Carbonate dual-phase improves the ionic conductivity and performance of mixed ionic and semiconductor single-layer fuel cell CURRENT STATUS: UNDER REVIEW

A mixed ionic and semiconducting composite in a single-layer configuration has been shown to work as a fuel cell at lower temperature (500-600 o C) than a traditional solid-oxide fuel cell. The performance of such a single-layer fuel cell (SLFC) is often limited by high resistive losses. Here, an eutectic mixture of alkali-carbonates were added to SLFC to improve the ionic transport. The dual-phase composite consisted of ternary carbonate (sodium lithium potassium carbonate) with gadolinium-doped cerium oxide (GDC) as the ionic conductor and lithium nickel zinc oxide as the semiconducting material. The SLFC prepared reached a high power density up to 582 mW/cm 2 (0.22 Scm -1 ) 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 the ohmic losses of the cell by more than 95%, whereas a binary carbonate (sodium lithium carbonate) showed a lower conductivity and performance (243 mW/cm 2 , 0.17 Scm -1 at 600°C). Adding carbonates to LNZ-GDC balanced the ratio of electronic and ionic conductivity which positively affected the performance.


Introduction
The single-layer fuel cell (SLFC) is a kind of solid-oxide fuel cell (SOFC) composed of a homogeneous layer of ionic conductor and semiconducting materials. It has been shown to include all necessary functionalities of a fuel cell in one single layer contrary to the conventional 3-layer fuel cell [1][2][3][4][5][6][7][8], which needs two electrodes and an electrolyte with different material compositions. The SLFC utilizes 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, a SLFC may reduce the interfacial resistance and avoid chemical and thermal mismatch of electrolyte and electrode materials [4]. A schematic of the principle of SLFC is presented in Figure 1.
The concept of SLFC was proposed by He et al. based on perovskite La 0.9 Sr 0.1 InO 3-d , but a modest power density of 3 mWcm -2 only was achieved due to low ionic conductivity (10 -2 Scm -1 ) [5]. The working mechanism of SLFC is likely related to creation of a heterojunction inside the material mixture by the oxides such as a p-n, Schottky, or bulk heterojunction, which blocks electron flow in the bulk material preventing short-circuiting and allowing ion conductance only [6][7] [8].
In a recent work [7], the performance of SLFC with LNZ-GDC was enhanced from 357 mWcm -2 to 801 mWcm -2 at 550 o C, when changing a non-catalytic Au current collector to catalytically active NCALcoated 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.
To improve the performance of 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 thickness of the SLFC, improving the conductivity or reducing the thickness of the layer would reduce the resistance of the cell. Here, we choose to investigate paths to improve the conductivity by adding a carbonate to the oxide phase to form a dual-composite material. Alkali-carbonates are known to show good ionic conductivity (both for O 2and H + ions) above the carbonate melting point [20] [21].
The components in the composite may provide different ionic pathways. The interface between the ceria and carbonate phase has been thought to provide a conducting pathway for O 2- [22] [23]. The SDC phase without carbonate shows negligible protonic (H + ) conductivity [24]. Adding (Li 0.52 Na 0.48 ) 2 CO 3 carbonate, the proton conductivity slightly increases above the melting point of the carbonate [25]. Even though the conducting path for O 2and H + in the dual-phase composite electrolytes is not conclusive, the incorporation of carbonate into the ceramic oxide indeed leads to rising ionic conductivity above the carbonate melting point. This conclusion has been verified by detecting water in both anode and cathode gas outlets [26]. In addition, a carbonate contributes to a balanced electronic and ionic conductivity in SLFC [15] [16]. Previous work has reported high ionic conductivities for binary (>0.4 Scm -1 at 600 o C) and ternary (>0.5 Scm -1 at 600 o C) carbonates [27], which indicate potential for their use in LNZ-GDC as well.
The aim of this study was to verify the benefits of eutectic binary and ternary alkali-carbonates for SLFC in the temperarture range 500-600 o C. The reference SLFC consisted of LNZ as catalytic material and GDC as ionic conductor to which a ternary carbonate (Na 2 CO 3 :Li 2 CO 3 :K 2 CO 3 , NLKC) or binary carbonate (Na 2 CO 3 :Li 2 CO 3 , NLC) was added. were prepared by pressing at 400 MPa for 2 min. 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 was 3:1, calcination was done for 60 min at 350 °C 5 and 600 °C for GDC-NLC and LNZ-GDC-NLC, respectively and the pellet pressing time was 5 min.

2.2.Electrochemical characterization
The cells were installed in a fuel cell reactor and heated to the operating temperature with a ramp of 5 °C/min. During the test, H 2 (70 ml/min) and air (200 ml/min) were supplied to two sides of the pellet as fuel and oxidant, respectively (unless otherwise stated). Both IV-curve and electrochemical impedance spectroscopy (EIS) measurements were done with Zahner Zennium impedance unit. The

