Potentiostatic Hold Research Articles

Electro-Chemo-Mechanical Evolution at the Garnet Solid Electrolyte-Cathode Interface.

Solid-state batteries promise higher energy density and improved safety compared with lithium-ion batteries. However, electro-chemomechanical instabilities at the solid electrolyte interface with the cathode and the anode hinder their large scale implementation. Here, we focus on resolving electro-chemo-mechanical instability mechanisms and their onset conditions between a state-of-the-art cathode, LiNi0.6Mn0.2Co0.2O2 (NMC622), and the garnet Li7La3Zr2O12 (LLZO) solid electrolyte. We used thin-film NMC622 on LLZO pellets to place the interfacial region within the detection depth of the X-ray characterization techniques. The experimental probes of the near-interface region included in operando X-ray absorption spectroscopy and ex situ focused ion beam scanning electron microscopy. Electrochemical degradation was not observable during cycling at room temperature with 4.3 V versus Li/Li+ charge voltage cutoff, or with stepwise potentiostatic hold up to 4.1 V versus Li/Li+. In contrast, secondary phases including reduced transition metal species (Ni2+, Co2+) were found after cycling up to 4.3 V versus Li/Li+ at 80 °C and during potentiostatic hold at 4.3 V versus Li/Li+ (Ni2+). Intergranular cracks between NMC622 grains and delamination at the NMC622|LLZO interface occurred readily after the first charge. These interface reaction products and mechanical failure lowered the capacity and cell efficiency due to partial loss of the NMC622 phase, partial loss of contact at the interface, and a higher polarization resistance. Electrochemical instability between delithiated NMC622 and LLZO could be mitigated by using a low charge voltage cutoff or cycling at lower temperature. Ways to engineer the mechanical properties to avoid crack deflection and delamination at the interface are also discussed for enhancing mechanical stability.

Significant Improvements to Si Calendar Lifetime Using Rapid Electrolyte Screening via Potentiostatic Holds

Silicon-based lithium-ion batteries exhibit severe time-based degradation resulting in poor calendar lives. This has been identified as the major impediment towards commercialization with cycle life considered a solved issue through nanosizing and protective coatings allowing over 1000 cycles of life to be achieved. In this work, rapid screening of sixteen electrolytes for calendar life extension of Si-rich systems (70 wt% Si) is performed using the voltage hold (V-hold) protocol. V-hold significantly shortens the testing duration over the traditional open circuit voltage reference performance test allowing us to screen electrolytes within a span of two months. We find a novel ethylene carbonate (EC) free electrolyte formulation containing lithium hexafluorophosphate (LiPF6) salt, and binary solvent mix of fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC) that extends calendar life of Si cells as compared to conventional EC based electrolyte. Our coupled experimental-theoretical analysis framework provides a decoupling of the parasitic currents during V-hold, allowing us to extrapolate the capacity loss to predict semiquantitative calendar lifetimes. Subsequently, cycle aging and oxidative stability tests of the EC free system also show enhanced performance over baseline electrolyte.

Electrochemical Evaluation of Al-5 wt% Zn Metal-Rich Primer for Protection of Al-Zn-Mg-Cu Alloy in NaCl

An intact and X-scribed Al-5wt%Zn-rich primer (AlRP) without pretreatment or topcoat was evaluated for its ability to suppress potential-dependent intergranular corrosion and intergranular stress corrosion cracking of peak-aged AA7075A-T651 in NaCl salt fog and full immersion. The ability of the primer to provide sacrificial anode-based cathodic prevention of peak-aged AA7075-T651 substrate was evaluated both under the primer coating and at scratches. The AlRP evaluated consisted an epoxy-based resin embedded with spherical Al-5wt%Zn pigment particles. Performance was evaluated under full immersion in 0.6 M NaCl solution and compared to ASTM B117 salt spray exposure using two approaches. These consisted of the University of Virginia (UVA) cycle test on intact coatings and the full immersion galvanic couple testing on simulated scratched panels created when intact coatings form bimetal couples with bare AA7075-T651. Focus was placed on the ability of the AlRP to achieve a targeted intermediate galvanic couple potential near a “prevention” potential which suppresses stress corrosion crack growth, intermetallic particle corrosion as well as intergranular corrosion. The long-term (24-h) open-circuit potential (OCP) of AlRP-coated AA7075-T651 in 0.6 M NaCl indicated that the AlRP provided less than 100 mV of cathodic potential shift of the intact coating from its OCP in 0.6 M NaCl. Electrochemical cycle testing conducted at a potentiostatic hold of –0.95 VSCE demonstrates that the AlRP did not enable sacrificial anode-based cathodic protection as the coupled potential remained at the corrosion potential of bare AA7075-T651. Furthermore, the current observed throughout galvanic corrosion experiments coupling of AlRP to AA7075-T651 indicated the AlRP coating was a cathode in the bimetal galvanic couple. ASTM B117 salt spray exposure of the AlRP revealed oxidation of the AA7075-T651 substrate below the primer detected as a continually growing oxygen signal at the primer-substrate interface that did not arrest corrosion over the exposure period.

