Isothermal Splitting of CO₂ to CO Using Cobalt-Ferrite Redox Looping

Anthropogenic carbon emissions continue to disrupt the carbon cycle leading to unwanted anthropogenic impact on the feedback between carbon cycle and the global inhabitability parameters such as temperature, mean sea level, and ocean acidity. While CO₂ emissions from anthropogenic activities have increased to 37 Gton/yr (global energy related CO₂ in 2017) [1], natural carbon sinks such as ocean and biosphere are expected to reach their saturation limit which will result in acceleration of climate change [2]. Continuously increasing fossil fuel consumption is the main reason for these unprecedented carbon emissions followed by deforestation. Energy and value-added chemicals are the commodities in demand that links to fossil fuel consumption [3]. Transition away from fossil fuels and toward renewable energy resources such as solar, biomass, wind, and geothermal energies provides mitigation from future carbon emissions. Realization of the future of fully renewable energy infrastructure assumes the time scale for catastrophic climatic activities to be later compared with this infrastructure time. But the climate models could include significant underestimation of the climate change and current atmospheric carbon content and additional carbon emissions that would be released from now until all renewables in the future. This alone could have disastrous effects on the global temperature and inhabitability of our planet. This potential risk calls for carbon capture and active sequestration, and utilization in parallel with efforts on renewable energy to effectively seek carbon negative pathway for the near future. The demand for carbon based energy, value-added chemicals, the need for renewable energy utilization and the necessity for CO₂ utilization can all be addressed together via solar-thermochemical conversion of CO₂ and H₂O into syngas (CO and H₂) which can then be utilized for various applications including carbon energy and chemicals production [4,5].

Thermochemical redox looping of metal oxides and mixed metal oxides at high temperature has been the most preferred pathway for CO₂ conversion as it requires only two steps and avoids the use of corrosive chemicals present in other thermochemical (CO₂/H₂O) conversion cycles [6,7]. These metal oxides are referred to as oxygen carriers (OC) and are classified into volatile and non-volatile cycles. The volatile cycles involve oxygen carriers with boiling point lower than operational temperatures such as ZnO/Zn, SnO/Sn while non-volatile cycles include solid oxygen carriers with no phase change during the cycles such as CeO₂, Fe₃O₄, and perovskites [8]. While volatile cycles provide thermodynamic advantage with higher spontaneity, the practicality of handling metal vapors and the need for rapid quenching makes them unattractive. Non-volatile cycles have been the most studied thermochemical redox cycles for CO₂ and H₂O splitting [8,9].

CeO₂ and its doped derivatives have been the most researched class of materials as oxygen carriers due to their significantly fast redox kinetics and high-temperature resiliency, and thus sustained cycle performance. But these materials typically require reduction temperature as high 1800 K along with significant requirement of low partial pressures of O₂ for comparable oxygen storage capacity [10]. Iron oxide-based materials were also proposed of which substituted ferrite-based materials were found to provide higher performance in terms of lowered reduction temperature and higher oxygen storage capacity [11–16].

Substituted ferrites are typically of the form M_xFe_3-xO₄. Different metals were studied at the substituent M to form a spinel structure. Co, Ni, Mn, Zn, and Fe are the most chosen substituents as they provided better thermodynamics by lowering the required reduction temperature, albeit at the cost of lowering spontaneity in their oxidation steps [8]. Co and Mn were found to show this behavior according to thermodynamic and experimental studies into Co and Mn ferrites especially for H₂O splitting [8,17]. Similar to other iron oxide-based materials, these ferrites were found to be prone to high-temperature sintering which lowered their cycle performance from the loss of active surface area. To avoid this, ferrite deposition on high-temperature resilient substrates such as ZrO₂, Al₂O₃, and yttrium stabilized zirconia (YSZ) were proposed and these studies showed formation of solid solution of Fe²⁺ with the substrate during thermal reduction and thus providing structural stability [8,12,15,18–20]. In the case of Al₂O₃, thermal reduction of M_xFe_3-xO₄ with Al₂O₃ led to the formation of aluminates and reduction of Fe³⁺ to Fe²⁺ and thus releasing O₂. The thermodynamic stability of these aluminates improved the spontaneity of reduction step and thus allowing for thermal reduction at relatively low temperatures compared with ferrites. These aluminate-based ferrite cycles are referred to as hercynite cycles in the literature due to the formation of hercynite intermediates. The improvement of reduction thermodynamics by incorporating Co ions and addition of Al₂O₃ substrates comes at the cost of lowered spontaneity in oxidation step where CO₂ reacts with the reduced state oxygen carrier to oxidize them and thus evolve CO. This resulted in the investigation into different compositions of substitution and alumina content so as to balance the advantages in reduction step with the disadvantages in oxidation step [8].