2.3.Microstructural characterization
The SLFCs were characterized with scanning electron microscope (FE-SEM, Zeiss Sigma VP) to study the cross-sectional image and surface properties of the cell. Transmission electron microscope (TEM), JEOL JEM-2800, was used to image the nanosized powder.
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 from above with another Si-plate. XRD-spectra of the powder, fresh pellet, and aged pellet are shown in Figure 3 normalized to the GDC (111) peak intensity. Characteristic peaks of GDC and LNZ are clearly visible from the spectra (pellet spectra contains also Au from the current collector). The relative intensity of LNZ peaks compared to GDC peaks decrease, when the pellet is aged. The intensity of LiZnO peaks normalized to GDC intensity decreases more than those of LiNiO during aging.   [27,35]. Furthermore, the molten carbonate phase did not affect the structural stability of the LNZ and GDC crystal structures. Figure 5a shows the electrochemical performance of the 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 shortcircuiting [15] [28]. An OCV of 0.75 V and maximum power density (P max ) of 352 mW/cm 2 was obtained for the best test cell at 500 °C. P max increased with temperature, e.g. at 550 °C P max = 510 mWcm -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 max reached 582 mWcm -2 , which is much higher than results reported in literature such as 150 mWcm -2 at 650 °C [15], 312 mWcm -2 at 550 °C [29] and 512 mWcm -2 at 600 °C [30]. The OCV is around 0.9 V at 600 °C, which is slightly below the Nernst theoretical voltage [31]. 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-d (SDC)-Na 2 CO 3 and Sr 2 Fe 1.5 Mo 0.5 O 6-d (SFM) [15]. 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. Our 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, which was outside the scope of the present study.  [35]. At 500 °C, the ohmic resistance of the pellet was 0.217 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. Increasing the temperature to 600 °C, the ohmic resistance reduced further to 0.136 Ω cm 2 , as the ionic conductivity increases with temperature. The electrode polarization resistance decreased slightly when raising the temperature from 500 °C to 600 °C. The polarization losses were reasonably low.

8
As LNZ-GDC-NLKC is a mixture of electronic and ionic conductive materials it is difficult to separate between the ionic and electronic conductivity strictly in this mixed material. As shown in Figure 5 (c), the conductivity of the material increases both in air and in nitrogen atmosphere with increasing temperature. The electrical conductivity of the component in air is about 0.22 Scm -1 at 600 o C, lower than that of pure GDC-NLKC material (about 0.55 Scm -1 ) due to addition of LNZ. In a N 2 atmosphere, the conductivity (mainly electronic conductivity) is around 0.088 Scm -1 at 600 o C, which is beneficial to avoid short-circuiting. 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 higher ionic conductive material alleviates the energy wasted by short circuiting [15].
A common concern with SLFC is its long-term stability [37]. 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 values of 0.92 V to 0.72 V after 7 hours after which it stabilized. The drop of the OCV was directly attributed to the cell resistance which increased from 0.15 to 0.22 Ω cm 2 Based on the XRD in Figure 3 and 4, the reason for the performance reaction 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 [27]. 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 during thermal cycling. All the measurements reported here were done in stable cell conditions.

Test with binary carbonate
We further investigated the effects of carbonates by changing the ternary to a binary carbonate (Na 2 CO 3 :Li 2 CO 3 , NLC). The binary carbonate cell LNZ-GDC-NLC had lower content of carbonate (25 wt%) as compared to the LNZ-GDC-NLKC cell (30 wt%). Furthermore, the eutectic temperature of the binary carbonate material is higher than that of the ternary carbonate material, which resulted in a lower ionic conductivity as compared to a ternary carbonate material, and hence also to a lower fuel cell performance shown in Figure 6. The LNZ-GDC-NLC test cell was characterized at 550 °C and 600°C under H 2 and air atmospheres (atm1: 50 ml/min H 2 and 125 ml/min air; atm2: 100 ml/min both).
The highest power density of LNZ-GDC-NLC fuel cell was 243 mWcm -2 , which is less than half of that of the LNZ-GDC-NLKC (582 mWcm -2 ), supporting the hypothesis that the ionic conductivity of the cell material limits the cell performance. It should though be noted that the binary carbonate cell was slightly thicker (1.5 mm) than the ternary cell (1 mm), but this does not alone explain the large difference.

Conclusions
A single-layer fuel cell consisting of gadolinium-doped cerium oxide (GDC) as ionic conductor and lithium nickel zinc oxide (LNZ) as semiconducting material, and a ternary carbonate Na 2 CO 3 :Li 2 CO 3 :K 2 CO 3 (NLKC) as an ionic-conduction enhancing additive was successfully fabricated and characterized. The fuel cell device reaches a power density of 510 mWcm -2 at 550 °C and 582 mWcm -2 at 600 °C. Adding carbonates to the GDC, improved the ionic conductivity 15-fold and the power density by more than 30-fold. Replacing the NLKC with a binary carbonate Na 2 CO 3 :Li 2 CO 3 (NLC), however, did not enhance the performance which is explained by the higher melting temperarture of the eutectic mixture, but also confirming the positive role of a carbonate to improve the ionic conductivity and performance of a single-layer fuel cell.
The cells tests showed some initial degradation in performance, which seemed to stabilize. However, long-term stability tests in controlled environment would be recommended for future work to verify the practical usefulness of the carbonate-adding strategy. At the same time, investigating alternative strategies for improvement of the fuel cell such as using advanced fabrication techniques (e.g. Plasma Layer Deposition) to produce thinner and mechanically rigid cells could be worthwhile to pursue.

Competing interests
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
Please contact the corresponding authors for data requests.

Author's contributiona
S-J and Y-X performed the experiments. J-E helped with the analysis of the samples. W-L and AM-R contributed to the sample preparation. M-A was responsible for making the experimental design. P-L was responsible for commenting and reviewing the manuscript. All authors contributed to the 11 manuscript. All authors have read and approved the final manuscript.