Electrocatalysis is central to electrified energy conversion technologies, including the production of fuels and chemicals from renewable electricity. In recent years, atomistic modeling based on density functional theory (DFT) has become an indispensable tool for catalysis researchers because it can provide a molecular-level perspective into catalytic reactivity that is otherwise inaccessible to experimental measurements. Computational electrocatalysis model inputs are intended to capture fundamental understanding of the complex interfacial environment of the catalytic active site, but rely on practical approximations that can affect the reliability of insights gained from DFT. This Perspective outlines a framework for rigorous and reproducible integration of theory and experiment in heterogeneous electrocatalysis research. We discuss state-of-the-art computational methods and sources of error in model development and interpretation. We describe opportunities for validation of theoretical models and experimental interpretations as it pertains to the structure of active sites, reaction energetics, and microkinetic modeling. We provide examples to help computational researchers develop models that move beyond obtaining qualitative agreement with reactivity trends, and toward the development of rigorous mechanistic insights that can be leveraged to predict promising new catalysts or operating conditions. Rigor and reproducibility are enhanced when theory and experiment have multiple nodes of connection, which enhances the longevity and robustness of conclusions and insights about electrocatalytic reactivity.

Active site ensembles on transition metal phosphides tune the selectivity of the nitrate reduction reaction and mitigate competing hydrogen evolution. Using Ni2P nanocrystals as a case study, we demonstrate evidence of a competitive Langmuir-Hinshelwood reaction mechanism that involves the co-adsorption of both H* and NOx* intermediates and the impact of co-adsorption on intermediate binding energetics and reaction selectivity toward NH3.

We demonstrate that active site ensembles on transition metal phosphides tune the selectivity of the nitrate reduction reaction. Using Ni2P nanocrystals as a case study, we report a mechanism involving competitive co-adsorption of H* and NOx* intermediates. A near 100% faradaic efficiency for nitrate reduction over hydrogen evolution is observed at -0.4 V, while NH3 selectivity is maximized at -0.2 V vs. RHE.

Electrocatalytic urea removal is a promising technology for artificial kidney dialysis and wastewater treatment. Urea electrooxidation was studied on a variety of nickel electrocatalysts modified with Cr, Mo, Mn, and Fe with varying electrochemically active surface and roughness. Mass transfer limits were observed for urea oxidation at physiological concentrations (10 mM). Urea oxidation kinetics were explored at higher concentrations (200 mM), showing improved performance during polarization, but lower currents per active site. A simplified dialysis model was developed to examine the relationship of mass transfer coefficients and extent of reaction on flowrate, composition, and pH of the reacting stream. For a nickel hydroxide catalyst, the model shows that a minimum electrode area of 1314 cm is needed for continuous operation. This research combines experimental data and a computational dialysis model for a simplified continuous dialysis system, highlighting the potential of these catalysts and paving the way for future improvements.

Nickel-chromium-molybdenum (NiCrMo) alloys are well-known for having exceptional corrosion resistance, but their electrocatalytic properties have not been extensively studied. In this paper, the development of electro-active nickel-oxyhydroxide (NiOOH) phases and kinetics of the oxygen evolution reaction (OER) have been examined on alloys G35, B3, and C276 in alkaline electrolyte at 25 °C. Reproducible oxide layers were grown by potential cycling between 0.85 and 1.52 V vs RHE up to 600 cycles, and the transition between Ni(OH) 2 and NiOOH was monitored by cyclic voltammetry throughout. Onset potentials, Tafel slopes, and turnover frequencies (TOF) were measured at OER overpotentials between 270 and 390 mV. Alloys with dissimilar Cr:Mo ratios had significantly higher electrochemical surface area and increased γ -NiOOH formation, suggesting higher metal dissolution rates. The equal Cr:Mo concentration alloy and pure Ni developed a primarily β -NiOOH surface, and had 1.8–2.0 times larger TOF values than those containing significant γ -NiOOH. The NiCrMo alloys required smaller overpotentials (54–80 mV) to produce 10 mA cm −2 of OER current, and had comparable Tafel slopes to pure Ni. The findings here indicate a β -NiOOH-developed surface to be more OER-active than a γ -NiOOH-developed surface, and suggest certain NiCrMo alloys have promise as OER electrocatalysts.

This work describes the electrocatalytic behavior of urea and creatinine on catalysts of nickel foam (NF), nickel hydroxy foam (NHF), and iron-doped nickel hydroxy foam (Fe-NHF) at room temperature in alkaline electrolyte at pH 14. This work is relevant to urea removal from urine and dialysate.

