Are SAXS and SANS suitable to extract information on the role of water for electric-double-layer formation at the carbon-aqueous-electrolyte interface?

This study reports on the applicability of X-ray transmission (XRT), small- and wide-angle X-ray scattering (SAXS/WAXS) and small-angle neutron scattering (SANS) for investigating fundamental processes taking place in the working electrode of an electric double-layer capacitor with 1 M RbBr aqueous electrolyte at different applied potentials. XRT and incoherent neutron scattering are employed to determine global ion- and water-concentration changes and associated charge-balancing mechanisms. We showcase the suitability of SAXS and SANS, respectively, to get complementary information on local ion and solvent rearrangement in nanoconfinement, but also underscore the limitations of simple qualitative models, asking for more quantitative descriptions of water-water and ion-water interactions via detailed atomistic modelling approaches.

In situ small-angle X-ray scattering reveals solution phase discharge of Li-O2 batteries with weakly solvating electrolytes.

Electrodepositing insulating lithium peroxide (Li2O2) is the key process during discharge of aprotic Li-O2 batteries and determines rate, capacity, and reversibility. Current understanding states that the partition between surface adsorbed and dissolved lithium superoxide governs whether Li2O2 grows as a conformal surface film or larger particles, leading to low or high capacities, respectively. However, better understanding governing factors for Li2O2 packing density and capacity requires structural sensitive in situ metrologies. Here, we establish in situ small- and wide-angle X-ray scattering (SAXS/WAXS) as a suitable method to record the Li2O2 phase evolution with atomic to submicrometer resolution during cycling a custom-built in situ Li-O2 cell. Combined with sophisticated data analysis, SAXS allows retrieving rich quantitative structural information from complex multiphase systems. Surprisingly, we find that features are absent that would point at a Li2O2 surface film formed via two consecutive electron transfers, even in poorly solvating electrolytes thought to be prototypical for surface growth. All scattering data can be modeled by stacks of thin Li2O2 platelets potentially forming large toroidal particles. Li2O2 solution growth is further justified by rotating ring-disk electrode measurements and electron microscopy. Higher discharge overpotentials lead to smaller Li2O2 particles, but there is no transition to an electronically passivating, conformal Li2O2 coating. Hence, mass transport of reactive species rather than electronic transport through a Li2O2 film limits the discharge capacity. Provided that species mobilities and carbon surface areas are high, this allows for high discharge capacities even in weakly solvating electrolytes. The currently accepted Li-O2 reaction mechanism ought to be reconsidered.

In Situ Tracking of Partial Sodium Desolvation of Materials with Capacitive, Pseudocapacitive, and Battery-like Charge/Discharge Behavior in Aqueous Electrolytes.

Aqueous electrolytes can be used for electrical double-layer capacitors, pseudocapacitors, and intercalation-type batteries. These technologies may employ different electrode materials, most importantly high-surface-area nanoporous carbon, two-dimensional materials, and metal oxides. All of these materials also find more and more applications in electrochemical desalination devices. During the electrochemical operation of such electrode materials, charge storage and ion immobilization are accomplished by non-Faradaic ion electrosorption, Faradaic ion intercalation at specific crystallographic sites, or ion insertion between layers of two-dimensional materials. These processes may or may not be associated with a (partial) loss of the aqueous solvation shell around the ions. Our work showcases the electrochemical quartz crystal microbalance as an excellent tool for quantifying the change in effective solvation. We chose sodium as an important cation for energy storage materials (sodium-based aqueous electrolytes) and electrochemical desalination (saline media). Our data show that a major amount of water uptake occurs during ion electrosorption in nanoporous carbon, while battery-like ion insertion between layers of titanium disulfide is associated with an 80% loss of the initially present solvation molecules. Sodiation of MXene is accomplished by a loss of 90% of the number of solvent molecules, but nanoconfined water in-between the MXene layers may compensate for this large degree of desolvation. In the case of sodium manganese oxide, we were able to demonstrate the full loss of the solvation shell.

Operando Scanning Small-/Wide-Angle X-ray Scattering for Polymer Electrolyte Fuel Cells: Investigation of Catalyst Layer Saturation and Membrane Hydration- Capabilities and Challenges.

Polymer electrolyte fuel cells are an essential technology for future local emission-free mobility. One of the critical challenges for thriving commercialization is water management in the cells. We propose small- and wide-angle X-ray scattering as a suitable diagnostic tool to quantify the liquid saturation in the catalyst layer and determine the hydration of the ion-conducting membrane in real operating conditions. The challenges that may occur in operando data collection are described in detail─separation of the anode and cathode, cell alignment to the beam, X-ray radiation damage, and the possibility of membrane swelling. A synergistic development of experimental setup, data acquisition, and data interpretation circumvents the major challenges and leads to practical and reliable insights.

