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Caltech engineers devise new thermochemical cycle for water splitting for H2; recyclable, non-toxic, non-corrosive and at lower temperatures

Schematic representation of the 4-step low-temperature, Mn-based thermochemical cycle. Xu et al. Click to enlarge.

Providing a possible new route to hydrogen-gas production, researchers at the California Institute of Technology (Caltech) have devised a new manganese-based thermochemical cycle with a highest operating temperature of 850 °C that is completely recyclable and does not involve toxic or corrosive intermediates.

The research group led by Mark Davis, the Warren and Katharine Schlinger Professor of Chemical Engineering at Caltech, describes the new, four-reaction process in an open access paper in the Proceedings of the National Academy of Sciences (PNAS).

The thermochemical production of hydrogen and oxygen from water via a series of chemical reactions is of interest because it directly converts thermal energy into stored chemical energy (hydrogen and oxygen), and thus can take advantage of excess heat given off by other processes. Research on thermochemical water splitting cycles largely began in the 1960s and 1970s and involved nuclear reactors and solar collectors as the energy sources, the team notes in their paper. (Davis’ first paper as a graduate student dealt with the sulfur-iodine low-temperature water-splitting cycle.)

Thermochemical cycles for water splitting can generally be grouped into two broad categories: high-temperature (above 1,000 °C) two-step processes; and low-temperature (below 1,000 °C) multi-step processes.

Low-temperature multistep processes, typically with the a highest operating temperature below 1,000 °C, allow for the use of a broader spectrum of heat sources, such as heat from nuclear power plants, and hence have attracted considerable attention. The majority of existing low-temperature processes produces intermediates that can be complex, corrosive halide mixtures. One of these processes, the sulfur-iodine cycle, has been studied extensively, and even piloted for implementation. This process produces strongly acidic mixtures of sulfuric and iodic acids that create significant corrosion issues, but requires only one high-temperature step at ca. 850 °C.

Two-step processes typically involve simpler reactions and intermediates, e.g., solid metal oxides, than the low-temperature multistep cycles. However, the temperatures required to close these types of cycles are well above 1,000 °C. Because of the requirement of high-temperature heat sources, these types of cycles have been investigated for use with solar concentrators. These cycles typically consist of one step that involves the oxidation of a metal [such as zinc] or a metal oxide [such as iron (II) oxide] by water to produce hydrogen, and a subsequent step to recover the starting material from its oxidized form (thermal reduction to produce oxygen).

The objective of our work is to create thermochemical water splitting cycles that involve non-corrosive solids and operate at below 1,000 °C. Essentially, we wish to create new cycles that take advantage of both the low-temperature multistep and high-temperature two-step cycles. Here, we show that more than two reactions will be necessary to perform thermochemical water splitting below 1,000 °C, and then introduce a new thermochemical water splitting cycle that involves non-corrosive solids that can operate with a maximum temperature of 850 °C.

—Xu et al.

The first thing postdoctoral scholar and lead author Bingjun Xu and graduate student Yashodhan Bhawe did was to perform a thermodynamic analysis demonstrating that it is unlikely, if not impossible, to split water with a two-step cycle where there is complete conversion between the oxidized and reduced forms at below 1,000 °C.

The new thermochemical cycle devised by the team has four main steps:

  1. Thermal treatment of a physical mixture of Na2CO3 (sodium carbonate) and Mn3O4 (manganese (II, III) oxide) to produce MnO (manganese (II) oxide), CO, and α-NaMnO2 (sodium manganate) at 850 °C;

  2. oxidation of MnO in the presence of Na2CO3 by water to produce H2, CO2, and α-NaMnO2 at 850 °C;

  3. Na+ extraction from α-NaMnO2 by suspension in aqueous solutions in the presence of bubbling CO2 at 80 °C; and

  4. recovery of Mn3O4 by thermally reducing the sodium ion extracted solid produced in step 3 at 850 °C.

The result is the stoichiometric splitting of water to hydrogen and oxygen without any by-product. The thermochemical system exhibits >90% yield for both hydrogen and oxygen evolution and shows no sign of deactivation during five cycles. The incorporation and extraction of Na+ into and out of the manganese oxides are the critical steps in lowering the temperature required for both the hydrogen evolution and the thermal reduction steps, the team reports.

To be shown to be commercially practical, the cycle will need to run thousands of times—experiments of this type are beyond the capabilities currently in the Davis lab.

Davis notes that the implementation of the cycle as a functioning water-splitting system will require clever engineering. For example, for practical purposes, engineers will want some of the reactions to go faster, and they would also need to build processing reactors that have efficient-energy flows and recycling amongst the different stages of the cycle.