Urea is one of the world’s most abundant waste products, commonly found in waste streams in varying concentrations. Urea is the compound in largest abundance in human urine (other than water), and creatinine is its second most abundant organic compound [1]. Typical concentrations of urea and creatinine in human urine are 0.22 M and 0.013 M, respectively. In treatment of human and animal urine streams, a method for reactive removal of organic compounds must include elimination of both urea and creatinine. A previous study by Schrank et al. [2] of the electrochemistry of urine compounds showed that, on a nickel cobaltite catalyst at urine-relevant concentrations, creatinine suppressed the reaction of urea at lower potentials, while creatinine oxidized preferentially over urea at higher potentials. In dialysate, however, urea and creatinine are present in lower concentration, typically 6 mM and 60 μM, respectively, and whether co-reaction of a urea/creatinine solution occurs is of strong interest in removal of uremic toxins. Electrochemical reaction of urea has been studied for a variety of electrodes and is currently receiving increased interest due to its potential in treating wastewater to produce hydrogen and ammonia [3–5]. Urea electrooxdiation is also suitable for a direct urea fuel cell [6]. To date, one of the most active electrodes is Ni(OH)2, which converts to the catalytically active (Ni3+) form of nickel oxyhydroxide (NiOOH) above 0.45 V vs. a Hg/HgO electrode at pH 14 [3]. Urea electrooxidation is a 6-electron process; the anode reaction in alkaline media is (NH2)2CO + 6 OH− → N2 + CO2 + 5 H2O + 6 e − and has a standard potential of −0.896 VHg/HgO (0.070 VRHE) at pH 14. Electrooxidation of creatinine is a 21-electron process C4H7N3O + 21 OH− → 3/2 N2 + 4 CO2 + 14 H2O + 21 e − and has a standard potential of −1.021 VHg/HgO (−0.055 VRHE) at pH 14. Materials and Methods Cyclic voltammetry (CV) and chronoamperometry were performed in a three-electrode cell (Pine electrochemistry) utilizing a Pt coil counter electrode (CE) and a Hg/HgO reference electrode (RE) appropriate for alkaline solutions. The working electrode (WE) for each experiment was either a polished Ni disk (Basi), Ni hydroxy foam (NHF), or NHF with 1% Fe (NHF-Fe). All solutions contained 1 M KOH as a supporting electrolyte, chosen to represent the strongly alkaline environment of an anion exchange membrane that would be used in a larger reaction cell. Urea and creatinine were added in concentration ranges typical of healthy and uremic human serum. Data collection and visualization were performed with a Solartron 1287A potentiostat coupled with CorrWare/CorrView software. All cyclic voltammetry was performed at a scan rate of 10 mV/s. Results Figure 1 shows CVs of blank solution, urea solution, and a urea/creatinine solution on a NHF-Fe electrode. The Ni2+/Ni3+transition [Ni(OH)2 + OH− ↔ NiOOH + H2O + e −] occurs over the range of 0.45–0.6 VHg/HgO for oxidation and 0.2–0.55 VHg/HgO for reduction. The oxygen evolution reaction (OER) occurs at potentials of 0.65 VHg/HgO and higher. Urea oxidation (in 10 mM urea) occurs over the range of 0.5–0.65 VHg/HgO , and its overlap with the Ni2+/Ni3+ transition demonstrates the catalytic nature of Ni3+. These results are in agreement with previous work. When 59 μM creatinine was added, the CV shows an increased oxidation peak in the urea oxidation region, attributed to co-oxidation of urea and creatinine. Chronoamperometry results (not shown) show that steady-state oxidation is established within one minute at 0.55 VHg/HgO. The steady-state oxidation current was 15 mA/cm2 for the urea/creatinine solution vs. 12 mA/cm2 for urea alone. Conclusion Electrocatalytic co-oxidation of urea and creatinine occur on a NHF-Fe electrode in alkaline (pH 14) media at potentials of 0.5–0.65 VHg/HgO. Demonstration of co-oxidation is of primary importance in reactive removal of organic compounds from dialysate and – by extension – from urine. Acknowledgements This work was supported by the University of Washington. References 1. D. F. Putnam, “Composition and concentrative properties of human urine,” McDonnell-Douglas Astronautics Company/NASA Contractor's Report (1971). 2. A. Schranck, R. Marks, E. Yates, and K. Doudrick, Environ. Sci. & Tech. 52, 8638–8648 (2018). 3. B. K. Boggs, R. L. King, and G. G. Botte, Chem. Comm. 32, 4859–4861 (2009). 4. R. L. King and G. G. Botte, J. Power Sources 196, 9579–9584 (2011). 5. V. Vedharathinam and G. G. Botte, Electrochim. Acta 108, 660–665 (2012). Figure 1

