Bioderived Calcite as Electrolyte for Solid Oxide Fuel Cells: A Strategy

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Bio-derived calcite as a novel electrolyte for solid oxide fuel cells: A strategy toward utilization of waste shells Yixiao Cai, Chen Xia, Baoyuan Wang, Wei Zhang, Yi Wang, and Bin Zhu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02406 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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ACS Sustainable Chemistry & Engineering

Bio-derived Calcite as a Novel Electrolyte for Solid Oxide Fuel Cells: A Strategy Toward Utilization of Waste Shells Yixiao Cai1,2, Chen Xia3, Baoyuan Wang2*, Wei Zhang2, Yi Wang4, Bin Zhu2,3* 1

NUS Environmental Research Institute, National University of Singapore, 1 Create Way,

Singapore 138602, Singapore 2

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Faculty

of Physics and Electronic Science, Hubei University, Wuhan, Hubei 430062, P.R. China 3

Department of Energy Technology, Royal Institute of Technology (KTH), Stockholm,

SE-10044, Sweden 4

Stuttgart Center for Electron Microscopy (StEM), Max-Planck-Institute for Solid State

Research (MPI-FKF), Heisenbergstr. 1, 70569 Stuttgart, Germany *Correspondence to: B. Zhu ([email protected]), B. Wang ([email protected])

ACS Paragon Plus Environment


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ABSTRACT:

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The excessive consumption of synthesized materials and enhanced

environmental protection protocols necessitate the exploitation of desirable functionalities to handle our solid waste. Through a simple calcination and composite strategy, this work envisages the first application of bio-calcite derived from the waste of crayfish shells as an electrolyte for solid oxide fuel cells (SOFCs), which demonstrates encouraging performances within a low temperature range of 450–550 °C. The single cell device, assembled from calcined waste shells at 600 °C (CWS600), enables a peak power density of 166 mW cm-2 at 550 °C, and further renders 330 and 256 mW cm-2 after compositing with perovskite La0.6Sr0.4Co0.8Fe0.2O3-δ

(LSCF)

and

layer-structured

LiNi0.8Co0.15Al0.05O2

(LNCA),

respectively. Notably, an oxygen-ion blocking fuel cell is used to confirm the proton-conducting property of CWS600 associated electrolytes. The practical potential of the prepared fuel cells is also validated when the cell voltage of the cell is kept constant value over 10 hours during a galvanostatic operation using a CWS600-LSCF electrolyte. These interesting findings may increase the likelihood of transforming our solid municipal waste into electrochemical energy devices, and also importantly, provide an underlying approach for discovering novel electrolytes for low-temperature SOFCs. KEYWORDS: Waste shells, Fuel cells, Waste to energy conversion, Composite electrolyte INTRODUCTION Envisioned as a clean and sustainable technology to generate electricity at high efficiencies, solid oxide fuel cells (SOFCs) have shown promise for future power generation.1 A typical SOFC device commonly employs a centrally positioned electrolyte, such as yttria-stabilized zirconia (YSZ) or doped-ceria. However, the road to commercialization for SOFC technology is fraught with challenges, as YSZ requires high operating temperatures (800–1000 °C) or precisely controlled thin-film technology to sustain a sufficient conductivity value, while ceria-based electrolytes cause serious energy loss due to their additional electronic conduction in reduced atmospheres.2 To eliminate these hindrances, efforts have been made to excavate alternative electrolyte materials with desirable ionic conductivities at reduced temperatures. A composite electrolyte strategy has been proposed based on the assembly of heterogeneous materials, including doped ceria/carbonates,3–8