Typical ferrite-based redox cycling involves reduction at high temperature 1600–1800 K in inert gas to produce O₂ followed by oxidation step at lower temperature (1100–1400 K), as in CO₂/H₂O conversion to CO/H₂. The reduction step is repeated to regenerate the lower oxidation state oxygen carrier to further repeat these cycles. While temperature swinging was the most studied pathway for redox looping, isothermal operation has also been investigated by cycling between inert gas and high CO₂ content input gas [21]. Thermodynamic studies on isothermal and temperature swinging operation revealed that while temperature swinging provided higher maximum solar-to-fuel efficiency, isothermal operation provides alleviation of complex solid–solid heat recovery, and thermal fatigue in the oxygen carrier and reactor materials and provides higher throughput via lower latency due to heating and cooling. Although required inert gas for isothermal operation is higher than temperature swinging, development of high-temperature oxygen separation, and improved design for gas–gas heat recuperation make isothermal operation preferable to temperature swinging [22].

Isothermal redox looping of hercynite cycle using CoFe₂O₄.3Al₂O₃ for H₂O splitting at 1623 K was demonstrated to reveal that it produced H₂ more than thrice compared with hercynite cycling from temperature swinging and almost 12 times compared with CeO₂ cycling from 1632 K to 1273 K [23]. This study provided the evidence on feasibility of isothermal operation and its higher yield without the need for temperature swinging. While isothermal redox looping has been studied for H₂O splitting, hercynite, and ferrite cycle conversion and its feasibility in isothermal redox looping is lacking in the current literature. The present paper addresses such lack of information into the feasibility study of CO₂ splitting in isothermal redox looping using ferrite spinel oxygen carriers and hercynite cycle. Cobalt-substituted ferrite and its equivalent hercynite were prepared via co-precipitation to understand the influence of ferrite-alumina oxygen carrier preparation compared with atomic layer deposition and other methods used in the literature, which included dry mixing ferrite with alumina [12]. Co-precipitation of ferrite with alumina provides even distribution and easier preparation technique with higher mixing between ferrite and alumina for hercynite cycle operation.

Co-ferrite (CF) and Co-ferrite-alumina (CFA) were prepared via constant pH co-precipitation technique utilizing respective nitrates as precursors. The materials utilized for co-precipitation were Co(NO₃)₂.6H₂O (99%, Acros Organics), Fe(NO₃)₃.9H₂O (ACS Reagent, ≥98%, Sigma Aldrich Inc.), Al(NO₃)₃.9H₂O (>99%, Acros Organics), NaOH (ACS reagent, ≥97%, Sigma Aldrich Inc.), and Na₂CO₃ (GR ACS, anhydrous).