Introduction Urea [CO(NH2)2] is one of the world’s most abundant waste products and is therefore commonly found in waste streams in varying concentrations. Human urine is 0.33 M urea while ruminant livestock urine has about half that concentration due to their herbivore diet [1]. Urea is also a versatile chemical that is industrially manufactured and commonly used. In effluent streams, urea hydrolyzes to form toxic ammonia (NH3) gas that can oxidize to form other pollutants like nitrates, nitrites, and nitrogen oxides [2]. Electrooxidation of urea for wastewater remediation is of interest whereby Eqn. (1) occurs at the anode of an electrochemical cell at a standard potential of E0 = –0.46 V vs. SHE at pH 14 [3]. Urea oxidation can be paired with concurrent reduction of water to produce H2 for energy storage, or reduction of O2 to directly produce energy in a direct urea fuel cell (DUFC) [4]. DUFCs and H2 production from urea both require high oxidation rates to make them economically viable. As a result, these studies typically use concentrations of 0.33 M urea to imitate human urine concentrations. Urea concentrations in wastewater will be diluted and therefore much lower, yet urea removal remains desirable for environmental reasons. At such low urea concentrations, transport to the electrode surface dominates the overall oxidation rate. The effects of low urea concentration on oxidation have yet to be fully studied, which is what this work aims to address. In an alkaline aqueous environment, nickel provides an active surface for urea oxidation due to the Ni2+/Ni3+ reversible redox couple described by Eqn. (2) at a standard electrode potential of E0= 0.49 V vs. SHE at pH 14 [3]. Subsequent urea oxidation occurs on Ni3+ and regenerates a Ni2+ site [Eqn. (3)]. At sufficiently anodic potentials, Eqns. (2–3) will occur in tandem and oxidize urea. Experimentally, urea oxidation is observed on Ni at potentials that favor Ni3+ which suggests that Eqn. (3) dominates over Eqn. (1). According to Eqn. (2), an alkaline environment is required for urea oxidation on Ni. To increase reaction rate, studies commonly use 1–5 M KOH, which is too harsh for an effluent stream. This work aims to investigate oxidation kinetics from pH 7–14. Materials and Methods Voltammetric studies were performed using a Solartron 1287A electrochemical interface coupled with CorrWare/CorrView software for data collection and visualization. Electrochemical experimental methods include cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronoamperometry (CA) in a three-electrode cell utilizing a 3.0 mm diameter Ni disk working electrode (WE), Hg/HgO reference electrode (RE), and a platinum coil counter electrode (CE). Urea is the analyte in all experiments, varying in concentration from 1.0 mM to 1.0 M. Supporting electrolytes include KOH and KHCO3 in varying concentrations to maintain pH between 7 and 14. Concentration and temperature variation are used to investigate transport and kinetic mechanism control, respectively. Results and Discussion Results will be discussed including electrochemical surface area (ECSA) and electrode characterization and the concentration regimes for both urea and KOH that define transport vs. kinetic reaction mechanism control. Initial experiments confirm mass transfer limited reaction at 15 mM urea and 0.1 M KOH at room temperature as seen in Figure 1. Experiments focusing on the temperature and concentration dependence are in progress. Significance Waste stream remediation is a major application of urea oxidation that focuses on removal of urea over maximizing current density. Further, it requires lower urea concentration and more neutral pH than typically studied. We lay the groundwork for mechanistic understanding over a range pH and urea concentrations relevant to such motivations. References Dijkstra, J., Oenema, O., Van Groenigen, J., Spek, J., Van Vuuren, A., and Bannink, A. Animal 7, 292 (2013). Ye, K., Wang, G., Cao, D. et al. Top Curr Chem 376, 42 (2018). Boggs, B. K., King, R. L., and Botte, G. Chem Comm 32, 4859 (2009). Singh, R. K., and Schechter, A. Acta 278, 405 (2018). Figure 1

Electrochemical reactors (ECR), such as fuel cells and electrolyzers, are ideal systems to include in chemical engineering design courses. Electrochemistry and electrochemical reaction engineering are not normally required in the chemical engineering curriculum, so use of an ECR in a capstone design course requires a brief introduction of the topic and a working model to assist the students in ECR design. Here, we focus on CO2 hydrogenation to make methanol and formic acid in competition with the hydrogen evolution reaction. This project was used in the Spring 2019 Process Design II class in Chemical Engineering at the University of Washington. A spreadsheet model was developed that estimates current densities of the three reactions occurring simultaneously. The model was derived from earlier versions that simulated H2/O2 fuel cells, in proton exchange membrane fuel cell (PEM) and solid oxide fuel cell (SOFC) configurations; and CH4-fed SOFCs with water gas shift and coking. The cathode reactions included in the model were: CO2+ 6 H++ 6 e − → CH3OH + H2O CO2+ 2 H++ 2 e − → HCOOH 2 H++ 2 e − → H2 and were coupled with the oxygen evolution anode reaction: H2O → 2 H++ 2 e − + 1/2 O2 Reactions are represented by a matrix of stoichiometric coefficients and physical properties of all species and reactions including molecular weight, gas and liquid phase viscosities, gas and liquid phase heat capacities, and heats and entropies of reaction. Activities are given by partial pressure for gaseous species and mole fraction for liquid species, under the assumptions of ideal gas and ideal solution. Reversible electrode potentials for non-unity species activities are calculated by the Nernst equation. The spreadsheet is a one-dimensional, along-the-channel model solved step-wise by the Eulerian method. The channel, in parallel or serpentine pattern, is divided into 100 segments and each segment solved by a double (outer and inner) iteration loop. Each segment is assumed to be gradientless, which makes the model a series of CSTRs. Iteration is by the Newton-Raphson method, and both iterations are open-loop. Five iterations are sufficient to achieve convergence in most cases. The outer iteration begins with a guessed initial current density, from which the model solves for the overpotentials of electrolyte resistance, mass transfer limitation, and anode kinetics. The single term (Tafel) form of the Butler-Volmer equation is sufficient for CO2 hydrogenation due to the high overpotentials involved. Individual reaction current densities are determined by the inner iteration, consisting of a guessed current density for methanol synthesis, from which the overpotential of that reaction is determined. Together with the reversible potentials of all three reactions, the cathode potential vs. RHE and the other cathode overpotentials can be determined. The inner iteration continues until the sum of all three reaction current densities equals the originally guessed current density. The outer iteration then proceeds until the outer-iterated cell potential agrees with the input cell potential. At this point, the total current density and individual reaction current densities are known, and the Faradaic efficiency can be determined. The mass balance is updated according to the extents of each reaction, and the solution procedure is repeated for the next segment. Modeling inputs are cell potential; temperature; and inlet pressures, flow rates, compositions, exchange current densities, and limiting current densities for cathode and anode. Physical parameters include cell dimensions (length x width), channel dimensions (width, height, rib), and choice of serpentine (single or multiple pass) or parallel flow fields. The screen shot in Fig. 1 shows a small portion of the control panel at left along with plots of CO2 and H2O conversion, current density, pressure, and species fluxes for the cell operating at −1.9 V cell potential and 70 °C. Serpentine flow fields are used for both cathode and anode with 1 and 2 passes, respectively. The average current density is 0.5 A/cm2, which is almost entirely due to methanol production, with a Faradaic efficiency of essentially 1. Current densities for formic acid and hydrogen evolution are less than 1 μA/cm2 in this case. The model enables solution of a complex, real-world problem with tools readily available and understandable by senior chemical engineering students. It can be adapted for a variety of electrochemical reactions making it a powerful tool in electrochemistry education. Figure 1