Corrosion Resistance, Composition, and Stratification of Passive Films: Ni-22Cr and Ni-22Cr-6Mo Alloys Passivated and Exposure Aged in Acidic Chloride Solutions

Ni-Cr based super-alloys have exceptional corrosion resistance, which is further improved with Mo alloying. The correlation between passive layer performance and composition was studied to gain a deeper mechanistic understanding of the role of Mo by comparing the behavior of Ni-22Cr to Ni-22Cr-6Mo (wt%) alloys. The passive layers were formed using galvanostatic holds to create fast and slow growth conditions using high and low current densities. A potentiostatic hold was added to initiate exposure aging. The passive film was characterized using electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), atomic emission spectro-electrochemistry (AESEC), and X-ray photoelectron spectroscopy (XPS). Combined electrochemical and XPS characterization offered insight in cation concentrations and stratification, bonding states (oxide, hydroxide), and their modulation as a function of electrochemical conditions and performance. Most importantly: (i) Mo addition enhanced Cr(III) bound in oxide, (ii) fast growth conditions resulted in less corrosion resistant films, and (iii) exposure aging increased Cr-enrichment and reduced stratification of Mo-cations. The correlation between passive film performance and Cr, Ni, and Mo oxidation states, bonding, oxide-hydroxide contributions, and stratification is discussed. Generally accepted correlations, such as Cr-cation concentration and performance of the passive layer, have to be reexamined in order to account for the complex chemical make-up of the passive layer.

Step Potential Electrochemical Spectroscopy (SPECS) Technique to Investigate the Ageing of Carbon Electrodes in Organic-Based Electrochemical Capacitors

Current research on electrode degradation in electrochemical capacitors (ECs) mainly focuses on the application of new materials and the establishment of their stability on the basis of various standards and qualitative tests. However, there exists limited research that focuses on the elucidation and determination of the degradation mechanisms and pathways of carbon electrodes. The proposed study uses an innovative approach with the use of the Step Potential Electrochemical Spectroscopy (SPECS) technique, which allows one to quantify the contribution of different charge-storing mechanisms, and successfully track any changes that take place after accelerated ageing, including structural changes of the electrode in the organic medium. Additionally, the role of oxygen content on the degradation of ECs was explored.In this report carbon material, namely binder-free Activated Carbon Cloth (ACC) Kynol® 507-20 was prepared in a furnace under a nitrogen atmosphere at different temperatures, namely 120, 500, and 1000ᵒC. This is so that the effect of the presence of different oxygen functional groups on the surface of carbon could be investigated. The material was selected as a self-standing electrode to eliminate any influence the binder would have on the electrode porosity. Subsequently, these materials were prepared as electrode materials and assembled in a symmetric two-electrode setup with 1M TEABF4/AN (tetraethylammonium tetrafluoroborate in Acetonitrile solvent). In each cell, the potential range of both electrodes was first determined in a 2-electrode Swagelok® cell with RE using galvanostatic charge/discharge (1 A g-1). The further procedure involved 3 CV cycles (1 mV s-1) at a predetermined potential for each electrode in a 3-electrode set-up, followed by SPECS measurement where 5-minute potentiostatic hold with 10 mV potential steps was applied. The obtained current vs. time plots were used to calculate the corresponding capacitance, diffusion and residual parameters. Floating tests at 2.7 V were then carried out for each system. According to the international standard (IEC 62391-1), system failure is reported when the initial capacitance drops below 80% of its initial value; thus floating experiments were halted after that value was reached for each system or when the resistance of the cell doubled in value. The electrochemical investigation of the individual aged electrodes was then repeated in the same manner.Our study allowed for a thorough examination of individual electrodes prior to and after floating experiments, which helped to elucidate the degradation causes and pathways in organic medium.

