Metal-solid Electrolyte Interface Research Articles

Cyclic electrochemical reactions on a Li-metal negative electrode having a low-resistance interface with a solid-state electrolyte

Li metal is a promising negative-electrode material for high-energy-density all-solid-state batteries. However, the surface of Li metal is prone to oxidation, which results in the formation of a contamination layer at the Li metal–solid electrolyte interface. This interfacial contamination layer is the root cause of short-circuiting and poor cycle stability, thus hindering the development of all-solid-state batteries. Prior studies have not quantitatively assessed the effect of the above layer on battery performance. Herein, the degradation mechanisms affecting the interface are investigated using alternating-current impedance measurements and Li plating–stripping cycle tests for a symmetric cell. A thin contamination layer results in a Li–electrolyte interface with a low resistance of 0.20 kΩ cm2 and stable Li plating–stripping behavior at a current density of 3 mA cm−2, whereas a thick contamination layer results in a high interfacial resistance of 2.0 kΩ cm2. The thinning of the contamination layer on Li metal enhances the stability of the Li–electrolyte interface and Li plating–stripping kinetics.

Operando Characterization of Plating and Stripping Dynamics of Li-Mg Alloy Anode and Sulfide Solid Electrolyte Interface

All-solid-state lithium metal batteries are projected to offer one of the highest specific energy among rechargeable batteries, positioning them as a front-runner for electric vehicle applications. Lithium metal anodes outperform conventional graphite anodes in terms of cell-level energy density. However, maintaining a conformal metal–electrolyte contact is a great challenge. The volumetric change of the lithium metal anode during stripping produces voids, which deteriorates the Li–electrolyte contact. To address this challenge, β-phase Li solid solution (i.e., Li-Mg alloy) has been shown to exhibit improved contact with the solid electrolyte during stripping. Moreover, Li solid solution has a similar electrochemical potential as Li, making it a potential replacement for Li metal anodes without sacrificing cell-level energy density. In this study, we show that the chemical properties of the alloy play an essential role in the morphological stability of metal–solid electrolyte interface. We compare the stripping behavior of pure Li and Li-Mg alloy anodes in argyrodite electrolyte and characterize the morphological evolution of the interface using operando scanning electron microscopy (SEM). We also analyze the electro-chemo-mechanical properties of Li and Li alloy anodes during stripping with galvanostatic electrochemical impedance spectroscopy (GEIS) to compare the influence of composition change and interfacial void on the electrode overpotential and interface resistance. Our research visualizes the morphological and compositional evolution of alloy metal anode during stripping and rationalizes their improved interfacial stability against solid electrolytes.Acknowledgment: This work was supported by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) under the Vehicle Technologies Program under Contract DE-EE0008864. Figure 1

Ultrahigh Areal Capacity Li Electrodeposition at Metal-Solid Electrolyte Interfaces under Minimal Stack Pressures Enabled by Interfacial Na-K Liquids.

The need for higher energy density rechargeable batteries has generated interest in metallic electrodes paired with solid electrolytes. However, impedance growth at the Li metal-solid electrolyte interface due to void formation during cycling at practical current densities and areal capacities, e.g., greater than 0.5 mA cm-2 and 1.5 mAh cm-2 respectively, remains a significant barrier. Here, we show that introducing a wetting interfacial film of Na-K liquid between the Li metal and the Li6.75La3Zr1.75Ta0.25O12 (LLZTO) solid electrolyte permits reversible stripping and plating of up to 150 μm of Li (30 mAh cm-2), approximately 10 times the areal capacity of today's lithium-ion batteries, at current densities above 0.5 mA cm-2 and stack pressures below 75 kPa, all with minimal changes in cell impedance. We further show that this increase in the accessible areal capacity at high stripping current densities is due to the presence of Na-K liquid at the Li stripping interface; this performance improvement is not enabled in the absence of the Na-K liquid. This design approach holds promise for overcoming interfacial stability issues that have heretofore limited the performance of solid-state metal batteries.

