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.

Resources

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