Assessing Electrolyte Fluorination Impact on Calendar Aging of Blended Silicon-Graphite Lithium-Ion Cells Using Potentiostatic Holds

Silicon-based lithium-ion batteries have started to meet cycle life metrics, but they exhibit poor calendar life. Here, electrolyte fluorination impact on calendar fade of blended silicon-graphite anodes is explored using a LiPF6 in EC:EMC:FEC electrolyte vs LiBOB in EC:EMC electrolyte. We utilize a combined experimental-modeling approach applying potentiostatic voltage holds (V-hold) to evaluate electrolyte suitability for calendar life in a shortened testing timeframe (∼2 months). Our theoretical framework deconvolutes the irreversible parasitic capacity losses (lithium lost to the solid electrolyte interphase) from the V-hold electrochemical data. Unfluorinated electrolyte (dominant LiBOB reduction) exhibits higher cell resistance as compared to fluorinated electrolyte (dominant FEC reduction). Both systems have similar irreversible capacities during the voltage hold duration with slower rate of parasitic capacity loss for the LiBOB system. Extrapolation of the parasitic losses to end of life capacity fade of 20% shows LiBOB electrolyte outperforming LiPF6 electrolyte in calendar life. The results demonstrate the applicability of the V-hold protocol as a rapid material screening tool providing semi-quantitative calendar lifetime estimates.

The role of chromium content in aqueous passivation of a non-equiatomic Ni38Fe20CrxMn21-0.5xCo21-0.5x multi-principal element alloy (x = 22, 14, 10, 6 at%) in acidic chloride solution

The effect of the Cr content on the corrosion behavior in a series of single-phase non-equiatomic Ni38Fe20CrxMn21-0.5xCo21-0.5x (6 < x < 22 at%) multi-principal element alloys (MPEAs) was investigated in acidified NaCl solutions. Comparisons were made with binary solid solution Co-Cr, Ni-Cr, and Fe-Cr alloys over a similar range of Cr contents. The corrosion behavior was evaluated using in-situ AC and DC electrochemical methods and ex-situ surface sensitive characterization techniques. Passivity and various levels of local corrosion resistance were obtained in the MPEA with 10 at% Cr or above. The binary Ni-Cr alloys with 12–30 at% Cr behaved similarly. The MPEA with 6 at% Cr and binary alloys with 5, 6, or 10 at% Cr were marginally passive or active and underwent localized corrosion during both linear sweep voltammetry and potentiostatic hold experiments. Passive films formed during potentiostatic hold experiments were characterized with X-ray photoelectron spectroscopy and atom probe tomography. Cr cation enrichment in the passive films was observed for all alloys and similar enrichment factors were obtained as a function of Cr content regardless of whether MPEA or binary alloy except for the Fe-Cr alloy. Moreover, passive current density was correlated with the Cr cation fraction in the passive film. The degree of Cr enrichment was attributed to a combination of thermodynamic and kinetic factors, such as selective chemical dissolution of alloying elements as well as limitation due to solute depletion at the metal oxide interface in alloys with a bulk Cr content ≤ 10 at%.

On the Specific Capacity and Cycle Stability of Si@void@C Anodes: Effects of Particle Size and Charge/Discharge Protocol

Silicon has the potential to be a high-performance anode material, but its practical application is impeded by huge volume expansion during lithiation. Many studies have revealed that the huge volume expansion problem can be mitigated by introducing engineered voids into Si/C core–shell structures. In this study, a Si/C core/shell structure with engineered voids, termed Si@void@C, is investigated for its specific capacity and cycle stability as a function of particle size and charge/discharge protocol. The study shows that finer Si@void@C particles result in higher specific capacities, but with little impact on the cycle stability. Further, lower and upper cutoff voltages in charge/discharge have a profound impact on the specific capacity and cycle stability. Importantly, cutoff voltages in formation cycles have long-lasting effects on the cycle stability, indicating the critical role of forming a robust solid electrolyte interphase (SEI) layer during formation cycles. Using a constant current charge followed by potentiostatic hold charge can further improve the cycle stability and minimize the sharp capacity decay in the first 20–40 cycles. With proper choices of charge/discharge protocols, the specific capacities of Si@void@C anodes at the electrode level are 66.8%, 38.2% and 22.7% higher than those of graphite anodes at the 1st, 300th and 500th cycles, respectively, proving that Si@void@C has promising potential to replace graphite anodes for practical applications in the future.