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doped ceria/semiconductor materials,9 proton conductor/semiconductor materials,10,11 and so forth. The developed composite electrolytes have shown tremendous potential as compared to conventional YSZ or doped ceria based electrolytes. Meanwhile, growing experimental evidence from these studies has proven that interfacial conduction plays a pivotal role in the resulting high ionic conductivities. This trend indeed facilitates material selection for SOFC technology and therefore, has encouraging prospects. Scientists and engineers constantly look to nature for inspiration to solve energy issues. Recent literature show that the use of natural materials enables effective conversion from thermal and chemical energies into electricity. Natural chalcopyrite12 and natural tetrahedrite13 are used as high-performance thermoelectric materials, while natural hematite ore is competitive for high-performing SOFCs.14–16 And likewise, a rare-earth mineral consisting of lanthanum, cerium, and praseodymium carbonates was reported to produce mixed rare-earth oxide electrolytes.17 Besides, through emerging nanoengineering methods, the nanostructured framework derived from marine and freshwater crustaceans can be employed as bio-inspired templates when constructing carbon based materials for different energy applications, including lithium-ion batteries,18,19 lithium-sulfur batteries,20 and super-capacitors,21,22 and so forth. These efforts spawned our interest in the exploitation of more natural materials for SOFC technologies and their roles in energy and chemical conversions. It has been reported that 6–8 million tonnes of waste crab, shrimp and lobster shell waste are produced globally.23 Yan et al.24,25 recently proposed the concept of “waste-shell-refinery”, which shows that each major component of shell waste could be transformed into a valuable chemical. Freshwater crayfish, also known as “little lobsters,” are a common source of food for human consumption. Crayfish exoskeletons comprise a hybrid composite structure of natural ceramic CaCO3/organic chitin.26 Because of the large specific surface areas, the high hydrophilicity, and biocompatible characteristics, nanosized CaCO3 (nanoCaCO3) has been used for enhancing the stability and catalytic activity of enzymes toward biosensing applications.27–29 Furthermore, to enhance the endurance of alkali carbonate electrolytes, the employment of alkaline-earth carbonates (e.g., CaCO3) as additives was considered as a



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common strategy in a molten-carbonate fuel cell (MCFC) system.30,31 Previous studies demonstrated that solid alkaline earth carbonates possess a certain amount of proton conduction.32–34 Considering this and inspired by the validity of a composite strategy toward low-temperature SOFCs, this work accesses the utility of bio-derived shell calcite. Such bio-derived calcite is prepared using a simple calcination process, before being incorporated with perovskite La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) and layer-structured LiNi0.8Co0.15Al0.05O2 (LNCA) to obtain composite materials. A series of fuel cells with symmetric geometry are then designed and evaluated within a low temperature range of 450–550 °C using the materials described above for the electrolyte layer and LiNi0.8Co0.15Al0.05O2 (LNCA) as the electrode material. Note that since the melting points in the carbonate phase are far above 450–550 °C,5,32 the electrolyte layers could be in a solid state during the operation. The results will serve as a prerequisite for the potential performance of our proposed fuel cell designs. It is anticipated that this work will provide an underlying strategy to develop novel cost-effective electrolytes for LT-SOFCs. EXPERIMENTAL SECTION Materials and Composite Fabrication Pristine shells were peeled from waste crayfish samples, which were obtained from a local marketplace (Wuhan, China). During pre-treatment, the remaining cooking oil and seasonings were carefully washed with acetone, ethanol and deionized water in the tank of an ultrasonic cleaner, before being desiccated naturally. The pristine samples were then broken into small pieces and thermally calcined at 300 °C, 600 °C, and 800 °C in air for 2 hours to obtain ash powders. This was done to investigate the effect of calcination temperatures on the structural and morphological changes. The resultant samples were ground thoroughly, and labeled as CWS300, CWS600 and CWS800 based on the temperatures listed above. The composite electrolyte samples were prepared via a wet-ball milling method. In brief, analytical

grade

La0.6Sr0.4Co0.8Fe0.2O3-δ

(LSCF,

Sigma-Aldrich,

USA)

and

LiNi0.8Co0.15Al0.05O2 (LNCA, Tianjin B&M Science and Technology Joint-Stock Co., Ltd, China) were obtained in a commercial form without further purification, and then mixed with


