Yimin Denga,
Shuo Lib,
Raf Dewilac,
Lise Appelsa,
Miao Yangb,
Huili Zhangd and
Jan Baeyens*ab
aKU Leuven, Department of Chemical Engineering, Process and Environmental Technology Lab, 2860, Sint-Katelijne-Waver, Belgium. E-mail: Jan.Baeyens@kuleuven.be
bBeijing University of Chemical Technology, Beijing Advanced Innovation Centre of Soft Matter Science and Engineering, 100029, Beijing, China
cUniversity of Oxford, Department of Engineering Science, Parks Road, Oxford, OX3 3PJ, UK
dBeijing University of Chemical Technology, School of Life Science and Technology, 100029, Beijing, China
First published on 2nd November 2022
Future energy systems must call upon clean and renewable sources capable of reducing associated CO2 emissions. The present research opens new perspectives for renewable energy-based hydrogen production by water splitting using metal oxide oxidation/reduction reactants. An earlier multicriteria assessment defined top priorities, with MnFe2O4/Na2CO3/H2O and Mn3O4/MnO/NaMnO2/H2O multistep redox cycles having the highest potential. The latter redox system was previously assessed and proven difficult to be conducted. The former redox system was hence experimentally investigated in the present research at the 0.5 to 250 g scale in isothermal thermogravimetry, an electrically heated furnace, and a concentrated solar reactor. Over 30 successive oxidation/reduction cycles were assessed, and the H2 production efficiencies exceeded 98 % for the coprecipitated reactant after these multiple cycles. Tentative economics using a coprecipitated reactant revealed that 120 cycles are needed to achieve a 1 € per kg H2 cost. Improving the cheaper ball-milled reactant could reduce costs by approximately 30 %. The initial results confirm that future research is important.
Currently, green electrolytic H2 can be produced using mostly wind or photovoltaic electricity. Its production requires an average electricity consumption of approximately 50 kW h kg−1 H2 at an overestimated efficiency of 80%.5 The 2019 global renewable energy production, without hydro-energy, was estimated at 2800 TW h. If these sources were devoted to solely produce H2, only 56 Mtons of electrolysis H2 could be generated, and this at a 2- to 4-fold production cost of the traditional petrochemical methods. Renewable technologies are therefore unable to meet the 1000 Mtons H2 goal.
It is therefore expected that fossil hydrocarbons will remain the important H2 sources in the near future, despite their environmental impact through CO2 emissions. Steam methane reforming yields approximately 10 kg of CO2 per kg H2, and is hence classified as “grey hydrogen” production method.3 The application of carbon capture and storage could make this technology “cleaner”, but involving significant costs to nearly double the price of the produced H2.6
Within the “green” H2 production methods,3 thermochemical water splitting by redox systems5 has a considerable potential, and is the subject of the present research. Alternative methods were, however, also investigated.7,8
These thermochemical reactions to produce H2 offer major advantages in comparison with common alkaline water electrolysis, which has a low efficiency of approximately 20% (30% for electricity and 65% for electrolysis). The high-temperature thermochemical H2 production efficiency is much higher, provided cheap renewable or waste high-temperature heat is available.
These water splitting systems have been proposed since 1964. The initial vanadium–chlorine cycle was, however, abandoned for its low efficiency, high cost, and the formation of toxic and hazardous products. The cycles were however further developed, and approximately 25 thermochemical cycles are currently proposed. Deng et al. examined and ranked these proposed redox reactions by multiple criteria assessment, including quantified parameters of thermal, chemical, environmental and economic nature.5
Very high temperature reactions (≫ 1000 °C) of metal–metal oxides/hydroxides, doped ceria, or perovskites were not considered since such high temperatures would necessitate the application of high-cost alloys to construct redox reactors. Selected redox systems should operate at temperatures that would limit reactor wall temperatures below 1000 °C, thus operating the redox material bed at maximum 800 °C.5 Based on this important target, 4 out of 24 redox reactions were finally selected, and included MnFe2O4/Na2CO3, Mn3O4/MnO/NaMnO2, U3O8/UO2CO3 and ZnO/Fe3O4/ZnFe2O4. The U3O8 cycles were discarded due to nuclear hazards. The ZnO cycles scored significantly lower than both remaining redox systems, and were not further investigated. The Mn3O4/MnO/NaMnO2 four-step redox process was assessed, but required an operation temperature of approximately 800 °C and suffered from poor reversibility of the redox cycles.1 The MnFe2O4/Na2CO3 cycle remained the selected system under scrutiny.
