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Apr 04, 2020
Solar Powered Hydrogen Production via a High Temperature Photocatalytic Water Splitting Cycle
PI: C. Huang (Florida Solar Energy Center) N. Muradov (Florida Solar Energy Center)
A. Raissi (Florida Solar Energy Center) A. Adebiyi (Florida Solar Energy Center)
Abstract Solar-driven thermochemical water splitting cycles (TCWSCs) provide an energy efficient and environmentally attractive method for generating hydrogen. Solar-powered TCWSCs utilize both thermal (i.e. high temperature heat) and light (i.e. quantum energy) components of the solar resource, thus boosting the overall solar-to-hydrogen energy conversion efficiency compared to those with heat-only input. At the Florida Solar Energy Center (FSEC), a new solar-powered TCWSC, sulfur dioxide (SO2)/sulfuric acid cycle, is under research and development. FSEC's cycle - a novel hybrid photo- thermochemical sulfur-ammonia (S-A) cycle, is a modification of the well-known Bowman-Westinghouse (B-W) hybrid cycle wherein the electrochemical step is replaced by a photocatalytic process. The main reaction (unique to FSEC's S-A cycle) is the light- induced photocatalytic production of hydrogen and ammonium sulfate from an aqueous ammonium sulfite solution. Ammonium sulfate product is processed to generate oxygen and recover ammonia and SO2. Ammonia and sulfur dioxide are then recycled and reacted with water to regenerate ammonium sulfite.
Introduction Thermochemical water splitting cycles (TCWSCs) can achieve high overall heat-to- hydrogen energy conversion efficiencies. Presently, the prospective high temperature heat sources suitable for thermochemical process interface include solar thermal concentrator and central receiver systems, and nuclear power plants (i.e. high temperature gas-cooled reactors, HTGR). U.S. DOE's Nuclear Energy Research Initiative program has funded several efforts aimed at hydrogen production using nuclear power. For example, the General Atomics (GA) Corp. has been developing the sulfur- iodine (S-I) cycle for many years. The GA cycle belongs to a group of TCWSCs known as the "sulfur family cycles." The objective of this research is to investigate a new sulfur ammonia cycle, which utilizes solar energy both for heating and for the photocatalytic oxidation process. It is potentially applicable to the production of solar hydrogen through thermochemical processes. The rationale developing a new solar powered TWSPC is to utilize both thermal and quantum energies to increase the solar to hydrogen energy conversion efficiency. For the B-W cycle, the solution pH can have a detrimental effect on the performance. In particular, at low pH, sulfur may form instead of hydrogen. To prevent sulfur formation, one must maintain a relatively high pH, which requires a reduction in acid concentration. However, high pH results in low hydrogen evolution rates. Additionally, low sulfuric acid concentration in the outlet of an electrochemical reactor would require a more energy- intensive and costly acid separation and concentration step. In order to mitigate these problems, a novel sulfur-ammonia (S-A) cycle has been developed at FSEC. In the
FSEC's S-A cycle, the Gibbs energy input to the reaction of sulfite oxidation is via photons (at wavelengths greater than 350 nm), thus making it compatible with a solar power source.
Background Despite the difficulties that challenge efficient electrolytic oxidation of sulfur dioxide, the Westinghouse hybrid sulfur cycle still remains as one of the most studied and promising thermochemical water splitting cycles for the production of hydrogen from water. The cycle is written as follows: SO2(g) + H2O(l) = H2(g) + H2SO4(aq) (electrolysis) 77oC (1) 2H2SO4(g) = 2SO2(g) + 2H2O(g) + O2(g) 850oC (2) The many advantages of the Westinghouse cycle (WC) have been widely reported and discussed in the literature. However, it is also well known that the cycle suffers from the low solubility of sulfur dioxide in water and challenges presented by the acidic environment during SO2 electrolytic oxidation process1. To date, many efforts have been made to improve the efficiency of the electrolytic process for oxidation of sulfur dioxide. Past activities have involved the use of a depolarized electrolyzer as well as the addition of a third process step - examples include S-I, S-Br and S-Fe cycles below: Ispra Mark 13 sulfur-bromine cycle: Br2(l) + SO2(g) + H2O(l) = 2HBr(aq) + H2SO4(aq) 77oC (3) 2H2SO4(g) = 2SO2(g) + 2H2O(g) + O2(g) 850oC (4) 2HBr(aq) = Br2(aq) + H2(g) (electrolysis) 77oC (5) General Atomics' sulfur-iodine cycle: I2 + SO2(g) + H2O(l) = 2HI(a) + H2SO4(aq) 100oC (6) 2H2SO4(g) = 2SO2(g) + 2H2O(g) + O2(g) 850oC (7) 2HI = I2(g) + H2(g) 450oC (8) Sulfur-iron cycle2: 2Fe2(SO4)3(aq)+SO2(g) + 2H2O(l) = 2FeSO4(aq) + 3H2SO4(aq) 25oC (9) 2H2SO4(g) = 2SO2(g) + 2H2O(g) + O2(g) (electrolysis) 850oC (10) 2FeSO4(aq) + 2HSO4(aq) = 2Fe2(SO4)3(aq) + H2(g) 25oC (11) Although these cycles solve some of the WC problems, especially with regard to the solubility of SO2 in water, they have many issues of their own. For example, separation of sulfuric acid from the reaction products such as HI, HBr or FeSO4 presents a challenge. Additionally, the pH factor remains a serious problem. In fact, this problem becomes more acute due to the generation of other acidic species such as HI and HBr. As an alternative to direct electrolysis of aqueous SO2, we have developed a new sulfur- ammonia (S-NH3) cycle3. This cycle was presented at the 15th World Hydrogen Energy Conference (WHEC-15), held in Yokohama, Japan in 2004 and received an Innovative Technology Award from the WHEC-15 scientific organizing committee - the only award given to the researchers from the United States of America.
