Low-temperature, manganese oxide-based, thermochemical water splitting cycle

Abstract

Thermochemical cycles that split water into stoichiometric amounts of hydrogen and oxygen below 1,000°C, and do not involve toxic or corrosive intermediates, are highly desirable because they can convert heat into chemical energy in the form of hydrogen. We report a manganese-based thermochemical cycle with a highest operating temperature of 850°C that is completely recyclable and does not involve toxic or corrosive components. The thermochemical cycle utilizes redox reactions of Mn(II)/Mn(III) oxides. The shuttling of Na þ into and out of the manganese oxides in the hydrogen and oxygen evolution steps, respectively, provides the key thermodynamic driving forces and allows for the cycle to be closed at temperatures below 1,000°C. The production of hydrogen and oxygen is fully reproducible for at least five cycles.hydrogen production | Na + extraction | multistep cycle T hermochemical production of hydrogen and oxygen from water involves a series of chemical reactions that convert water into stoichiometric amounts of hydrogen and oxygen using heat as the only energy source. Thermochemical water splitting is of interest because it directly converts thermal energy into stored chemical energy (hydrogen and oxygen). Research on thermochemical water splitting cycles largely began in the 1960s and 1970s and involved nuclear reactors (1, 2) and solar collectors (3) as the energy sources. Numerous reviews of the thermochemical cycles proposed and experimentally investigated are available, e.g., (4). A large number of thermochemical cycles for splitting water has been proposed, and generally can be grouped into two broad categories: high-temperature two-step processes (5, 6) and lowtemperature multistep processes (7,8). One of us (M.E.D.) has conducted previous research on low-temperature multistep processes in the 1970s (9).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 (8). 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 (10, 11). These cycles typically consist of one step that involves the oxidation of a metal [such as zinc (12)] or a metal oxide [such as iron (II) oxide (6, 13)] by water to produce...

Solar thermochemical splitting of water to generate hydrogen

“…Generation of H 2 occurs at 850°C in this cycle ( Fig. 1) and the cycle is devoid of corrosive products (8). Thermal oxidation of Mn 3 O 4 to Mn 2 O 3 is thermodynamically unfavorable, but the use of Na 2 CO 3 drastically decreases the ΔG and forms MnO(s) and α-NaMnO 2 (s) at 850°C with the evolution of CO 2 (g), as shown in step 1 (Eq.…”

“…In this context, the thermochemical cycle based on Mn 3 O 4 /MnO oxides proposed by Davis and coworkers is noteworthy (8). The four reactions in this cycle are shown in Fig.…”

“…Thermal oxidation of Mn 3 O 4 to Mn 2 O 3 is thermodynamically unfavorable, but the use of Na 2 CO 3 drastically decreases the ΔG and forms MnO(s) and α-NaMnO 2 (s) at 850°C with the evolution of CO 2 (g), as shown in step 1 (Eq. S3) (8,9). The CO 2 evolution temperature can be decreased drastically (∼600°C) with the use of nanoparticles of Mn 3 O 4 and Na 2 CO 3 obtained with ball milling (Fig.…”

“…During step 2 (Eq. S4), introduction of H 2 O(g) at 850°C oxidizes MnO to α-NaMnO 2 with the evolution of H 2 (g) (8). The main hurdle in this cycle is the slow H 2 evolution.…”

“…The two-step process involving the thermal decomposition of metal oxides followed by reoxidation by reacting with H 2 O to yield H 2 is an attractive and viable process that can be rendered to become an isothermal process. Thermochemical splitting of H 2 O at low temperatures (<1,000°C) is accomplished by a minimum of three steps as dictated by thermodynamic energy constraints (7,8). In this paper we present the highlights of recent investigations of H 2 O splitting by the low-temperature multistep process as well as the high temperature two-step process.…”