Reactivity and efficiency of ceria-based oxides for solar CO2 splitting

Article pubs.acs.org/EF

Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Reactivity and Efficiency of Ceria-Based Oxides for Solar CO2 Splitting via Isothermal and Near-Isothermal Cycles Liya Zhu and Youjun Lu* State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF), Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China S Supporting Information *

ABSTRACT: In this work, different doping strategies for ceria were investigated in isothermal and near-isothermal cycles for solar fuel production. The system efficiency was evaluated by a more realistic model with the whole oxidation process concerned. In terms of fuel productivity, doping ions with lower valences seems to be a disadvantageous strategy due to formation of inert oxygen vacancies. Zr doped ceria exhibited the highest productivity under all of the reaction conditions investigated. The efficiency comparison of pure and Zr doped ceria is different from theoretical prediction, and the influence of oxidation rate is highlighted. The advantage of higher productivity caused by adding Zr is traded off by its drawbacks in kinetics for isothermal cycles at 1300 and 1400 °C. Benefit in efficiency only could be observed in an isothermal cycle at 1500 °C. Compared to the isothermal cycle, the near-isothermal cycle with a small temperature swing showed improvement in both fuel productivity and efficiency. Pure ceria showed better performance than 10% Zr doped ceria in 1500 °C/1300 °C near-isothermal cycles.

1. INTRODUCTION Fossil fuel combustion over the past two hundred years has caused serious environmental and climatic problems. Also, the fossil fuel reserves are declining rapidly. Clean and renewable energy research has become a hot issue, and the conversion of solar energy is particularly valued. Solar fuel production via a two-step thermochemical cycle is viewed as a promising solution to the greenhouse gas emission and energy crisis.1−8 As depicted in Figure 1, the cycle consists of, first, reduction of the metal oxide using concentrated solar radiation as the energy source of the high temperature process and, second, oxidation of the reduced oxide by CO2/H2O to produce CO/H2. These two steps are endothermic and exothermic, respectively. Metal oxides as the medium are recycled, and both oxygen and gas

fuels are produced in two separate processes, bypassing the gas separation problem. The gas fuels produced could be used in fuel cells or synthesized into liquid fuels via mature industrial methods, such as Fischer−Tropsch synthesis.9,10 Many redox pairs, such as ZnO/Zn,11,12 SnO2/SnO,12,13 Fe3O4/FeO,14,15 and CeO2−δ,16−18 have been investigated for the cycle. Among them, ceria (CeO2−δ) has shown to be a promising candidate for its good stability, prominent oxygen storage capacity, and fast kinetics.16−18 Complete reduction of CeO2 to Ce2O3 calls for a temperature as high as 2000 °C,19 and thus a nonstoichiometric change at lower temperatures was suggested.16−18 It takes advantage of the defect equilibrium of oxygen vacancies under different (T and pO2) conditions. The reactions in each step can be expressed as follows: reduction:

CeO2 − δox → CeO2 − δred +

δred − δox O2 2

(1)

oxidation: CeO2 − δred + (δred − δox )CO2 → CeO2 − δox + (δred − δox )CO

(2a)

or CeO2 − δred + (δred − δox )H 2O → CeO2 − δox + (δred − δox )H 2

(2b)

Doping has been widely investigated as an effective method for ceria modification. Compared to pure ceria, besides the improvement on fuel productivity, the reduction enthalpy would to some degree be reduced, which could lead to a benefit Received: October 25, 2017 Revised: December 12, 2017 Published: December 13, 2017

Figure 1. Schematic of solar fuel production via two-step thermochemical cycle. © XXXX American Chemical Society