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the CWS600 powders at a weight ratio of 3:7. Ball milling of the composites was performed using a lab planetary ball mill (XQM-0.4 L) for 10 hours, during which ethanol was used as the dispersing medium. Finally, the resultant mixtures were sintered at 500 °C for 2 hours to obtain CWS600-LSCF and CWS600-LNCA composite samples. The sintered powders were ground adequately before being pressed into pellet form. Material Characterizations Inductively coupled plasma analysis optical emission spectroscopy (ICP-OES) was employed to detect metals and some nonmetals in calcined samples from waste shells. The crystal structure analyses were using a Bruker D8 Advanced X-ray diffractometer (XRD) with Cu Kα radiation (λ= 1.54060 Å) as the source, tube voltage of 40 kV, a tube current of 40 mA, and a count time of 0.2 s per 0.02° in the range of 10–90°. The thermal decomposition of the pristine waste shell and prepared composite materials were evaluated using thermos-gravimetric analysis/differential thermal analysis (TGA/DTA), which was conducted on a Shimadzu TGA-50 analyzer at a heating rate of 10 °C min-1 in air-flow of 50 ml min-1, ranging from an ambient temperature to 800 °C. The testing sample was kept on an alumina crucible during the TGA experiments. The particle morphology, elemental composition and internal structure were evaluated using a field emission scanning electron microscope (FE-SEM, JEOL JSM7100F, Japan) that was equipped with an energy dispersive spectrometer (EDS) that operated at 15 kV. The high resolution transmission electron microscopy (HRTEM), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and corresponding energy dispersive spectroscopy (EDS) mapping were performed using a JEOL JEM0ARF200F TEM/STEM with a spherical aberration corrector (Talos F00X, FEI Co., USA). Fuel Cell Construction The fuel cells were constructed by sandwiching CWS600, CWS600-LSCF and CWS600-LNCA between two pieces of Ni foam pasted with LNCA, and then dry-pressing them under a uniaxial load of 200 MPa to form disc cells that were designated as cell #1, cell #2 and cell #3, as shown in Scheme 1. The assembled cylindrical cells have the same



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symmetric configuration as LNCA-Ni/sample/LNCA-Ni with an active area of 0.64 cm2 and are 1.5 mm in thickness. All fuel cells went through an in situ pre-heating treatment at 560 °C for 1 hour before their performance was evaluated. Note that LNCA is an excellent electrode material with high conductivity (approximately 10 S cm-1 at 500–600 °C).35,36

Scheme 1. Schematic illustration of the formation process of proposed fuel cells. Electrochemical Measurements The fuel cells were operated at 450-550 °C with humidified hydrogen as fuel (120–140 ml min-1) and air flow as the oxidant (120 ml min-1). To evaluate their performance, the cell voltage and current were collected based on the programmable electronic load, which was measured by a computerized instrument (IT8511, ITECH Electrical Co., Ltd.). The electrochemical impedance spectra (EIS) were probed in a hydrogen/air atmosphere with silver electrodes using an electrochemical work station (Gamry Reference 3000, USA). The measurement was performed under open circuit voltage (OCV) conditions, and the applied frequency range was 1 M Hz to 0.1 Hz with a voltage amplitude of 10 mV. The results were further summarized and analyzed using commercial software (Z-View, Scribner Associates). RESULTS AND DISCUSSION The XRD pattern from the pristine waste shell sample (inset of Figure 1a) shows two sharp peaks at 2θ of 9.2° and 19.6°, which are typical of crystalline structure of chitin.26,37 In the pattern of CWS300, the absence of above peaks suggested the removal of chitin, whereas


