construction of a new hot alkali water electrolyzer, a study of an anode and a cathode attaining the mechanistically highest activities even at 6,000 Am-2 in 30 wt % KOH solution at 90°C is in progress.
Methanation of carbon dioxide
Catalyst for COmethanation
Catalyst for mass production
As has been reviewed above, it was confirmed by laboratory investigations that nickel supported by the tetragonal ZrO2-type double oxide formed from the amorphous Ni-Zr alloy precursors was an efficient catalyst for CO2 methanation. However, in general, amorphous alloys are not suitable for mass production of an industrial catalyst. As mentioned above, the essential factor for the high catalytic activity of CO2 methanation was not the amorphous structure but the presence of the tetragonal ZrO2-type multiple oxide containing extra cations of a lower valence than Zr4+. Hence, we synthesized the tetragonal ZrO2-type double oxide by the calcination of a mixture of zirconia hydrosol with a salt of rare earth elements and then by the impregnation of nickel on the tetragonal ZrO2-type double oxide. The catalyst prepared in such way was proved to act satisfactorily as a catalyst for CO2 methanation .
Aiming at a further simplification of the catalyst preparation process, we calcined a mixture of zirconia hydrosol, rare earth element salt, and nickel salt to obtain a mixture of the tetragonal ZrO2-type triple oxide and NiO. After this, surface NiO was reduced by H2 to prepare metallic nickel supported by the tetragonal ZrO2 type oxide. The Ni supported by the tetragonal Zr1 x - ySm x Ni y O2 − 0.5x − y catalysts possessed better catalytic activity than the catalysts formed from amorphous alloys .
In conclusion, the proposed procedure starting from the calcination of a mixture of zirconia hydrosol, Ni salt, and a salt of cation stabilizing the tetragonal ZrO2 structure to form an oxide mixture of NiO and a tetragonal ZrO2 type multiple oxide, and finishing with a final reduction of surface NiO to form metallic nickel on the surface has proven to be ideal and promising for the mass production of the desired catalyst for CO2 methanation.
In this connection, the use of rare elements such as rare earth elements is not favorable for supplying fuel to the whole world. Instead of rare earth elements, the stabilization of the tetragonal ZrO2-type multiple oxide could be realized by the addition of Ca2+ or Mg2+. The authors have already found that the CO2 methanation activity of the best Ni-Zr-Ca catalyst prepared by the calcination of a mixture of zirconia hydrosol with CaSO4 and NiSO4 is higher than that of the best Ni-Zr-Sm catalyst.
Toward the industrialization of global carbon dioxide recycling
The basic technologies for realizing global CO2 recycling have already been in our hands. However, natural gas is readily available as a primary energy source nowadays but our methane is a quaternary energy formed by the conversion from solar energy via electricity and hydrogen. Each conversion requires cost, and hence our methane cannot compete at present with natural gas. However, as has been pointed out at the beginning of this review, the exhaustion of energy resources on the Earth is inevitable in the foreseeable future. Therefore, it is anticipated that in 15 to 20 years, countries possessing oil resources will start imposing limitations on the exports of their resources for their own survival. To face this severe reality straightforwardly, we have to develop alternative means to acquire energy on a large scale to sustain prosperity of our society. To the authors' knowledge, the proposed global CO2 recycling system is the most convenient alternative to acquire massive amounts of energy from renewable resources without being dependent on fossil or fissile mineral resources.
The authors will continue to invest research efforts to improve the key materials and systems for the global CO2 recycling and to develop large-scale industrial plants based on this concept. Japanese and foreign firms have formed a joint development for the industrialization of global CO2 recycling plants in which they pay a particular attention to work without precious metals and rare elements. For a worldwide spread of technologies for the solution of global energy and related problems, we should never use precious metals and rare elements, considering always the total world reserves of the necessary elements for the spread of systems. Furthermore, we have succeeded in obtaining the maximum amount of methane from biomass by a combination of CO and CO2 methanation as well as a water-gas shift reaction of biomass-based syngas on our catalyst.
This is the review of work done by the groups of KH for 25 years, to which several tens of scientists and engineers have been contributing. The names of authors listed in this article are now mostly contributing to the work and they read and approved the final manuscript.
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