Constructing a pathway for mixed ion and electron transfer reactions for O2 incorporation in Pr0.1Ce0.9O2−x

Abstract

In interfacial charge-transfer reactions, the complexity of the reaction pathway increases with the number of charges transferred, and becomes even greater when the reaction involves both electrons (charge) and ions (mass). These so-called mixed ion electron transfer (MIET) reactions are crucial in intercalation/insertion electrochemistry, such as those occurring in oxygen reduction/evolution electrocatalysts and lithium-ion battery electrodes. Understanding MIET reaction pathways, particularly identifying the rate-determining step (RDS), is crucial for engineering interfaces at the molecular, electronic, and point defect levels. Here we develop a generalizable experimental and analysis framework for constructing the O 2 (g) incorporation reaction pathway in Pr 0.1 Ce 0.9 O 2-x. We converge on four candidate RDS (dissociation of neutral 31 oxygen adsorbate) out of more than 100 possibilities by measuring the current density-32 overpotential curves while controlling both oxygen activity in the solid and the oxygen gas 33 partial pressure, and quantifying the chemical and electrostatic driving forces using operando 34 ambient pressure X-ray photoelectron spectroscopy. 35 36 3 Mixed ion and electron transfer (MIET) reactions involve the transfer of both ionic and electronic charges across interfaces. They are substantially more complex than electron transfer and proton-coupled electron transfer reactions because the ionic charge also crosses the electrochemical double layer. 1 The net reactions are usually chemical in nature (i.e., no net charge transfer). Examples include H + intercalation in layered hydroxides and Li + insertion in 41 metal oxides (Fig. 1a,b). 2 Another ubiquitous example is the oxygen incorporation reaction (OIR) 42 occurring at the solid/gas interface (Fig. 1c). It is rate-determining for many energy-and 43 environment-related technologies, including oxygen storage materials for emission control, 3 44 solid oxide fuel cells (SOFCs), 4 electrolysis cells, 5 thermochemical water splitting cycles, 6 and oxygen permeation membranes. 7 The OIR is expressed as − − + → 2 2 O 4e 2O. (1) 47 Understanding the OIR reaction pathway is crucial for engineering and discovering catalysts, 48 typically oxides, with high activity and stability. 8,9 Mixed ionic-electronic conductors (MIECs) 49 have received widespread interests because they expand the effective OIR site to the gas/solid 50 double-phase boundary beyond the traditional triple phase boundary between gas, ionic and 51 electronic conductors. 10,11 There, oxygen ions and electrons react with oxygen adsorbates at the same active site, resulting in a reaction that involves the transfer of two oxygen ions and four electrons. The number of charges transferred during OIR has made it challenging to isolate the ratedetermining step (RDS). Most experimental work has focused on measuring the exchange coefficients 12 using tracer diffusion, 13 conductivity and mass relaxation, 14 and impedance spectroscopy, 15 as well as their reaction order with respect to oxyge...

Understanding electrochemical switchability of perovskite-type exsolution catalysts

“…Therein, O O , v ÁÁ O , Fe Fe , and Fe = Fe denote regular lattice oxygen (i.e., oxide ions), oxygen vacancies, regular lattice iron (i.e., Fe 3+ ), and reduced iron as a polaron (i.e., Fe 2+ ), respectively. In contrast to liquid-phase electrochemistry, the redox reaction at the LSF surface is not driven by an electrostatic potential difference 28,30 . Rather, the overpotential at the WE (η WE ) causes a defect chemical polarization of the LSF phase.…”

Investigating oxygen reduction pathways on pristine SOFC cathode surfaces by in situ PLD impedance spectroscopy

“…OER reaction rates can be measured by different experimental techniques such as tracer diffusion, 15–17 relaxation experiments 18–20 and impedance spectroscopy 12,21–24 performed in different atmospheres and at different temperatures. All these measurement techniques yield different surface exchange coefficients (which are themselves often not straightforwardly interpretable 25 ).…”

Performance modulation through selective, homogenous surface doping of lanthanum strontium ferrite electrodes revealed by in situ PLD impedance measurements

“…According to many literature studies on similar materials (LSF, LSC, and LSCF), the main arc in the impedance spectra is related to the oxygen exchange kinetics on the electrode surface, 17,24,33,40 as long as electronic in-plane conductivity and ionic across-plane conductivity are sufficiently high, which is the case for our films. In the medium frequency range, sometimes a shoulder was observed, which is most likely associated with the ion transfer across the electrode/electrolyte interface.…”

“…Here, mostly perovskite-type mixed ion and electron conducting (MIECs) cathode materials, such as La 1-x Sr x CoO 3-δ (LSC), [9][10][11][12][13][14][15][16] La 1-x Sr x FeO 3-δ (LSF), [17][18][19][20][21] La 1-x Ba x CoO 3-δ (LBC), 22,23 Sm 1-x Sr x CoO 3-δ (SSC) [24][25][26] , La 1-x Sr x Co 1-y Fe y O 3-δ (LSCF) [27][28][29][30][31][32] , SrTi 1-x Fe x O 3-δ , (STF) [33][34][35][36] or Ba 1-x Sr x Co 1-y Fe y O 3-δ (BSCF) [37][38][39] were investigated. In addition, studies on Pr 1-x Ce x O 2-δ (PCO) 40,41 , NdNi 2 O 4+δ (NNO) 42,43 or La 2 NiO 4+δ (LNO) 44,45 have been conducted.…”

“…On MIEC electrodes, the elementary steps of the OER (in cathodic direction) are oxygen gas diffusion, oxygen adsorption on the surface, dissociation and ionization at active sites as well as incorporation of the reduced oxygen species into an oxygen vacancy, followed by ion transport towards the electrolyte. 21,40,[46][47][48][49][50][51] One important goal of mechanistic OER studies on MIEC electrodes is to identify the rate limiting step and to understand its dependence on the properties of the electrode material and its surface, since this is the basis for a knowledge-driven material improvement. Such knowledge has the potential to greatly contribute to the development of optimized MIEC electrodes for intermediate temperature SOFCs.…”