A

DOI: 10.1021/acs.energyfuels.7b03284 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels on energy conversion efficiency.20 A lot of doping ions have been studied with different success.21−37 However, results from different doping strategies are hardly comparable because of different reaction conditions. The reduction was usually conducted at a fixed temperature and the oxygen pressures were not clearly stated. Nevertheless, Zr and Hf were shown to be the most promising alternatives for their significant improvement in fuel productivity.29−37 However, considering their drawback in kinetics, compromise between productivity and reaction rate should be made when choosing the optimal doping concentration.34,38 Solar to fuel efficiency is particularly concerned in expectation of commercialization. Different thermodynamic models have been developed for the whole system and the influence of reaction conditions and technical schemes has been widely discussed.38−43 However, most of the models are equilibriumbased, and their conclusions are slightly different from each other due to different assumptions on some energy cost terms. The efficiency could be quite high due to optimiztic assumptions/parameters involved in the models. Heat recovery of both gas and solid (for nonisothermal cycle) phases is necessary for considerable efficiency. Gas heat loss is from the energy input to heat gas species to the reaction temperature, especially when substantial inert gas is used in the reduction step to keep a low oxygen partial pressure. The inert gas demand predicted by counter flow arrangement of redox material and inert flow is much lower than that predicted by a perfect mixed model.39−41 However, a more realistic model reveals that the reduction in inert gas demand caused by counter flow arrangement is much smaller than the previous prediction.44 Replacing inert gas with vacuum pumps was proposed to be a promising method.41,43,44 Solid heat loss comes from the temperature swings between two steps. Considerable sensible heat could be lost when cooling the redox material from reduction temperature to oxidation temperature. Its recovery calls for solid−solid heat transfer between redox materials from reduction and oxidation chambers, and meanwhile the gas from two chambers should not be mixed. These requirements make it quite technically challenging. Some reactor configurations have been designed for solid heat recovery. However, they are either tested with partial success or still concepts in papers.45−49 Considering the significant impact of solid heat loss on efficiency and difficulties in solid heat recovery, a new reaction scheme was proposed, namely isothermal cycle.50 Temperature swings between two steps were demonstrated to be unnecessary, and a difference in the oxygen partial pressure was used to provide the driving force. The cycle could thus proceed isothermally, eliminating solid heat loss. Some other benefits, such as simplification in reactor design and avoidance of thermal shock, could also be expected in an isothermal system. Solar-to-fuel efficiency ≥10% was predicted for both H2O and CO2 splitting at 1500 °C provided efficient gas-phase heat recovery.51 Isothermal cycles based on pure ceria were experimentally demonstrated in CO2 and H2O splitting, respectively.52,53 However, the isothermal cycles were conducted only at 1500 °C, and the potential of doped ceria in isothermal cycle was not experimentally investigated. Theoretical analysis shows that isothermal cycles eliminate solid heat loss at the cost of higher oxidizing gas consumption and lower fuel productivity.52−54 It is due to the thermodynamic property of both redox material and oxidizing gas. Nonisothermal cycle with a small temperature difference, which could be noted as

near-isothermal cycle, was suggested to be beneficial even without solid heat recovery.41,54 However, so far there is no experimental study on near-isothermal cycle either. In this study, we offer experimental investigations and kinetic-based efficiency evaluation of pure and doped ceria in both isothermal and near-isothermal cycles. Ceria-based solid solutions doped with ions with different valences (Li+, Ca2+, Pr3+, and Zr4+) were synthesized and tested in isothermal (1300, 1400, and 1500 °C) and near-isothermal (1500 °C/ 1300 °C) cycles. Further doping of low-valence ions in Zr−Ce solid solutions was also investigated. A kinetic-based model with the whole oxidation process considered was established to evaluate the influence of both materials and reaction schemes on solar-to-fuel efficiency. Considering practical challenge in solid heat recovery and impossibility of complete heat recovery of oxidizing gas with gas heat exchanger, recovering solid heat and oxidation heat via further heating the preheated oxidizing gas was also evaluated as an aspect of efficiency.

2. APPROACH 2.1. Experiment. 2.1.1. Material Synthesis. All of the solid solutions were synthesized via a chemical solution method. The process is similar to that described in Hao et al.’s work.37 Ce(NO3)3· aH2O(AR), LiCl·bH2O(AR), Ca(NO3)2·cH2O(AR), Pr(NO3)3·dH2O(AR), and ZrOCl2·eH2O(AR) were used as metal precursors. Before synthesis, the content of crystal water in each metal precursor was determined by calcination in air at 500 °C for 2 h. The values of a, b, c, d, and e were determined to be 5.80, 1.08, 4.01, 6.00, and 7.72, respectively. Citric acid and ethylenediaminetetraacetic acid (EDTA) were used as complexing agent. The molar ratio of metal precursor, critic acid, and EDTA was set to be 1:1.5:1.5. For each sample, the aiming weight was 3 g. The precursors of Ce and dopants, citric acid and EDTA, were weighed and dissolved in ∼150 mL of deionized water under continuous stirring. A total of ∼20 mL ammonium hydroxide was then gradually poured into the mixture to get a light brown aqueous solution. The solution was heated in water bath at 90 °C to get a transparent wet gel. The wet gel was dried in an electric furnace at 120 °C for 2 h and then calcined at 600 °C for 2 h to eliminate organics. Finally, the powder was ground in an agate mortar and collected for thermochemical tests and characterizations. For convenience, the doped samples are referred to as “Mx” in the following text, in which “M” represents the doped element and “x” is the doping fraction, for example, Zr10 is ceria doped with 10% of Zr. 2.1.2. Thermochemical Cycle. The synthesized samples were tested in two-step cycles in the setup shown in Figure 2. Isothermal cycles were conducted at 1300, 1400, and 1500 °C, respectively. Argon (Ar, 99.999%) was used as sweeping gas during reduction with the flow rate

