Adsorption of Natural Gas in Metal-Organic Frameworks: Selectivity, Cyclability, and Comparison to Methane Adsorption.

Evaluation of metal-organic frameworks (MOFs) for adsorbed natural gas (ANG) technology employs pure methane as a surrogate for natural gas (NG). This approximation is problematic, as it ignores the impact of other heavier hydrocarbons present in NG, such as ethane and propane, which generally have more favorable adsorption interactions with MOFs compared to methane. Herein, using quantitative Raman spectroscopic analysis and Monte Carlo calculations, we demonstrate the adsorption selectivity of high-performing MOFs, such as MOF-5, MOF-177, and SNU-70, for a methane and ethane mixture (95:5) that mimics the composition of NG. The impact of selectivity on the storage and deliverable capacities of these adsorbents during successive cycles of adsorption and desorption, simulating the filling and emptying of an ANG tank, is also demonstrated. The study reveals a gradual reduction in the storage performance of MOFs, particularly with smaller pore volumes, due to ethane accumulation over long-term cycling, until a steady state is reached with substantially degraded storage performance.

Energy storage emerging: A perspective from the Joint Center for Energy Storage Research.

Energy storage is an integral part of modern society. A contemporary example is the lithium (Li)-ion battery, which enabled the launch of the personal electronics revolution in 1991 and the first commercial electric vehicles in 2010. Most recently, Li-ion batteries have expanded into the electricity grid to firm variable renewable generation, increasing the efficiency and effectiveness of transmission and distribution. Important applications continue to emerge including decarbonization of heavy-duty vehicles, rail, maritime shipping, and aviation and the growth of renewable electricity and storage on the grid. This perspective compares energy storage needs and priorities in 2010 with those now and those emerging over the next few decades. The diversity of demands for energy storage requires a diversity of purpose-built batteries designed to meet disparate applications. Advances in the frontier of battery research to achieve transformative performance spanning energy and power density, capacity, charge/discharge times, cost, lifetime, and safety are highlighted, along with strategic research refinements made by the Joint Center for Energy Storage Research (JCESR) and the broader community to accommodate the changing storage needs and priorities. Innovative experimental tools with higher spatial and temporal resolution, in situ and operando characterization, first-principles simulation, high throughput computation, machine learning, and artificial intelligence work collectively to reveal the origins of the electrochemical phenomena that enable new means of energy storage. This knowledge allows a constructionist approach to materials, chemistries, and architectures, where each atom or molecule plays a prescribed role in realizing batteries with unique performance profiles suitable for emergent demands.

Discovery of Salt Hydrates for Thermal Energy Storage.

Thermal energy storage (TES) has the potential to improve the efficiency of many applications but has not been widely deployed. The viability of a TES system depends upon the performance of its underlying storage material; improving the energy density of TES materials is an important step in accelerating the adoption of TES systems. For applications in thermochemical energy storage, salt hydrates are a promising class of materials due to their relatively high energy densities and their reversibility. Despite their promise, relatively few salt hydrates have been characterized, presenting the possibility that new hydrate compositions with superior properties may exist. Here, the energy densities, turning temperatures, and thermodynamic stabilities of 5292 hypothetical salt hydrates are predicted using high-throughput density functional theory calculations. The hydrates of several metal fluorides, including CaF2, VF2, and CoF3, are identified as stable TES materials with class-leading energy densities and operating temperatures suitable for use in domestic heating and intermediate-temperature applications. The promising performance of these materials is demonstrated at the system level by parameterizing an operating model of a solar thermal TES system with data from the new hydrates. Finally, machine learning models for salt hydrate thermodynamics are developed and used to identify design guidelines for maximizing the energy density. In total, the new materials and design rules reported here are expected to nurture the implementation of TES systems.

Microscale Determination of Binary Gas Adsorption Isotherms in MOFs.

The experimental determination of mixed gas isotherms is challenging and thus rarely performed. Nevertheless, characterizing the performance of adsorbents toward mixtures of gases is critical in most adsorptive separations. Here, the utility of Raman spectroscopy in determining binary gas adsorption isotherms on the microscale with metal-organic framework (MOF) single crystals is demonstrated for quantifying C2H6/CH4 selectivity. The influence of pore size on sorption selectivity is determined experimentally. The technique also allows determination of kinetics of methane adsorption in MOFs, which is critical for refueling times in adsorbed natural gas storage.