Recent advances in carbon capture create new opportunities for recycling CO2 into liquid fuels to store intermittent electrical energy1. One option is to co-electrolyze CO2 and H2O at high temperature using solid oxide electrolysis cells (SOECs). Although promising, the factors controlling rates of CO2 and H2O reduction in SOECs are not well understood, hampering development2,3. Traditional electrochemical techniques have difficulty resolving the mechanisms and kinetic parameters of the individual steps governing CO2 and H2O reduction. This limitation can potentially be overcome using linear and non-linear electrochemical impedance spectroscopy (EIS and NLEIS) in conjunction with dynamic measurement of gas-phase species using differential electrochemical mass spectrometry (DEMS). Regarding co-electrolysis, mixed ionic electronic conductors (MIEC) have gained interest as alternatives to nickel-yttria stabilized zirconia (Ni-YSZ) because the active region is not limited to the triple phase boundary and they are more stable in reducing environments3. The MIEC studied here, gadolinia-doped ceria (GDC), has been well characterized as an electrolyte3 and is a promising cathode in SOECs. Unlike Ni-YSZ, it does not coke in carbon environments or oxidize completely in a feed of water and CO2. We have performed NLEIS and EIS measurements of CO2 reduction, water reduction, and co-electrolysis on button cells composed of GDC as the working electrode with electrolyte YSZ under various temperatures and gas compositions. Gas atmospheres surrounding the cells contain mixtures of water, CO2, CO, H2, and carrier gas such as Ar or N2, used to manipulate the oxygen partial pressure of the system and thus the oxygen vacancy concentration of the surface and bulk electrode. The data we have collected on CO2 and water reduction on GDC have been compared with model NLEIS spectra developed in our previous work3,4 in the context of a reaction mechanism and a rate determining step. Additionally, these mechanisms were cross-checked through measuring a gas phase response in DEMS and a micro-kinetic model was developed to predict product distributions corresponding to the mechanism that most closely matched the results. References Graves, C., Ebbesen, S. D., Mogensen, M., & Lackner, K. S. (2011). Sustainable hydrocarbon fuels by recycling CO2 and H2O with renewable or nuclear energy. Renewable and Sustainable Energy Reviews, 15(1), 1–23. https://doi.org/10.1016/j.rser.2010.07.014 Graves, C., Chatzichristodoulou, C., & Mogensen, M. B. (2015). Kinetics of CO/CO2 and H2/H2O reactions at Ni-based and ceria-based solid-oxide-cell electrodes. Faraday Discussions, 182(0), 75–95. https://doi.org/10.1039/C5FD00048C Valdés-Espinosa, H., Stuve, E. M., & Adler, S. B. (2015). Modeling Water Reduction on 10 Mole% Gadolinia-Doped Ceria (GDC10) Porous Electrodes. ECS Transactions, 66(2), 229–251. https://doi.org/10.1149/06602.0229ecst Witt, J., Stuve, E.M., & Adler, S.B. (2017). Modeling CO2 Electrolysis on Gadolinia Doped Ceria Porous Electrode using a 1-D Macro-Homogeneous Model and Linear and Non-Linear Electrochemical Impedance Spectroscopy. ECS Transactions (accepted and awaiting publication).