Quantification of PEFC Catalyst Layer Saturation via In Silico, Ex Situ, and In Situ Small-Angle X-ray Scattering.

The complex nature of liquid water saturation of polymer electrolyte fuel cell (PEFC) catalyst layers (CLs) greatly affects the device performance. To investigate this problem, we present a method to quantify the presence of liquid water in a PEFC CL using small-angle X-ray scattering (SAXS). This method leverages the differences in electron densities between the solid catalyst matrix and the liquid water filled pores of the CL under both dry and wet conditions. This approach is validated using ex situ wetting experiments, which aid the study of the transient saturation of a CL in a flow cell configuration in situ. The azimuthally integrated scattering data are fitted using 3D morphology models of the CL under dry conditions. Different wetting scenarios are realized in silico, and the corresponding SAXS data are numerically simulated by a direct 3D Fourier transformation. The simulated SAXS profiles of the different wetting scenarios are used to interpret the measured SAXS data which allows the derivation of the most probable wetting mechanism within a flow cell electrode.

Exclusive Solution Discharge in Li-O2 Batteries?

Capacity, rate performance, and cycle life of aprotic Li-O2 batteries critically depend on reversible electrodeposition of Li2O2. Current understanding states surface-adsorbed versus solvated LiO2 controls Li2O2 growth as surface film or as large particles. Herein, we show that Li2O2 forms across a wide range of electrolytes, carbons, and current densities as particles via solution-mediated LiO2 disproportionation, bringing into question the prevalence of any surface growth under practical conditions. We describe a unified O2 reduction mechanism, which can explain all found capacity relations and Li2O2 morphologies with exclusive solution discharge. Determining particle morphology and achievable capacities are species mobilities, true areal rate, and the degree of LiO2 association in solution. Capacity is conclusively limited by mass transport through the tortuous Li2O2 rather than electron transport through a passivating Li2O2 film. Provided that species mobilities and surface growth are high, high capacities are also achieved with weakly solvating electrolytes, which were previously considered prototypical for low capacity via surface growth.

On the nanoscale structural evolution of solid discharge products in lithium-sulfur batteries using operando scattering.

The inadequate understanding of the mechanisms that reversibly convert molecular sulfur (S) into lithium sulfide (Li2S) via soluble polysulfides (PSs) formation impedes the development of high-performance lithium-sulfur (Li-S) batteries with non-aqueous electrolyte solutions. Here, we use operando small and wide angle X-ray scattering and operando small angle neutron scattering (SANS) measurements to track the nucleation, growth and dissolution of solid deposits from atomic to sub-micron scales during real-time Li-S cell operation. In particular, stochastic modelling based on the SANS data allows quantifying the nanoscale phase evolution during battery cycling. We show that next to nano-crystalline Li2S the deposit comprises solid short-chain PSs particles. The analysis of the experimental data suggests that initially, Li2S2 precipitates from the solution and then is partially converted via solid-state electroreduction to Li2S. We further demonstrate that mass transport, rather than electron transport through a thin passivating film, limits the discharge capacity and rate performance in Li-S cells.

A Facile One-Pot Synthesis of Hierarchically Organized Carbon/TiO2 Monoliths with Ordered Mesopores.

Sol-gel processing combined with soft templating and gelation-induced phase separation is very sensitive to the precursor sol composition. In this work we present a straightforward synthesis towards hierarchically structured, macroporous carbon/titania monoliths with ordered mesopores derived from resorcinol/formaldehyde monoliths and a glycolated titanium precursor. We demonstrate the influence of various reaction solvents, where diol-based media and the proportion of the catalyst seem to be essential in controlling spinodal decomposition, obtaining similar monolithic structures under different synthesis conditions. Based on these observations, we further homogeneously incorporated TiO2 into the carbon structure by an in situ synthesis approach, obtaining structural features similar to pure carbon materials with surface areas of about 400 m2 g-1 , periodically arranged mesopores with a mean distance of 10-11 nm and cellular macroporosity.

Mechanism of mediated alkali peroxide oxidation and triplet versus singlet oxygen formation.