New Insights into Pt Dissolution Mechanisms from SFC-ICP-MS Measurements for Well-Defined Surfaces

In the last years, polymer electrolyte membrane fuel cell (PEMFC) technology has progressed notably, allowing its extended implementation in the transport sector. For example, the fuel cell cars currently being commercialized use PEMFCs with Pt nanoparticles for the oxygen reduction reaction (ORR) at the cathode, primarily because of their better long-term stability in comparison to other electrocatalysts. However, even pure Pt catalysts degrade under real-life conditions, since a PEMFC is expected to resist hundreds of thousands of load cycles and tens of thousands of start-up/shut-down cycles during its lifetime, leading to a high number of platinum oxidation/reduction cycles and resulting in its extensive degradation.1 This degradation of Pt catalysts is mainly linked to electro-oxidation and dissolution processes, which have been investigated for a long time in polycrystalline and supported nanoparticle catalysts. More recently, investigations with Pt single crystal electrodes have been carried out, which offer the possibility of a more detailed understanding of these processes at the atomic level. These works include studies using in-situ surface X-ray diffraction (SXRD) and on-line inductively coupled mass spectrometry (ICP-MS), which have for example shown differences in the onset potential for anodic dissolution on Pt(100) and Pt(111) that have their origin in the different atomic structures of the initial oxide.2 The present work focuses on the dissolution behaviour of the well-defined surfaces of the three Pt basal planes investigated by scanning flow cell inductively coupled mass spectrometry (SFC-ICP-MS). Further investigations providing new information about the dissolution mechanisms are carried out by varying a wide variety of parameters: upper potential limit, scan rate, potential holding times, electrolyte composition, pH, purging gas and cooling atmosphere. Some of the results will be presented in combination with SXRD measurements in order to explain different aspects of the restructuring and dissolution mechanisms for these surfaces. For example, the dissolved amounts for Pt(111), Pt(100) and Pt(110) in 0.1 M HClO4 and 0.1 M H2SO4 after cyclic voltammetry at a scan rate of 0.05 V s-1 for three different upper potential limits, 1.20 V, 1.40 V and 1.60 V vs. RHE, were obtained. In all cases dissolution follows the order Pt(110) > Pt(100) > Pt(111), which is in agreement with the previous works performed in 0.1 M HClO4.3 Higher dissolution is always observed in the case of H2SO4 electrolyte. Potentiostatic hold experiments for Pt(100) have been carried out, allowing the separation of the anodic and cathodic dissolution peaks. It can be observed that the cathodic dissolution increases proportionally with the increase in the oxidation potential. The combination of this data with SXRD measurements suggests that there are specific species in the oxide structure at high potentials that are the responsible for the majority of the cathodic dissolution in the Pt(100) surface.References M. K. Debe, Nature, 486 (2012) 43-51.Fuchs, T.; Drnec, J.; Calle-Vallejo, F.; Stubb, N.; Sandbeck, D.; Ruge, M.; Cherevko. S.; Harrington, D. A.; Magnussen, O. M. Nature Catalysis 3 (2020) 754-761.Sandbeck, D. J. S.; Brummel, O.; Mayrhofer, K. J. J.; Stubb, N.; Libuda, J.; Katsounaros, I.; Cherevko. S. ChemPhysChem 20 (2019) 2997-3003

Operando monitoring of activated carbon electrodes operating with aqueous electrolytes

Remarkable changes in the volume of activated carbon electrodes operating with aqueous electrolytes were measured with scanning electrochemical microscopy (SECM) and electrochemical dilatometry (ECD) techniques. It appears that the electrolyte volume absorbed by the carbon electrode strongly depends on the polarization direction and the electrode potential. It is assumed that these changes result from different ion adsorption properties and the volume of the electrolyte. Furthermore, the electrode surface functionalities appear to change. This has been confirmed by operando contact angle measurements. In addition to the observations of volume changes, the method of preconditioning electrochemical capacitors is proposed, which is an outcome of the observed electrode dilatations and modulated wettability. The conditioning method promotes deep electrochemical impregnation of the electrodes by an electrolyte. In fact, remarkably better wetting of the electrodes is reflected in significant changes in the potential distribution between electrodes. Moreover, the internal pressure build-up during the potentiostatic hold test is reduced. Finally, in situ observations of the electrodes with X-ray tomography suggest the precipitation of insoluble deposits at the surface of the positive electrode near the separator.