(Invited) How Does One Enable High Energy Li Metal Batteries a Case Study with Lipon

The next generation of high energy batteries using Li metal as an anode and utilizing a solid electrolyte requires good interfacial stability, reasonable ionic conductivity, and most importantly the ability to eliminate dendrites.i While most solid-state electrolytes suffer from both interfacial instability and Li penetration resulting in cell shorting,ii,iii The solid electrolyte Lipon has a remarkable ability to suppress dendrites, and enable solid state batteries that can cycle for more than 1000 cycles and support current densities as high as 10 mA/cm2.iii,iv Recent efforts have started to illuminate the origin of Lipon’s ability to enable high-energy solid-state Li metal batteries. This presentation will highlight the unique structure of Lipon and Lipon like materials, the interfacial structure of Lipon with Li metal, and the mechanics that enable high quality performance in Lipon based solid-state batteries.v,vi,vii Finally, the presentation will connect these findings from Lipon to outline key underlying principles that answer the question, how do we enable high energy Li metal batteries? Acknowledgements: Work for this presentation was funded by ARPA-E under contract #DE-AR0000775, and the US Department of Energy Office of Energy Efficiency and Renewable Energy for the Vehicle Technologies Office’s US-German Cooperation on Energy Storage: Interfaces and Interphases In Rechargeable Li-metal based Batteries Program and the Battery Materials Research Program under program managers Tien Duong and Simon Thompson. P. Albertus et al., “Challenges for and pathways toward Li-metal-based all-solid-state batteries”, ACS Energy Letters, 6, 4, 1399-1404, Mar. 2021.Fudong Han, Andrew S Westover, Jie Yue, Xiulin Fan, Fei Wang, Miaofang Chi, Donovan N Leonard, Nancy J Dudney, Howard Wang, Chunsheng Wang, “High electronic conductivity as the origin of lithium dendrite formation within solid electrolytes”, Nature Energy, 1, Jan. 2019. Andrew S Westover, Nancy J Dudney, Robert L Sacci, Sergiy Kalnaus, “Deposition and Confinement of Li Metal along an Artificial Lipon–Lipon Interface”, ACS Energy Letters, 4, 651-655, Feb. 2019.Juchuan Li, Cheng Ma, Miaofang Chi, Chengdu Liang, and Nancy J. Dudney. "Solid electrolyte: the key for high‐voltage lithium batteries." Advanced Energy Materials5, 4, 1401408, 2015. Andrew S Westover* Valentina Lacivita,* , Andrew Kercher, Nathan D Phillip, Guang Yang, Gabriel Veith, Gerbrand Ceder, Nancy J Dudney, “Resolving the amorphous structure of lithium phosphorus oxynitride (Lipon)”, Journal of the American Chemical Society 140 (35), 11029-11038, July 2018. Andrew S Westover, Robert L Sacci, Nancy Dudney, “Electroanalytical measurement of interphase formation at a Li metal–solid electrolyte interface”, ACS Energy Letters, 5, 12, 3860-3867, Nov. 2020.Sergiy Kalnaus, Andrew S Westover, Mordechai Kornbluth, Erik Herbert, Nancy J Dudney, “Resistance to fracture in the glassy solid electrolyte Lipon”, Journal of Materials Research, 36, 4, 787-796, Feb. 2021.

Heterogeneity-Driven Interface Instability in Solid-State Batteries

While solid-state batteries promise to enhance the energy density and power density of conventional lithium-ion batteries, the morphological instability of the lithium metal anode persists to be a major challenge. During plating and stripping, the dynamic evolution of the lithium metal-solid electrolyte interface is governed by a coupled set of electrochemical-mechanical-transport processes. This mechanistic interaction is influenced by several factors including the mechanical properties of the solid electrolyte and lithium, presence of heterogeneities such as grain boundaries, defects and voids, and external pressure/temperature. In this presentation, the underlying role of microstructural and interfacial heterogeneities on the reaction kinetics, morphological growth, and the origin of electrochemical-mechanical failure will be examined. A fundamental analysis on how such heterogeneity-driven implications scale with plating and stripping currents, and their mechanistic dependence on the interfacial stress-temperature fields will be presented.