In recent years, lithium-air batteries have attracted significant interest due to increased energy density compared to currently available technologies such as lithium-ion batteries. The discharge process involves oxygen reduction to form LiO2, followed by further reduction to Li2O2 1. Meanwhile, oxygen evolution from the discharge deposits occurs during charge. The poor conductivity of Li2O2 2 requires the use of high overpotentials during charge, leading to side reactions that hinder battery performance1. A significant amount of work has been devoted at improving battery performance via cathode3 and electrolyte engineering4. In addition, several experimental and computational studies have attempted to understand the nature of the charge and discharge reactions through the use of electrochemical cells2,4-5. However, the complexity of electrochemical cells makes it difficult to gain fundamental insight of these processes. An alternative involves the use of ultrahigh vacuum (UHV) studies, where the ability to examine the influence of electrode potential on surface reactions is limited. This issue can be resolved by focusing on the effect of electric field on surface reactions since electric field is directly related to electrode potential. An example of a UHV technique is field ionization microscopy (FIM). In FIM, a Pt field emitter tip of approximately 350 Å radius is used as the substrate. The electric field on the surface is easily controllable by biasing the tip at moderate voltages (around 5 kV), to produce fields of up to 5 V/Å6. Reactions are carried out by adsorption of the species of interest (Li, O2, and a solvent representative of the electrolyte) followed by application of a baseline field. Adsorbed reaction products and their morphology can be monitored by FIM and field electron microscopy (FEM). Reaction intermediates and products can be detected by pulsed field desorption time of flight mass spectrometry (PFD-MS). In PFD, reactions products are formed under a base field, followed by desorption when a field pulse is applied. By varying the base field magnitude, pulse repetition frequency, and reaction temperature, it is possible to obtain information such as activation energies and possible reaction steps7. In this work, the kinetics of LiOx consumption in acetonitrile, a low DN solvent, will be studied using PFD and FIM. Namely, the focus of this work will be to characterize the effect of electric field, temperature, and reaction time on the rate of Li2O2 consumption during charge, as well as characterizing the lifetime of the LiO2 intermediate during charge. References Girishkumar, G; McCloskey, B; Luntz, A. C; Swanson, S; Wilcke, W. “Lithium-air battery: Promise and challenges”. Phys. Chem. Lett. 2010, 1, 2193–2203 Viswanathan, V; Thygesen, K. S; Hummelshøj, S; Nørskov, J. K; Girishkumar, G; McCloskey, B. D; Luntz, A. C. “Electrical Conductivity in Li2O2 and its role in determining capacity limitations in non-aqueous Li-O2 batteries”. J Chem. Phys. 2011, 135, 214704 Kwak, W. J; Lau, K. C; Shin, C. D; Amine, K; Curtiss, L. A; Sun, Y. K. “AMo2C/Carbon Nanotube Composite Cathode for Lithium-Oxygen Batteries with High Energy Efficiency and Long Cycle Life”. ACS Nano. 2015, 9, 4129–4137 Laoire, C. O; Mukerjee, S; Abraham, K. M; Plichta, E. J; Hendrickson, M. A. “Influence of Nonaqueous Solvents on the Electrochemistry of Oxygen in the Rechargeable Lithium-Air Battery”. Phys. Chem. C. 2010, 114, 9178–9186 Laoire, C. O; Mukerjee, S; Abraham, K. M; Plichta, E. J; Hendrickson, M. A. “Elucidating the Mechanism of Oxygen Reduction for Lithium-Air Battery Applications”. Commun. 2011 , 47, 9438-9440 Rothfuss, C. J; Medvedev, V. K; Stuve, E. M. “The influence of the surface electric field on water ionization: a two-step dissociative ionization and desorption mechanism for water ion cluster emission from a platinum field emitter tip”. Surface Science. 2003, 554-555, 133-143 Kruse, N. “Surface reaction kinetics on the atomic scale: studies by means of pulsed field desorption mass spectrometry”. Materials Science and Engineering A. 1999, 270, 75-82

Recent advances in carbon capture create new opportunities for recycling CO2 into liquid fuels to store intermittent electrical energy1. One option is to co-electrolyze CO2 and H2O at high temperature using solid oxide electrolysis cells (SOECs). Although promising, the factors controlling rates of CO2 and H2O reduction in SOECs are not well understood, hampering development2,3. Traditional electrochemical techniques have difficulty resolving the various factors and mechanisms governing CO2 and H2O reduction. This limitation can potentially be overcome through the use of linear and non-linear electrochemical impedance spectroscopy (EIS and NLEIS) in conjunction with dynamic measurement of gas-phase species using mass spectrometry. In regards to co-electrolysis, mixed ionic electronic conductors (MIEC) have gained interest as alternatives to nickel-yttria stabilized zirconia (Ni-YSZ) because the active region is not limited to the triple phase boundary and they are more stable in reducing environments3. Gadolinia-doped ceria coupled with perovskite La1-xSrxCr1-yMnyO3-δ (GDC-LSCM) has been a particularly well studied MIEC3. We are currently conducting NLEIS and EIS measurements of CO2 reduction, water reduction, and co-electrolysis reduction experiments on button cells composed of GDC and LSCM as the working electrode with electrolyte YSZ under various temperatures and gas compositions. Gas atmospheres surrounding the cells contain various mixtures of water, CO2, CO, H2, and carrier gas such as Ar or N2. Currently hypothesized mechanisms/models on H2O and CO2 reduction are considered and scrutinized against NLEIS data in order to further reveal the nature of water and CO2 reduction. References Graves, C., Ebbesen, S. D., Mogensen, M., & Lackner, K. S. (2011). Sustainable hydrocarbon fuels by recycling CO2 and H2O with renewable or nuclear energy. Renewable and Sustainable Energy Reviews, 15(1), 1–23. https://doi.org/10.1016/j.rser.2010.07.014 Graves, C., Chatzichristodoulou, C., & Mogensen, M. B. (2015). Kinetics of CO/CO2 and H2/H2O reactions at Ni-based and ceria-based solid-oxide-cell electrodes. Faraday Discussions, 182(0), 75–95. https://doi.org/10.1039/C5FD00048C Valdes-Espinosa, H., Stuve, E. M., & Adler, S. B. (2015). Modeling Water Reduction on 10 Mole% Gadolinia-Doped Ceria (GDC10) Porous Electrodes. ECS Transactions, 66(2), 229–251. https://doi.org/10.1149/06602.0229ecst

