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Hydrogen titanates (HTOs) form a diverse group of metastable, layered titanium oxides with an interlayer containing both water molecules and structural protons. We investigated how the chemistry of this interlayer environment influenced electrochemical Li+-insertion in a series of HTOs, H2TiyO2y+1·nH2O (y= 3, 4, and 5). We correlated the electrochemical response with the physical and chemical properties of HTOs using operando X-ray diffraction, in situ differential electrochemical mass spectroscopy, solid-state proton nuclear magnetic resonance, and quasi-elastic neutron scattering. We found that the potential for the first reduction reaction trended with the relative acidity of the structural protons. This mechanism was supported with first-principles density functional theory (DFT) calculations. We propose that the electrochemical reaction involves reduction of the structural protons to yield hydrogen gas and formation of a lithiated hydrogen titanate (H2–xLixTiyO2y+1). The hydrogen gas is confined within the HTO lattice until the titanate structure expands upon subsequent oxidation. Our work has implications for the electrochemical behavior of insertion hosts containing hydrogen and structural water molecules, where hydrogen evolution is expected at potentials below the hydrogen reduction potential and in the absence of electrolyte proton donors. This behavior is an example of electrochemical electron transfer to a nonmetal element in a metal oxide host, in analogy to anion redox.

Advances in lithium-ion battery (LIB) research have strived for high-energy, safe, and sustainable materials. Li-rich Li1.2Ni0.2Mn0.6O2(LRNM) cathodes have shown great promise with high voltages and excellent specific capacities. Obstacles preventing the viability and commercialization of LRNM are the initial capacity loss of roughly 30% and rapid voltage decay upon cycling. Herein, lithium oxalate (Li2C2O4) is investigated as a pre-lithiation additive to utilize the first-cycle lithium re-intercalation losses of LRNM to compensate the irreversible lithium consumption in graphite and high-capacity a-Si on copper silicide nanowire (a-Si/CuSi NW) anodes. Specifically, the decomposition process of Li2C2O4as well as the interaction between the electrode components inside the cell are comprehensively examined. This concept allows us to extend the cycle life of graphite||LRNM cells at 1 C from less than 500 cycles (142 mAh g−1, 78%) to more than 900 cycles (143 mAh g−1, 82%) before reaching the 80% capacity retention threshold. Finally, LRNM electrodes with a precisely balanced concentration of Li2C2O4are prepared in order to compensate the high irreversible losses of a-Si/CuSi NW anodes, achieving a capacity retention of almost 50% with a remaining specific capacity of 82 mAh g−1after 400 cycles in high-energy a-Si/CuSi NW||LRNM lithium-ion cells.

Utilization of high-capacity alloying anodes is a promising yet extremely challenging strategy in building high energy density alkali-ion batteries (AIBs). Excitingly, it was very recently found that the (de-)sodiation of tin (Sn) can be a highly reversible process in specific glyme electrolytes, enabling high specific capacities close to the theoretical value of 847 mA h g–1. The unique solid electrolyte interphase (SEI) formed on Sn electrodes, which allows highly reversible sodiation regardless of the huge volume expansion, is herein demonstrated according to a series of in situ and ex situ characterization techniques. The SEI formation process mainly involves NaPF6decomposition and the polymerization/oligomerization of the glyme solvent, which is induced by the catalytic effect of tin, specifically. This work provides a paradigm showing how solvent, salt, and electrode materials synergistically mediate the SEI formation process and obtains new insights into the unique interfacial chemistry between Na-alloying electrodes and glyme electrolytes, which is highly enlightening in building high energy density AIBs.

