Carbonate dual-phase improves the performance of single-layer fuel cell made from mixed ionic and semiconductor composite CURRENT

A mixed ionic and semiconducting composite in a single-layer configuration has been shown to work as a fuel cell at a lower temperature (500-600 oC) than a traditional solid-oxide fuel cell. The performance of a single-layer fuel cell (SLFC) is often limited by high resistive losses. Here, an eutectic mixture of alkali-carbonates was added to SLFC to improve the ionic conductivity. The dual-phase composite ionic conductor consisted of a ternary carbonate (sodium lithium potassium carbonate, NLKC) mixed with gadolinium-doped cerium oxide (GDC). Lithium nickel zinc oxide (LNZ) was used as the semiconducting material. The LNZ-GDC-NLKC SLFC reached a high power density, 582 mW/cm2 (conductivity 0.22 S/cm) at 600 °C, which is more than 30 times better than without the carbonate. The best results were obtained with the ternary carbonate which decreased the ohmic losses of the cell by more than 95%, whereas the SLFC with a binary carbonate (sodium lithium carbonate, NLC) showed a lower conductivity and performance (243 mW/cm2, 0.17 S/cm at 600°C). It is concluded that adding carbonates to LNZ-GDC will improve the ionic conductivity and positively contribute to the cell performance. These results suggest a potential path for further development of SLFCs, but also imply the need for efforts on up-scaling and stability to produce practical applications with SLFC.


Introduction
Single-layer fuel cell (SLFC) is an intriguing innovation that consists of one homogenous mixed layer of an ionic conductor and a semiconductor. It has been shown to include all the necessary functionalities of a fuel cell in this single-layer [ whereas conventional fuel cells, such as solid oxide fuel cells (SOFCs), require a threelayer structure with two electrodes and an electrolyte [9][10] [11]. The SLFC utilizes 3 nanocomposite materials and nano-redox reactions [1] [2] [3] and may operate below 600 o C, making it easier to fabricate and less prone to mechanical stresses. Also, the singlelayer structure avoids interfacial resistances and chemical and thermal mismatches between the layers. Thus, the issues related to e.g. interfacial loss factors [12] are avoided. Schematics showing the structures and working principles of a three-layer fuel cell (SOFC as an example) and a SLFC are presented in Figure 1. Working principle of a SLFC (b). Electrode reactions are also shown. Gas and particle flow directions are marked with arrows.
In a recent work [7], the performance of a SLFC with LNZ-GDC material was enhanced from 357 mW/cm 2 to 801 mW/cm 2 at 550 o C, when a non-catalytic Au current collector was changed to catalytically active NCAL-coated Ni-foam. In that context it was found out that H + dominated the ionic charge transfer process, while O 2ions provided a minor contribution only. However, due to catalytic activity of NCAL, it is possible that the NCALcoated Ni-foams acted as electrodes. Indeed, in [32] and [33] devices with a semiconductor -ionic conductor layer sandwiched between two NCAL-coated Ni-foams were described as three-layer fuel cells without a traditional electrolyte.
To improve the performance of a SLFC, the resistive losses of the cell need to reduced.
Outgoing from the definition of the surface specific resistance R ~ s/L, where s is the conductivity and L the thickness of the SLFC, improving the conductivity or reducing the thickness of the layer will reduce the resistance of the cell. In this study, the conductivity was improved by adding alkali carbonates to GDC, creating a dual-phase ionic conductor.
Alkali carbonates are known to show good ionic conductivity (both for O 2and H + ions) above the carbonate melting point [34] [35].
The components in the composite may provide different ionic conduction pathways. The interface between the ceria and carbonate phases has been thought to provide a conducting pathway for O 2- [36] [37]. The SDC phase without carbonate shows negligible protonic (H + ) conductivity [37]. Adding (Li 0.52 Na 0.48 ) 2 CO 3 carbonate, the proton conductivity increases above the melting point of the carbonate [38]. Even though the conducting path for O 2and H + in the dual-phase composite electrolytes is not conclusive, the incorporation of the carbonate into the ceramic oxide indeed leads to rising ionic conductivity above the carbonate melting point [36] [39]. In addition, the carbonates contribute to balancing the electronic and ionic conductivity in SLFC [28] [29]. In the SLFC field, typically a single carbonate, Na 2 CO 3 , has been added to the doped ceria [8][14] [15] [16] [28]. Previous experiments with three-layer fuel cells have reported high ionic conductivities for binary (>0.4 S/cm at 600 o C) [40] and ternary (>0.5 S/cm at 600 o C) carbonates [41]. Thus investigating the effect of these carbonate mixtures to the SLFC performance deserves attention.
The aim of this study was to verify the benefits of applying eutectic binary and ternary alkali carbonates to SLFC in the temperarture range of 500-600 o C. The SLFC configuration utilized LNZ as a catalytic material, chosen based on previous studies and the simplicity of its synthetizing procedure [6][7] [14]. The ionic conductor was based on GDC to which a ternary carbonate (Na 2 CO 3 :Li 2 CO 3 :K 2 CO 3 , NLKC) or a binary carbonate (Na 2 CO 3 :Li 2 CO 3 , NLC) was added. The carbonates were eutectic mixtures, i.e. the mass ratios were chosen so that the melting temperature was minimized. Afterwards, the pellets were sintered at 700 °C for 2 h. Gold paste was deposited on the pellets for improved current collection. The thickness of the LNZ-GDC-NLKC pellets (10 samples) was about 1mm.
For the LNZ-GDC-NLC cells, a modified recipe was used. Ball milling was done at 250 rpm for 60 min in acetone, weight ratio of GDC and eutectic NLC (57:43 w-% Na 2 CO 3 :Li 2 CO 3 , melting point 497 °C [40]) was 3:1, calcination was done for 60 min at 350 °C and 600 °C for GDC-NLC and LNZ-GDC-NLC respectively and the pellet pressing time was 5 min.  X-ray diffraction (XRD) was used for phase identification and grain size analysis of the powder, fresh pellet, and aged pellet. Rigaku SmartLab X-ray Diffractometer, equipped with a 9 kW rotating Cu anode (0.154 nm) and a HyPix-3000 2D single photon counting detector in 1D mode was used in parallel beam mode with a monochromator in θ/2θ scan geometry. High-temperature XRD characterization was done with Rikagu SmartLab X-ray diffractometer and Ge-220 monochromator. Scan rate was 6°/min for all these measurements. Prior to measurements, the powder samples were put on a Si-plate and smoothened by pressing gently from above with another Si-plate.