In recent years, lithium-air batteries have attracted significant interest due to increased energy density compared to currently available technologies such as lithium-ion batteries. The increased capacity in lithium-air batteries is due to charge/discharge reactions involving the formation of lithium oxide (LiOx) species1 However, issues such as loss of capacity due to irreversible oxidation reactions2, low capacity compared to the maximum theoretical capacity, high charging overpotentials, and the presence of parasitic reactions have limited their commercial deployment. Most of these issues occur in the cathode, where the lithium-oxygen deposits form. Recent work has focused in solving these issues via cathode engineering3-4 and improved electrolyte formulations5. In addition, there has been an increasing body of work aiming to understand the processes occurring in the cathode6-7. Most of these studies involve the use of electrochemical cells with subsequent characterization of the electrode. However, the complexity of electrochemical cells makes it difficult to gain insight regarding the nature of the cathode reactions. Instead, ultrahigh vacuum (UHV) surface science studies can be used to gain further insight regarding the surface reactions. In UHV studies, the ability to examine the influence of electrode potential on surface reactions is limited. This issue can be resolved by focusing on the effect of electric field on surface reactions since electric field is directly related to electrode potential. The effects of electric field on kinetics can be studied in a UHV system based on field ionization microscopy (FIM). A Pt field emitter tip of approximately 350 Å radius is used as the substrate. The electric field on the surface is easily controllable by biasing the tip at moderate voltages (around 5 kV), to produce fields of up to 5 V/Å8-9. The experiment is detailed in Fig. 1. Reactions are carried out by adsorption of the species of interest (Li, O2, and a solvent representative of the electrolyte) followed by application of a baseline field for a certain time and temperature. Adsorbed reaction products (LiOx) and their morphology can be monitored by FIM and field electron microscopy (FEM). Reaction intermediates and products can be detected by pulsed field desorption time of flight mass spectrometry (PFD-MS) and ramped field desorption (RFD). PFD-MS enables a survey scan of all species, while RFD probes the effects of electric field and temperature on surface reactions. In this work, the formation of LiOx deposits as a function of field, temperature, and coverage are studied with FIM and RFD. Reactions are conducted on both Pt and carbon-coated Pt tips, the latter to simulate a Li-O2 cathode. The differing field dependencies of LiOx deposit formation for different x will be used to estimate relative rates of LiOx formation in the Li-O2 battery. References Girishkumar, G; McCloskey, B; Luntz, A. C; Swanson, S; Wilcke, W. “Lithium-air battery: Promise and challenges,” J. Phys. Chem. Lett. 2010, 1, 2193–2203 2. Christensen, J; Albertus, P; Sánchez-Carrera, R. S; Lohmann, T; Kozinsky, B; Liedtke, R; Ahmed, J; Kojic, A. A Critical Review of Li/Air Batteries. J Electrochem Soc. 2012, 159, R1-R30 Kwak, W. J; Lau, K. C; Shin, C. D; Amine, K; Curtiss, L. A; Sun, Y. K. “AMo2C/Carbon Nanotube Composite Cathode for Lithium-Oxygen Batteries with High Energy Efficiency and Long Cycle Life,” ACS Nano. 2015, 9, 4129–4137 Lu, Y. C; Xu, Z; Gasteiger, H. A; Chen, S; Hamad-Schifferli, K; Shao-Horn, Y. “Platinum-Gold Nanoparticles: A Highly Active Bifunctional Electrocatalyst for Rechargeable Lithium-Air Batteries,” J. Am. Chem. Soc. 2010, 132, 12170–12171 Laoire, C. O; Mukerjee, S; Abraham, K. M; Plichta, E. J; Hendrickson, M. A. “Influence of Nonaqueous Solvents on the Electrochemistry of Oxygen in the Rechargeable Lithium-Air Battery,” J. Phys. Chem. C. 2010, 114, 9178–9186 Laoire, C. O; Mukerjee, S; Abraham, K. M; Plichta, E. J; Hendrickson, M. A. Elucidating the Mechanism of Oxygen Reduction for Lithium-Air Battery Applications. Chem. Commun. 2011 , 47, 9438-9440 Black, R; Oh, S. H; Lee, J. H; Yim, T; Adams, B; Nazar, L. F. “Screening for Superoxide Reactivity in Li-O2 Batteries: Effect on Li2O2/LiOH Crystallization,” J. Am. Chem. Soc. 2012, 134, 2902–2905 Rothfuss, C. J; Medvedev, V. K; Stuve, E. M. “The influence of the surface electric field on water ionization: a two-step dissociative ionization and desorption mechanism for water ion cluster emission from a platinum field emitter tip,” Surface Science. 2003, 554-555, 133-143 Rothfuss, C. J; Medvedev, V. K; Stuve, E. M. “Cluster formation and distributions in field ionization of coadsorbed methanol and water on platinum,” Surface Science. 2015, http://dx.doi.org/10.1016/j.susc.2015.12.023 Figure 1

Pure and mixed clusters of methanol and water were examined with pulsed field desorption time-of-flight mass spectrometry (TOF-MS) as a function of adlayer composition varying from pure water to nominally pure methanol. The experiments were performed on a Pt tip at 165K and total pressure of approximately 5 × 10–6 Torr. Protonated clusters of up 7 water molecules and up to 4 methanol molecules were detected. For mixed adlayers, mixed clusters involving 1 or 2 water and methanol molecules were observed. The hydronium cluster (H2O)H+ exhibited unusual behavior in that its maximum intensity occurred for an approximately equimolar mixture. This was attributed to direct ionization of a methanol monohydrate species, (CH3OH⋅H2O). Water production was observed in methanol-rich layers and ascribed to scission of the C–O bond to produce CH3 and OH. The TOF-MS data exhibited significant time lags for most higher mass clusters. The time lags for pure H2O were analyzed in terms of a two-step mechanism involving a trade-off of ion cluster emission and growth, from which the rate constant for cluster growth was estimated as 9 × 10–6 s–1.