Going forward, the team plans to study further the chemistry of the cycle at the molecular level. They have already learned that shuttling sodium in and out of the manganese oxide is critical in lowering the operating temperature, but they want to know more about what exactly is happening during those steps. They hope that the enhanced understanding will allow them to devise cycles that could operate at even lower maximum temperatures.

What we’re trying to ask is, ‘Where are the places around the world where people are just throwing away energy in the form of heat?’ The lower the temperature that we can use for driving these types of water-splitting processes, the more we can make use of energy that people are currently just wasting.

—Mark Davis

The work was funded by a donation from Mr. and Mrs. Lewis W. van Amerongen.


  • Bingjun Xu, Yashodhan Bhawe, and Mark E. Davis (2012) Low-temperature, manganese oxide-based, thermochemical water splitting cycle PNAS doi: 10.1073/pnas.1206407109



Many "water splitting for H2" articles sound promising.

I hope this article draws some ME, Chem, thermo,.. engineer comments.

Besides this article, what could:

".. And provided the dried zeolite material is prevented from coming into contact with water, it can store the heat for an unlimited amount of time.." mean to our energy future?


850°C is in the temperature range of waste heat from high-temperature fuel cells, and could be reached by molten-salt reactors.

Reactions with solids looks like it might be problematic, though.  Getting intimate mixtures to produce the necessary products might mean slow reactions and large and expensive equipment.  Maybe this could be done in solution of a carrier salt that doesn't participate in the reaction.


Clearly a CCC - Cataclysmic Conglomeration of Corruption. If you believe this, I got property to sell you in Nicaragua.


Waste heat mostly comes in at a few hundred C, not 850. The authors recognize this and are trying to get the temperature down. They might actually consider solar heat. I assume they don't need any particular step to go to 100% completion, thus the rather intermittant nature of solar heat generation may not be too detrimental.

If this works, what would they do with the hydrogen? I'd use it as a chemical feedstock to create liquid fuels rather than as a fuel itself.


Well, Im not a chemical engineer but my impression is that this is another typical exmple of redundant and superfluous research, or the left hand not knowing what the right hand is doing and vice versa.


BK4, if hydrogen was used to deoxygenate biomass, the available carbon would be sufficient to replace most fossil fuels.


Or at least petroleum motor fuels (after thinking about it I'm sure I'm mis-remembering my calculations).


That kind of process and associated equipments can be miniaturized and put into a hydrogen fuelcell car and you made the hydrogen while driving or when parked and plugged to a 110 volts socket.


@EP - The figure that I've heard for what biomass could supply is 25% of current motor fuels, without impinging heavily on food production. I.e., when restricted to being grown on marginal, non-irrigated lands. But I think that's for BTL processes in which the biomass provides all the energy for the conversion. Driving the conversion as you suggest -- using hydrogen to deoxygenate biomass -- would double that to 50%.

@Kelly - The ScienceDaily article you asked about made a complete hash of the description of how the "heat storage" would work. I'd have to read the actual research paper to judge whether it has any real significance. I can pretty much guarantee, however, that it has no relevance to thermal splitting of water. 850 C would have been the temperature used to thoroughly dehydrate the zeolytes. There's no way that subsequent readsorbtion of water vapor could generate temperatures anywhere close to that.

The figure that I've heard for what biomass could supply is 25% of current motor fuels
That assumes that all the energy comes from the biomass; adding hydrogen changes the game.

The USA burns about 140 billion gallons of gasoline/gasohol and another 50 billion gallons of diesel per year.  At 6.2 lb/gal for gasoline, ~7 for diesel and about 84% carbon for both, that's about 1 trillion pounds or 500 million short tons of carbon.  The Billion-Ton Vision found 1.3 billion tons of available biomass; at 45% carbon, that's about 580 million tons.  That's definitely in the ballpark.


And of course, if plug-in hybrids replace 2/3 of liquid fuels with electricity, it's almost easy.

Roger Pham

Welcome back, Silverthorn, after a long hiatus.

The LDV fleet will undergo major increase in fuel efficiency, possibly doubling current level. Trucks can't get much more efficient, but electric hybrid trucks can run on electricity from overhead wires on main routes that will cut way back on petroleum consumption. These means that fuel consumption in the transportation sector can be reduced by half in the next decade or two.

I think that the thermo-chemo scheme for H2 generation is quite complicated here, and the many steps involved will be costly equipment-wise and efficiency will be negatively impacted with the 4 steps involved. High-temperature steam electrolysis is a much simpler alternative that can also use heat energy efficiently, although the durability of the oxygen-emitting electrode must be further improved before commercialization.


Correct Roger. Which is why we made the wry remark we did. Dan Nocera's artificial leaf powered by sunlight is a whole lot more elegant. And with the advent of very low cost electric energy shortly steam electrolysis will be far more efficient.

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