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the CWS300 sample had two broader fringes, echoing the common features for amorphous calcium carbonate 38. The XRD of CWS600 revealed a well-crystallized structure, as verified by the occurrence of intense and narrow peaks. The strongest diffraction intensity of the CWS600 was at peak (104), which is attributed to calcite (JCPDS File No. 29-0306), the most stable polymorph of calcium carbonate.39 In the pattern of CWS800, major 2θ peaks of CWS800 appear at 32.2°, 37.3° and 53.9°, which correspond to the (111), (200), and (220) planes of CaO (JCPDS File No. 82-1691). This authenticates the transformation of the waste shell sample from CaCO3 to CaO at 800 °C. Additionally, as seen in Figure 1b, the patterns of CWS600-LSCF and CWS600-LNCA reveal complete peaks from each phase, thus indicating that there is no chemical interaction during the mixing and sintering processes of composites. Table S1 shows the ICP results. Ca and C were found as the major constituents for CWS600, whereas the C content significantly decreases in case of CWS800. This is consistent with the XRD results. Besides, trace metals including K, Mg, Sr, Ba, Al, Cu, Zn, Mn, Ni, Co and Cr were detected in both samples and there were no certified values for Pt, Pd and Ag. The TGA/DTA profiles were investigated under a flowing air environment at room temperatures up to 800 °C and a heating rate of 10 °C min-1. For the pristine waste shell samples, there are four weight-loss stages during the entire heating process up to 800 °C, as shown in Figure 1c. Initially, the weight loss reflected the evaporation of the remaining water. The further two losses occurred between 250–550 °C with a significant weight loss of 16.7%, and an exothermic peak centered at 300 °C on the DTG curve that corresponds to the combustion of the organic matrix, especially the chitin

40,41

. The sharp weight loss of 23.6%

began near 600 °C and continued until 750 °C, which corresponded to the decomposition of CaCO3. When the calcination temperature was above 750 °C, the weight loss appeared to be almost constant. Meanwhile, the weight loss of CWS600-LSCF and CWS600-LNCA was not detected in the cell operational temperature range of 450-550 °C as indicated in Figure 1d, which implies that the composite electrolyte layer would exist steadily within the operational temperature range. It was emphasized previously that the stability of composite electrolyte materials was of paramount importance to ensure a durable fuel cell performance.42



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Figure 1. XRD patterns of (a) the pristine sample and samples at different calcination temperatures; (b) LSCF, LNCA, the as-obtained CWS600-LSCF, and CWS600-LNCA samples. TGA/DTA profiles of (c) the pristine sample and (d) the as-obtained CWS600-LSCF and CWS600-LNCA samples. SEM was used to observe the temperature-induced microstructural transitions. Figure 2a shows that the pristine waste shell had an interlaced multi-layered architecture. When the calcination temperature was 300 °C, the emergence of ellipse-type macropores revealed that partial organic substances (e.g., chitin) was removed from the surface. Interestingly, a further increase in the calcination temperature altered the intrinsic architecture of the crayfish shells, as the CWS600 sample showed a good distribution of crystallized particles, while the CWS800 sample appeared to be made up of more condensed particles with finer distributions. Moreover, the resultant CWS600-LSCF and CWS600-LNCA composite samples were found to be heterogeneous and consisted of two phases. The detailed microstructure of the CWS600-LSCF sample was investigated by TEM. The mapping result shown in Figure 2g confirmed all the elements including Ca, O, C, La, Sr, Co and Fe, which is in good agreement


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with XRD results, and demonstrated the interfacial region between the CWS600 and LSCF particles.

Figure 2. SEM images of (a) a pristine sample from the waste crayfish shell and calcined samples at (b) 300, (c) 600, and (d) 800 °C; composite samples: (e) CWS600-LSCF and (f) CWS600-LNCA. (g) HAADF-STEM image of CWS600-LSCF and mapping images of the elements of Ca, O, C, La, Sr, Co, and Fe. Figure 3 shows the current-voltage (I-V) and current-power (I-P) curves. It can be observed that cell #1 (CWS600) reached an OCV of up to 0.98 V and a peak power density of 166 mW cm-2 at 550 °C. At this temperature, this performance is superior to a previously reported CaCO3 electrolyte-based SOFC

32

and comparable to a recently reported thin-film

electrolyte SOFC, which used a YSZ/GDC bi-layer.43 More encouragingly, after compositing