3.1.
Structure and morphology

3.2.
Electrochemical test with ternary carbonate Figure 6a shows the electrochemical performance of a SLFC at different temperatures with H 2 and air as fuel and oxidant respectively. Due to the mixed electronic and ionic conductive characteristic of the single layer material, it is not disclosed that some loss in the OCV is due to electronic short-circuiting [28] [42]. An OCV of 0.75 V and maximum power density (P m ax ) of 352 mW/cm 2 were obtained for the best test cell at 500 °C. P m ax increased with temperature, e.g. at 550 °C P m ax = 510 mW/cm 2 , which is much higher than the respective value for pure LNZ-GDC material (shown in Figure S1). The main reason for this improvement is attributed to the enhanced ionic conductivity from the carbonate, which results in decreased ohmic losses in the cell. At 600 °C, P m ax reached 582 mW/cm 2 , which is higher than results reported in the literature including 150 mW/cm 2 at 650 °C [28], 312 mW/cm 2 at 550 °C [43] and 512 mW/cm 2 at 600 °C [44]. The OCV is around 0.9 V at 600 °C and, contrary to expected behavior of OCV vis-à-vis temperature, the OCV increased here with increasing temperature. The same phenomenon has been observed in other studies with SLFC, e.g. with Ce 0.8 Sm 0.2 O 2-δ (SDC)-Na 2 CO 3 and Sr 2 Fe 1.5 Mo 0.5 O 6-δ (SFM) [28]. The reason for this behavior could be in the enhancement of electronic and ionic conductivities, the catalytic activity of the functional electrodes and the carbonate in the SLFC. A preliminary study shows that the difference between the ionic conductivity and the electronic conductivity increases with increasing temperature (shown in Figure S2 and S3 in the Supplementary Information), which perhaps reduces the electronic losses in the cell, resulting in a higher OCV. A more systematic study would be necessary to explain the cause in detail, but it was outside the scope of the present study.   [48]. In this study, the focus was on the ionic conductivity of the SLFC and thus the quantitative EIS analysis is limited to the ohmic resistance. At 500 °C, the ohmic resistance of the pellet was 0.22 Ω cm 2 , which is much lower than that of the LNZ-GDC material (approx. 0.5 Ω cm 2 at 550 °C [7]) due to the enhanced ionic conductivity of adding the carbonate. When the temperature was increased to 600 °C, the ohmic resistance was reduced further to 0.14 Ω cm 2 since the ionic conductivity increases with temperature. The electrode polarization resistance decreased slightly when increasing the temperature from 500 °C to 600 °C. The polarization losses were reasonably low.
As LNZ-GDC-NLKC is a mixture of electronic and ionic conductive materials, it is difficult to separate the ionic and electronic conductivity strictly in this mixed material. As shown in The combination of LNZ-GDC and carbonates balanced the ratio of electronic and ionic conductivity reflecting positively to the performance. The balance of ionic and electronic conductivity by adding a material with higher ionic conductivity alleviates the energy wasted by short circuiting [28].
A common concern with SLFC is its long-term stability [49]. A 10-h test of the LNZ-GDC-NLKC cell was performed at OCV at 600 °C to test the new material combination. Indeed the OCV dropped from its initial value, 0.92 V, to 0.72 V after 7 hours after which it stabilized. Meanwhile, the ohmic resistance of the cell increased from 0.15 to 0.22 Ω cm 2 Based on the XRD in Figure 4 and 5 the reason for the performance reduction is probably not in the chemical interaction between LNZ, GDC, and carbonates, but in the instability of the amorphous carbonate, which is in molten state at this temperature [40].
A long-term test was outside the scope of the present study, but would be beneficial to identify other possible sources for degradation and mechanical stability, as we observed cell cracking in some cells during thermal cycling. All the measurements reported here were done in stable conditions.

Test with binary carbonate
The effect of the ionic conductor composition was further investigated by changing the ternary to a binary carbonate (Na 2 CO 3 :Li 2 CO 3 , NLC) and reducing the carbonate content of the ionic conductor from 30 w-% to 25 w-%. Similar 3:1 GDC-NLC mixture was reported as an electrolyte in a three-layer fuel cell [40]. shown to be faster than linear when the pellet thickness is increased [7]. Increased thickness and reduced ionic conductivity together can explain the lower performance of the LNZ-GDC-NLC cell when compared to the LNZ-GDC-NLKC cell. However, it should be noted that there may be some other factors resulting from the manufacturing procedures that have a minor contribution to the results obtained.  The authors have no competing interests.

Funding
The research work was primarily supported by Academy of Finland (Grant No. 13279204).
China Scholarship Council provided financial support through a scholarship. Hubei Talent 100 programme and Academy of Finland (Grant No. 329016) also supported the work.

Availability of data and materials
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