Surface Analysis of Pristine and Cycled NMC/Graphite Lithium-Ion Battery Electrodes: Addressing the Measurement Challenges.

Lithium-ion batteries are the most ubiquitous energy storage devices in our everyday lives. However, their energy storage capacity fades over time due to chemical and structural changes in their components, via different degradation mechanisms. Understanding and mitigating these degradation mechanisms is key to reducing capacity fade, thereby enabling improvement in the performance and lifetime of Li-ion batteries, supporting the energy transition to renewables and electrification. In this endeavor, surface analysis techniques are commonly employed to characterize the chemistry and structure at reactive interfaces, where most changes are observed as batteries age. However, battery electrodes are complex systems containing unstable compounds, with large heterogeneities in material properties. Moreover, different degradation mechanisms can affect multiple material properties and occur simultaneously, meaning that a range of complementary techniques must be utilized to obtain a complete picture of electrode degradation. The combination of these issues and the lack of standard measurement protocols and guidelines for data interpretation can lead to a lack of trust in data. Herein, we discuss measurement challenges that affect several key surface analysis techniques being used for Li-ion battery degradation studies: focused ion beam scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and time-of-flight secondary ion mass spectrometry. We provide recommendations for each technique to improve reproducibility and reduce uncertainty in the analysis of NMC/graphite Li-ion battery electrodes. We also highlight some key measurement issues that should be addressed in future investigations.

The Role of Tungsten Oxide in Enhancing the Carbon Monoxide Tolerance of Platinum-Based Hydrogen Oxidation Catalysts.

Significant reductions in total cost of ownership can be realized by engineering PEM fuel cells to run on low-purity hydrogen. One of the main drawbacks of low-purity hydrogen fuels is the carbon monoxide fraction, which poisons platinum electrocatalysts and reduces the power output below useful levels. Platinum-tungsten oxide catalyst systems have previously shown high levels of CO tolerance during both ex situ and in situ investigations. In this work, we explore the mechanism of enhanced tolerance using in situ electrochemical attenuated total reflection-infrared (ATR-IR) and Raman spectroscopy methods and investigate, using a mixture of Pt/C and WO3 powders, the role of the WV/WVI redox couple in the oxidation of adsorbed CO.

Multihole water oxidation catalysis on haematite photoanodes revealed by operando spectroelectrochemistry and DFT.

Water oxidation is the key kinetic bottleneck of photoelectrochemical devices for fuel synthesis. Despite advances in the identification of intermediates, elucidating the catalytic mechanism of this multi-redox reaction on metal-oxide photoanodes remains a significant experimental and theoretical challenge. Here, we report an experimental analysis of water oxidation kinetics on four widely studied metal oxides, focusing particularly on haematite. We observe that haematite is able to access a reaction mechanism that is third order in surface-hole density, which is assigned to equilibration between three surface holes and M(OH)-O-M(OH) sites. This reaction exhibits low activation energy (Ea ≈ 60 meV). Density functional theory is used to determine the energetics of charge accumulation and O-O bond formation on a model haematite (110) surface. The proposed mechanism shows parallels with the function of the oxygen evolving complex of photosystem II, and provides new insights into the mechanism of heterogeneous water oxidation on a metal oxide surface.

La5 Ti2 Cu0.9 Ag0.1 S5 O7 Modified with a Molecular Ni Catalyst for Photoelectrochemical H2 Generation.

The stable and efficient integration of molecular catalysts into p-type semiconductor materials is a contemporary challenge in photoelectrochemical fuel synthesis. Here, we report the combination of a phosphonated molecular Ni catalyst with a TiO2 -coated La5 Ti2 Cu0.9 Ag0.1 S5 O7 photocathode for visible light driven H2 production. This hybrid assembly provides a positive onset potential, large photocurrents, and high Faradaic yield for more than three hours. A decisive feature of the hybrid electrode is the TiO2 interlayer, which stabilizes the oxysulfide semiconductor and allows for robust attachment of the phosphonated molecular catalyst. This demonstration of an oxysulfide-molecular catalyst photocathode provides a novel platform for integrating molecular catalysts into photocathodes and the large photovoltage of the presented system makes it ideal for pairing with photoanodes.

Energy transfer and photoluminescence properties of lanthanide-containing polyoxotitanate cages coordinated by salicylate ligands.