Mechanism of Void Formation in Lithium Metal Solid-State Batteries

Solid-state batteries (SSBs) with lithium metal anodes promise higher energy and power density over conventional lithium-ion batteries. But the development of SSBs faces fundamental challenges related to the electro-chemo-mechanics and stability of solid-solid interfaces. Among these, the formation and growth of voids at the lithium metal-solid electrolyte interface during stripping persists to be a major limitation. In this work, we analyze the nature of metal electro-dissolution and its critical implication on the evolution of solid-solid point contacts. The competing nature of lithium self-diffusion, reaction kinetics and ionic transport at the solid-solid interface, dependent on factors such as temperature and surface roughness are examined. Hotspots in the dissolution morphologies are identified and systematically connected to the electrochemical signature and onset of failure in SSBs.

(Invited) Designing Fault-Tolerant Interfaces Between Metal Electrodes and Solid Electrolytes

The use of alkali metal electrodes is widely considered to be an enabler for the next generation of high energy-density rechargeable batteries [1]. In all-solid-state systems, the most critical interface appears to be that between the alkali metal and the solid electrolyte, from which metal-filled cracks can initiate and grow into single-crystal, polycrystal, and glassy electrolytes alike [2] under sufficiently high electrochemical stress. However, failure can be mitigated by softening of the metal electrode, whether through increases in temperature (including melting) or changes in composition (including changing alkali metals [3]). Here, we discuss semi-solid metal electrode design approaches in which a minor liquid phase fraction is deliberately introduced to produce a self-healing function that enables high current densities [3]. A bulk semi-solid electrode approach is demonstrated using Na–K alloys with controlled liquid fraction between the state-of-charge limits; these show potassium ion critical current densities (using the a K-β″-alumina electrolyte) that exceed 15 mA cm‒2. An interfacial wetting approach uses a thin interfacial film of Na–K liquid between a Li metal electrode and an LLZTO solid electrolyte; here the critical current density is doubled, and cyclable areal capacities exceed 3.5 mAh cm‒2. Moreover, evidence from both approaches suggest that void formation in solid metal electrodes during cycling at practical current densities (>0.5 mA/cm2) [4], manifested as impedance growth at the metal-solid electrolyte interface, can be largely mitigated through these semi-solid design strategies.Support from the US Department of Energy, Office of Basic Energy Science, through award no. DE-SC0002633 (J. Vetrano, Program Manager), is gratefully acknowledged.[1] Albertus, P., Babinec, S., Litzelman, S. & Newman, A. Nat. Energy 3, 16–21 (2018).[2] Porz, L. et al. Adv. Energy Mater. 7, 1701003 (2017).[3] Park, R.J-Y. et al. Nat Energy 6, 314–322 (2021).[4] Kasemchainan, J. et al. Nat. Mater. 18, 1105–1111 (2019). Figure 1

(Invited) Semi-Solid Alkali Metal Electrodes Enabling High Critical Current Densities and Accessible Areal Capacities in Solid Electrolyte Batteries