Aprotic alkali metal-O2 batteries face two major obstacles to their chemistry occurring efficiently, the insulating nature of the formed alkali superoxides/peroxides and parasitic reactions that are caused by the highly reactive singlet oxygen (1O2). Redox mediators are recognized to be key for improving rechargeability. However, it is unclear how they affect 1O2 formation, which hinders strategies for their improvement. Here we clarify the mechanism of mediated peroxide and superoxide oxidation and thus explain how redox mediators either enhance or suppress 1O2 formation. We show that charging commences with peroxide oxidation to a superoxide intermediate and that redox potentials above ~3.5 V versus Li/Li+ drive 1O2 evolution from superoxide oxidation, while disproportionation always generates some 1O2. We find that 1O2 suppression requires oxidation to be faster than the generation of 1O2 from disproportionation. Oxidation rates decrease with growing driving force following Marcus inverted-region behaviour, establishing a region of maximum rate.

Persistent and reversible solid iodine electrodeposition in nanoporous carbons.

Aqueous iodine based electrochemical energy storage is considered a potential candidate to improve sustainability and performance of current battery and supercapacitor technology. It harnesses the redox activity of iodide, iodine, and polyiodide species in the confined geometry of nanoporous carbon electrodes. However, current descriptions of the electrochemical reaction mechanism to interconvert these species are elusive. Here we show that electrochemical oxidation of iodide in nanoporous carbons forms persistent solid iodine deposits. Confinement slows down dissolution into triiodide and pentaiodide, responsible for otherwise significant self-discharge via shuttling. The main tools for these insights are in situ Raman spectroscopy and in situ small and wide-angle X-ray scattering (in situ SAXS/WAXS). In situ Raman confirms the reversible formation of triiodide and pentaiodide. In situ SAXS/WAXS indicates remarkable amounts of solid iodine deposited in the carbon nanopores. Combined with stochastic modeling, in situ SAXS allows quantifying the solid iodine volume fraction and visualizing the iodine structure on 3D lattice models at the sub-nanometer scale. Based on the derived mechanism, we demonstrate strategies for improved iodine pore filling capacity and prevention of self-discharge, applicable to hybrid supercapacitors and batteries.

Author Correction: Persistent and reversible solid iodine electrodeposition in nanoporous carbons.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Willow Bark for Sustainable Energy Storage Systems.

Willow bark is a byproduct from forestry and is obtained at an industrial scale. We upcycled this byproduct in a two-step procedure into sustainable electrode materials for symmetrical supercapacitors using organic electrolytes. The procedure employed precarbonization followed by carbonization using different types of KOH activation protocols. The obtained electrode materials had a hierarchically organized pore structure and featured a high specific surface area (>2500 m2 g-1) and pore volume (up to 1.48 cm3 g-1). The assembled supercapacitors exhibited capacitances up to 147 F g-1 in organic electrolytes concomitant with excellent cycling performance over 10,000 cycles at 0.6 A g-1 using coin cells. The best materials exhibited a capacity retention of 75% when changing scan rates from 2 to 100 mV s-1.

Towards Real-Time Ion-Specific Structural Sensitivity in Nanoporous Carbon Electrodes Using In Situ Anomalous Small-Angle X-ray Scattering.

Current in situ techniques to study ion charge storage and electrical double-layer formation in nanoporous electrodes are either chemically sensitive to element-specific concentration changes or structurally sensitive to rearrangements of ions and solvent molecules; but rarely can they cover both. Here we introduce in situ anomalous small-angle X-ray scattering (ASAXS) as a unique method to extract both real-time structural and ion-specific chemical information from one single experiment. Using a 1 M RbBr aqueous electrolyte and a hierarchical micro- and mesoporous carbon electrode, we identify different charging mechanisms for positive and negative applied potentials. We are able not only to track the global concentration change of each ion species individually, but also to observe their individual local rearrangement within the pore space.

In Situ Measurement of Electrosorption-Induced Deformation Reveals the Importance of Micropores in Hierarchical Carbons.

Dimensional changes in carbon-based supercapacitor electrodes were investigated using a combination of electrochemical dilatometry and in situ small-angle X-ray scattering. A novel hierarchical carbon material with ordered mesoporosity was synthesized, providing the unique possibility to track electrode expansion and shrinkage on the nanometer scale and the macroscopic scale simultaneously. Two carbons with similar mesopore structure but different amounts of micropores were investigated, employing two different aqueous electrolytes. The strain of the electrodes was always positive, but asymmetric with respect to positive and negative applied voltages. The asymmetry strongly increased with increasing microporosity, giving hints to the possible physical origin of electrosorption induced pore swelling.