Mixed nitrate solutions of Co(NO₃)₂.6H₂O, and Fe(NO₃)₃.9H₂O were prepared for Co-ferrite, and Co(NO₃)₂.6H₂O, Fe(NO₃)₃.9H₂O, and Al(NO₃)₃.9H₂O for Co-ferrite-alumina. Incorporation of Co into ferrite structure increases the extent of reduction at the loss of spontaneity toward oxidation by CO₂. The literature revealed that the ratio of Co/Fe of 0.5 (which leads to CoFe₂O₄) provided one to obtain maximum oxygen storage capacity. Higher Co content can increase the extent of reduction while high-temperature oxidation could compensate for loss in oxidation potential via increased kinetics and thus allowing for isothermal operation. In this paper Co/Fe of 0.76 was utilized which effectively resulted in Co_1.3Fe_1.7O₄ (called herein as CF). For the preparation of Co_1.3Fe_1.7O₄, nitrate solution with appropriate Co:Fe ratio were mixed to obtain a total 1 M solution. In a continuous stirring beaker containing 0.2 M Na₂CO₃ and pH meter, the nitrate solution was added dropwise using burette along with 5 M NaOH solution to maintain a pH of 10. This resulted in the formation of precipitates, which was stirred overnight. Thus, the obtained precipitates were filtered and washed using de-ionized water until the pH was neutral followed by drying at 353 K for 24 hrs. The dried powder was then calcined at 823 K in air for 5 hrs. The calcined powder was ground to uniform size before testing.

For the preparation of CFA, appropriate nitrate solution was prepared to effectively obtain Co_1.3Fe_1.7O₄.3Al₂O₃. Similar procedure as given above for Co_1.3Fe_1.7O₄ was utilized for co-precipitation.

The prepared oxygen carriers were tested for their isothermal redox capability in a horizontal furnace equipped with alumina reactor tubing. 1 g of the ground sample was placed in the reactor tube and secured using alumina fiber wool (Zircar ceramics: Type ALBF). For reduction and oxidation steps, the total flowrate of gases was maintained at 500 sccm. Initially the reactor was heated to 1673 K in the presence of N₂ (airgas 99.998%) and maintained at this temperature for 1 h to allow for reduction step. This was followed by oxidation step for which, the flowing gases were changed to 25% CO₂ and 75% N₂ at total flowrate of 500 sccm. The evolving gases were analyzed using gas analyzer (Horiba PG-350 with NDIR) for CO₂ and CO. The oxidation step was maintained for 2 h to obtain CO. The composition of CO in the evolved gas was utilized to calculate the net CO yield, reaction rate and maximum CO flowrate. These redox cycles at 1673 K were repeated multiple times to obtain cycle stability. After three cycles, the samples were analyzed using FEI Quanta 200 scanning electron microscope (SEM) operated at 30 kV of accelerating voltage and scanning secondary electrons for its morphology in comparison with the initial sample to understand the influence of high-temperature redox operating conditions on the oxygen carrier’s surface and its possible relation with cycle performance.

CO evolution was the main parameter for comparative purposes for these studies and the CO obtained from isothermal operation of CF and CFA were compared with the performance from the literature to reveal the relevance of the present study and its importance providing better performance in isothermal mode using two different types of materials.

Figure 1 shows the evolution of CO flowrate during isothermal redox looping at 1673 K. One of the apparent features of CO evolution from CF is the continuous evolution of CO for long time periods (relatively long for complete conversion) compared with CeO₂-based oxygen carriers. The maximum CO yield rate from CF was ∼14 µmol/g·min, but it increased to 20 µmol/g·min in the second cycle followed by a decrease to 10 µmol/g·min. This variation in the maximum flowrate corresponds to the availability of surface area for reaction with CO₂. Increase from cycle #1 to cycle #2 was possibly due to incomplete calcining which led to lower flowrate in cycle #1. Then, the drop of maximum flowrate from cycle #2 to cycle #3 could be due to sintering of the oxygen carrier sample which lowered its surface area and thus lowering the surface reactions. But, as the surface reactions completed in ∼30–40 min, the CO flowrate reached a plateau behavior, which suggests the reaction being limited by diffusion of gas reactants/products through CF’s lattice for oxidation reaction.