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with LNCA and LSCF, cell #2 (CWS600-LSCF) obtained an OCV of 1.02 V at 550 °C with an improved maximum power density of 330 mW cm-2. In the case of cell #3 (CWS600-LNCA), the OCV was above 1.01 V and the maximum power density was 256 mW cm-2. Quite significant here is the fact that CWS600-LSCF and CWS600-LNCA gained considerably enhanced ionic conductivities through the composite effect, leading to an improved fuel cell performance. In practice we also investigated lower temperature operations of these cells, showing cell #1 exhibited a sharply decreased power density at 500 °C and below. This is mainly due to the reduced ionic conductivity of CWS600 and subdued electrode catalytic activities of LNCA electrodes with the temperature decreases. However, cells #2 and #3 attained appreciable power outputs at lower temperatures, reaching 118 mW cm-2 and 150 mW cm-2 at 450 °C, respectively.

Figure 3. Current density-voltage and power density characteristics for (a) cell #1 (CWS600), (b) cell #2 (CWS600-LSCF), and (c) cell #3 (CWS600-LNCA). Based on the investigation described above, it is likely that the electrical properties of CWS600 played a key role in this mechanism. To test this hypothesis, a series of fuel cells were constructed with BaZr0.1Ce0.7Y0.2O3-δ (BZCY) in a unique configuration of LNCA /


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BZCY / testing material / BZCY / LNCA. The BZCY powders were prepared using a previous reported method.44 Figure S1 demonstrates the mapping results after operation, which clearly showed five layers in a BZCY / CWS600 / BZCY-based cell. Here, BZCY is known as a state-of-the-art perovskite proton conductor that only allows protons to be transported with negligible oxygen and electron conductivity.45,46 Such an engineered tri-layer membrane would, therefore, block electron and oxygen ions, whilst transporting protons dynamically. Figure 4b shows that the BZCY / CWS600 / BZCY-based cell exhibited a maximum power density of 129 mW cm-2 with an OCV of 1.02 V at 550 °C, while the cells assembled with CWS600-LSCF and CWS600-LNCA composites exhibited maximum power densities of 240 mW cm-2 and 200 mW cm-2 with an OCV of 1.16 V and 1.11 V, respectively. Comparatively, the testing oxygen-ion blocking cells exhibited slightly weaker electrochemical performances than cell #1, #2, and #3 in each case. This is chiefly because, with thicker tri-layer configuration, the centered electrolyte membrane would induce more ohmic losses as the tri-layer configuration becomes thicker.

Figure 4. (a) A cross-sectional SEM image of the prepared oxygen-ion blocking fuel cell and (b) I-V and I-P plots of the BZCY-associated proton-transport fuel cells at 550 °C. Figure 5a presents the EIS curves of CWS600, which were acquired in H2/air with a dewelling time of 30 min at each measuring point to stabilize the cell. The EIS of CWS600-LSCF and CWS600-LNCA obtained in the same atmosphere, as shown in Figure S2. For better comparison, Figure 5b plots the EIS curves of CWS600, CWS600-LSCF, and CWS600-LNCA tested at 550 °C. All impedance spectra consisted of a compressed



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semicircle in the high-medium frequency followed by a tail (partial arc) in the low frequency. The EIS curves are fitted with an empirical equivalent circuit model Rb(RgbCPEgb)(ReCPEe) (inset in Figure 5a), in which Rb, Rgb, Re and CPE stand for bulk resistance, grain boundary resistance, electrode polarization resistance, and constant phase element, respectively. The simulated results are summarized in Table S2. Normally, the different impedance arcs are allocated to the corresponding behavior based on their characteristic capacitances (C), which is determined from the following formula 47: 1/ ( CPEi − P )