Computational Identification and Experimental Demonstration of High-Performance Methane Sorbents.

Remarkable methane uptake is demonstrated experimentally in three metal-organic frameworks (MOFs) identified by computational screening: UTSA-76, UMCM-152 and DUT-23-Cu. These MOFs outperform the benchmark sorbent, HKUST-1, both volumetrically and gravimetrically, under a pressure swing of 80 to 5 bar at 298 K. Although high uptake at elevated pressure is critical for achieving this performance, a low density of high-affinity sites (coordinatively unsaturated metal centers) also contributes to a more complete release of stored gas at low pressure. The identification of these MOFs facilitates the efficient storage of natural gas via adsorption and provides further evidence of the utility of computational screening in identifying overlooked sorbents.

Optimizing Hydrogen Storage in MOFs through Engineering of Crystal Morphology and Control of Crystal Size.

Metal-organic frameworks (MOFs) are promising materials for hydrogen storage that fail to achieve expected theoretical values of volumetric storage density due to poor powder packing. A strategy that improves packing efficiency and volumetric hydrogen gas storage density dramatically through engineered morphologies and controlled-crystal size distributions is presented that holds promise for maximizing storage capacity for a given MOF. The packing density improvement, demonstrated for the benchmark sorbent MOF-5, leads to a significant enhancement of volumetric hydrogen storage performance relative to commercial MOF-5. System model projections demonstrate that engineering of crystal morphology/size or use of a bimodal distribution of cubic crystal sizes in tandem with system optimization can surpass the 25 g/L volumetric capacity of a typical 700 bar compressed storage system and exceed the DOE targets 2020 volumetric capacity (30 g/L). Finally, a critical link between improved powder packing density and reduced damage upon compaction is revealed leading to sorbents with both high surface area and high density.

Synthesis of Antiperovskite Solid Electrolytes: Comparing Li3SI, Na3SI, and Ag3SI.

Prior calculations have predicted that chalcohalide antiperovskites may exhibit enhanced ionic mobility compared to oxyhalide antiperovskites as solid-state electrolytes. Here, the synthesis of Ag-, Li-, and Na-based chalcohalide antiperovskites is investigated using first-principles calculations and in situ synchrotron X-ray diffraction. These techniques demonstrate that the formation of Ag3SI is facilitated by the adoption of a common body centered cubic packing of S2- and I- in the reactants and products at elevated temperatures, with additional stabilization achieved by the formation of a solid solution of the anions. The absence of these two features appears to hinder the formation of the analogous Li and Na antiperovskites.

Kinetic Stability of MOF-5 in Humid Environments: Impact of Powder Densification, Humidity Level, and Exposure Time.

Metal-organic frameworks (MOFs) are an emerging class of microporous, crystalline materials with potential applications in the capture, storage, and separation of gases. Of the many known MOFs, MOF-5 has attracted considerable attention because of its ability to store gaseous fuels at low pressure with high densities. Nevertheless, MOF-5 and several other MOFs exhibit limited stability upon exposure to reactive species such as water. The present study quantifies the impact of humid air exposure on the properties of MOF-5 as a function of exposure time, humidity level, and morphology (i.e., powders vs pellets). Properties examined include hydrogen storage capacity, surface area, and crystallinity. Water adsorption/desorption isotherms are measured using a gravimetric technique; the first uptake exhibits a type V isotherm with a sudden increase in uptake at ∼50% relative humidity. For humidity levels below this threshold only minor degradation is observed for exposure times up to several hours, suggesting that MOF-5 is more stable than generally assumed under moderately humid conditions. In contrast, irreversible degradation occurs in a matter of minutes for exposures above the 50% threshold. Fourier transform infrared spectroscopy indicates that molecular and/or dissociated water is inserted into the skeletal framework after long exposure times. Densification into pellets can slow the degradation of MOF-5 significantly, and may present a pathway to enhance the stability of some MOFs.

Thermodynamics, Adhesion, and Wetting at Li/Cu(-Oxide) Interfaces: Relevance for Anode-Free Lithium-Metal Batteries.