Figure 2. Schematic of experimental setup for the thermochemical cycle. B


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Energy & Fuels of 300 sccm. At the beginning of the oxidation step, Ar flow was switched to 200 sccm and CO2 was added to the gas stream at a flow rate of 100 sccm. Both reduction and oxidation durations were 1 h. For comparison, near-isothermal cycles were also investigated for pure ceria and Zr10. Reduction and oxidation temperatures were 1500 and 1300 °C, respectively. Both heating rate and cooling rate were set to be 10 °C/min. The temperatures in and out of the furnace tube at the heating zone were measured by two B-type thermocouples. The concentration of O2 and CO produced in each step was detected in real time by the O2 detector (ENVITEC, OOM202) and gas mass spectrometer (HIDEN, QIC-20), respectively, both of which were daily calibrated and showed perfect linearity. For all of the redox modes investigated, 1 g of each sample was tested in 3 cycles. The oxygen content in the furnace tube during reduction was higher than that in the Ar gas resource because of diffusion of oxygen from air. Prior to cycling test, the oxygen concentrations in furnace tube at 1300, 1400, and 1500 °C were determined by O2 detector to be 10, 43, and 78 ppm. The system pressure was normal pressure (1 bar), which was verified by the pressure gage. 2.1.3. Material Characterizations. To confirm the lattice structure of the samples before and after cycles, X-ray diffraction (XRD) was performed using a PANalytical X’pert MPD Pro diffractometer with Ni-filtered Cu Kα irradiation (wavelength 1.5406 Å). All samples were scanned in the 2θ range from 10° to 90° with a step size of 0.2°/s. For a rough estimation on sintering effect, crystallite size of samples after synthesis (600 °C) and thermochemical cycles (1500 °C) were calculated from the Scherer equation based on the strongest peak (1 1 1) Kλ D= β cos θ

m = nCO

T , pO

2

CO2 ↔ CO +

1 O2 2

(6)

The standard equilibrium constant is

pCO /p0 · (pO /p0 )1/2 2

K CO2(T ) =

pCO /p0

(7)

2

It was obtained from NIST and polynomial fitted (see Table S1). 2.2.2. Efficiency. The solar-to-fuel efficiency is defined as ηfuel =

Q fuel Q solar

(8)

where Qfuel (J) is the energy stored in the gas fuel. In this case it is obtained by

Q fuel = nCO(t )HHVCO

(9)

To conduct a kinetic-based analysis, here nCO (mol) is given in a time dependent form. It is obtained from the CO profiles measured in the experiment

(3)

δ O2 2

(5)

where MCeO2 is molar mass of CeO2. According to the models, δred and δox could be obtained by the temperature and oxygen partial pressure in reduction and oxidation steps, respectively. For reduction, as described in the Experiment section, the oxygen partial pressure was determined by values measured by the oxygen sensor. For oxidation, the oxygen partial pressure was calculated from equilibrium of direct CO2 splitting:

where D, K, λ, β, and θ are the grain size, dimensionless shape factor, X-ray wavelength, line broadening at half the maximum intensity, and Bragg angle, respectively. The crystallite micrograph was obtained from a field emission scanning electron microscope (SEM, JSM7800F) at an accelerating voltage of 3 kV. 2.2. Thermodynamic Model. 2.2.1. Productivity. The oxygen releasing/absorbing reaction of ceria could be expressed as CeO2 ←⎯⎯→ CeO2 − δ +

106(δred − δox ) MCeO2

nCO(t ) = α(t )nmat(δred − δox )

(10)

where α(t) is the oxidation extent and nmat is the molar amount of the redox material. Qsolar is the energy input of the whole system. Solar energy is the original source of all energy consuming/storing terms, which is calculated by Q solar = Q rerad + Q surf + Q pump + Q mat + Q CO + Q ox + Q fuel

(4)

2

(11)

in which deviation from stoichiometry, δ, is determined by both temperature (T) and oxygen partial pressure (pO2). Several analytical models have been developed for predicting δ by (T and pO2). For example, Schelffe et al.20 developed a thermodynamic model considering both ideal solution behavior and defect interactions of all different species in the lattice. The natural logarithm of equilibrium constants of both vacancy formation and defect interaction reactions were shown to have reciprocal temperature dependence for both pure and doped ceria. Thus, the reduction extent at high temperatures could be predicted by extrapolation from equilibrium constants measured at low temperatures. Bulfin et al.55 developed an Arrheniusbased model considering both O2 releasing and absorbing processes. It was deduced based on the fact that there should be an equilibrium between releasing and absorbing reactions when the compositions in ceria lattice stay stable under certain (T and pO2) conditions. Parameters were determined by fitting the model to equilibrium data from literatures. A statistic thermodynamic model was also developed by Bulfin et al.,56 in which dependence of reduction enthalpy on reduction extent (oxygen nonstoichiometry) was taken into account. Details of these models could be found in the references. As a verification of the experiment method, the productivities of pure ceria in different reaction modes measured in experiment are compared with predictions of different models. In thermochemical cycles, fuel production is realized by introducing a difference of oxygen nonstoichiometry (δ) between reduction and oxidation steps, Δδeq = δred − δox. From eqs 1, 2a, and 2b, the fuel productivity (μmol/g) could be obtained by

with all of the right terms illustrated in the following text. The reactor is assumed to be a blackbody receiver for simplicity.39,51 The absorption efficiency of the reactor is ηabs = 1 −