The manufacturing methods are diverse, with main objectives to produce MnFe2O4 particles of small size, high crystallinity, and large specific surface area. The comparison of the manufacturing methods and the relevant physicochemical properties is given in Table 1. The manufacturing methods used in the present research, i.e. by simple ball-milling and by co-precipitation, are included in the table.
Synthesis method | Reaction T (°C) | Time (h) | Particle size (nm) | Surface area (m2 g−1) | Ref. |
---|---|---|---|---|---|
Polyol | 210 | 3 | 7 | 165.39 | 15 |
Co-precipitation | 70 | 2 | 20–80 | 84.5 | 16 |
95 | 2 | 30 | — | 17 | |
50 | 3 | 24 | — | 18 | |
70 | 1 | 36 | — | 19 | |
75–80 | 6 | 14 | 0.293 (ref. 20) | 21 | |
80 | 6 | 80 | — | 22 | |
Sol–gel | 70 | 2 | 45 | — | 19 |
Ball milled | 900 | 1 | 6.78–8.06 | 23 | |
Thermal decomposition | 295 | 1 | 7 | — | 24 |
270 | 1.5 | 18.9 | — | 25 | |
Solution combustion | 300 | — | 30–35 | 33 | 26 |
Hydrothermal | 200 | 12 | 30–50 | — | 27 |
220 | 10 | 14.5 | — | 28 | |
180 | 28 | 280 | 32.19 | 29 | |
180 | 16 | 16 | — | 19 | |
Solvothermal | 200 | 24 | 8.6 | — | 30 |
180 | 12 | 60 | 70 | 31 | |
500–600 | 24 | 12–22 | — | 32 | |
200 | 8 | 250–260 | — | 33 | |
Co-precipitation | 80 | 2 | 40 | 132 | This work |
Ball milled | 20 | 1 | 100 | 4.92 | This work |
Whereas ball-milled reactant appears less performance due to its coarse particle size and low surface area, coprecipitated reactant seems to meet the main property targets. Our coprecipitation differs from other cited synthesis methods through its use of metal chlorides as precursors, rather than the previously favoured metal nitrates or sulphates.
The crystalline of both ball-milled and coprecipitated MnFe2O4, is determined from XRD analysis. The diffractograms of both reactants are shown in Fig. 1.
In both cases, diffraction peaks were matched with the respective references of the ICDD cards. The main peaks correspond to the space group Fdm (spinel ferrite). The crystallite size for each phase was determined from the Sherrer's equation,34 accounting for the most intense XRD-peak. The crystal size, δ, is determined as:
The fairly novel application in water splitting relies on the thermochemical cycle of MnFe2O4/Na2CO3, as described by the following reactions:1
2MnFe2O4 (s) + 3Na2CO3 (s) + H2O (g) → 6Na(Mn1/3Fe2/3)O2 (s) + 3CO2 (g) + H2 (g) |
6Na(Mn1/3Fe2/3)O2 (s) + 3CO2 (g) → 2MnFe2O4 (s) + 3Na2CO3 (s) + 0.5O2 (g) |
The optimal operation temperature of both reactions is approximately 700 to 750 °C.35 H2 production at lower temperatures shows slower kinetics, and complete regeneration cannot be fully accomplished.36,37 The reduction step is problematic and less effective, although adding Fe2O3 could help to overcome the problem.38 The reduction step seems to pose a main problem for all the redox pairs that were reported,35,39 and the reaction kinetics are not fully defined. The problems in the oxygen-releasing step are amplified along with the increasing experimental scale.
Although literature is still scarce, earlier research was reported by Murmura et al.,39 and by Varsano et al.35 Both Murmura et al.39 and Varsano et al.35 used about 40 mg MnFe2O4/Na2CO3 at the lab-scale for temperatures between 700 and 800 °C, or 600 and 800 °C, respectively. The H2 yield obtained after 1 h of oxidation varied from 81% (700 °C) to 86% (800 °C).
