## 2 Materials and Methods  
In the present work, the tape calendering method was utilised to prepare the entire PCFC, which was then sintered in a single step. In order to achieve successful results, it was necessary to take several factors into consideration: (a) the electrolyte must be fully densified, while the electrodes should have a sufficient porosity; (b) all the functional materials need to exhibit similar thermomechanical properties; (c) no significant chemical interactions should occur at the anode/electrolyte and electrolyte/cathode interfaces. Taking these limitations into account, BaCe\({}_{0.5}\)Zr\({}_{0.3}\)Dy\({}_{0.2}\)O\({}_{3-8}\) + 0.5 wt% CuO (BCZD) was selected as a proton-conducting electrolyte [28; 32], while Pr\({}_{1.9}\)Ba\({}_{0.1}\)NiO\({}_{4+5}\) (PBN) was used to form a cathode [29]. In detail, BCZD is characterised by good protonic conductivity levels, coupled with a high chemical stability in highly moistursised atmospheres. A small amount of copper oxide was added as an aid [33; 34] to promote BCZD sinterability. PBN was selected due to its good chemical and thermal compatibilities with BCZD, even at 1350 \({}^{\circ}\)C [29]. The PCFC was fabricated according to the principal scheme presented in Figure 1. First, four individual powders were prepared for the corresponding functional layers:  
1. An anode supporting layer (ASL) was prepared by thorough the mixing of the NiO, BCZD and pore former (starch) powders in a weight ratio of 60:40:20;
2. The anode functional layer (AFL) consisted of NiO and BCZD (without starch) powders mixed in a weight ratio of 55:45;
3. An electrolyte layer (EL) was formed using the BCZD powder only;
4. A cathode functional layer (CFL) was prepared by thorough mixing of the PBN, BCZD and starch in a weight ratio of 70:30:10. The second stage included the preparation of mixtures of the corresponding powders in an organic binder (components and their concentrations are provided elsewhere [30]). These mixtures were dried overnight to remove evaporating solvents (acetone, benzene). The dried residues were rolled using a Durtson rolling mill to fabricate the corresponding film layers with the required thicknesses. Since the minimal gap between the rolls was \(\sim\)100 \(\upmu\)m, the PCFC with a 30 \(\upmu\)m-thick electrolyte was prepared by rolling the 100 \(\upmu\)m-thick films several times: first EL with CFL, and then the obtained film with AFL to produce the 100 \(\upmu\)m-thick structure AFL]EL[CFL. The green ASL]AFL]EL]CFL cell was prepared by co-rolling the latter film with a 500 \(\upmu\)m-thick ASL film using a larger roll gap. Finally, the green cell was co-sintered according to the following regime: heating up to 600 \({}^{\circ}\)C with a heating rate of 1 \({}^{\circ}\)C min\({}^{-1}\), heating up to 1350 \({}^{\circ}\)C with a heating rate of 3 \({}^{\circ}\)C min\({}^{-1}\), isothermal soaking at 1350 \({}^{\circ}\)C for 5 h, and cooling down to room temperature with a cooling rate of 3 \({}^{\circ}\)C min\({}^{-1}\). Scanning electron microscopy (SEM, TSCAN MIRA 3 LMU microscope, TESCAN, Czech Republic) coupled with energy-dispersive X-ray spectroscopy (EDX, INCA Energy 350 microanalysis system, Oxford Instruments Analytical, UK) was used to analyse the microstructural parameters of the PCFC. The fabricated PCFC was electrochemically characterised utilising a complex of Amel 2550 potentiostat/galvanostat (Amel, Italy) and a Materials M520 frequency material response analyser (Amel, Italy) to obtain data on volt-ampere and impedance dependences. The impedance spectra were collected in a frequency range of 3-10\({}^{4}\)-5-10\({}^{-2}\) Hz with an acceleration voltage of 30 mV and a cell bias corresponding to open circuit voltage (OCV) or maximal power density (P\({}_{\text{max}}\)) conditions. Humid air with water vapour partial pressure (pH\({}_{2}\)O) of 0.30 atm was used as an oxidant, while wet hydrogen (pH\({}_{2}\)O = 0.03 atm) was used as a fuel.