AσTred 4 GC

(12) −2

−4

where A = 1 is the absorptivity, σ = 5.67−8 wm K is Stefan− Boltzmann constant, G = 1 kw is the nominal solar flux incident, and C = 3000 is the solar concentration ratio. The reradiation loss from the aperture is

Q rerad = (1 − ηabs)Q solar

(13)

Some of the energy absorbed by the reactor could be lost from the reactor walls through heat transfer. A factor is used to consider this:

Q surf = F(Q solar − Q rerad)

(14) 39,51

F is set to be 0.2 as suggested in previous studies. In the experiment, reduction was conducted in Ar for ease of operation. The use of inert gas during reduction was shown to be an inefficient strategy due to large energy penalty caused by gas production and heating.39,43 Its function could be easily realized by a vacuum pump, which could bring advantages in both kinetics and efficiency.43,57 An efficiency analysis based on vacuum reduction seems to make more sense for evaluating the potential of different materials and reaction modes. In the ideal case, the energy needed for pumping gas from a low pressure to normal pressure is C


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Energy & Fuels Wideal = ngasRT ln

p0 pvac

(εgas) was thus introduced. The CO concentration in the gas mixture is so low that the heat capacity of the outflow is simply considered to be the same as that of CO2. In addition, as has been illustrated before, the CO2 could be further heated by the solid sensible heat and oxidation reaction heat. Another recovery factor εsr is involved in calculation to consider both of them. So the energy for heating CO2 from room temperature to oxidation temperature is expressed as

(15)

where ngas, R, pvac, and p0 are the molar amount of gas, gas constant, vacuum pressure, and normal pressure, respectively. However, the real mechanical pumps do not work in the ideal case in practice. The pump efficiency is thus introduced as the ratio between the ideal work and the real work consumed by mechanical pump

ηpump

Q CO

(16)

2

The efficiency of the commercial mechanical pump is obtained from the envelope function developed by Brendelberger et al.58 (see Table S1). Another factor, ηelec, was used to account for the thermal to electricity efficiency. The solar energy due to pumping O2 out of the reduction chamber will be

nO2(t )RT ln Q pump =

p0 pO

2,red

ηpumpηelec

nCO2(t ) = nCO ̇ 2t

(17)

1 nCO(t ) 2

∫T

Tred

(18)

c p,mat(T ) dT

(23)

3. RESULTS AND DISCUSSION 3.1. Isothermal Cycle. The CO-producing profiles of pure ceria in all cycles are shown in Figure 3. For all cycles, the oxidation could be 90% complete in about 6 min. Although reduction is more favored at high temperature, the effect of reverse reaction (reduction) cannot be observed here, since the overall reaction rate monotonically increases as temperature

Here nO2 is also a function of oxidation time because, for a fixed reduction condition, the amount of O2 can be released during reduction is determined by the oxidation duration (oxidation extent) of the previous cycle. In nonisothermal cycles, the redox material should be heated from Tox to Tred before (or along) reduction, the corresponding energy cost is

Q mat = nmat

(22)

The heat capacity of CO2, cp,CO2, is obtained from NIST and polynomial fitted (see Table S1). nCO2(t) is the CO2 consumption when the oxidation duration is t. It is calculated from CO2 flow rate during the experiment:

Here ηelec is assumed to be 0.2,59 and pO2,red is the oxygen pressure during reduction. Considering that the oxygen may be not totally cooled, the working temperature of the pump was set to be 200 °C. The amount of oxygen is obtained by nO2(t ) =

Tox ⎧ c p,CO2(T ) ⎪(1 − εgas)nCO2(t ) T0 ⎪ = ⎨ dT − ε (Q + Q ox ), if > 0 sr mat ⎪ ⎪ 0, if < 0 ⎩

∫

W = ideal Wreal

(19)

ox

here cp,mat is the heat capacity of the redox material. The heat capacity of ceria was shown to be dependent on both temperature and oxygen nonstoichiometry in the literature.60 However, the effect of this variation on efficiency is negligible. Based on the experiment results reported,60 here the heat capacity of both pure and doped ceria is set to be 80 kJ mol−1 K−1 for simplicity. Here it can be seen that solid− solid heat recovery is not involved in this model due to the coherent technical difficulty. Instead, the energy released during solid cooling, together with the thermal energy released during oxidation reaction, could potentially be used for further heating of the preheated oxidizer. This may be a more realistic method for energy saving and is included in the following part. The energy cost for driving the reduction reaction is

Q red = nO2(t )

δred

1 − δox(t )

∫δ

δred

ox(t )

−Δh(δ) dδ

(20)

where Δh(δ) is the partial molar enthalpy of reduction. It is obtained from thermogravimetric studies37,61 and polynomial fitted with δ < 0.1 (see Table S1). Qred could be seen as chemical energy stored in the redox material during reduction, most of which will be converted to the chemical energy in CO during oxidation, and the rest of it will be released as thermal energy: Q ox = Q red − Q fuel

Figure 3. CO producing rate (open plot) of pure ceria in all isothermal cycles. Isothermal cycles were conducted at 1300, 1400, and 1500 °C, and the corresponding oxygen partial pressures during reduction are 1, 4.3, and 7.8 Pa, respectively. During oxidation, 100 sccm CO2 mixed with 200 sccm Ar was used as oxidizing gas. Three cycles were conducted at each temperature. Reaction extent (line) vs time of the third cycle at 1500 °C was also lined, showing that the oxidation could be >90% complete in ∼6 min.