 Ri ( CPEi − T )  Ci =  Ri

where the value of CPE-P indicates the similarity of constant phase element (CPE) to a true capacitor, and R denotes the value of resistance which is in parallel with the CPE. As a result, the intercept of high frequency impedance arc of the EIS on the real axis represents the bulk resistance; the semi-circle located at intermediate frequencies is attributed to the grain-boundary behavior; while the arc at low frequencies represents the electrode polarization behavior according to their characteristic capacitances.36,48,49 As seen in Figure 5a, the Rb of the pellet remained unchanged as the temperature increased, while the Rgb decreased noticeably. This was because the ability of ions to migrate at grain boundaries is thermally activated at grain boundaries. The intuitive comparison in Figure 5b reveals that the EIS for CWS600-LSCF and CWS600-LNCA have significantly reduced intercepts (at high frequencies) and shrunken semicircles, compared to that of CWS600. This is an indication of both decreased Rb and Rgb for the two composite samples. As shown in Table S2, the Rb of CWS600 varied slightly with elevated temperatures, and the Rgb decreased from 10.656 Ω cm2 to 2.810 Ω cm2. With respect to CWS600-LSCF and CWS600-LNCA, their Rb and Rgb exhibit smaller values as compared to CWS600. This is attributed to the introduction of highly electrically conductive LSCF and LNCA to the composite materials. Furthermore, the electrode polarization resistances (Re) of cell #2 and cell #3 are less than that of cell #1, thus reinforcing the electrochemical performance of the fuel cell. According to the fitting results, the electrical conductivity (σ) was calculated by the


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following relation: σ =

L where L is the thickness of the pellet, S denotes the effective R×S ,

area, and R represents the total resistance including the bulk resistance and grain boundary contribution. Figure 5c represents the temperature dependence of the electrical conductivity within the temperature range of 450–550 °C. As a result, the calculated ionic conductivity of CWS600 is 0.0036 S cm-1 at 500 °C, reaching as high as 0.0126 S cm-1 at 550 °C. This conducting performance is comparable to that of typical oxygen ion electrolyte YSZ and proton electrolyte BZCY 50. With the addition of electronic conduction, CWS600-LSCF and CWS600-LNCA reveal enhanced conductivity of 0.143 and 0.062 S cm-1 at 550 °C, respectively.

Figure 5. (a) EIS of CWS600 acquired under H2/air atmosphere and corresponding equivalent circuit; (b) EIS of CWS600, CWS600-LSCF and CWS600-LNCA under H2/air atmosphere at 550°C and (c) corresponding conductivity derived from EIS.



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It is commonly recognized that hydrogen bonding is involved in almost all species of proton conductors, mainly because it is considered a precursor for proton transfer reactions.51 Furthermore, long-range proton transport requires dynamic hydrogen bond rearrangements, including rapid bond breaking and forming processes that are expected to occur in a meta-stable hydrogen bonded system.52 Considering this, we propose an empirical speculation to interpret the proton conduction process in our CWS600 based fuel cell, as illustrated in Scheme 2. In

the initial stage, when protons approach the electrolyte layer from the anode, it may develop meta-stable hydrogen bonds with oxygen ions from CO32- in a type of transitional HCO3− state. As the operational temperature arises to 450–550 °C, the mobility and rotation of the CO32- are activated, thus accelerating the breaking and formation of the hydrogen bonds with CO32- in close proximity. In this manner, the protons can transport via the temporal bonding of H + + CO 32- → HCO

3

and HCO 3- → H + + CO 32 - . This leads to an effective proton

transportation that is driven by the established hydrogen concentration gradient in the device. Under this circumstance, CO32- on the surface of CWS600 could serve as a “stepping-stone” for protons to hop from one CO32- to another, thereby facilitating proton transport. After reaching the cathode, the protons meet the O2- and finally generate H2O to complete the fuel cell reaction.