Varsano et al.40 repeated the tests in a 1 kW concentrated solar facility at 750 and 800 °C and obtained H2 yields of approximately 20 to 37% at 750 °C, and ∼72% at 800 °C. The lower H2 efficiency in the solar-driven reactor was attributed to a nonuniform temperature within the solar reactor, and to a too coarse particle size (0.5–2 mm) used in the solar reactor.41 Such coarse particles have a very low active surface area. In general, the previous studies paid insufficient attention to the individual steps (oxidation and reduction), to the reaction kinetics, and to the long-duration and multicycle operation.
In the ball-mill preparation,42 analytical grade MnCO3 and Fe2O3 were used without further treatment. They were mixed for 40 minutes at a molar ratio of 3:2 in a 1000 rpm Simoloyer CM01 mixer/mill at ambient conditions and after the addition of ethanol (96%). 5 mm diameter stainless steel balls were used at a weight ratio of 6 with respect to the reactant's mix. Ethanol was evaporated at 378 K. The dried reactant was calcined in N2 atmosphere at 973 K for 1 hour, and re-milled in a Retsch mill. The calcination under N2 is needed to activate the reactants before their first use.
For the coprecipitated MnFe2O4, an aqueous solution of 0.5 mol MnCl2 and 1 mol FeCl3 were mixed at 60 °C with continuous stirring at 250 rpm. Subsequently, a 0.64 mol NaOH solution was added. The solution was maintained at 80 °C for 1 h. The precipitates were centrifuged and wasted 5 times with distilled water at 80 °C (to remove excess NaOH and formed NaCl). After drying at 105 °C and calcination at 450 °C during 2 h, the powder was milled in a Retsch mill into fine particles (<50 μm). The chemical reaction is presented as
MnCl2 + 2FeCl3 + xNaOH + H2O → MnFe2O4 + (x − 8)NaOH + 8NaCl + 3H2O |
For the experiments in the vertical and solar furnaces, inert olivine was added to form a porous fixed bed, or to be able to operate the solar reactor in an isothermal fluidized mode by improving the heat transfer from the reactor wall to the fluidized bed of reactants. Malvern laser-diffraction and confirming SEM-imaging were used to determine the particle size distributions, which were mostly Gaussian with a narrow size distribution. This is illustrated in Table 2 for the feedstock particles. The near-spherical olivine particles (Mg, Fe-silicates) had a Waddell sphericity factor ψ of 0.8–0.9.41 The sphericity of all reactants was considered close to 0.84. These particles were further milled and processed into smaller particles. The pulverized mixtures had an ultrafine particle size, well below 2 μm, with smaller particles of approximately 150 nm.
Chemicals | dv (μm) | dsv (μm) | σ (μm) |
---|---|---|---|
MnCO3 | 10.9 | 9.2 | 5 |
Na2CO3 | 394.5 | 331.3 | 36.5 |
MnCl2 | 6.4 | 5.9 | 0.5 |
Fe2O3 | 5.6 | 5.3 | 0.3 |
Olivine (100–150 mesh) | 167.9 | 142.7 | 18.9 |
Particle size was measured by diffractometry, and the BET surface area (m2 g−1; Brunauer–Emmett–Teller) was determined in a Micromeritics instrument by low-temperature (−196 °C) nitrogen adsorption. SEM images are shown in Fig. 2 for the prepared MnFe2O4 compound reactants (ball-milled and coprecipitated). The ball-milled reactant particles have an agglomerated particle diameter of ∼60 μm (Fig. 2a) and are composed of smaller (∼0.1 μm) grains. Higher magnifications of coprecipitated reactant (Fig. 2b) show that smaller particles of 50 to 150 nm size are obtained. The coprecipitated reactants have a higher specific surface area (132 m2 g−1) than the ball-milled reactants (4.9 m2 g−1).
N2 (carrier gas) and CO2 feeds are set by mass flow meters. H2O is added by a syringe pump. The N2/H2O (oxidation cycle) or CO2 (reduction cycle) flows are preheated before being added into the water splitting reactor. The reactors are either vertical furnaces or TGA cells. After the reaction, the exhaust gas is cooled and dehumidified before being sent to a GC-MS for component monitoring. CO2 was removed from the gas by absorption in a 1.5 g L−1 Ca(OH)2 solution. The reactors contained appropriate quantities of reactant particles. A full description of the setups and the applied experimental procedures are given in ESI-3.† Olivine was sometimes added to increase the porosity and flowability of the reactants. The ball-milled reactant was prepared by milling 7.05 g ferrite and 4.87 g Na2CO3. For coprecipitation reactant, 53.94 g of MnFe2O4 was mixed with 37.18 g of Na2CO3. The bed heights in the electrically heated furnace were 15 cm, against 25 cm in the solar reactor.