(21)

During oxidation, 200 sccm Ar was included in the oxidizing gas due to the limit of MS detection. Pure CO2 could be used in practice; thus, Ar used during oxidation is not concerned in efficiency calculation. This could be seen as a modest option since both fuel productivity and oxidation rate would be slightly higher in pure CO2. The CO2 could be preheated by the gas mixture after oxidation. A gas heat recovery factor D


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Energy & Fuels goes higher. A maximum transient rate of 0.519 μmol s−1g−1 could be obtained at 1500 °C, which is slightly lower than that reported in previous studies.52,53 For example, the maximum transient CO producing rate was reported to be about 0.8 μmol s−1g−1 in a CO2 flow of 95 sccm at 1500 °C.53 This could be attributed to the diluted oxidizing gas used here. The oxidation rate decreases significantly as the reaction progresses. The last 10% may take more than 10 min with a very low reaction rate. The average oxidation rate could decrease dramatically after the initial period, which may make the cycle inefficient due to the energy penalty caused by heating excess oxidizing gas. This implies the importance of a kinetics based efficiency analysis, which is more realistic and could help to optimize the oxidation duration.53 CO productivities were obtained by integration and shown in Figure 4. It should be noted here that no obvious decrease in

Figure 5. Productivity of pure ceria and 10% M (M = Li, Ca, Pr, and Zr) doped ceria in isothermal cycles at 1300, 1400, and 1500 °C. CO yields were averaged in three cycles.

°C, respectively. The effect of ions with lower valences seems to be more complicated. Slight improvement could be observed at 1300 and 1400 °C. This is due to defect interaction and ordering, which lower the reduction enthalpy.62−67 On the contrary, the reduction enthalpy of pure ceria is much higher at low δ.61 Thus, for doped ceria, the oxygen releasing reaction could start at a relatively lower temperature and more oxygen could be released. The improvement of fuel productivity diminishes as the temperature goes high. The productivity of doped samples is even lower than pure ceria at 1500 °C. This seems to be inconsonant with some previous investigations on low-valence doped ceria21−27 but is consistent with the observations in some more recent studies.34,68 This is due to introduction of inert oxygen vacancies. Based on electric neutrality, dopants with lower valences than Ce4+ could introduce considerable oxygen vacancies. For example, adding 10% of Pr3+ to ceria could bring an oxygen nonstoichiometry of δ = 0.05, which is comparable to that induced by the most reductive condition investigated here (1500 °C, pO2 = 7.8 × 10−5 bar). However, these vacancies are not capable of CO2/ H2O splitting since they are created just by doping rather than reduction of Ce4+. To get a better understanding, here we express the formation of oxygen vacancies (eq 4) in a Kröger− Vink notation:

Figure 4. Productivity of pure ceria in isothermal cycles at 1300, 1400, and 1500 °C. CO yields were averaged in three cycles. Results from different models are also shown for comparison.

productivity was observed in three cycles, which indicates excellent reactivity of ceria. The fuel productivity increases as temperature goes high. Results of thermodynamic models from Bulfin et al.55,56 and Schellfe et al.20 were also plotted for comparison. The experiment results are very close to the prediction based on Bulfin et al.’s model, in which dependence of reduction enthalpy on reduction extent (oxygen nonstoichiometry) was taken into account. On the contrary, the model based on fits of equilibrium constants of vacancy and defect cluster formation reactions seems to underestimate the effect of pO2 on δ. However, the agreement between experiment and models seems to be acceptable, which also could be seen as a validation of the experiment method. The productivities of pure and M10 (M = Li, Ca, Pr, and Zr) doped ceria in isothermal cycles at different temperatures are compared in Figure 5. CO yields were averaged in three cycles to minimize the effect of measuring errors (the first cycle at 1300 °C was excluded to eliminate the effect of organic residuals). CO yields in all conducted cycles were listed in the Supporting Information (see Table S2). Obviously, Zr doped ceria has a higher productivity than both pure ceria and other single doped ceria at all temperatures investigated here. Compared to pure ceria, the CO productivity increases by 95.29%, 78.68%, and 37.77% for Zr10 at 1300, 1400, and 1500

