Development of Electrode-Supported Proton Conducting Solid Oxide Cells and their Evaluation as Electrochemical Hydrogen Pumps

Abstract

Protonic ceramic solid oxide cells (P-SOCs) have gained widespread attention due to their potential for operation in the temperature range of 300-500 °C, which is not only beneficial in terms of material stability but also offers unique possibilities from a thermodynamic point of view to realize a series of reactions. For instance, they are ideal for the production of synthetic fuels by hydrogenation of carbon dioxide and nitrogen, upgradation of hydrocarbons, or dehydrogenation reactions. However, the development of P-SOC is quite challenging because it requires a multifront optimization in terms of material synthesis and fabrication procedures. Herein, we report in detail a method to overcome various fabrication challenges for the development of efficient and robust electrode-supported P-SOCs (Ni-BCZY/BCZY/Ni-BCZY) based on a BaCe_0.2Zr_0.7Y_0.1O_3-δ (BCZY271) electrolyte. We examined the effect of pore formers on the porosity of the Ni-BCZY support electrode, various electrolyte deposition techniques (spray, spin, and vacuum-assisted), and thermal treatments for developing robust and flat half-cells. Half-cells containing a thin (10-12 μm) pinhole-free electrolyte layer were completed by a screen-printed Ni-BCZY electrode and evaluated as an electrochemical hydrogen pump to access the functionality. The P-SOCs are found to show a current density ranging from 150 to 525 mA cm^-2 at 1 V over an operating temperature range of 350-450 °C. The faradaic efficiency of the P-SOCs as well as their stability were also evaluated.

Keywords: Ni-BCZY electrode-supported cell; electrochemical hydrogen pump; pinhole-free BCZY electrolyte; protonic ceramic solid oxide cell (P-SOC); vacuum-assisted coating.

Figure 1

Fabrication process for an asymmetric protonic solid oxide cell (P-SOC) from precursor powders.

Figure 2

Proton conducting solid oxide cell test and evaluation setup.

Figure 3

SEM images of a NiO-BCZY271 support electrode presintered at 1300 °C for 5 h with different types of pore formers: (ai) surface of the electrode without any pore former, (bi) with Iotect as a pore former, (ci) with dextrin I as a pore former, (di) with dextrin II as a pore former, (ei) with Prolabo as a pore former, and (aii–eii) the fractured cross-section images of the same samples.

Figure 4

SEM images of the half-cells sintered for 10 h at a temperature of (a) 1300, (b) 1450, and (c) 1500 °C. The surface of the membrane is shown in ai–ci, and the cross-section of the half-cells with the membrane and the support electrode is shown in aii–cii.

Figure 5

Influence of sintering temperature on the grain size of BCZY271 sintered for 10 h at (a) 1300, (b) 1450, and (c) 1500 °C. SEM images are of the same scale and magnification.

Figure 6

Effect of time duration on the grain growth of BCZY271 sintered at 1500 °C for (a) 1, (b) 5, and (c) 10 h.

Figure 7

(a) XRD patterns for BCZY271 sintered at 1500 °C for 1, 5, and 10 h compared with the commercial powder, (b) estimation of the full width half-maximum for the 110, 200, and 211 peaks.

Figure 8

Fractured cross-section SEM image of a protonic ceramic cell: (a) the printed electrode–electrolyte-support electrode, (b) magnified image of the BCZY271 electrolyte layer, (c) printed electrode–electrolyte interface, (d) electrolyte–support electrode interface, (e) magnified image of the printed electrode, and (f) magnified image of the support electrode.

Figure 9

Nyquist plots for the EIS measurements performed near open circuit potential at (a) 350, (b) 400, and (c) 450 °C under humidified hydrogen conditions (10 vol % H₂) in helium at the negatrode | humidified hydrogen (60 vol % H₂) in helium at the positrode. (d) Distribution of relaxation time (DRT) analysis of the P-SOC using discretization parameter, lambda order: 1 × 10^–4. P1, P2, and P3 are associated with the charge transfer reaction, formation of nickel–hydrogen bonding, and hydrogen adsorption and desorption processes, respectively.

Figure 10

(a) I–V polarization characteristics and (b) faradaic efficiency between 350 and 450 °C for the P-SOC evaluated as a hydrogen pump under humidified helium at the negatrode | humidified hydrogen (60 vol % H₂) in helium at the positrode.

Figure 11

Aging behavior of the P-SOC operated for 100 h at 320 mA cm^–2 at 450 °C under humidified helium at the negatrode | humidified hydrogen (60 vol % H₂) in helium at the positrode.