Electrochemical Modeling and Experimental Verification of Lithiation Gradients in Oxide Cathodes of Lithium-Ion Cells

Lithiated nickel-cobalt-manganese oxides, such as NCM523, are used in the positive electrode (cathode) of Li-ion cells. Using operando X-ray diffraction profilometry, lithiation gradients in the cathode matrix can be observed and quantified by expansion into Legendre polynomials with time-dependent weights. These weights (referred to as gradients) increase in magnitude when electric current flows through the cell, decrease during potentiostatic hold and finally relax to zero when the current is interrupted during open circuit rest. Both physics-based electrochemical models and operando X-ray experiments suggest that the time constants for gradient growth and abatement are primarily determined by ionic diffusion in the oxide particles, which in turn depends on their lithium content. In contrast, the magnitude of gradients depends mainly on the applied current. The X-ray profilometry provides a way of directly probing the formation and disappearance of Li gradients across the cathode during fast cycling, which can help to diagnose the effects of material degradation in the cells.

Predicting the impact of formation protocols on battery lifetime immediately after manufacturing

Predicting the impact of formation protocols on battery lifetime immediately after manufacturing

Dealloying Behavior in Ni-Cr-Fe Alloys As a Precursor to SCC in Boiling Caustic Environments

Experiments have shown that the susceptibility of Ni-Cr-Fe Alloys to dealloying, and subsequently stress corrosion cracking (SCC), highly depends on alloy composition and electrochemical potential. Dealloying is defined as the selective dissolution of the more active elements in an alloy (e.g., Cr and Fe in a Ni-based alloy). A dealloying process results in a remnant nano-porous layer on the surface that could be a precursor to SCC by a mechanism known as film-induced cleavage (FIC) where cracks propagate through this layer with sufficient velocity to promote energy transmission into the underlying metal resulting in cleavage fracture. Presence of a threshold amount of a more noble element could theoretically suppress this phenomenon by surface diffusion. This was investigated further by previous researchers, and the percolation theory of materials was hypothesized that states that the more noble element in an alloy creates a percolating mass that would limit the ability to retain a connected pathway of the reactive elements from the surface given it reaches a certain threshold of the mass of the material. This percentage was estimated to be between 50-60% for most alloying systems and termed the parting limit. In this study, percolation theory and dealloying is further explored by exposing several Ni-based and Fe-based alloys with varying percentages of Ni to boiling caustic solutions (50% NaOH) and evaluating their dealloying behavior.As mentioned, dealloying highly depends on electrochemical potential and is believed to occur above a threshold potential known as the dealloying critical potential. The dealloying critical potential is defined as the condition at which a transition occurs from a “passivated” alloy surface to the sustained formation of a bi-continuous porous structure. It can be identified by running an anodic polarization scan and initially matching anodic peaks to a pH-potential diagram (Pourbaix diagram) to evaluate the potential dissolution behavior. Bench-top electrochemical measurements were conducted to find the dealloying critical potential and make a correlation between this potential and alloys’ composition. The figure attached shows the anodic polarization scan results for all alloys included in the study in 50% NaOH at 140°C. It was observed that for the samples with high Ni content (i.e., Alloy 600 and Alloy 690) there was only a single active peak, which indicates transitioning to passivation and inhibition of dealloying. All other materials were susceptible to dealloying. Electron microscopy techniques were used to analyze all the samples used in the figure after exposure to 50% NaOH at 140°C at open circuit potential (OCP) and during a potentiostatic hold at the dealloying critical potential. Scanning electron microscopy (SEM) imaging followed by energy dispersive X-ray spectroscopy (EDS) revealed varying degrees of dealloying, with low Ni-containing alloys showing greater severity.The role of dealloying as a precursor to SCC was also evaluated by utilizing U-bend samples for Alloy 690, Alloy 800, and SS 304. The findings were in-line with previous dealloying result; the alloys with Ni content below the parting limit, SS304 and the Alloy 800, both underwent SCC after 48 hours of exposure to 50% NaOH solution at 135 °C, while the Alloy 690 sample (~60Ni) did not show any signs of cracking. A peculiar phenomenon was also noted for alloys containing Mo where it is seen that their dissolution current is approximately two folds higher than their counterparts with approximately the same Ni content. For example, when comparing SS 304 and SS 316 or Alloy C-276 and Alloy G-35. A possible explanation for that is the electrochemical behavior of Mo in caustic conditions where it is shown in its pourbaix diagram that dissolution occurs over a wider range of pH and potential in comparison to the other alloying elements. This earlier dissolution can possibly create more terrace sites on the surface that could accelerate the dissolution of iron and chromium. However, further studies are needed before a conclusion can be reached. Further work has also been conducted on Alloy 800 under the same conditions to evaluate the possibility of controlling the geometry of the dealloyed layer. The results so far indicate that the time of exposure and the electrochemical potential are critical in controlling the geometry of the dealloyed layer. Parallel studies are being conducted on SS 304 to evaluate the effect of Pb on extending the dealloying regime. The results so far have shown that the addition of Pb would shift the OCP more positively towards a region of passivity. It was also noted that dealloying kinetics is hindered in comparison to pure caustic conditions. However, SCC may still be possible with dissolved Pb present. Figure 1