T , pO

1 O2 (24) 2 4+ × × ·· where CeCe, OO, CeCe ′ , and VO are lattice cerium (Ce ), lattice oxygen, polaron (Ce3+), and oxygen vacancy, respectively. Intrinsic oxygen vacancies introduced by doping could cause a reduction of lattice oxygen but an increment of oxygen vacancy, which could be disadvantageous for Ce4+ reduction according to the equilibrium of vacancy formation. This could help to clarify the ambiguity in the doping strategy aiming at this application. Since a higher reduction extent of Ce4+ is preferred and the enhancement on reduction at low temperatures is far from satisfactory, doping low-valence ions seems not to be an effective method. XRD patterns of pure and 10% M (M = Li, Ca, Pr, and Zr) doped ceria before and after cycles are shown in Figure 6. Cubic fluorite structure was confirmed and no impurity peak × × 2CeCe + OO ←⎯⎯→ 2Ce′Ce + V ··O + 2

E


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Figure 7. Transient oxidation rates of pure ceria and Zr10 in isothermal cycles at 1500 °C. Reduction was conducted at 1500 °C with pO2 = 7.8 Pa; 100 sccm CO2 mixed with 200 sccm Ar was used during oxidation. Three cycles were conducted.

oxidation rate of pure ceria stay stable in three cycles, while the oxidation rate of Zr10 kept decreasing. This is attributed to a decrease of surface area and reduction extent. The former is due to sintering at high temperatures. The latter is due to low kinetics of Zr10 during reduction. The duration of reduction was set to be 1 h in the experiment, which is enough for all of the samples but Zr10. This was observed during the experiment. Incomplete reduction led to a slightly lower δred than equilibrium; thus, the oxidation rate was lowered in the following step. However, it should be noted that the slow kinetics of Zr10 during reduction is due to heating rate and mass transfer and could be dramatically improved in vacuum reduction.57 Here it still could be seen that the oxidation rate of Zr10 at 1500 °C could be higher or at least comparable to that of pure ceria. The results imply an advantage of Zr doped ceria when oxidized at high temperatures. As have been illustrated before, the priority of Zr doped ceria over pure ceria in two-step thermochemical cycle calls for a second thought when considering the drawback of Ce−Zr solutions in kinetics and oxidizer consumption. Or at least compromise between productivity and reaction rate should be made in choosing a suitable material for thermochemical cycle fuel production.38,57 However, here we can see the oxidation rate of Zr doped ceria could be higher than pure ceria at high oxidation temperatures. Since the oxidation reaction are suggested to be surfacelimited,57 this is attributed to the high oxidation temperature and better thermal stability of Zr10. In a more realistic view, the gaining in kinetics at a high oxidation temperature may be more beneficial to efficiency than a higher productivity with a low reaction rate. Ce−Zr solutions further doped with low-valence ions were also investigated in expectation of better redox properties. Different from some results reported before,69−71 no improvement in fuel productivity was observed (see Figure S2 and Table S2). This is consistent with the analysis above. Further doping Ce−Zr with low-valence ions was shown to be effective on oxidation kinetics.57 However, it cannot be clearly distinguished here since only three cycles were conducted and the oxidation rates of all samples in the last cycle were very close to each other (see Figure S3). XRD results show

Figure 6. XRD patterns of pure ceria and 10 mol % M (M = Li, Ca, Pr, and Zr) doped ceria (a) after synthesis (600 °C, 2 h) and (b) after cycles (1500 °C).

was detected in the samples, indicating successful formation of solid solutions. Compared to XRD patterns before cycling, the peaks grow stronger after cycling, which means a growth of crystalline grains during the high temperature process. This could be confirmed from the SEM images (see Figure S1), in which significant sintering could be seen for materials after cycles. Crystallite size of different samples after synthesis (600 °C) and thermochemical cycles (1500 °C) were calculated from Scherrer equation based on the strongest peak (1 1 1). The results were listed in Table 1. Zr doped ceria has a smaller grain size, indicating better thermal stability. Figure 7 gives the transient oxidation rate of ceria and Zr10 samples in isothermal cycles at 1500 °C. Both peak and average Table 1. Crystallite Size of Different Samples after Synthesis (600°C) and Thermochemical Cycles (1500 °C)a before cycle after cycle

CeO2

Li10

Ca10

Pr10

Zr10

186 1519

200 1836

126 1301

185 1672

137 1109

a

The values were calculated from the Scherer equation based on the strongest peak (1 1 1). F


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Energy & Fuels

ature in isothermal cycles increases the energy efficiency. The maximum efficiencies of Zr10 at 1300 and 1400 °C are just comparable to that of pure ceria. Its advantage over pure ceria only could be observed when the oxidation is conducted longer than the optimal time. This is due to its slow oxidation rate, which traded off the benefits gained in productivity and reduction enthalpy by adding Zr. However, the drawback in kinetics may be turned off or at least alleviated when the isothermal cycle is conducted at 1500 °C. The maximum efficiency at 1500 °C was calculated to be 3.42% for Zr10, which is slightly higher than that of pure ceria. To get a better understanding of energy consumption, the fractions of all energy losing/storing terms when the oxidation is conducted the optimal oxidation time are shown in Figure 10. The lower

successful synthesis of solid solutions and close thermal stability (see Figure S4 and Table S3). 3.2. Near-Isothermal Cycle. Figure 8 gives the transient oxidation rate of pure ceria and Zr10 in near-isothermal cycles