Under similar conditions, maximum CO flowrate from CeO₂ reached almost 60 µmol/g·min. But in CeO₂-based isothermal redox looping, the CO flowrate dropped to 0 in 4 min that provided an average of 48 µmol/g · min over the first 90 s [24]. Although the maximum CO flowrate in the current case was lower, due to its continued reaction and isothermal nature of the reaction, longer cycle times can provide with significantly higher CO yields compared with CeO₂. Operating at higher cycle duration in isothermal conditions alleviates the concerns of this lower kinetics and allowed for higher CO yields without any significant losses. Similarly, in the case of H₂O splitting using Co-hercynite cycles at 1623 K provided maximum H₂ flowrate of 30 µmol/g · min which rapidly dropped to 6 µmol/g · min in 5–7 min and to 0 in a total of 15 min [23]. Although this provided faster cycles, under isothermal conditions, cycle duration does not affect efficiency of the process and with adjusted reactor design, it does not affect throughput. So, CF can provide constant CO flowrate allowing for easier CO capture.

Figure 2 shows cumulative CO yield with time from isothermal redox looping of CF. Cycle #1 provided with CO yield as high as 1250 µmol/g over a span of 120 min. With consecutive cycles the yield dropped to ∼750 µmol/g due to sintering. Under similar conditions but for H₂O splitting, Co-hercynite in the literature yielded 102 µmol/g. Although this was obtained over relatively short cycle time, it has an overall storage capacity of the hercynite in the literature while in the current case of CF, increase in cycle duration can increase the CO yield.

Comparing with temperature swinging looping of oxygen carriers in the literature, the results shown in Fig. 2 on CO yield are significantly higher. Swinging between 1673 and 1323 K, non-stoichiometric Mn doped and other perovskites yielded only 200–250 µmol/g over 1 hr [21,25]. Some YSM perovskites reached CO yield as high as 750 µmol/g but needed temperature swing of 500 K from 1673 K as the reduction temperature which makes practicality of such reactors to decrease due to low efficiency. Different kinds of CeO₂ derived oxygen carriers also yielded less than 250 µmol/g. Only some materials such as Co_0.8Zr_0.1Fe₂O₄/SiO₂ provided higher yields of around 800 µmol/g over 30 min but carried out by swinging between 1673 and 1373 K [15]. Hercynites are the only ones that provide comparable results to the current case, but they were looped via temperature swinging. This suggests that although the current material may not be the best performing, it brings the significance of isothermal testing to focus as literature lacks such information. From Fig. 3, one can also observe that CF is prone to sintering. Figure 3(a) shows fresh sample morphology to contain a distribution of various particle sizes which includes particles smaller than 10 µm. But when this sample was subjected to three high-temperature CO₂ splitting cycles, the morphology changed to larger particles with significantly lowered porosity, see Fig. 3(b). This also shows that although CF provided high CO yields, due to its sintering potential, improvement of this material is still necessary for sustainable CO₂ splitting for longer durations needed in industrial scale.

With the pursuit of obtaining improved oxygen carrier material properties in terms of high-temperature resilience, Co-ferrite-alumina (CFA) was prepared. The composition of CFA can be effectively written as Co_1.3Fe_1.7O₄.3Al₂O₃ which is similar to the above CF except for the presence of alumina. As per literature incorporation of alumina into ferrite cycle allows for hercynite cycling, which improves the reduction performance. The lack of literature on isothermal redox looping of hercynite for CO₂ splitting also motivated these examinations to gain insight into the feasibility of isothermal mode for hercynite cycling.

Figure 4 reveals the CO evolution from isothermal cycling of CFA. Comparison of these results with Fig. 1 directly reveals lower CO yields from CFA cycles. But it reveals that even though the flowrate is relatively lower, it is still competitive with the results in literature suggesting that this is an appropriate pathway to obtain high CO yield while gaining the advantages of isothermal operation.

