Water Splitting by MnOx/Na2CO3 Reversible Redox Reactions: History
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Contributor:
Jia Liu
,
Shuo Li
,
Raf Dewil
,
Maarten Vanierschot
,
Jan Baeyens
,
Yimin Deng

Hydrogen is referred to as a secondary energy carrier, made from a primary energy source (nowadays, ~95% from fossil fuels). The global transition to hydrogen as an energy carrier accompanies the rise in required “green” energy. The H2 production by the different MnOx/Na2CO3 reactants was repeated in the solar reactor. Temperatures of the outer reactor wall were limited to below ~1000 °C (strength limitation of the Incoloy construction material). Due to the inertia of the heliostat focusing, temperatures in the bed varied between about 760 and 790 °C (average 775 °C) and between 815 and 835 °C (average 825 °C). Mostly, the first H2 production cycle was investigated. For the cold mix Mn3O4/Na2CO3, the reactants after H2 production were regenerated for 6 h at an average 825 °C using pure CO2.

  • H2 yield
  • pilot-scale
  • thermochemical water splitting
  • redox reactions

1. The Need to Develop H2 Production

The global energy production is influenced by the growing shortage of traditional fossil fuels and the threat of disturbing the ecological balance due to climate change. Conferences like COP21 and COP26 set goals to reduce greenhouse gases such as CO2. These COPs proclaimed the leading role of renewable energy in solving climate problems. The wider use of hydrogen was advocated [1][2] through, e.g., hydrogen-enriched natural gas [3], the direct reduction of iron ore [4], the use of hydrogen in the minerals’ processing [5], and the production of CO2-based synthetic natural gas [6], among others. It is important to assess the ways that hydrogen production can help to meet the renewable energy goals.
Since there are no significant resources of hydrogen on earth, hydrogen is referred to as a secondary energy carrier, made from a primary energy source (nowadays, ~95% from fossil fuels) [1][3]. The global transition to hydrogen as an energy carrier accompanies the rise in required “green” energy.
In 2020, the annual production of hydrocarbons reached 14 billion tons of oil-equivalent [7]. To affect the global CO2 emissions, at least 10% of hydrocarbons should be replaced by renewable energy resources and/or by hydrogen. If considering hydrogen as a major potential renewable energy carrier, roughly 1 billion tons of H2 produced from non-fossil sources are required to accomplish this. In 2020, only 87 million tons of hydrogen were produced worldwide [8], mainly used in ammonia production and oil refining. This is more than 10 times below the required quantity to replace 10% of hydrocarbons.
The main method currently considered in producing green hydrogen is electrolysis using electricity from renewable sources (wind turbines, photovoltaics). It theoretically takes about 50 kWh to produce electrolysis H2 at an overestimated efficiency of 80% [1], without taking other losses into account. The annual global energy production from renewable sources (excluding hydro-energy) was estimated to be 2800 TWh in 2019, which would translate into 56 million tons of H2 annually, if these sources were solely used to produce H2. This is lower than the current hydrogen production. In addition, the production cost of the renewable-based H2 is two to four times higher than when using traditional petrochemical pathways, and it is not deemed practical to expand these renewable technologies to meet the goal of 1 billion tons of H2 [7].
The global electricity production of hydropower in 2019 was around 38 Exajoules (104 GWh) [9], which allows an annual production of 200 million tons of H2 if no electricity is sent to the power grid, which is only three times the current production. Since hydropower is already realized for over 20% of its potential, it is therefore unable to solve the problem [9]. The same reason applies to nuclear energy, where the energy production is only 25 Exajoules (even less than hydropower). With the often negative public attitude towards nuclear energy and the limited reserves of Uranium, nuclear power will not sustain a future for hydrogen production.
According to the arguments above, fossil hydrocarbons are deemed to remain a valid source for hydrogen production in the near future. However, the production of hydrogen with Steam Methane Reforming (SMR) results in the emission of 10 kg CO2/kg H2, and such hydrogen is classified as “gray hydrogen” according to ecological standards [1]. It is environmentally unattractive and does not contribute to the reduction of CO2 emissions. An additional process to make this technology “cleaner” is carbon capture and storage (CCS). The major disadvantage of this process is that the total cost rises because the process requires more energy. CCS would not be sufficient to have an environmental impact. With the rise of the hydrogen cost to almost the double, it is not a viable solution [10].