A rechargeable battery that employs a Li metal anode requires that Li be plated in a uniform fashion during charging. In "anode-free" configurations, this plating will occur on the surface of the Cu current collector (CC) during the initial cycle and in any subsequent cycle where the capacity of the cell is fully accessed. Experimental measurements have shown that the plating of Li on Cu can be inhomogeneous, which can lower the efficiency of plating and foster the formation of Li dendrites. The present study employs a combination of first-principles calculations and sessile drop experiments to characterize the thermodynamics and adhesive (i.e., wetting) properties of interfaces involving Li and other phases present on or near the CC. Interfaces between Li and Cu, Cu2O, and Li2O are considered. The calculations predict that both Cu and Cu2O surfaces are lithiophilic. However, sessile drop measurements reveal that Li wetting occurs readily only on pristine Cu. This apparent discrepancy is explained by the occurrence of a spontaneous conversion reaction, 2 Li + Cu2O → Li2O + 2 Cu, that generates Li2O as one of its products. Calculations and sessile drop measurements show that Li does not wet (newly formed) Li2O. Hence, Li that is deposited on a Cu CC where surface oxide species are present will encounter a compositionally heterogeneous substrate comprising lithiophillic (Cu) and lithiophobic (Li2O) regions. These initial heterogeneities have the potential to influence the longer-term behavior of the anode under cycling. In sum, the present study provides insights into the early stage processes associated with Li plating in anode-free batteries and describes mechanisms that contribute to inefficiencies in their operation.

Thermodynamic screening of metal-substituted MOFs for carbon capture.

Metal-organic frameworks (MOFs) have emerged as promising materials for carbon capture applications due to their high CO2 capacities and tunable properties. Amongst the many possible MOFs, metal-substituted compounds based on M-DOBDC and M-HKUST-1 have demonstrated amongst the highest CO2 capacities at the low pressures typical of flue gasses. Here we explore the possibility for additional performance tuning of these compounds by computationally screening 36 metal-substituted variants (M = Be, Mg, Ca, Sr, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, W, Sn, and Pb) with respect to their CO2 adsorption enthalpy, ΔH(T=300K). Supercell calculations based on van der Waals density functional theory (vdW-DF) yield enthalpies in good agreement with experimental measurements, out-performing semi-empirical (DFT-D2) and conventional (LDA & GGA) functionals. Our screening identifies 13 compounds having ΔH values within the targeted thermodynamic window -40 ≤ ΔH ≤ -75 kJ mol(-1): 8 are based on M-DODBC (M = Mg, Ca, Sr, Sc, Ti, V, Mo, and W), and 5 on M-HKUST-1 (M = Be, Mg, Ca, Sr and Sc). Variations in the electronic structure and the geometry of the structural building unit are examined and used to rationalize trends in CO2 affinity. In particular, the partial charge on the coordinatively unsaturated metal sites is found to correlate with ΔH, suggesting that this property may be used as a simple performance descriptor. The ability to rapidly distinguish promising MOFs from those that are "thermodynamic dead-ends" will be helpful in guiding synthesis efforts towards promising compounds.

Lithium peroxide surfaces are metallic, while lithium oxide surfaces are not.

The thermodynamic stability and electronic structure of 40 surfaces of lithium peroxide (Li(2)O(2)) and lithium oxide (Li(2)O) were characterized using first-principles calculations. As these compounds constitute potential discharge products in Li-oxygen batteries, their surface properties are expected to play a key role in understanding electrochemical behavior in these systems. Stable surfaces were identified by comparing 23 distinct Li(2)O(2) surfaces and 17 unique Li(2)O surfaces; crystallite areal fractions were determined through application of the Wulff construction. Accounting for the oxygen overbinding error in density functional theory results in the identification of several new Li(2)O(2) oxygen-rich {0001} and {1 ̅100} terminations that are more stable than those previously reported. Although oxygen-rich facets predominate in Li(2)O(2), in Li(2)O stoichiometric surfaces are preferred, consistent with prior studies. Surprisingly, surface-state analyses reveal that the stable surfaces of Li(2)O(2) are half-metallic, despite the fact that Li(2)O(2) is a bulk insulator. Surface oxygens in these facets are ferromagnetic with magnetic moments ranging from 0.2 to 0.5 µ(B). In contrast, the stable surfaces of Li(2)O are insulating and nonmagnetic. The distinct surface properties of these compounds may explain observations of electrochemical reversibility for systems in which Li(2)O(2) is the discharge product and the irreversibility of systems that discharge to Li(2)O. Moreover, the presence of conductive surface pathways in Li(2)O(2) could offset capacity limitations expected to arise from limited electron transport through the bulk.