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Scheme 2. The speculated ionic transport pathways for the proposed CWS600-based fuel cells. This paper notes that the anodic LNCA can be reduced to a Co-Ni alloy in an H2 atmosphere; by introducing semiconductors (p-type) LSCF and LNCA into the electrolyte, the metal can generate a Co-Ni/semiconductor Schottky junction in the device.35,53 This leads to a built-in field in the junction directing from the metal to the semiconductor, which is able to prevent the electrons in H2 supply side from passing through the junction.53 Concerning the improved fuel cell power outputs, the significant interface region was established between the two phases that may provide sufficient proton transport pathways, as shown in Figure 6. Moreover, benefiting from the mixed-conductive property of the composite materials, the trip phase boundary (TPB) of the electrode could be extended,54 thus promoting a reduction that reduces oxygen. This can be confirmed by the lower polarization resistances obtained by the CWS600-LSCF and CWS600-LNCA based cells compared with the CWS600-based cell.

Figure 6. (a) TEM image and (b) polycrystalline nature of the sample was confirmed by an electron diffraction (SAED) pattern in a selected area. (c) HRTEM image of the CWS600-LSCF sample. Durability is another important indicator to evaluate the utility of proposed fuel cells. The cells were operated under the galvanostatic mode with a current density of 80 mA cm-2, as illustrated in Figure 7a. The voltage of cell #1 decreased significantly after 1 hour of



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operation at 550 °C, while the operations of cell #2 were apparently more stable when the voltage was maintained at approximately 0.95 V during the whole operation over 10 hours. Since the electrode materials and cell architectures were identical in both testing cells, the difference can be ascribed mainly to the selection of electrolytes. One rational explanation is that the ionic conduction origin of the CWS600 could hardly sustain a continuous proton transport and sufficient electrode reaction during cell operation, while through interfacial conduction, CWS600-LSCF attained sufficient ionic conductivity and possessed better oxygen reduction reaction (ORR) catalysis to realize a relatively stable electrochemical performance. Next, we examined the integrity of the best-performing fuel cell after 10 hours of operation. An SEM cross-sectional image (Figure 7b) of cell #2 after 10 hour operation showed no signs of evident pores or cracks in the CWS600-LSCF layer. Figure 7c shows the EDX elemental mapping of the anode area. The electrolyte/cathode interface was clear and uniform for each element, which suggested there was no element migration or segregation. Overall, these results indicated that the calcined crayfish shells could be utilized as a feasible SOFC electrolyte material via a composite strategy.

Figure 7. (a) Durability comparison of fuel cells over 10 hours of continuous operation; (b) a


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best-performing single cell #2 in hand and corresponding SEM cross-sectional image after stability test and (c) EDX elemental mapping. CONCLUSIONS This paper aimed to demonstrate a few interesting designs of fuel cells by adopting calcined waste crayfish shell as a viable electrolyte. This was the first report on the possibility of utilizing natural shells for fuel cell applications. Our investigation showed that the calcined crayfish waste sample exhibited a conductivity of 0.0126 S cm-1 at 550 °C, and reached a peak power density of 166 mW cm-2 at 550 °C for the corresponding cell. We managed to promote cell performance through a composite strategy with semiconducting materials, showing that CWS600-LSCF and CWS600-LNCA-based fuel cells demonstrated 330 mW cm-2 and 256 mW cm-2 at 550 °C, respectively. Also importantly, the proton-conducting property of the electrolytes was verified using an oxygen-ion blocking fuel cell. These findings may lead to a greater number of practical waste-to-energy technologies, and other interesting properties could be anticipated in new composites through the judicious introduction of different functional materials. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. ICP-OES; EDS mapping images; EIS plots and corresponding fitting parameters; and conductivities. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions #

Y.C. and C.X. contributed equally to this work.



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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The financial support provided for this work by the Natural Science Foundation of Hubei Province (Grant No. 2015CFA120) and Hubei Provincial 100-Talent Distinguished Professor Grant are gratefully appreciated. REFERENCES (1)

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Table of Contents (TOC)

Bio-derived calcite from waste crayfish shells could be utilized to fabricate a series of novel solid oxide fuel cells, providing a sustainable strategy for recycling waste shell based materials.


Bioderived Calcite as Electrolyte for Solid Oxide Fuel Cells: A Strategy

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