In recent years, there has been an increasing interest in understanding electrochemical reactions on oxide surfaces. The advent of techniques such as ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) has helped to provide a better understanding of aspects such as the nature of the surface1-2 and the species involved in the reactions3-4. However, a quantitative connection between macroscopic rates and the surface structure and dynamics remains elusive. Among these reactions, water electrolysis has attracted interest due to favorable kinetics when compared to low temperature electrolysis5. Doped ceria, a mixed ionic and electronic conductor, is a potential candidate for this reaction due to higher stability and more efficient use of the electrode area compared to currently available materials, such as nickel/yttria-stabilized zirconia cermet6. Recent spectroscopic studies of water reduction/oxidation on doped ceria have shed valuable new insights about this reaction, but uncertainties remain in the interpretation of these measurements (in terms of a rate-determining step) and the connection to macroscopic rates3,4. In order to better connect spectroscopic observations to macroscopic rates, we have developed a thermodynamically self-consistent elementary kinetic model for H2/H2O exchange on the surface of ceria. This model considers both the bulk defect thermodynamics, as well as the effects of cation reconstruction/segregation at the surface and space charge effects, both of which are expected to alter the defect chemistry and concentrations of reactive intermediates. We have used this model to explore a variety of mechanisms and rate-limiting steps proposed in the literature3,4, and to predict the expected nonlinear electrochemical rate law. This includes prediction of the nonlinear electrochemical impedance (NLEIS) response of thin film ceria electrodes, in an approach similar to our previous studies of O2 reduction kinetics on perovskite thin films8. References Mueller, D. N.; Machala, M. L.; Bluhm, H; Chueh, W. C. Redox activity of surface oxygen anions in oxygen-deficient perovskite oxides during electrochemical reactions. Nature Comm. 2015, 6, 6097 Chueh, W.C.; McDaniel, A. H.; Grass, M. E.; Hao, Y; Jabeen, N; Liu, Z; Haile, S. M.; McCarthy, K. F.; Bluhm, H; El Gabaly, F. Highly Enhanced Concentration and Stability of Reactive Ce3+ on Doped CeO2 Surface Revealed in Operando. Chem. Mater. 2012, 24, 1876-1882 Zhang, C; Yu, Y; Grass, M. E; Dejoie, C; Ding, W; Gaskell, K; Jabeen, N; Hong, Y. P; Shavorskiy, A; Bluhm, H; Li, W. X; Jackson, G. S; Hussain, Z; Liu, Z; Eichhom, B. W. Mechanistic Studies of Water Electrolysis and Hydrogen Electro-Oxidation on High Temperature Ceria-Based Solid Oxide Electrochemical Cells. J. Am. Chem. Soc. 2013 , 135, 11572-15579 Feng, Z. A; El Gabaly, F; Ye, X; Shen, Z. X; Chueh, W. C. Fast vacancy-mediated oxygen ion incorporation across the ceria-gas electrochemical interface. Nature Comm. 2014 , 5, 4374 Brisse, A; Chefold, J; Zahid, M. High Temperature Water Electrolysis in Solid Oxide Cells. International Journal of Hydrogen Energy. 2008, 33, 5375-5382 Nakamura, T; Yashiro, K; Kaimai, A; Otake, T; Sato, K; Kawada, T; Mizusaki, J. Determination of the Reaction Zone in Gadolinia-Doped Ceria Anode for Solid Oxide Fuel Cell. J Electrochem Soc. 2008, 155, B1244-B1250 Wilson, J. R; Schwartz, D. T; Adler, S. B. Nonlinear electrochemical impedance spectroscopy for solid oxide fuel cell cathode materials. Electrochimica Acta. 2006, 51, 1389-1402 Wilson, J.R.; Sase, M; Kawada, T; Adler, S. B. Measurement of Oxygen Exchange Kinetics on Thin-Film La0.6Sr0.4CoO3-δ Using Nonlinear Electrochemical Impedance Spectroscopy. Electrochemical and Solid State Letters. 2007, 10, B81-B86