The need for higher energy-density rechargeable batteries has generated interest in alkali metal electrodes paired with solid electrolytes [1]. However, metal penetration and electrolyte fracture at low current densities [2], as well as impedance growth at the metal-solid electrolyte interface due to void formation during cycling at practical current densities (> 0.5 mA cm-2), have emerged as fundamental barriers [3]. Here we demonstrate two semi-solid electrode architectures in which the presence of a minor liquid phase enables high current densities while it preserves the shape retention and packaging advantages of solid electrodes [4]. First, biphasic Na–K alloys selected for controlled liquid fraction between the state-of-charge limits show K+ critical current densities (with a K-β″-Al2O3 electrolyte) that exceed 15 mA cm‒2. Second, introducing a wetting interfacial film of Na–K liquid between a Li metal electrode and Li6.75La3Zr1.75Ta0.25O12 solid electrolyte doubles the critical current density and permits cycling at areal capacities that exceed 3.5 mAh cm‒2. These design approaches hold promise for overcoming electrochemomechanical stability issues that have heretofore limited the performance of solid-state metal batteries.We acknowledge support from the US Department of Energy, Office of Basic Energy Science, through award no. DE-SC0002633 (J. Vetrano, Program Manager).[1] Albertus, P., Babinec, S., Litzelman, S. & Newman, A. Nat. Energy 3, 16–21 (2018).[2] Porz, L. et al. Adv. Energy Mater. 7, 1701003 (2017).[3] Kasemchainan, J. et al. Nat. Mater. 18, 1105–1111 (2019).[4] Park, R.J-Y. et al. Nat Energy 6, 314–322 (2021). Figure 1

Electroanalytical Measurement of Interphase Formation at a Li Metal–Solid Electrolyte Interface

The interphase between Li metal and solid electrolytes dictates performance in Li metal batteries. For the solid electrolyte Lipon, we electroanalytically measure a capacity loss of 1.03 ± 0.16 μAh...

Microstructure Ramifications on Electrodeposition for Solid-State Electrolytes

Safe Li-ion batteries with enhanced energy and power density rely on utilization of lithium metal anodes in conjunction with non-flammable solid electrolytes that inhibit dendritic growth through mechanical rigidity. The kinetics, transport, mechanics complexations are strongly dependent on the microstructure of the solid electrolyte. In particular, garnet type electrolytes exhibit crystalline structure consisting of grain, grain boundaries and voids while glassy sulfide type electrolytes exhibit an amorphous structure with voids; with prominent dependence of the transport properties (ionic conductivity) and mechanical properties (shear modulus, fracture toughness) on the structural arrangement. The Li metal-solid electrolyte interface also shows a wide degree of stochasticity due to the imperfect solid-solid contact delineating the importance of surface roughness effects on kinetics and interlayer design to promote surface uniformity. In this work, we investigate these coupled interactions and delineate electrodeposition stability for aforementioned class of inorganic solid electrolytes cognizant of microstructural considerations.

Correlating the interface resistance and surface adhesion of the Li metal-solid electrolyte interface

Solid electrolytes could enable stable cycling of metallic Li anodes, which can offer drastic increases to the capacity of Li-ion batteries. However, little is known about the mechanics of the Li-solid electrolyte interface. This study combines electrochemical and mechanical characterization to correlate interface kinetics with adhesive strength. Cubic garnet with the Li6·25Al0·25La3Zr2O12 (LLZO) formulation was selected as a model solid electrolyte based on its high conductivity and stability against Li metal. Symmetric Li-LLZO cells were tested using electrochemical impedance spectroscopy to determine the interfacial resistance, Rint, and the adhesive strength of the Li-LLZO interface, σadh, was measured using a unique tensile test in an inert atmosphere. It was determined that the Rint is directly correlated to the adhesive strength of Li on LLZO. At the highest Rint in this study, 330 k·cm2 the σadh was 1.1 kPa and at the lowest Rint in this study, 7 ·cm2, σadh was 8 MPa. Furthermore, by optimizing the surface chemistry the wettability of LLZO was enhanced resulting in σadh exceeding the ultimate tensile strength of Li metal. The relationship demonstrated provides a deeper understanding of the mechanical properties of the Li-electrolyte interface, which will play an important role in the design of batteries employing metallic Li anodes.

