Metal Oxide Thermochemical Water Splitting Cycles
Using energy from the sun is seen as a viable path in which to produce renewable energy. A feasible method in which to utilize energy from the sun is to use thermochemical water splitting cycles to generate hydrogen. One such method uses metal oxides with a generic cycle consisting of two steps:
MxOy → MxOy-1 + ½O
MxOy-1 + H
2O → MxOy + H
For a metal oxide thermochemical cycle to proceed the only inputs needed are water and heat from solar energy. Oxygen and hydrogen are released in separate steps, thus eliminating a difficult hydrogen/oxygen separation that would be necessary in directly splitting water. Metal oxide water splitting cycles each contain at least two steps, one of which is a high temperature endothermic step. The reduction step (Equation 1) will always be endothermic, but with the oxygen dissociation occurring at a temperature significantly lower than directly splitting water. Having the reaction proceed at a lower temperature will alleviate some reactor materials of construction issues. Additionally, the lower temperatures will decrease re-radiation losses leading to higher reactor efficiencies for a given solar concentration. The hydrogen generating step (Equation 2) is exothermic and occurs at a lower temperature than the reduction step. Products from the hydrogen generating step have a natural tendency to separate leading to easy hydrogen recovery. Metal oxide water splitting cycles are an attractive method to produce hydrogen and should be studied further.
Though simple in nature, metal oxide water splitting cycles have challenges that must be overcome. Reaction components are non-renewable materials and must be recycled to make the cycles truly renewable. It is essential that water and heat are the only inputs into a cycle . Along with the non-renewable material issue, having more than one reaction step will reduce the process efficiency due to irreversibilities, which will decrease the yield of hydrogen relative to the amount of solar irradiation driving the process[9, 13]. As a result, it is important to look at cycles that have a minimal number of steps (two to three step cycles are typically considered) and those steps need maximized efficiency [8, 9, 11-13]. A final hurdle that must be overcome when considering thermochemical water splitting cycles is the bulk handling of solids. The bulk movement of solids is difficult, and so cycles with a minimal movement of bulk solids are considered to be most feasible. In light of these restrictions, there are very few metal oxide cycles that have been studied in depth. With that our group has identified ZnO/Zn and Mn2O3/MnO as two potentially successful metal oxide cycles.
ZnO/Zn Thermochemical Cycle
The two-step ZnO/Zn thermochemical water splitting cycle has been extensively studied, because of its perceived simplicity (Figure 1) [1-5].
|Figure 1 ZnO/Zn Thermochemical Cycle
The dissociation of ZnO to Zn is the high temperature endothermic reaction which requires temperatures in excess of 2250K to achieve complete dissociation. At this temperature zinc vaporizes, so it is then rapidly quenched to induce a simple gas/solid separation to remove O
2. Zinc metal is then reacted with steam (i.e. H
2O) to reform ZnO and H
2. The H
2 generation reaction is a straight forward reaction, employing only steam and zinc particles. This reaction is carried out at temperatures near 700 K. After which ZnO is recycled back to a solar reactor to be reduced again.
The temperature at which ZnO dissociation occurs is considerably higher than that required for other multi-step cycles; however the Zn/ZnO cycle should have higher process efficiency due to having only two steps. Additionally, the zinc product vaporizes during the dissociation reaction, which upon quenching creates high surface area nanoparticles[6, 7]. This is a distinct advantage as the use of zinc nanoparticles increases the rate of the H
2 generation reaction along with the extent of reaction. Finally the relatively low temperature and simplicity of the H
2 generation reaction make this a very attractive cycle.
The original manganese oxide thermochemical water splitting cycle was developed in the 70’s as part of “The Thermochemical Hydrogen Program” at Los Alamos Scientific Laboratory. Only waste heat from a nuclear reactor was available, so the dissociation product was Mn
3/MnO Thermochemical Cycle
4 as opposed to MnO. By reducing Mn
3 further, the efficiency of the cycle will improve as the reactant mass flow per mole of H
2 generated would be reduced by a factor of three.The manganese oxide thermochemical water splitting cycle comprises three steps (Figure 2).
|Figure 2 Mn
3/MnO Thermochemical Cycle
The high temperature dissociation reaction occurs at temperatures in excess of 1673K. It is an endothermic reaction and manganese oxide remains a solid throughout the extent of the dissociation. Reduced manganese oxide is reacted with NaOH at temperatures between 900 and 1100K; the products of this reaction are solid sodium manganate and H
2 gas. Because of the phase difference, the reaction products should be easy to separate. The cycle is then closed with an oxidizing reaction. Sodium manganate is reacted with an excess of water to reform Mn
3 and NaOH in a slightly exothermic reaction. An excess of water is used because Mn
3 is not soluble in water while NaOH is. Excess water is beneficial in the separation of components.
The manganese oxide thermochemical cycle has one more step than the zinc cycle, making it presumably less efficient. However, the temperature at which the manganese oxide dissociation reaction occurs is significantly lower than that of the zinc oxide two-step thermochemical cycle. Mn
3 dissociation is favored in the forward direction at a temperature nearly 400K. This reduced reactor temperature could compensate for the added complication of three steps. Additionally, the phase separation of MnO from dissociated O
2 appears simple because MnO remains as a solid at the favorable reaction temperature. This is advantageous as it reduces a potential recombination reaction.
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- Muller, R. and A. Steinfeld, H
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- Brown, L.C., G.E. Besenbruch, K.R. Schultz, A.C. Marshall, S.K. Showalter, P.S. Pickard, and J.E. Funk. Nuclear production of hydrogen using thermochemical water-splitting cycles. in International Congress on Advanced Nuclear Power Plants. 2002. Hollywood, FL.
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