Polyoxotitanate (POT) cages have attracted considerable attention recently; much of this from the fact that they can be considered to be structural models for the technologically important semiconductor TiO2. Among the reported POT cages, lanthanide-containing (Ln-POT) cages are of particular interest owing to the fascinating luminescence properties of Ln3+ ions and the versatile coordination environments that they can adopt. In the present study, we report the energy transfer mechanism and photoluminescence properties of a series of isostructural Ln-POT cages coordinated by salicylate ligands, of general formula [LnTi6O3(OiPr)9(salicylate)6] (Ln-1, Ln = La to Er excluding Pm). Both visible (for Pr-1, Sm-1, Eu-1, Ho-1 and Er-1) and near-infrared (for Nd-1 and Er-1) Ln3+-centred photoluminescence can be sensitised in solution, and most importantly, their excitation bands all extend well into the visible region up to 475 nm. With the assistance of steady-state and time-resolved photoluminescence spectroscopy, an energy-transfer mechanism involving the salicylate-to-Ti4+ charge-transfer state is proposed to account for the largely red-shifted excitation wavelengths of these Ln-1 cages. The photoluminescence quantum yield of Nd-1 upon excitation via the charge-transfer state reaches 0.30 ± 0.01% in solution, making it among the highest reported values for Nd3+-complexes in the literature.

Tuning Product Selectivity for Aqueous CO2 Reduction with a Mn(bipyridine)-pyrene Catalyst Immobilized on a Carbon Nanotube Electrode.

The development of high-performance electrocatalytic systems for the controlled reduction of CO2 to value-added chemicals is a key goal in emerging renewable energy technologies. The lack of selective and scalable catalysts in aqueous solution currently hampers the implementation of such a process. Here, the assembly of a [MnBr(2,2'-bipyridine)(CO)3] complex anchored to a carbon nanotube electrode via a pyrene unit is reported. Immobilization of the molecular catalyst allows electrocatalytic reduction of CO2 under fully aqueous conditions with a catalytic onset overpotential of η = 360 mV, and controlled potential electrolysis generated more than 1000 turnovers at η = 550 mV. The product selectivity can be tuned by alteration of the catalyst loading on the nanotube surface. CO was observed as the main product at high catalyst loadings, whereas formate was the dominant CO2 reduction product at low catalyst loadings. Using UV-vis and surface-sensitive IR spectroelectrochemical techniques, two different intermediates were identified as responsible for the change in selectivity of the heterogenized Mn catalyst. The formation of a dimeric Mn0 species at higher surface loading was shown to preferentially lead to CO formation, whereas at lower surface loading the electrochemical generation of a monomeric Mn-hydride is suggested to greatly enhance the production of formate. These results emphasize the advantages of integrating molecular catalysts onto electrode surfaces for enhancing catalytic activity while allowing excellent control and a deeper understanding of the catalytic mechanisms.

Electrocatalytic and Solar-Driven CO2 Reduction to CO with a Molecular Manganese Catalyst Immobilized on Mesoporous TiO2.

Electrocatalytic CO2 reduction to CO was achieved with a novel Mn complex, fac-[MnBr(4,4'-bis(phosphonic acid)-2,2'-bipyridine)(CO)3 ] (MnP), immobilized on a mesoporous TiO2 electrode. A benchmark turnover number of 112±17 was attained with these TiO2 |MnP electrodes after 2 h electrolysis. Post-catalysis IR spectroscopy demonstrated that the molecular structure of the MnP catalyst was retained. UV/vis spectroscopy confirmed that an active Mn-Mn dimer was formed during catalysis on the TiO2 electrode, showing the dynamic formation of a catalytically active dimer on an electrode surface. Finally, we combined the light-protected TiO2 |MnP cathode with a CdS-sensitized photoanode to enable solar-light-driven CO2 reduction with the light-sensitive MnP catalyst.

Precious-metal free photoelectrochemical water splitting with immobilised molecular Ni and Fe redox catalysts.

Splitting water into hydrogen and oxygen with molecular catalysts and light has been a long-established challenge. Approaches in homogeneous systems have been met with little success and the integration of molecular catalysts in photoelectrochemical cells is challenging due to inaccessibility and incompatibility of functional hybrid molecule/material electrodes with long-term stability in aqueous solution. Here, we present the first example of light-driven water splitting achieved with precious-metal-free molecular catalysts driving both oxygen and hydrogen evolution reactions. Mesoporous TiO2 was employed as a low-cost scaffold with long-term stability for anchoring a phosphonic acid-modified nickel(ii) bis-diphosphine catalyst (NiP) for electrocatalytic proton reduction. A turnover number of 600 mol H2 per mol NiP was achieved after 8 h controlled-potential electrolysis at a modest overpotential of 250 mV. X-ray photoelectron, UV-vis and IR spectroscopies confirmed that the molecular structure of the Ni catalyst remains intact after prolonged hydrogen production, thereby reasserting the suitability of molecular catalysts in the development of effective, hydrogen-evolving materials. The relatively mild operating conditions of a pH 3 aqueous solution allowed this molecule-catalysed cathode to be combined with a molecular Fe(ii) catalyst-modified WO3 photoanode in a photoelectrochemical cell. Water splitting into H2 and O2 was achieved under solar light illumination with an applied bias of >0.6 V, which is below the thermodynamic potential (1.23 V) for water splitting and therefore allowed the storage of solar energy in the fuel H2.