Systematic Modeling for Triple Phase Boundary of Ni/ZrO2 SOFC Anode from First Principles

Introduction In solid electrolyte-based electrochemical devices such as fuel cells and gas sensors, the devices include hetero interfaces between solid electrolyte and metal, which play an important role for the device working mechanism. The interface between solid-electrolyte and solid-metal can be recognized as a two phase boundary, and the two phase boundary exposed to vacuum (i.e., gas phase) is called as triple (or three) phase boundary (TPB). TPB in electrochemical devices is a key boundary for electrochemical reactions because the boundary is an active site for charge transfer reactions, and thus for the production of electricity. In addition, a rate limiting step in electrochemical devices is often identified in the electrochemical reactions at TPB, although a specific investigation is required in each system. In this study, we carried out first principles electronic structure calculations for the precise understanding of electrochemical processes on Ni-YSZ TPB, and show how TPB models should be constructed in the atomistic level, and what is the suitable index for the characterization of the electrochemical activity of TPB. Method For the investigation of electrochemical reactions in the atomistic level, first principles electronic structure method is a powerful theoretical tool, but the calculation procedures are not so simple when the target system includes complicated interface TPBs. The TPB atomistic model must include the following ingredients: 1) metal region (i.e., slab), 2) solid electrolyte region (slab), 3) the metal-solid electrolyte interface, and 4) the interface exposed to vacuum [1,2]. When we construct a TPB model holding these conditions, the number of atoms included in the computational cell becomes large, and reaction analysis based on the large models is sometimes found to be inappropriate depending on the computational costs. Therefore, theoretical calculations for reaction analysis on TPB were sometimes carried out using small metal-cluster deposited on solid electrolyte slab. However, the electronic structures of metal-slab and metal-cluster are very different because of the finite size effect of the metal cluster model. At the first part of this study, we will present how the metal cluster models on solid electrolyte inappropriately affect computational results at TPB, and how an acceptable TPB model should be constructed based on metal-slab layers (see an example for TPB structure in Figure). The second step is the selection of the stoichiometry and the determination of impurity positions. For example in yttrium stabilized zirconia (YSZ) as solid electrolyte, the numbers of dopant Y and oxygen vacancy are determined depending on the stoichiometry and dopant concentration. Thus, we usually fix the system stoichiometry and dopant concentration at first (e.g., stoichiometric or oxygen-rich or oxygen poor in YSZ), and in turn determine the position of dopants from the calculated total energies. The reaction analysis using the atomistic models are thereby carried out as the final step by using first principles calculations with the nudged elastic band method. Since the standard calculations are done along the procedure, the computational results are usually characterized in terms of the system stoichiometry. However, in electrochemical reactions in TPB, the system stoichiometry is sometimes not a good index for the characterization of TPB activity. At the second part of this study, we will present the calculated reaction energies at TPB are highly scattered when those are classified with the system stoichiometry, but clearly characterized when those are classified with a new index, TPB local stoichiometry. We will present the detail of the new concept for the TPB local stoichiometry. [1] Tomofumi Tada, Shusuke Kasamatsu, and Satoshi Watanabe, First principles study on electronic structures of Ni/H/ZrO2 triple phase boundary, ECS Transactions 16(51) , 265 (2009). [2] Tomofumi Tada and Satoshi Watanabe, Chemically softened boundary of metal/vacuum/solid-electrolyte from first principles, J. Phys. Chem. C 113 , 17780 (2009). Figure 1

Oxygen Role in the Electrochemical Response of a Gas Sensor Using Ideally Polarizable Electrodes

In an attempt to explain the electrochemical effects observed on an original potentiometric gas sensor, the influence of oxygen pressure and temperature on the electrochemical response of the device were largely investigated. The sensor consists of a solid electrolyte associated with two different electrodes, one in platinum the other in gold, located in the same gas mixture. A thermodynamic model is proposed, based first on the competitive adsorption of two oxygen species on the metals and second on the capacitance effects which may occur at the metal solid electrolyte interface. The simulation of the model provides a satisfactory solution if one of the oxygen species is considered as a metastable particle responsible for the electrostatic potential that appears at the metal‐solid electrolyte interface. Additional electrostatic experiments confirm the role of oxygen in the potentials of platinum and gold which appear as ideally polarizable electrodes. © 1999 The Electrochemical Society. All rights reserved.