Dynamics of Hydroxyl Anions Promotes Lithium Ion Conduction in Antiperovskite Li2OHCl.

Li2OHCl is an exemplar of the antiperovskite family of ionic conductors, for which high ionic conductivities have been reported, but in which the atomic-level mechanism of ion migration is unclear. The stable phase is both crystallographically defective and disordered, having ∼1/3 of the Li sites vacant, while the presence of the OH- anion introduces the possibility of rotational disorder that may be coupled to cation migration. Here, complementary experimental and computational methods are applied to understand the relationship between the crystal chemistry and ionic conductivity in Li2OHCl, which undergoes an orthorhombic to cubic phase transition near 311 K (≈38 °C) and coincides with the more than a factor of 10 change in ionic conductivity (from 1.2 × 10-5mS/cm at 37 °C to 1.4 × 10-3 mS/cm at 39 °C). X-ray and neutron experiments conducted over the temperature range 20-200 °C, including diffraction, quasi-elastic neutron scattering (QENS), the maximum entropy method (MEM) analysis, and ab initio molecular dynamics (AIMD) simulations, together show conclusively that the high lithium ion conductivity of cubic Li2OHCl is correlated to "paddlewheel" rotation of the dynamic OH- anion. The present results suggest that in antiperovskites and derivative structures a high cation vacancy concentration combined with the presence of disordered molecular anions can lead to high cation mobility.

High capacity hydrogen storage materials: attributes for automotive applications and techniques for materials discovery.

Widespread adoption of hydrogen as a vehicular fuel depends critically upon the ability to store hydrogen on-board at high volumetric and gravimetric densities, as well as on the ability to extract/insert it at sufficiently rapid rates. As current storage methods based on physical means--high-pressure gas or (cryogenic) liquefaction--are unlikely to satisfy targets for performance and cost, a global research effort focusing on the development of chemical means for storing hydrogen in condensed phases has recently emerged. At present, no known material exhibits a combination of properties that would enable high-volume automotive applications. Thus new materials with improved performance, or new approaches to the synthesis and/or processing of existing materials, are highly desirable. In this critical review we provide a practical introduction to the field of hydrogen storage materials research, with an emphasis on (i) the properties necessary for a viable storage material, (ii) the computational and experimental techniques commonly employed in determining these attributes, and (iii) the classes of materials being pursued as candidate storage compounds. Starting from the general requirements of a fuel cell vehicle, we summarize how these requirements translate into desired characteristics for the hydrogen storage material. Key amongst these are: (a) high gravimetric and volumetric hydrogen density, (b) thermodynamics that allow for reversible hydrogen uptake/release under near-ambient conditions, and (c) fast reaction kinetics. To further illustrate these attributes, the four major classes of candidate storage materials--conventional metal hydrides, chemical hydrides, complex hydrides, and sorbent systems--are introduced and their respective performance and prospects for improvement in each of these areas is discussed. Finally, we review the most valuable experimental and computational techniques for determining these attributes, highlighting how an approach that couples computational modeling with experiments can significantly accelerate the discovery of novel storage materials (155 references).

Predicting hydrogen storage in MOFs via machine learning.

The H2 capacities of a diverse set of 918,734 metal-organic frameworks (MOFs) sourced from 19 databases is predicted via machine learning (ML). Using only 7 structural features as input, ML identifies 8,282 MOFs with the potential to exceed the capacities of state-of-the-art materials. The identified MOFs are predominantly hypothetical compounds having low densities (<0.31 g cm-3) in combination with high surface areas (>5,300 m2 g-1), void fractions (∼0.90), and pore volumes (>3.3 cm3 g-1). The relative importance of the input features are characterized, and dependencies on the ML algorithm and training set size are quantified. The most important features for predicting H2 uptake are pore volume (for gravimetric capacity) and void fraction (for volumetric capacity). The ML models are available on the web, allowing for rapid and accurate predictions of the hydrogen capacities of MOFs from limited structural data; the simplest models require only a single crystallographic feature.

Simultaneous segregation at coherent and semicoherent heterophase interfaces.