Figure 5 shows cumulative CO yields, which reveal that over 2 hr cycles, CO yield of ∼200 µmol/g while over 1 hr, it was only ∼100 µmol/g. These results were similar to hercynite cycle data reported in the literature, which showed almost ∼100 µmol/g of H₂ produced from isothermal mode [23]. Although this is not as high as the potential yield that can be obtained from hercynite cycles, it is competitive with CeO₂ and higher than temperature swinging of the same hercynite between 1673 and 1173 K [23]. This calls for further optimization of the operating conditions and the CFA composition to obtain the best possible CO yields from these materials in isothermal mode.

Figure 6 shows the promise of CFA compared with CF in terms of high-temperature resiliency and morphological stability compared with results shown in Fig. 3. Figure 6(a) showing the morphology of fresh CFA reveals similarity with Fig. 3(a) in terms of distribution of particle size, due to the utilization of co-precipitation for preparation. A comparison of Figs. 6(a) and 6(b) reveals mold increases in the number of larger size particles, but the morphology in Fig. 6(b) suffered insignificant sintering issues and maintained the structure over high-temperature operation. Compared with Fig. 3, this shows the impact of Al₂O₃ presence in providing high-temperature morphological resiliency. This also provides promise of lessened loss of CO yield over multiple cycles without the need for testing over multiple cycles. Considering this stability, development of these hercynite materials with optimized composition and isothermal temperature is necessary to obtain sustainable CO yields, while the presence of Al₂O₃ provided the morphological stability. This paper demonstrates the appreciable feasibility of ferrite class and hercynite class oxygen carriers to isothermally convert CO₂ to CO under similar gas flow conditions as temperature swinging operation. Furthermore, it provides the important advantage of isothermal operation for ease in practical application.

Isothermal redox looping for CO₂ splitting provides practical advantages of faster reaction, throughput, lessened losses from solid–solid heat recovery and time taken for temperature swinging along with lowered thermal fatigue in oxygen carriers and reactor materials. The literature does not reflect this importance and contains significant knowledge gap in this field. This paper demonstrates the importance of isothermal redox looping for CO₂ conversion using Co-ferrite and Co-ferrite-alumina (hercynite cycle) along with appropriate comparison from the literature with other isothermally tested oxygen carrier materials to reveal the performance of different oxygen carriers in temperature-swing redox looping. Co-ferrite provided significantly high CO yields and flowrate compared with those reported in the literature. CO yields as high as 750 µmol/g from only 25% CO₂ flow (125 sccm) in isothermal mode offers very promising results which are higher compared with the performance reported in the literature on temperature swinging CeO₂, perovskites and isothermal CeO₂ and hercynite. Similar testing of Co-ferrite-alumina revealed relatively lower CO yields compared with its ferrite counterpart, but the results were similar to the isothermal hercynite cycle reported in the literature on H₂O splitting. The potential for lowering of reduction temperature suggests that further optimization of isothermal operational conditions and the composition of this hercynite oxygen carrier requires further examination. Morphological studies revealed significant promise of Co-ferrite-alumina in maintaining its surface area and thus CO yield over multiple redox cycles compared with Co-ferrite which showed significant decrease in the porosity and surface area. This reveals the need for further studies into isothermal operation of hercynite cycles for enhanced CO production and the development of isothermal reactors prior to pilot-scale demonstration using renewable energy such as concentrated solar power to easily scalable CO₂ conversion and CO production.

This research was supported by the Office of Naval Research (ONR) and is gratefully acknowledged. The authors acknowledge the assistance provided by Mr. Ramsey Stuart at Combustion Lab, University of Maryland, College Park in the preparation of oxygen carrier samples. The authors also acknowledge the assistance provided by Center for Advanced Life Cycle Engineering (CALCE) group at the University of Maryland, College Park and their graduate student, Mr. Saurabh Saxena in SEM acquisition. Kiran G. Burra received Hulka Energy Research Fellowship from the University of Maryland and it is gratefully acknowledged.

There are no conflicts of interest.