2. Hydrogen Production by Water Splitting in Thermal Redox Systems

An earlier research by the researchers [11] assessed and classified different thermo-chemical water splitting redox reactions. A redox system ranking was developed by applying multiple assessment criteria, including energy efficiency, H2 yield, conversion, cyclic operation, production cost, and environmental and safety impacts.
Several very high temperature reactions (≥1000 °C) were not taken into consideration, e.g., involving. metal–metal oxides/hydroxides, perovskites, or doped ceria. At such high temperatures, concentrated solar energy would be the potential heat source, but at the expense of having to use extravagant construction materials for the solar receiver-reactor. Selected systems should operate at lower temperatures, preferably below 1000 °C for the reactor wall temperature. Deng et al. [11] finally selected 4 out of 24 redox reactions that meet the systems’ selection priorities. These systems included Mn3O4/MnO/NaMnO2, MnFe2O4, U3O8/UO2CO3 and ZnO/Fe3O4/ZnFe2O4 redox reactions. The U3O8 system was discarded for nuclear hazard reasons, and the ZnO system scored 30% lower than both other redox reactions and was hence eliminated from a priority investigation. Both the MnFe2O4 and Mn3O4/MnO/NaMnO2 redox processes are of a multi-step nature, where water is decomposed into H2 and O2 via a medium-temperature two-step (MnFe2O4) or a medium-temperature four-step process (Mn3O4), respectively, that forms a closed cycle [11]. This process was previously fostered as having a high potential [12] and involves four reaction steps, as shown below.

3. MnOx/Na2CO3 Cycle

Xu et al. [12] comprehensively studied the Mn3O4/MnO/Na2CO3 system. Theoretical considerations were validated by experiments on a small scale (200 mg) in a thermogravimetric analyzer. The Mn-based reduction and oxidation reactions required an operating temperature in excess of 850 °C. This four-step redox cycle is given by the equations below.
3 Na 2 CO 3   ( s ) + 2 Mn 3 O 4 ( s ) → 4 NaMnO 2 ( s ) + 2 CO 2 ( g ) + 2 MnO ( s ) + Na 2 CO 3   ( s )
2 MnO ( s ) + Na 2 CO 3   ( s ) + H 2 O ( v ) → H 2 ( g ) + CO 2 ( g ) + 2 NaMnO 2 ( s )
6 NaMnO 2 ( s ) + ayH 2 O ( v ) + ( 3 + b ) CO 2 ( g ) → 3 Na 2 CO 3   ( s ) + aH x MnO 2 · yH 2 O ( s ) + bMnCO 3   ( s ) + cMn 3 O 4 ( s )
aH x MnO 2 · yH 2 O ( s ) + bMnCO 3   ( s ) → ( 2 − c ) Mn 3 O 4 ( s ) + ayH 2 O ( v ) + bCO 2 ( g ) + 0.5 O 2 ( g )
The results of Xu et al. [12] exhibited a more than 90% yield for both the H2 and O2 evolution for five successive redox cycles. The key recyclability feature of the Mn-based system was demonstrated to be the complete in/out shuttling of Na+ of the manganese oxides, where the reactions to form α-NaMnO2 are thermodynamically favorable. It was demonstrated that the Na+ extraction is enhanced by the mobility of Na+ in the layered structure when intercalated by water and is promoted by CO2, thus driving the reaction equilibrium to a mixture of Mn3O4, and MnCO3. After thermal reduction at 850 °C, Mn3O4 is formed, and this closes the thermo-chemical cycle.
The cycles consist of multiple stages, operating from 50 °C to 850 °C [12][13]. Dissolved Na2CO3 must be recovered from the aqueous solution before the needed re-heating to 850 °C, and it will impose a considerable energy cost. The wide range of the temperatures in the different steps requires adequate heat management and makes the process more complex.
The possible loss of volatile Na species at high temperature can be responsible for the progressive loss of process cyclability [14]. The hydrolysis reaction, however, drives the formation of Na-birnessite. Bayón et al. demonstrated that the surface area of Mn3O4 is the main factor, while the crystalline domain size has a secondary influence [15]. Xu et al. [16] performed the multistep reactions on 200 mg Mn3O4/Na2CO3 below 850 °C and demonstrated that the rates of H2 releasing and Na+ extraction depend on the redox properties of metals in ferrite-based oxides and the facility of intercalating alkali cations. Bayón et al. [13] also studied the feasibility of the MnO/Na2CO3 cycle, with a maximum conversion of 47% and a productivity of 20.1 μmol H2 min−1 g−1. Alonso et al. [17] studied the Mn2O3/Mn3O4/MnO cycles at >1835 K using solar-driven thermogravimetry and derived the overall kinetic rate equations.