By employing a combination of three-dimensional atom-probe tomography and first-principles calculations, significant qualitative and quantitative differences in solute segregation at coherent and semicoherent interfaces bounding a single θ^{'} precipitate in an Al-Cu-based alloy are found. Qualitatively, localized segregation is observed at the semicoherent interface, whereas delocalized behavior is present at the coherent facets. Quantitatively, segregation at the semicoherent interface is a factor of 2 greater than at the coherent interface, resulting in a decrease in interfacial energy that is more than 5 times greater than that observed at the coherent facet. These observations illustrate unambiguously the strong couplings among interface structure, chemical composition, and energetics.

Modeling the Interface between Lithium Metal and Its Native Oxide.

Owing to their high theoretical capacities, batteries that employ lithium (Li) metal as the negative electrode are attractive technologies for next-generation energy storage. However, the successful implementation of lithium metal batteries is limited by several factors, many of which can be traced to an incomplete understanding of surface phenomena involving the Li anode. Here, first-principles calculations are used to characterize the native oxide layer on Li, including several properties associated with the Li/lithium oxide (Li2O) interface. Multiple interface models are examined; the models account for differing interface (chemical) terminations and degrees of atomic ordering (i.e., crystalline vs amorphous). The interfacial energy, formation energy, and strain energies are predicted for these models. The amorphous interface yields the lowest interfacial formation energy, suggesting that it is the most probable model under equilibrium conditions. The work of adhesion is evaluated for the crystalline interfaces, and it is found that the O-terminated interface exhibits a work of adhesion more than 30 times larger than that of the Li-terminated model, implying that Li will strongly wet an oxygen-rich Li2O surface. The electronic structure of the interfaces is characterized using Voronoi charge analysis and shifts in the Li 1s binding energies. The width of the Li/Li2O interface, as determined by deviations from bulklike charges and binding energies, extends beyond the region exhibiting interfacial structural distortions. Finally, the transport of Li ions through the amorphous oxide is quantified using ab initio molecular dynamics. Facile transport of Li+ through the native oxide is observed. Thus, increasing the percentage of amorphous Li2O in the solid electrolyte interphase may be beneficial for battery performance. In total, the phenomena quantified here will aid in the optimization of batteries that employ high-capacity Li metal anodes.

Low-temperature paddlewheel effect in glassy solid electrolytes.

Glasses are promising electrolytes for use in solid-state batteries. Nevertheless, due to their amorphous structure, the mechanisms that underlie their ionic conductivity remain poorly understood. Here, ab initio molecular dynamics is used to characterize migration processes in the prototype glass, 75Li2S-25P2S5. Lithium migration occurs via a mechanism that combines concerted motion of lithium ions with large, quasi-permanent reorientations of PS43- anions. This latter effect, known as the 'paddlewheel' mechanism, is typically observed in high-temperature crystalline polymorphs. In contrast to the behavior of crystalline materials, in the glass paddlewheel dynamics contribute to Lithium-ion mobility at room temperature. Paddlewheel contributions are confirmed by characterizing spatial, temporal, vibrational, and energetic correlations with Lithium motion. Furthermore, the dynamics in the glass differ from those in the stable crystalline analogue, γ-Li3PS4, where anion reorientations are negligible and ion mobility is reduced. These data imply that glasses containing complex anions, and in which covalent network formation is minimized, may exhibit paddlewheel dynamics at low temperature. Consequently, these systems may be fertile ground in the search for new solid electrolytes.

Predicting Wettability and the Electrochemical Window of Lithium-Metal/Solid Electrolyte Interfaces.