Figure 8. Transient oxidation rate of pure ceria (solid) and Zr10 (open) in three near-isothermal cycles (1500 °C/1300 °C). Reduction was conducted at 1500 °C with pO2 = 7.8 Pa; 100 sccm CO2 mixed with 200 sccm Ar was used during oxidation. Insert: productivity of pure ceria and Zr10 in near-isothermal cycle (1500 °C/1300 °C). CO yields were averaged in three cycles and productivity in isothermal cycle at 1500 °C was also included for comparison.

Figure 10. Fractions of all energy losing/storing terms in isothermal cycles based on pure ceria and Zr10.

(1500 °C/1300 °C). Productivity at equilibrium was compared with that in isothermal cycles at 1500 °C. For both pure ceria and Zr10, the productivity almost doubled after introducing a small temperature swing (200 °C). Zr10 still exhibited a better performance in aspect of productivity. However, different from the isothermal cycle at 1500 °C, the oxidation rate of Zr10 was much lower than that of pure ceria due to a lower oxidation temperature. This could make Zr doping inefficient if a constant flow of CO2 is used in practice. 3.3. Efficiency. As illustrated before, a kinetic-based model was used to evaluate the efficiency of both isothermal and nearisothermal cycles. Here we go to isothermal cycles first. Figure 9 gives the efficiency of pure and 10% Zr doped ceria in isothermal cycles at different temperatures. Increasing temper-

Qox for Zr10 compared to pure ceria reveals the benefit in reduction enthalpy gained by doping Zr. Both Qrerad and Qsurf take considerable shares in energy cost. According to the model, they are temperature dependent and constant, respectively. Consistent with the analysis based on thermogravimetric results,37,38,41,43 even with gas heat recovery considered, the energy cost for heating CO2 to the oxidation temperature account for a large proportion of the solar energy input. This indicates the importance of efficient gas heat recovery. The efficiency of near-isothermal cycles is shown in Figure 11. For both pure and Zr doped ceria, the near-isothermal cycle even with no solid heat recovery showed higher efficiency than the isothermal cycle, which is consistent with the previous analysis.43,54 However, pure ceria is shown to be more efficient than Zr10 in the near-isothermal cycle (4.05% compared to 3.77%). This is attributed to the low oxidation rate of Zr10, which may cause larger CO2 consumption than pure ceria. Together with the efficiency of isothermal cycles, it could be seen that the efficiency comparison of pure and doped ceria is very different from theoretical results, in which Zr doped materials were predicted to be advantageous in most reaction modes and a large temperature swing between reduction and oxidation was suggested for them.38,72 The difference derives from involvement of the oxidation rate in the model. This is more realistic since usually the reactor is an open system and the reaction process is nonequilibrium.53 Since solid−solid heat recovery is very technically challenging, taking solid−solid heat recovery into calculation seems to make less sense in getting a practical view of the process efficiency. However, when the oxidizing gas flows into the reactor, it may be heated up by the redox material and the

Figure 9. Solar-to-fuel efficiency vs oxidation duration for pure ceria (solid) and Zr10 (open) in isothermal cycles at 1300, 1400, and 1500 °C. G


Article

Energy & Fuels

challenging problem, the low efficiency is mainly due to gas heat loss and low productivity. Future work in this area may be focused on these aspects. For example, coupling another redox cycle which works at a lower temperature, serving as an oxygen pump, is potentially capable to highly improve the reduction extent, thus the fuel productivity in both isotheral and nonisothermal cycles. What is more, the use of inert gas could be avoided, and the process heat of the H2O/CO2 splitting cycle may be used in the pump cycle, which is also beneficial to the system efficiency.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b03284. Figure S1. SEM images of pure ceria before (a) and after (b) thermochemical cycles and of Zr10 before (c) and after (d) thermochemical cycles. Figure S2. CO productivity of Zr10 and 2.5% M (M = Li, Ca, and Pr) doped Zr10 in isothermal cycles at 1300, 1400, and 1500 °C. CO yields were averaged in three cycles. Figure S3. Transient oxidation rate of Zr10 and 2.5% M (M = Li, Ca, and Pr) doped Zr10 in isothermal cycles at 1500 °C. Reduction were conducted at 1500 °C with pO2 = 7.8 Pa; 100 sccm CO2 mixed with 200 sccm Ar was used during oxidation. Only oxidation rate in the third cycle are shown here for simple comparison. Figure S4. XRD patterns of Zr10 and 2.5 mol % M (M = Li, Ca, Pr, and Zr) doped Zr10 (a) after synthesis (600 °C, 2 h) and (b) after cycles (1500 °C). Table S1. Polynomial fits of parameters involved in the paper. Table S2. CO yields in all conducted cycles. Table S3. Crystallite size of Zr10 and 2.5 mol % M (M = Li, Ca, and Pr) doped Zr10 after synthesis (600 °C) and thermochemical cycles (1500 °C). The values were calculated from the Scherer equation based on the strongest peak (1 1 1). (PDF)