Electrochemical hydrogen adsorption and desorption, and the state of adsorbed hydrogen (*H) on metal surfaces are of fundamental interest as well as practical importance for the hydrogen evolution reaction (HER) and electrochemical hydrogenation reactions including CO2and CO electroreduction. Here, we report a previously undiscovered phenomenon whereby *H desorbs as H2during an anodic potential sweep at potentials anodic of (more positive than) the equilibrium potential of U0(H2/H+), hence at potentials where hydrogen desorption would be expected as H+. Using electrochemistry – online mass spectrometry, we observe, quantify, and characterize the phenomenon on two different materials in two different environments – Ru(0001) in acid and polycrystalline Cu in alkaline. For both Ru and Cu, the anodic H2formation seems to coincide with *OH adsorption, which would be consistent with a displacement mechanism. We propose that a high barrier for the Volmer step causes some of the displaced *H to desorb as H2(Tafel step) rather than the thermodynamically more favorable desorption as H+(Volmer step).

In order to learn more about the effect of H2 fuel starvation on the degradation of Pt anode catalysts and in particular on the role of the support therein, we have investigated the impact of a simulated fuel starvation procedure on three different polymer electrolyte fuel cell anode catalysts, Pt/C, Pt/TiO2 and Pt/TiO2+C, by differential electrochemical mass spectrometry (DEMS) measurements. Monitoring the potential as well as the mass spectrometric signal transients of the possible reaction products CO2 (carbon oxidation reaction - COR) and O2 (oxygen evolution reaction - OER) upon switching from H2-saturated to N2-saturated electrolyte under galvanostatic oxidation conditions, we find a rapid increase of the potential to values at about 1.75 V upon electrolyte exchange, together with the onset of CO2 formation and O2 evolution, where the latter dominates at high potentials. The ratio of O2 evolution and carbon support oxidation differs significantly for different catalyst supports, with the COR contributing more for the carbon-containing catalysts in the initial stage of the fuel starvation period, and the OER prevailing for the Pt/TiO2 catalyst over the whole starvation time. The complex interplay between different catalyst degradation processes during fuel starvation is discussed.

The electrochemical activation of Li2MnO3domains in Li- and Mn-rich layered oxides (LRLO) is highly important, and can be tuned by surface modification of the active materials to improve their cycling performance. In this study, citric acid was employed as a combined organic acid, reducing agent, and carbon precursor in order to remove surface residues from the calcination process, implement an oxygen deficient layer on the surface of the primary LRLO particles, and cover their surface with a carbon-containing coating after a final annealing step. A broad selection of bulk and surface sensitive characterization methods was used to characterize the post-treated spherical particles, providing the evidence for successful creation of an oxygen deficient near-surface region, covered by carbon-containing deposits. Post-treated materials show enhanced electrochemical discharge capacities after progressive Li2MnO3activation, reaching maximum capacities of 247 mAh g−1. Gassing measurements reveal the suppression of oxygen release during the first cycle, concomitant with an increased CO2formation for the carbon-coated materials. The voltage profile analysis in combination with post-mortem characterization after 300 cycles provide insights into the aging of the treated materials, which underlines the importance of the relationship between structural changes during scalable post-treatment and the electrochemical performance of the powders.

Photoelectrochemical water oxidation and photocatalytic H2evolution on planar nanocrystalline Au/TiO2catalyst film electrodes were studied by combined electrochemistry and online mass spectrometry, allowing for the detection and quantification of the volatile reaction products O2and H2, and thus, by comparison with the photocurrent, to determine selectivities and to map out contributions from side reactions. The catalysts were prepared by a photodeposition method and deposited on conductive glass substrates, and measurements were performed under defined mass transport conditions in a photoelectrochemical thin-layer flow cell. The role of Au nanoparticles as cocatalysts in these reactions was explored by varying the Au loading and the light intensity, respectively. On the basis of these data, it is demonstrated that the Au nanoparticles affect the above reactions in different ways, by (i) catalyzing the recombination of photogenerated holes and electrons as well as parasitic side reactions (O2reduction) in the photoelectrochemical water oxidation reaction, by (ii) affecting the selectivity in that reaction, and by (iii) improving the separation of photogenerated electrons and holes by catalyzing the reaction with sacrificial agents such as methanol, thus enhancing the photocatalytic H2evolution reaction.