The GC-MS continuously monitored and recorded the H2 concentration in the oxidation step, and the O2 concentration in the reduction step.
The behaviour of the activated MnFe2O4 reactant was assessed for 3 subsequent oxidation–reduction steps each at 700 °C. The reverse step used pure CO2 for 3 h. Time-dependent H2 production values are illustrated in Fig. 3 for coprecipitated MnFe2O4, and in Fig. 4 for ball-milled MnFe2O4. The results were cumulated and expressed in mol H2 per mol MnFe2O4. The coprecipitated reactant significantly performs better than the reactant produced by ball milling.
Since it was seen that the ball-milled reactant had a significantly lower H2-yield than its coprecipitated alternative, these tests were terminated after 60 min. It was moreover evident that reduction times longer than 3 h were needed. According to Chen et al.,43 the oxygen release (step 2 of the reaction cycle) between layered Na(Mn1/3Fe2/3)O2 and CO2 is fairly slow and requires >3 h to be completed.44,45 This was further investigated by TGA, where it was demonstrated that the reduction step preferably requires 4.5 to 6 h. Stoichiometrically, the yield should be 0.5 mol H2 per mol MnFe2O4, which is only closely achieved for the coprecipitated reactant. Regrinding of the reactants between the cycles slightly increased the H2 yield. It was hence tentatively presumed that agglomeration or sintering of the reactant took place between cycles during different days. The effect of agglomeration can be mitigated by grinding or reactivating the reactants.
For the ball-milled reactant, the poor performance can be explained by the incomplete reaction of Fe2O3, MnCO3 and Na2CO3. Although some MnFe2O4 can be recovered after one cycle, some iron is segregated and forms Fe2O3 phase. The Fe2O3 phase will react with Na2CO3 and form NaFeO2, thus causing a decrease in H2 productivity between the first and second cycle due to difficulties in regeneration step. The unreacted Fe2O3 when synthesizing the ball-milled catalysts also exacerbated the decrease in H2 yield between cycles compared to the coprecipitated ones.
The solid–solid contact and ions transportation are equally important for the regeneration step. According to Chen et al.,43 smaller particle size with moderate crystallinity is beneficial to maintain its structure stability and can lead to a better ionic transport within the crystals than through the grain boundaries in order to finish the whole cycle. This also counts for the difference in H2 yield between the first two cycles. The particles smaller than 30 nm are good for the contact with Na2CO3 to release H2 in the first step, but hamper removing Na+ from the lamellar Na(Mn1/3Fe2/3)O2 oxide considering the low layered structure stability.
The cyclic O2/H2 production was however achieved. Fig. 5 compares typical SEM images of the ball-milled and coprecipitation reactants after water splitting at 700 °C. The particles synthesized by ball-milling (Fig. 5a) formed agglomerates consisting of 0.1 μm dense grains. The co-precipitation reactant maintains a size in the order of 50–150 nm (Fig. 5b). The porous morphology of the MnFe2O4 reactants is maintained even after calcination and reaction at 700 °C.
The time-dependent conversion allows the study of the reaction kinetics. To evaluate appropriate kinetic models,26 data were transformed in terms of reaction progress, α. These data were normalized against the conversion at α = 50%. All data transformations revealed similar fitting profiles, as illustrated in Fig. 6.
The MnFe2O4 water splitting is controlled by different reaction models. A 2D geometrical-contracting model (R2) seems appropriate when α < 0.5. For α > 0.5, a first-order (F1) or 2D diffusion model (D2) seems more appropriate. The morphology of Mn seems essential to the H2 generation reaction kinetics. This is also the case for MnOx-based water splitting reactions.43 An additional kinetics study was carried out based upon the TGA results, as reported below.
The experimental results of Fig. 7 clearly demonstrate that a 3 hour CO2-induced reduction cycle is insufficient to restore the initial activity of the reactant. The reaction was hence stopped after 7 cycles. This finding confirms the electrically heated furnace results.