(http://dbase.fsec.ucf.edu/pls/operation/press_display?pressid=2158). We have carried out extensive experimental and analytical (i.e. thermodynamic and process flowsheet analyses using AspenPlus™ and HYSYS chemical process simulation platforms) work on this concept and a patent application based on our findings is in the works. The following provides a brief description of the concept. We have modified WC by adding ammonia to reaction (1). The reaction steps of our new cycle can be written as follows: SO2(g) + 2NH3(g) + H2O(l) → (NH4)2SO3(aq) 25oC (chemical absorption) (12) (NH4)2SO3(aq)+H2O → (NH4)2SO4(aq) + H2(g) 80oC (photochemical step) (13) (NH4)2SO4(aq) →2NH3(g) + H2SO4(l) 300oC (thermochemical step) (14) H2SO4(l) → SO2(g) + H2O(g) + 1/2O2(g) 850oC (thermochemical step) (15) Overall Reaction: H2O + solar energy = H2 + 0.5 O2 The main advantages of the S-NH3 are as follows: Reaction (12) produces only one product (ammonium sulfite) so no separation stage is necessary. And it can be viewed as the step for extracting and separating oxygen from SO2. This reaction is more efficient than the conventional O2 separation process used in the WC. Unlike SO2, both (NH4)2SO3 and (NH4)2SO4 have very high solubility (about 6M at 30oC) in water and thus more suitable (than SO2) for the production of hydrogen using a depolarized electrolyzer. The pH of (NH4)2SO3(aq) and (NH4)2SO4(aq) are about 8.50 and 6.00, respectively, indicating that reactions (12) and (13) can proceed in neutral pH. Additionally, (NH4)2SO3 and (NH4)2SO4 can easily decompose at temperatures of 80oC and 300oC, respectively. Furthermore, S-NH3 cycle employs readily available and low cost chemicals. The flow diagram is shown in Figure 1. We have proposed and developed five variations of the reaction (13) according to the form of the energy input into the process (i.e. electrical energy versus photonic energy) and the use of photocatalysts as follows: a. (NH4)2SO3 + H2O + electricity → H2 + (NH4)2SO4 b. (NH4)2SO3 + H2O + photocatalyst + sunlight → H2 + (NH4)2SO4 c. (NH4)2SO3 + H2O + UV light → H2 + (NH4)2SO4 d. (NH4)2SO3 + H2O + UV light + TiO2 → H2 + (NH4)2SO4 e. (NH4)2SO3 + H2O + photoelectrode + sunlight → H2 + (NH4)2SO4
photocatalyt ic reactor
NH3 recovery mixunit
sulfuric acid decomp.
Figure 1. Flow diagram of solar powered S-NH3 thermochemical water splitting cycle
Experimentally we have verified the viability of the reactions (b-d). In particular, we have developed and successfully tested an efficient photocatalyst for the reaction (b) that absorbs in the visible region of the solar spectrum. Further experimental work is still in progress for full characterization of reactions (a) and (e). To address the application of the S-NH3 both on lunar surface and on earth surface, we focus our efforts on the practical realization of the reaction (b), (c) and (d) – photolytic and photocatalytic processes of ammonium sulfite aqueous solution for the production of hydrogen. Potentially, this approach will allow utilization of the ultraviolet and visible light on the lunar surface for directly convert solar photonic energy into hydrogen chemical energy. The solar thermal energy is used for the production of oxygen through the decomposition of sulfuric acid shown in Reaction (15). In short, we propose to combine FSEC developed technology described above with a solar concentrator to develop a state-of-the-art cycle for the cost-effective production of hydrogen from water.
Part I: UV Light Photolytic Hydrogen Production from Ammonium Sulfite Aqueous Solution
Introduction One o