Figure 11. Energy efficiency of CeO2 (solid) and Zr10 (open) in 1500 °C isothermal cycle and 1500 °C/1300 °C near-isothermal cycle. Efficiency of near-isothermal cycle with solid heat and oxidation heat recovered by further heating oxidizing gas (CO2) was also plotted and compared.

redox material could be slightly cooled at the same time. This solid−gas heat recovery seems to be more practical. The heat released during oxidation reaction may also be recovered this way. Thus, here we assume 80% of them to be recovered by further heating the preheated CO2. From the results it could be seen, due to lower oxidation rate, the Zr doped ceria still shows lower efficiency compared to pure ceria. Solar-to-fuel efficiency of ∼5% could be expected for pure ceria (CeO2_1500/1300_R in Figure 11).

4. CONCLUSIONS AND PERSPECTIVES In this work, different doping strategies for ceria were investigated in isothermal and near-isothermal cycles for solar fuel production. The system efficiency was evaluated by a much more realistic model with the whole oxidation process concerned. Compared to the other ions investigated here, Zr doped ceria exhibited the highest productivity under all of the reaction conditions investigated here. For the other doping ions (Li+, Ca2+, and Pr3+), only slight improvement of CO productivity could be observed when the temperature is relative low (1300 and 1400 °C). Further doping of Li+, Ca2+, and Pr3+ in Zr−Ce solid solution showed no improvement on both productivity and oxidation rate. In terms of fuel productivity, doping ions with lower valences seems to be a suboptimal strategy due to formation of inert oxygen vacancies. The efficiency comparison of pure and Zr doped ceria is different from theoretical prediction, and the influence of oxidation rate is highlighted. The advantage of higher productivity caused by adding Zr is traded off by its drawbacks in kinetics for isothermal cycles at 1300 and 1400 °C. Benefit in efficiency only could be observed in the isothermal cycle at 1500 °C. Compared to the isothermal cycle, the nearisothermal cycle with a small temperature swing showed improvement in both fuel productivity and efficiency. Pure ceria showed better performance than 10% Zr doped ceria in 1500 °C/1300 °C near-isothermal cycles. Above all, the efficiency predicted here is still much lower than the targeted efficiency for this approach of solar fuel production.42 It may be drawn that, with present technologies, solar fuel production in this path is still far from satisfying. Besides solid heat loss, which has been illustrated as a

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 86-029-82664345. Fax: 86-029-82669033. E-mail: yjlu@ mail.xjtu.edu.cn. ORCID

Liya Zhu: 0000-0002-0096-987X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Fundamental Research Funds for the Central Universities and the National Nature Science Foundation of China through Contract Nos. 51676158 and 91634109.

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NOMENCLATURE T = temperature Tred = reduction temperature Tox = oxidation temperature px = partial pressure of gas species, x = O2, CO2, and CO cp,x = heat capacity of species, x = CO2, CeO2, and Zr10 nx = molar amount of species, x = CO, CO2, O2, CeO2, and Zr10 DOI: 10.1021/acs.energyfuels.7b03284 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels ṅCO2 = molar flow rate of CO2 p0 = normal pressure δ = oxygen nonstoichiometry δox = oxygen nonstoichiometry after oxidation δred = oxygen nonstoichiometry after reduction Δδeq = oxygen nonstoichiometry difference between oxidation and reduction at equilibrium nmCO,eq = CO productivity at equilibrium D = grain size K = dimensionless shape factor λ = X-ray wavelength β = line broadening at half the maximum intensity θ = Bragg angle MCeO2 = molar mass of ceria HHVCO = high heat value of CO α = reaction extent Qfuel = energy stored in fuels Qsolar = overall solar energy cost Qrerad = reradiation loss Qpump = pump consumption Qsurf = surface loss Qmat = solid sensible loss QCO2 = gas heat loss caused by heating CO2 Qox = reaction heat released during oxidation Qred = energy stored during reduction Wideal = energy consumption for a Wreal = real pump consumption pvac = vacuum pressure ηfuel = solar-to-fuel efficiency ηabs = absorption efficiency of the reactor ηpump = pump efficiency ηelec = thermal-to-electricity efficiency F = surface loss factor εgas = gas heat recovery factor εsr = recovery factor of solid heat and oxidation heat A = absorptivity of reactor σ = Stefan−Boltzmann constant G = nominal solar flux incident C = solar concentration ratio R = gas constant Δh(δ) = partial molar enthalpy of reduction

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Reactivity and efficiency of ceria-based oxides for solar CO2 splitting

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