Electrochemical promotion of IrO2 catalyst activity for the gas phase combustion of ethylene

The catalytic activity for the gas phase combustion of ethylene of an IrO2 film, used as electrode in a solid electrolyte cell, can be increased by polarizing the metal–solid electrolyte interface. The catalytic activity remains higher even after current interruption. To explain the increase in the open circuit rate after polarization we speculate that the IrO2 catalyst is oxidized at the catalyst–solid electrolyte interface, forming IrO2+delta, which is transported by surface and/or bulk diffusion to the gas exposed catalyst, thus modifying its catalytic activity.

The double layer structure at the metal-solid electrolyte interface

The Gouy-Chapman-Stern model (GCS) has been extended to the case of the metal-solid electrolyte interface in which the positions of charges in the electrolyte are discrete. Our model allows mobile positive and negative charges to move freely parallel to the metal electrode surface, but constrains the charges to lattice planes at fixed distances Δx, 2 Δx, 3 Δxnormal to the metal surface. The non-linearised Poisson-Boltzmann equation for this system was solved to compute the potentials and charges on each plane and interfacial capacitance-potential curves ( C DL − E) were constructed. We compare these C DL − E curves with those predicted by the GCS model. Significant deviations from GCS were found when the condition kΔxθ 0 ⪢ 1 was not satisfied. Here k is the reciprocal of the Debye Length, Δx is the lattice plane spacing and θ 0 ( = FΦ 0/ RT) is the normalised interfacial potential drop ( Φ 0). The condition kΔx ⪡ 1 was not sufficient because at large θ 0 the charge decays more rapidly with distance than the potential and effects of discretisation are still observed. The single mobile carrier case was treated and good agreement with the Mott-Schottky depletion layer approximation was obtained. We have also treated cases in which there is a limit to the amount of charge which can be accommodated on a single lattice plane; the effect of this restriction is to introduce a maximum in the C DL − E curve as the potential is moved away from the potential of zero charge. Incorporation of a disordered and therefore active first layer in which the density of carriers is larger than in the bulk results in the ‘washing out’ of the diffuse layer minimum. The model was extended to include specific adsorption of anions in a plane halfway between the metal surface and the first lattice plane. When charge occupancy of the planes was allowed to reach high values (> 10 μC cm −2) the C DL− E curve shows only a transition between two Helmholtz-like capacitances. Restricting the charge occupancy on the planes results in a peak in the C DL − E curve at a potential determined by the free energy of adsorption and a positive shift in the potential of the diffuse layer minimum. This shift disappears as the charge occupancy on the planes is decreased further.

95/01862 Study of heat transfer augmentation in a high-temperature field by a radiation promoter generating a secondary flow

Solid electrolytes can be used as active catalyst supports to dramatically and reversibly alter the catalytic properties of porous metal films by electrically polarizing the metal-solid electrolyte interface. The common features of this effect, termed Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA), are summarized in the present work, together with the origin of the effect, which is due to the controlled change in catalyst work function and to the concomitant changes in the strength of chemisorptive bonds

Electrochemical promotion in catalysis: non-faradaic electrochemical modification of catalytic activity

The catalytic activity and selectivity of the gas-exposed electrode surface of metal electrodes in solid electrolyte cells is altered dramatically and reversibly upon polarizing the metal—solid electrolyte interface. The induced steady-state change in catalytic rate can be up to 9000% higher than the normal (open-circuit) catalytic rate and up to 3 × 10 5 higher than the steady-state rate of ion supply. This new effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) has been already demonstrated for more than 30 catalytic reactions on Pt, Pd, Rh, Ag, Au and Ni surface by using O 2−, F −, Na + and H + conducting solid electrolytes. There is also a recent demonstration for an aqueous electrolyte system. In this paper the common features of previous NEMCA studies are summarized and the origin of the effect is discussed in light of recent in situ work function and XPS measurements which showed that: (1) solid electrolyte cells with metal electrodes are work function probes and work function controllers, via potential application, for their gas-exposed electrode surfaces; and (2) NEMCA is due to an electrochemically driven and controlled spillover of ions from the solid electrolyte onto the gas-exposed electrode surface. These spillover ions establish an effective electrochemical double layer and act as promoters for catalytic reactions. This interfacing of electrochemistry and catalysis offers several exciting theoretical and technological possibilities.