This entry is adapted from the peer-reviewed paper 10.3390/su14137597

References

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  2. Abdin, Z.; Zafaranloo, A.; Rafiee, A.; Mérida, W.; Lipiński, W.; Khalilpour, K.R. Hydrogen as an energy vector. Renew. Sustain. Energy Rev. 2020, 120, 109620.
  3. Deng, Y.; Dewil, R.; Appels, L.; Van Tulden, F.; Li, S.; Yang, M.; Baeyens, J. Hydrogen-enriched natural gas in a decarbonization perspective. Fuel 2022, 318, 123680.
  4. Li, S.; Zhang, H.; Nie, J.; Dewil, R.; Baeyens, J.; Deng, Y. The Direct Reduction of Iron Ore with Hydrogen. Sustainability 2021, 13, 8866.
  5. El-Emam, R.S.; Gabriel, K.S. Synergizing hydrogen and cement industries for Canada’s climate plan—Case study. Energy Sources, Part A Recover. Util. Environ. Eff. 2021, 43, 3151–3165.
  6. Ipsakis, D.; Varvoutis, G.; Lampropoulos, A.; Papaefthimiou, S.; Marnellos, G.E.; Konsolakis, M. Τechno-economic assessment of industrially-captured CO2 upgrade to synthetic natural gas by means of renewable hydrogen. Renew. Energy 2021, 179, 1884–1896.
  7. Dudley, B. Statistical Review of World Energy; BP Statistical Review: London, UK, 2021.
  8. Antweiler, W. What Role Does Hydrogen Have in the Future of Electric Mobility? Available online: https://wernerantweiler.ca/blog.php?item=2020-09-28 (accessed on 25 May 2022).
  9. BP, p.l.c. Hydroelectricity. Available online: https://www.bp.com/en/global/corporate/energy-economics/statistical-review-of-world-energy/hydroelectricity.html (accessed on 25 May 2022).
  10. Chao, C.; Deng, Y.; Dewil, R.; Baeyens, J.; Fan, X. Post-combustion carbon capture. Renew. Sustain. Energy Rev. 2021, 138, 110490.
  11. Deng, Y.; Dewil, R.; Appels, L.; Li, S.; Baeyens, J.; Degrève, J.; Wang, G. Thermo-chemical water splitting: Selection of priority reversible redox reactions by multi-attribute decision making. Renew. Energy 2021, 170, 800–810.
  12. Xu, B.; Bhawe, Y.; Davis, M.E. Low-temperature, manganese oxide-based, thermochemical water splitting cycle. Proc. Natl. Acad. Sci. USA 2012, 109, 9260–9264.
  13. Bayón, A.; de la Peña O’Shea, V.A.; Serrano, D.P.; Coronado, J.M. Exploring the alternative MnO-Na2CO3 thermochemical cycle for water splitting. J. CO2 Util. 2020, 42, 101264.
  14. Kreider, P.B.; Funke, H.H.; Cuche, K.; Schmidt, M.; Steinfeld, A.; Weimer, A.W. Manganese oxide based thermochemical hydrogen production cycle. Int. J. Hydrogen Energy 2011, 36, 7028–7037.
  15. Bayón, A.; de la Peña O’Shea, V.A.; Coronado, J.M.; Serrano, D.P. Role of the physicochemical properties of hausmannite on the hydrogen production via the Mn3O4–NaOH thermochemical cycle. Int. J. Hydrogen Energy 2016, 41, 113–122.
  16. Xu, B.; Bhawe, Y.; Davis, M.E. Spinel Metal Oxide-Alkali Carbonate-Based, Low-Temperature Thermochemical Cycles for Water Splitting and CO2 Reduction. Chem. Mater. 2013, 25, 1564–1571.
  17. Alonso, E.; Hutter, C.; Romero, M.; Steinfeld, A.; Gonzalez-Aguilar, J. Kinetics of Mn2O3 –Mn3O4 and Mn3O4–MnO Redox Reactions Performed under Concentrated Thermal Radiative Flux. Energy Fuels 2013, 27, 4884–4890.
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