The electrooxidation of formic acid on a Au film electrode in perchloric acid solution was investigated under controlled and continuous mass transport conditions, employing rapid scan in situ ATR-FTIR spectroscopy measurements with 25 ms time resolution. Comparison of the dynamics and the potential dependence of adsorbed spectator species (perchlorate) and of adsorbed bridge-bonded formates and evaluation of kinetic isotope effects on the reaction characteristics provides detailed information on the reaction mechanism.

The interaction of CO, HCOOH, HCHO, and CH3OH molecules with a Pt surface initially covered with a layer of oxide under open circuit potential (OCP) is studied by combined infrared and mass spectroscopy. We found that after switching to the fuel containing solution and concomitantly switching off the electrode potential control at 1.2 V, (i) the OCP decays from 1.2 V down to values of 0.58 V (CO), 0.12 V (HCOOH), 0.08 V(HCHO) and 0.24 V(CH3OH); (ii) CO is the only adsorbate formed at the Pt surface from the fuels; (iii) the rates for the decay of OCP and for the buildup of COadadlayer decrease in the order of CO > HCHO > HCOOH > CH3OH; (iv) the rate of CO2production and the total amount of CO2produced decreases in the order of CO > HCOOH > HCHO > CH3OH; and (v) a significant amount of HCOOH is formed for the case with HCHO and the main by product from CH3OH is HCHO. Our results indicate that (i) the change in OCP is determined by the change of net charge at the electrode/electrolyte interface due to the production of electrons from fuel oxidation and the consumption of electrons by Oad/OHadreduction; and (ii) even without an externally potential control, the reactions occurring at the interface are controlled by the electrochemical potential of the respective reactants and products. The implication from OCP transient to the local potential distribution and local activity in practical fuel cells are discussed.

A combination of in situ Fourier transform infrared spectroscopy in an attenuated total reflection configuration (ATR-FTIRS) and online differential electrochemical mass spectrometry (DEMS) was used to investigate the electrooxidation of 1-propanol on Pt film electrodes, focusing on the unambiguous confirmation of adsorbed propionyl as reaction intermediate and on its role in the adsorption/oxidation reaction. Using unlabeled and 13C-labeled 1-propanol, with the C1 atom being labeled, and employing mainly potentiostatic transient and adsorbate stripping experiments, we arrive at the following conclusions: (i) The shift of the band at 1640 cm–1, which was tentatively assigned to adsorbed propionyl, to 1610 cm–1upon 13C-labeling confirms the above assignment. (ii) Adsorbed propionyl is the main precursor for COadformation, at least for COadfrom the carbon atom carrying the OH group (C1 atom). (iii) Upon dissociative adsorption of 1-propanol at 0.2 V, only the C1 carbon atom forms COad; further reaction of the alkyl chain as second propionyl decomposition product is inhibited. (iv) At 0.4 and 0.6 V, COadformation from the alkyl chain (12COadformation) is possible but proceeds more slowly than from the functional group. Mechanistic trends are summarized in a reaction scheme.

The electrooxidation of saturated CO adlayers on Pt/Vulcan and polycrystalline Pt has been studied by potential step techniques combined with differential electrochemical mass spectrometry (DEMS) and kinetic Monte Carlo (KMC) simulations. DEMS was used to selectively monitor the COadelectrooxidation, via the CO2formation rate, without interference from the pseudocapacitive double-layer charging and electrode surface oxidation, while the KMC simulations were employed to understand the mechanism and kinetics of COadelectrooxidation at the molecular level. Our DEMS data show that the current transients of COadelectrooxidation on polycrystalline Pt and Pt/Vulcan exhibit an initial spike immediately after the potential step, followed by a slow current decay and finally a broad main peak. The temporal evolution of the transients depends strongly on the oxidation potential applied, resulting in the overlap of the initial spike and the main peak for high potentials. A model is proposed to account for the observed phenomena. On the basis of this model, we developed a kinetic Monte Carlo simulation code specific to the electrooxidation of adsorbed CO on Pt. The simulations reproduce the experimental data very well, confirming the robustness of our model.