Two-Step Electrochemical Route to Synthesize Nanostructured Cu2O Thin Films

The conversion of solar energy to chemical fuels via a photoelectrochemical (PEC) cell is one promising approach to produce clean energy. A PEC cell consists of two light absorbing electrodes with appropriate optoelectronic properties. Depending upon the band alignment, one of two electrodes is a photoanode producing O2, and the other one is a photocathode which produces H2. One such suitable photocathode for a PEC cell is Cu2O; an intrinsic p-type semiconductor with a direct bandgap of 2.0 eV and a high absorption coefficient of 104 cm-1 in the visible range. Its conduction band minimum (CBM) lies 0.7 V negative of water reduction potential and therefore, can act as a photocathode. However, one major challenge associated with Cu2O is its poor minority carrier diffusion length resulting in a large recombination rate in bulk and lower overall cell efficiency. In this aspect, nanostructured materials are of great use owing to their inherent properties. Orthogonal charge separation, enhanced light absorption, and large electrode-electrolyte are few advantages of the nanostructured materials which can be used to overcome the issue of minority carrier diffusion length. However, the synthesis of nanostructured Cu2O thin films is quite challenging as it involves multiple steps and/or require high-temperature processing.In this work, we present a low temperature, and straightforward two-step synthesis method to obtain nanostructured Cu2O with 10-30 nm thick nano-walls on FTO coated glass substrate. First, the Cu2O thin films are electrodeposited at a potential of -0.35 V vs. Ag/AgCl in a pH 12 lactate stabilized copper sulfate electrolyte at 55 °C. The electrodeposited Cu2O films are 1.3 µm thick and show pyramidal grains of size up to 2 µm on the surface after a deposition period of 15 mins. Later, these films are subjected to voltage cycling in a potential range of -0.6 V to 0.6 V (vs. Ag/AgCl) in a pH 5 electrolyte comprised of 0.5 M Na2SO4 and 0.1 M K2HPO4 at RT. This causes top 0.5 µm of deposited films to reconstruct to 10-30 nm thick nano-walled porous morphology after 20 cycles of voltage cycling. The voltammogram shows that reconstruction is a concomitant etching of Cu2O as Cu2+ ions along with oxidation to CuO. In fact, a potentiostatic hold at anodic potentials leads to porous CuO. This porous CuO is reduced back to Cu2O during the cathodic part of the voltage cycling. Interestingly, the number of cycles performed during voltage cycling is found to affect the phase purity of the material. The XRD scans reveal that Cu2O films are phase pure only up to 50 cycles. However, phase purity is no longer preserved as Cu impurity is detected in Cu2O films post 50 cycles. A comparison of linear sweep voltammetry (LSV) scans of pristine Cu2O, and CuO thin films show that reduction of Cu2O to Cu is a slow process in this potential range, and hence, Cu-impurity is not observed up to 50 cycles. The bare nanostructured Cu2O film show a photocurrent of -2.0 mA-cm-2 at a potential of 0.3 vs. RHE in a pH 5 media, which is almost 1.5 times of the dense Cu2O. This enhancement in photocurrent is attributed to efficient separation of photogenerated charge carriers in the orthogonal direction, increased electrode-electrolyte contact area, and enhanced light trapping. The photocurrent can further be improved via coating with wide bandgap metal oxide layers such as AZO/TiO2. Such a facile fabrication technique can be utilized to prepare nanostructures of various metal oxides on a large scale for their application in PEC cells and other optoelectronic devices. Figure 1

