## 3 Results and Discussion  
The electrode-supported PCFC fabricated via the tape calendering method followed by one-step sintering and was characterised in terms of its microstructure. According to Figure 2, the thicknesses of AFL, EL and CFL were estimated to be 40, 30 and 20 \(\upmu\)m, respectively; while the thickness of the entire PCFC was \(\sim\)600 \(\upmu\)m. The layers exhibit essential properties, including a dense state for the electrolyte and a porous structure for the functional cathode and anode; moreover, there is a good interface contact (adhesion) for both AFL/EL and EL/CFL couples. These results imply that the combination of the used methods, temperature regimes and material compositions was quite well optimised. Here we should stress that the present example of an asymmetric PCFC, co-sintered during a single stage, represents one of the initial reports, achieving successful results comparable to those obtained via conventional sintering at such high temperatures as 1350 \({}^{\circ}\)C. In other cases [26; 27], asymmetric-type PCFCs were  
Figure 1: Principal scheme of protonic ceramic fuel cells (PCFCs) fabricated by tape calendering method and one-step sintering. fabricated at lower sintering temperatures (below 1250 \({}^{\circ}\)C), which, as a rule, are insufficient for the required densification of Zr-containing (especially, Zr-enriched) BaCeO\({}_{3}\) materials. Figure 3 shows the volt-ampere (U-j) characteristics of the PCFC recorded under OCV conditions (when j = 0) to \(\sim\)100 mV with a voltage step of 25 mV. The high current density conditions, corresponding to a voltage range of 0-100 mV, were used in order to prevent significant heating of the cell. Analysing the free-current conditions, the cell generates OCVs varying from 1.040 V at 550 \({}^{\circ}\)C to 0.935 V at 700 \({}^{\circ}\)C. At low temperatures, these OCVs are very close to the theoretical values (1.056 V at 550 \({}^{\circ}\)C), but start to deviate from them (by \(\sim\)80 mV at 700 \({}^{\circ}\)C) at higher temperatures, indicating the electron transport of the BCZD membrane [35]. Here, the theoretical levels were estimated on the basis of a Nernstian equation for a pure protonic conductor (E\({}_{\rm H}\) = RT/2F\(\cdot\)In[p\({}^{\prime}\)H\({}_{2}\)/p\({}^{\prime\prime}\)H\({}_{2}\)]). The PCFC load results in the generation of P\({}_{\rm max}\), which increases from 290 mW cm\({}^{-2}\) at 550 \({}^{\circ}\)C to 470 mW cm\({}^{-2}\) at 700 \({}^{\circ}\)C.  
The EIS analysis was further utilised to reveal the main factors affecting the overall fuel cell performance. For this purpose, the impedance spectra were recorded in two conditions. One of these corresponds to the conventional measurements at OCV, while the other is related to realising P\({}_{\rm max}\). Despite the P\({}_{\rm max}\) conditions describing the real operation of a fuel cell, this function has yet to be analysed in detail in the literature. Figure 3: Volt–ampere and power density curves for the fabricated PCFC at different temperatures. Figure 2: Cross-section of the fabricated PCFC with the 30 \(\upmu\)m-thick BaCe\({}_{0.5}\)Zr\({}_{0.3}\)Dy\({}_{0.2}\)O\({}_{3-\delta}\) (BCZD) electrolyte and corresponding energy-dispersive X-ray spectroscopy (EDX) analysis. The EIS results presented in Figure 4 indicate that the spectra have different shapes depending on the measurement temperatures and/or cell bias. In detail, at least three parts (described by RQ elements, where R is the resistance, Q is the constant phase element) can be clearly distinguished at low temperatures; these merge to enable the detection of at least two (low- and high-frequency) processes at higher temperatures. Due to the revealed complexity, only the overall polarisation resistance (R\({}_{\rm p}\)) was calculated in order to analyse the electrochemical behaviour of the PCFC electrodes.