Computational design of cobalt-free mixed proton–electron conductors for solid oxide electrochemical cells

Abstract

Proton-conducting solid-oxide electrolyzer and fuel cells (PC-SOECs/FCs) represent viable, intermediate-temperature green technologies for H2 production and conversion. While PC ceramics have been extensively investigated as electrolytes for PC-SOECs/FCs, the development of corresponding single-phase electrode components has been hindered by difficulties in finding efficient mixed proton-electron conductors (MPECs), with also effective catalytic activity toward oxygen reduction and evolution reactions (ORR/OER). To address this challenge, we applied first-principles methods (PBE+U) to design new perovskite-oxide MPEC electrodes based on the known BaZrO3 PC ceramic. Our strategy has been to modify the parent material by substituting Zr with earth abundant transition metals, namely Mn and Fe. We found Zr:Mn and Zr:Fe ratios of 0.75:0.25 to be sufficient to obtain electronic structural features that can enable electric conductivity. We also investigated other relevant processes for MPEC-based electrodes: hydration, proton migration, and ORR/OER electrocatalysis. From calculations of key descriptors associated with these processes, we found that Zr substitution with Mn or Fe delivers in both cases promising PC-SOEC/FC electrodes. Moreover, our first-principles results highlight the specific qualities of Mn and Fe: the first provides better electronic features and electrocatalytic activities, whereas the latter allows for better hydration and proton migration. In perspective, our findings present clear indications for the experimental implementation and test of new low-cost materials for solid-oxide electrochemical cells

Mixed‐Conducting Perovskites as Cathode Materials for Protonic Ceramic Fuel Cells: Understanding the Trends in Proton Uptake

“…Density functional theory (DFT) calculations yielded higher proton transfer barriers of 0.5–0.6 eV for BaZr 0.75 Co 0.25 O 3‐ δ compared with BaZrO 3 treated with the same methodology (0.3 eV), but these barriers also include defect association . For BaZr 0.75 Fe 0.25 O 3‐ δ and BaZr 0.75 Mn 0.25 O 3‐ δ , slightly decreased proton migration barriers of about 0.2 eV were found compared with BaZrO 3 for configurations in which the initial as well as final proton position is located between one Zr and one Fe/Mn (i.e., absence of defect association effects) . For Sr 2 Fe 1.5 Mo 0.5 O 6‐ δ and Ba 0.25 Sr 1.75 Fe 1.5 Mo 0.5 O 6‐ δ double perovskites, proton migration barriers of 0.5 and 0.3–0.4 eV have been obtained from DFT calculations .…”

Tailoring cations in a perovskite cathode for proton-conducting solid oxide fuel cells with high performance

“…Therefore, to avoid this problem and to have an understanding about the effect of K-doping on proton mobility, DFT calculations, which have been demonstrated to be a powerful tool for predicting proton migration in oxides in many previous studies, 38,[47][48][49][50] were used to investigate and compare proton migration in BSCF and BKSCF. Figure 3 shows a schematic of the proton migration process, which involves a hopping step and a rotating step.…”

Analysis of the Phase Stability of LiMO2 Layered Oxides (M = Co, Mn, Ni)

“…We used the DFT + U method [23,24], which has been extensively validated for correcting the large self-interaction error in transition metal oxides [25][26][27], caused by the approximate form of the standard exchange-correlation density functional when applied to strongly localized unpaired electrons, as in the d manifold of Co, Ni and Mn. Ab initio derived U values depend on the number of unpaired d electrons; thus, the reported U parameters for Ni, Mn and Co are 6.0, 4.0 and 3.3 eV, respectively [28,29].…”