Non-Faradaic electrochemical modification of catalytic activity 5. Oxygen chemisorption on silver

The technique of isothermal surface titration was used to investigate the chemisorptive properties of porous Ag films deposited on Y 2O 3-doped ZrO 2 in a solid electrolyte cell. It was found that the chemisorptive properties of Ag can be influenced in a pronounced and reversible manner upon polarizing electrochemically the metal-solid electrolyte interface. Decreasing catalyst work function has very little effect on subsurface oxygen but strengthens significantly the Ag-atomic oxygen chemisorptive bond, as inferred both by the observed pronounced increases in the rate of oxygen adsorption and in the equilibrium oxygen coverage and by the observed significant decrease in the rate of atomic oxygen desorption. The results are consistent with the previously proposed interpretation of the NEMCA effect and show that solid electrolytes can be used as active catalyst supports to alter significantly the chemisorptive and catalytic properties of metals.

Study of the NEMCA effect in a single-pellet catalytic reactor

It has been recently found that solid electrolytes can be used as active catalyst supports to dramatically alter the catalytic properties of porous metal catalyst films by polarizing the metal-solid electrolyte interface. This new effect of non-Faradic electrochemical modification of catalytic activity (NEMCA) has been demonstrated using both O{sup 2{minus}} and Na{sup +} conducting solid solid electrolytes. It has been attributed to the controlled change in catalyst work function and to the concomitant changes in the strength of chemisorptive bonds due to ion spillover onto the catalyst surface created at the three-phase boundaries upon polarization of the metal-solid electrolyte interface. NEMCA can induce reversible catalytic rate changes in product selectivity. The increase in catalytic rate can be up to a factor 3 {times} 10{sup 5} higher than the rate of ion transfer to or from the catalyst film. In this work the authors examine the possibility of inducing NEMCA during C{sub 2}H{sub 4} oxidation on Pt utilizing a stabilized ZrO{sub 2} pellet with both sides exposed to a reacting C{sub 2}H{sub 4}/O{sub 2} mixture so that all three electrodes (catalyst, counter, and reference) are exposed to the reacting mixture. This reaction has been recently investigated in a fuel-cell type reactormore » and found to exhibit a pronounced NEMCA effect. Furthermore, they examine the existence of differences in NEMCA behavior for different arrangements of the counter and reference electrodes relative to the catalyst-working electrode.« less

Non-Faradaic electrochemical modification of catalytic activity: the work function of metal electrodes in solid electrolyte cells

The catalytic activity and selectivity of metal films used as electrodes in solid electrolyte cells can be altered dramatically and reversibly by polarizing the metal-solid electrolyte interface. This effect, termed non-Faradaic electrochemical modification of catalytic activity (NEMCA) leads to steady state catalytic rate increases up to 3×10 5 times higher than the steady state rate of removal or supply of ions. Catalytic rates can be enhanced reversibly up to 7000%. The NEMCA effect has been already demonstrated with O 2− and Na + conducting solid electrolytes using Pt, Pd, Ag, Ni, Rh and Au electrodes. In the present work we summarize some of the common experimental findings of previous studies and discuss the origin of NEMCA which lies in the controlled variation of the catalyst surface work function upon polarization of the catalyst-solid electrolyte interface. We show both experimentally, using a Kelvin probe, and theoretically that in solid electrolyte cells there exists a one-to-one correlation between ohmic-drop-free electrode potential and gas-exposed electrode surface work function. Thus solid electrolyte cells with metal electrodes can be used both to measure and to control the gas-exposed electrode work function. This controlled variation in catalyst-electrode work function is due to ion spillover and results in significant variations in the binding strength of chemisorbed species, thus causing the NEMCA effect.