Currently, the most widely considered material for solid oxide fuel and electrolysis cells is the nickel/yttria-stabilized zirconia (Ni/YSZ) cermet1. However, issues such as low resistance to poisoning, coking, and changes in the microstructure have pushed for the use of new materials. Mixed ionic conductors2 alleviate the shortcomings of Ni/YSZ and can achieve more efficient use of the electrode material, since a three-phase boundary is not required for reaction. Among these materials, doped ceria has shown promise, yet little is known regarding the reaction mechanism for water electrolysis. The advent of techniques such as ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) has helped to provide a better picture of the surface, as well as the species involved in the reaction. Unfortunately, there is still controversy regarding the nature of the rate-determining step3,4. In addition, most work has made use of electrochemical impedance spectroscope (EIS), a linear technique that cannot differentiate among rate determining scenarios. The use of nonlinear techniques such as non-linear EIS (NLEIS) has been shown to be a powerful tool that is sensitive to different reaction mechanisms5. In order to elucidate these rate limiting phenomena, porous 10% doped gadolinia-doped ceria /YSZ electrochemical cells were analyzed at 750 and 800 °C under different H2-H2O gas environments using both EIS and NLEIS. The experimental results were then analyzed using a macro-homogeneous model6 involving oxygen vacancies and the chemical potential of electrons7. Multiple modeling scenarios were considered including one-dimensional and multidimensional transport, as well as the presence or absence of surface diffusion of hydroxyl species in the surface. References Nakamura, T; Kobayashi, T; Yashiro, K; Kaimai, A; Otake, T; Sato, K; Mizusaki, J; Kawada, T. Electrochemical Behaviors of Mixed Conducting Oxide Anodes for Solid Oxide Fuel Cell. J Electrochem Soc. 2008, 155, B563-B569 Nakamura, T; Yashiro, K; Kaimai, A; Otake, T; Sato, K; Kawada, T; Mizusaki, J. Determination of the Reaction Zone in Gadolinia-Doped Ceria Anode for Solid Oxide Fuel Cell. J Electrochem Soc. 2008, 155, B1244-B1250 Zhang, C; Yu, Y; Grass, M. E; Dejoie, C; Ding, W; Gaskell, K; Jabeen, N; Hong, Y. P; Shavorskiy, A; Bluhm, H; Li, W. X; Jackson, G. S; Hussain, Z; Liu, Z; Eichhom, B. W. Mechanistic Studies of Water Electrolysis and Hydrogen Electro-Oxidation on High Temperature Ceria-Based Solid Oxide Electrochemical Cells. J. Am. Chem. Soc. 2013 , 135, 11572-15579 Feng, Z. A; El Gabaly, F; Ye, X; Shen, Z. X; Chueh, W. C. Fast vacancy-mediated oxygen ion incorporation across the ceria-gas electrochemical interface. Nature Comm. 2014 , 5, 4374 Wilson, J. R; Schwartz, D. T; Adler, S. B. Nonlinear electrochemical impedance spectroscopy for solid oxide fuel cell cathode materials. Electrochimica Acta. 2006, 51, 1389-1402 Adler, S. B; Lane, J. A; Steele, B. C. H. Electrode Kinetics of Porous Mixed-Conducting Oxygen Electrodes. J. Electrochem Soc. 1996, 143, 3554-3564 Wang, S; Kobayashi, T; Dokiya, M; Hashimoto, T. Electronic and Ionic Conductivity of Gd-Doped Ceria. J Electrochem Soc. 2000, 147, 3606-3609

Lithium-ion batteries store electrical energy in the form of chemical potential, the same as that in primary batteries; however, the charge-discharge process in lithium-ion batteries is more complex as it involves not only Faradaic reactions at the interface between electrode and electrolyte, but also mass and charge transport and volume change of electrodes that commonly possess low electrical conductivity. One strategy for improving battery performance is the use electrodes away from thermodynamic equilibrium. These include nanostructures with high surface energy, poor-crystalline materials, and materials with significant surface or bulk defects. Such materials are in a higher energy state and more readily undergo phase transformation and nucleation. Their less closely packed structures enhance mass transport, lithium-ion accommodation, and tolerance to volume change. As an example, Figure 1 shows cycling performance of V2O5 electrodes in pure form (a), after Sn doping (b), and in the nano-belt configuration (c) [1,2]. Both the pure and Sn-doped electrodes exhibit an initial drop in specific capacity followed by an activation period that is somewhat longer for the Sn-doped electrode than for the pure electrode. The specific capacity of the pure electrode decays to about 180 mAh g–1 within 50 cycles. Notably, the Sn-doped electrode retains nearly its maximum capacity of 320 mAh g–1 throughout 50 cycles. Given the similar morphologies of these two electrodes, shown in Fig. 2(a) and (b), the change in capacity is most likely due to a chemical or electronic structure effect brought about by Sn-doping. In contrast, the Na400 nano-belt electrode, prepared by reaction of V2O5 with H2C2O4 and NaNO3 and subsequent calcining to 400 °C, exhibits a steady yet slightly increasing capacity with cycling. The capacities of the pure V2O5 electrode (a) and the nano-belt electrode (c) become similar at about 40 cycles. This similarity may reflect changes in surface area of the pure electrode. Initially, the nano-belt electrode has, due to its well-formed belt structure [Fig. 2(c)], lower surface area than does the pure electrode. With cycling time the pure electrode eventually loses surface area, reaching a point similar to that of the nano-belt. The extent of other factors, such as surface and bulk defects and volume change, are currently under investigation. This presentation will consider V2O5 electrodes and a new lithium titanate electrode as two model materials to illustrate the influences of doping, surface defects and carbon coating, and nanostructures on the lithium-ion intercalation properties. 1. Y. Li, J. Yao, E. Uchaker, M. Zhang, J. Tian, X. Liu, and G. Cao, J. Phys. Chem. C, 117, 23507 (2013). 2. S. Liang, T. Chen, A. Pan, D. Liu, Q. Zhu, and G. Cao, ACS Appl. Mater. Interfaces, 5, 11913 (2013).