The development of solid electrolytes (SEs) is expected to enhance the safety of lithium-ion batteries. Additionally, a viable SE could allow the use of a Li-metal negative electrode, which would increase energy density. Recently, several antiperovskites have been reported to exhibit high ionic conductivities, prompting investigations of their use as an SE. In addition to having a suitable conductivity, phenomena at the interface between an SE and an electrode are also of great importance in determining the viability of an SE. For example, interfacial interactions can change the positions of the band edges of the SE, altering its stability against undesirable oxidation or reduction. Furthermore, the wettability of the SE by the metallic anode is desired to enable low interfacial resistance and uniform metal plating and stripping during cycling. The present study probes several properties of the SE/electrode interface at the atomic scale. Adopting the antiperovskite SE Li3OCl (LOC)/Li-metal anode interface as a model system, the interfacial energy, work of adhesion, wettability, band edge shifts, and the electrochemical window are predicted computationally. The oxygen-terminated interface was determined to be the most thermodynamically stable. Moreover, the large calculated work of adhesion for this system implies that Li will wet LOC, suggesting the possibility for low interfacial resistance. Nevertheless, these strong interfacial interactions come at a cost to electrochemical stability: strong interfacial bonding lowers the energy of the conduction band minimum (CBM) significantly and narrows the local band gap by 30% in the vicinity of the interface. Despite this interface-induced reduction in electrochemical window, the CBM in LOC remains more negative than the Li/Li+ redox potential, implying stability against reduction by the anode. In sum, this study illustrates a comprehensive computational approach to assessing electrode/electrolyte interfacial properties in solid-state batteries.

Thermodynamic Assessment of Coating Materials for Solid-State Li, Na, and K Batteries.

The development of all-solid-state batteries (ASSBs) presents a pathway to enhance the energy density and safety of conventional Li-ion batteries that use liquid electrolytes. However, one of the more promising categories of solid electrolytes (SEs), sulfides, are generally unstable in contact with common electrode materials, resulting in SE decomposition and high interfacial resistance. Recent studies have indicated that the application of coatings can, in some cases, stabilize the electrode/SE interface, reducing the likelihood for harmful interfacial reactions. Here, stable coatings for Li, Na, and K ASSBs are identified. In total, the stability windows for 1112 ternary alkali-metal-based compounds were assessed, including fluorides, chlorides, oxides, sulfides, phosphides, and nitrides. In general, the fluorides and chlorides exhibit the highest oxidative stability, suggesting that they are good choices for stabilizing SE/cathode interfaces. In contrast, sulfides, phosphides, and nitrides exhibit much lower oxidative stabilities, with many of these materials predicted to decompose above 2 V. At the anode/SE interface, nitrides and oxides are predicted to be the most effective coatings, as they are generally the most stable against reductive decomposition. As expected, sulfides and phosphides are the least stable class of materials under reducing conditions. Overall, oxides appear to be the most versatile class of coating materials: several oxides are predicted to exhibit stability windows ranging from 0 to 3 V with respect to Li/Li+, Na/Na+, or K/K+. Examples of promising oxides for stabilizing the SE/anode interface include Li5AlO4, Li4SiO4, NaAlO2, Na3PO4, KAlO2, and K3PO4. Similarly, promising compounds for stabilizing the SE/cathode interface include NaPO3 and KPO3. Finally, the possibility for kinetic stabilization suggests that additional ternary oxides (e.g., based on Ga, Nb, Sb, and Ta) may be viable coatings at the SE/cathode interface.

Thermodynamic Overpotentials and Nucleation Rates for Electrodeposition on Metal Anodes.

Rechargeable batteries employing metal negative electrodes (i.e., anodes) are attractive next-generation energy storage devices because of their greater theoretical energy densities compared to intercalation-based anodes. An important consideration for a metal's viability as an anode is the efficiency with which it undergoes electrodeposition and electrodissolution. The present study assesses thermodynamic deposition/dissolution efficiencies and associated nucleation rates for seven metals (Li, Na, K, Mg, Ca, Al, and Zn) of relevance for battery applications. First-principles calculations were used to evaluate thermodynamic overpotentials at terraces and steps on several low-energy surfaces of these metals. In general, overpotentials are observed to be the smallest for plating/stripping at steps and largest at terrace sites. The difference in the coordination number for a surface atom from that in the bulk was found to correlate with the overpotential magnitude. Consequently, because of their low bulk coordination, the body-centered alkali metals (Li, Na, and K) are predicted to be among the most thermodynamically efficient for plating/stripping. In contrast, metals with larger bulk coordination such as Al, Zn, and the alkaline earths (Ca and Mg) generally exhibit higher thermodynamic overpotentials. The rate of steady-state nucleation during electrodeposition was estimated using a classical nucleation model informed by the present first-principles calculations. Nucleation rates are predicted to be several orders of magnitude larger on alkali metal surfaces than on the other metals. This multiscale model highlights the sensitivity of the nucleation behavior on the structure and composition of the electrode surface.