(Invited) Automotive Proton Exchange Membrane Fuel Cells Subzero Performance

Subzero automotive cold-starts are a challenge for proton exchange membrane fuel cell (PEMFC) stacks, in that both the thermal and mass fluxes from the cathode oxygen reduction reaction must be balanced. The heat generated, from both the exothermic reaction and resistive heating, enables rapid thermal responses of the interior cells of a stack (>300-cells) under high applied loads (galvanostatic loads above 300 mA cm-2 or potentiostatic loads below 0.5 V). Stacks of greater than 20-cells are required for -30°C cold-starts to overcome the thermal losses to the endplates [1]. The Faradic water production rate trends with the magnitude of the applied load, requiring the internal temperature of the stack to rapidly be elevated above 0°C. This circumvents oxidant starvation due to ice accumulation and failed cold-start. The U.S. Department of Energy’s subzero cold-start targets for PEMFC stacks include starts from -30°C, limited parasitic shut-down/start-up energy of 5 MJ, and time of 20 sec to reach 50% rated power [2].Presently, the majority of the published subzero cold-start PEMFC studies are performed on single cells and are dominated by isothermal water fill tests (WFT), due to the cost of stack testing [3]. As opposed to subzero stack cold-starts used under high applied loads to evacuate product water, isothermal WFT are performed under low galvanostatic loads (£ 150 mA cm-2) to quantify the water storage capacity of the cathode. To bridge the gap between subzero isothermal WFT and cold-starts, quasi-adiabatic PEMFC single-cell fixtures have been designed to insulate the cell from the endplates and emulate adjacent cell heating that occurs in a stack [4, 5].In Figure 1, three -20°C cold-starts are compared in terms of initial water content (l) and applied load type (600 mA cm-2 and 0.1 V). The higher water content results in an initially enhanced (A) power density output and (B) thermal response. The potentiostatic hold increases the initial usable power. The isothermal WFT under an applied load of only 10 mA cm-2 was able to store 1.8 mg cm-2 of water before freezeout after 32 min on load (complete data not shown), while the cold-start for the same of 3.2 produced the same amount of water within 48 sec and had warmed up to above 0°C. Khandelwal, M., S. Lee, and M.M. Mench. J. Power Sources, 2007. 172(2): p. 816-830. www.energy.gov/eere/fuelcells/doe-technical-targets-fuel-cell-systems-and-stacks-transportation-applications.Rice, C. J. Electrochem. Soc., 2021. (Accepted).Pistono, A.O. and C.A. Rice. Molecules, 2020. 25(6): p. 1410.Balliet, R.J. and J. Newman. J. Electrochem. Soc., 2011. 158(8): p. B939-B947. Figure 1

Intrinsic Kinetic Limitations in Substituted Lithium-Layered Transition-Metal Oxide Electrodes.

Substituted Li-layered transition-metal oxide (LTMO) electrodes such as LixNiyMnzCo1-y-zO2 (NMC) and LixNiyCo1-y-zAlzO2 (NCA) show reduced first cycle Coulombic efficiency (90-87% under standard cycling conditions) in comparison with the archetypal LixCoO2 (LCO; ∼98% efficiency). Focusing on LixNi0.8Co0.15Al0.05O2 as a model compound, we use operando synchrotron X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) spectroscopy to demonstrate that the apparent first-cycle capacity loss is a kinetic effect linked to limited Li mobility at x > 0.88, with near full capacity recovered during a potentiostatic hold following the galvanostatic charge-discharge cycle. This kinetic capacity loss, unlike many capacity losses in LTMOs, is independent of the cutoff voltage during delithiation and it is a reversible process. The kinetic limitation manifests not only as the kinetic capacity loss during discharge but as a subtle bimodal compositional distribution early in charge and, also, a dramatic increase of the charge-discharge voltage hysteresis at x > 0.88. 7Li NMR measurements indicate that the kinetic limitation reflects limited Li transport at x > 0.86. Electrochemical measurements on a wider range of LTMOs including Lix(Ni,Fe)yCo1-yO2 suggest that 5% substitution is sufficient to induce the kinetic limitation and that the effect is not limited to Ni substitution. We outline how, in addition to a reduction in the number of Li vacancies and shrinkage of the Li-layer size, the intrinsic charge storage mechanism (two-phase vs solid-solution) and localization of charge give rise to additional kinetic barriers in NCA and nonmetallic LTMOs in general.

Decomposition of Phosphorus-Containing Additives at a Charged NMC Surface through Potentiostatic Holds

Multiple phosphorus-containing compounds were evaluated as electrolyte additives for their reactivity at the cathode surface using LiNi0.5Mn0.3Co0.2O2 (NMC532) // Li4Ti5O12 (LTO) cells with both cycling and high voltage potentiostatic holds. We surveyed additives including phosphite and phosphate derivatives with either trifluoroethyl, ethyl, or trimethylsilyl groups. Phosphite additives with the same substituents showed lower Coulombic efficiency (CE) and higher oxidation current during the potentiostatic hold. Regardless of substitution group, all phosphates showed slightly higher CE than the additive-free electrolyte (baseline). However, no additive significantly decreases oxidation reactions over the course of the potentiostatic hold, indicating a non-passivated surface. Post-mortem X-ray photoelectron spectroscopy analysis of the cathodes indicates that the additive with trimethylsilyl groups produces significantly more oxygen and phosphorus on the cathode surface for both phosphites and phosphates. Atomistic simulations indicate that these additives are susceptible to electrochemical and chemical oxidation, however chemical oxidation is much more likely for the phosphite additives. The identity of ligands on the phosphorus-containing additive can dramatically affect both the decomposition current and the cathode surface after the potentiostatic hold.