Fig. 7 Weight evolution of the oxidation (3 h) and reduction cycles (3 h). Seven cycles were examined. |
Long-duration, multicycle experiments with reduction cycles of 4.5 and 6 h were performed, and the results (Fig. 8a and b) clearly demonstrated that a longer reduction cycle enhances the recyclability.
All results for the first 7 cycles (with different total cycle times) are represented in Fig. 8c.
Finally, the system H2 efficiency is calculated as follows, and presented in Fig. 8d:
The efficiency of the combined (3 + 6) h cycles exceeded 98% after 33 cycles, again 95% only for the (3 + 4.5) h operation. It is hence recommended to combine a 3 h oxidation (H2 release) with 6 h of reduction (O2 release).
The TGA results enable the calculation of the apparent reaction rate constant (k). Since α ≫ 0.5, first-order kinetics can be applied. Table 3 summarizes the results for the given numbers of cycles and a specific reduction time (3, 4.5, and 6 h).
Reaction rate constant (min−1) | Reduction time | ||
---|---|---|---|
3 h | 4.5 h | 6 h | |
Cycle 1 | 2.23 × 10−2 | 3.17 × 10−2 | 3.54 × 10−2 |
Cycle 6 | 1.68 × 10−2 | 2.72 × 10−2 | 3.07 × 10−2 |
Cycle 30 | — | 1.77 × 10−2 | 2.22 × 10−2 |
The reaction rate decreases with the number of cycles for a given duration of the reduction, corresponding to a progressive but limited loss of activity of the solid reactant. The rate constant however increases when the reduction time is increased. It is hence important to either extend the oxidation cycle beyond 3 h as cycling proceeds, or to re-activate the reactants more frequently by calcination and/or re-milling after a certain number of cycles.
The reaction mechanism of the water splitting has been investigated by several researchers, both in electrocatalytic and thermochemical routes. Zhou et al.46,47 studied the electrocatalytic reduction reaction (ORR), also called the hydrogen evolution reaction (HER), and oxygen evolution reaction (OER). Four elementary reaction steps are proposed, including OOH and OH species. Spin-polarized DFT simulations demonstrate that the reaction step are reported to be slow.48 These 4 steps were confirmed by computational screenings.49 The thermochemical mechanism was assessed by Chen et al.43 and Angotzi et al.50 with special emphasis on the slow oxygen release (over 3 h) of the Na(Mn1/3Fe2/3)O2 intermediate. In the hydrogen release step, lamellar Na(Mn1/3Fe2/3)O2 oxide is formed by intercalating a Na+ layer into two adjacent oxygen interspaces.51 Both intercalation of Na+ into adjacent oxygen interspaces and the Jahn–Teller effect lead to a more stable Na(Mn1/3Fe2/3)O2 oxide. The full mechanism is presented in Chen et al.43
Further to the TGA observations and prior to conducting additional cycling experiments that are now taking place in the season of high direct normal irradiance (DNI), the rig was adapted to conduct oxidation and reduction steps in parallel in the single cavity, as illustrated in Fig. 10.
The H2 upgrading will apply a sequence of steps, involving the removal of excess H2O vapour; the removal of CO2 by vacuum swing adsorption6 on appropriate adsorbents, e.g. zeolite, activated carbon, or others;52,53 a membrane permeation of H2, e.g. using a Matrimid 521854 or P84 membranes55 with over 95% H2 purity and 96% recovery. The use of sintered metal fibre filters at the reactor exhausts limits the loss of reactant.56
The heating costs are not considered since the system is supposed to use excess photovoltaic or wind turbine electricity, or to operate on concentrated solar heat. Heat recovery will be maximized by good heat management. The number of required cycles (Nc), to break even can be calculated for a proposed selling cost of H2, since:
CO2 should be separated, stored, and used in the reverse reaction. It is also proposed to use membrane modules to produce very pure H2.54,55
The solar energy balance is currently being assessed. At 4 € per kg H2 and 500 € per ton of reactants, 30 cycles should be realized to break even. To reach 1 € per kg H2, 120 cycles should be achieved, and this is presumed possible in view of the obtained cycling results. If the cheaper ball-milled reactant could be improved, the number of required cycles to break even will be reduced.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ra05319e |
This journal is © The Royal Society of Chemistry 2022 |