Communication—Effect of Lower Cutoff Voltage on the 1st Cycle Performance of Silicon Electrodes

Silicon electrodes were cycled once to determine the effect of lower cutoff voltage (LCV) on the irreversible capacity and Li15Si4 phase presence for a protocol that incorporated a potentiostatic hold at the LCV. Cells lithiated below 90 mV showed indications of the crystalline Li15Si4 peak, which was absent for cells with lower cutoff voltages above 90 mV. Irreversible capacity loss was independent of the LCV used. While higher LCVs led to lower capacities, this capacity could be recovered with longer potentiostatic holds. These longer holds did not lead to Li15Si4 formation.

Effect of electrolyte composition on rock salt surface degradation in NMC cathodes during high-voltage potentiostatic holds

Surface degradation of lithium-ion cathodes at high voltages represents a significant barrier to increasing the useable energy of conventional cathodes. Though much work has been focused on mitigating the detrimental effects of such phenomena, the complex and correlated mechanisms involved have made progress difficult. Herein, the effect of electrolyte identity on surface degradation is examined. LiNi0.5Mn0.3Co0.2O2 (NMC532)/Li4Ti5O12 (LTO) full cells were constructed using three different electrolytes: a baseline organic carbonate electrolyte, a baseline organic carbonate electrolyte with an oxidizable additive, and a fluorinated carbonate electrolyte. Each system was subjected to a 60 h potentiostatic hold at 3.05 V (4.6 V vs. Li+/Li) and subsequently deconstructed for characterization via transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Molecular simulations were carried out on each distinct component of the three electrolytes in contact with crystal surfaces of an NMC532 particle. Results indicate an affinity for the additive-containing electrolyte to extract atomic oxygen from the cathode surface, whereas the fluorinated electrolyte components have negligible interactions. Hydrogen abstraction from the baseline electrolyte is facilitated by the NMC532 surface. TEM analysis of cathode surfaces, after the potentiostatic holds, revealed that the NMC532 particles cycled with the additive-containing electrolyte underwent significant surface degradation, including changes to local crystal structure in a region penetrating 20–40 nm from the surface; consistent with oxygen and lithium loss from the cathode material. However, the NMC532 in contact with the fluorinated electrolyte showed a disordered layer of just a few nm in thickness, similar to the depth of crystal disorder observed in pristine NMC532 primary particles. EELS data of the oxygen K-edge also indicated transition metal (TM) reduction, consistent with oxygen loss and changes in crystal structure extending into the cathode surface for each of the electrolytes surveyed. These surface reactions, including oxygen loss, are shown to correlate well with elevated oxidation currents and surface reconstructions of cathode particles.

Calendar-life versus cycle-life aging of lithium-ion cells with silicon-graphite composite electrodes

The use of blended silicon-graphite (Si-Gr) negative electrodes increases the energy density of lithium-ion cells over those containing only graphite (Gr) electrodes. However, volume changes in the Si particles that occur during cycling cause deterioration of the solid-electrolyte interphase (SEI) layer on the particles resulting in further electrolyte reduction that immobilizes Li+ ions and, therefore, capacity fade. Because the volume changes are not expected to occur during a potentiostatic hold (referred to as calendar-life aging), by comparison of cycle-life and calendar-life aged cells one can expect to assess the role of volume changes in the deterioration of cell performance. To this end, cells with Si-Gr and Gr negative electrodes (and Li1.03(Ni0.5Co0.2Mn0.3)0.97O2 containing positive electrodes) were assembled, tested, and compared using cycle-life and calendar-life aging protocols. As expected, the capacity loss of the cycle-life aged cells was higher than that of the calendar-life aged cells. However, the measurable capacity loss for the calendar-life aged cells indicates continued immobilization of Li+ ions. Furthermore, electrolytes extracted from the calendar-life aged cells showed more LiPF6 hydrolysis products than those extracted from the cycle-life aged cells. We discuss possible mechanistic causes for the observed aging behaviors in this article.