Combination of electron impact ionization (EI-MS) and electrospray ionization mass spectrometry (ESI-MS) allowed us for simultaneous quantitative online detection of both volatile (carbon dioxide) and non-volatile (formic acid and formaldehyde) reaction products during continuous methanol electrooxidation. The overall reaction rate was monitored via the Faradaic current, carbon dioxide formation was followed mass spectrometrically (electron impact mass spectrometry) via a membrane inlet system in a thin-layer flow-cell, while formaldehyde and formic acid were detected by ESI-MS after the online derivatization-extraction-separation procedure described previously (W. Zhao et al., Anal. Chem. 82, 2472 (2010)).

Aiming at the investigation of spillover and transport effects in electrocatalytic reactions on bimetallic catalyst electrodes, we have prepared novel, nanostructured electrodes consisting of arrays of homogeneously distributed pairs of Pt and Ru nanodisks of uniform size and with controlled separation on planar glassy carbon substrates. The nanodisk arrays (disk diameter ≈ 60 nm) were fabricated by hole-mask colloidal lithography; the separation between pairs of Pt and Ru disks was varied from −25 nm (overlapping) via+25 nm to +50 nm. Morphology and (surface) composition of the Pt/Ru nanodisk arrays were characterized by scanning electron microscopy, energy dispersive X-ray analysis, and X-ray photoelectron spectroscopy, the electrochemical/electrocatalytic properties were explored by cyclic voltammetry, COadmonolayer oxidation (“COadstripping”), and potentiodynamic hydrogen oxidation. Detailed analysis of the COadoxidation peaks revealed that on all bimetallic pairs these cannot be reproduced by superposition of the peaks obtained on electrodes with Pt/Pt or Ru/Ru pairs, pointing to effective Pt−Ru interactions even between rather distant pairs (50 nm). Possible reasons for this observation and its relevance for the understanding of previous reports of highly active catalysts with separate Pt and Ru nanoparticles are discussed. The results clearly demonstrate that this preparation method is perfectly suited for fabrication of planar model electrodes with well-defined arrays of bimetallic nanodisk pairs, which opens up new possibilities for model studies of electrochemical/electrocatalytic reactions.

Transport effects may not only influence the rate of electrocatalytic reactions, but also the product distribution. The underlying effects were studied under well-defined reaction and transport conditions, using nanostructured Pt/glassy carbon electrodes, which consist of ordered arrays of electrocatalytically active Pt nanostructures on a planar glassy carbon substrate. The effect of varying the density of the Pt nanostructures or the electrolyte flow rate on the reaction characteristics and product distribution was investigated for apparently simple electrocatalytic reactions such as CO oxidation, methanol oxidation and oxygen reduction. It is shown that reducing the Pt coverage leads to an increase of the relative amount of the reaction intermediates. Similar effects are obtained for increasing the electrolyte flow rate. The results are discussed on a molecular scale in terms of the 'desorption - re-adsorption - reaction' model which we had introduced recently (Y.E. Seidel et al., Faraday Discuss. 140, 167 (2008)).

The electrooxidation of methanol, ethanol and ethylene glycol on a Pt/Vulcan catalyst was investigated over a wide temperature range [20 to 100oC] at 3 bar overpressure and under continuous electrolyte flow by differential electrochemical mass spectrometry [DEMS]. Oxidation of methanol, ethanol and ethylene glycol at 0.6 V at ambient temperatures yields CO2 current efficiencies of ~40, 5 and 4%, which increase to 100, 24 and 33% at 100oC, respectively. The apparent activation energies for the overall reaction and for oxidation to CO2 were found to be 58 and 64 kJ mol-1 for methanol oxidation, for ethanol / ethylene glycol they were 41 / 32 kJ mol-1 and 73 / 53 kJ mol-1, respectively. The mechanistic and kinetic implications of these data and the importance of model studies under realistic reaction conditions will be discussed.

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Electrochemical Quartz Crystal Microgravimetry (EQCM) studies were carried out in a new type of electroless copper plating solution containing Co(II)-ethylenediamine species as a reducing agent for Cu(II)-ethylenediamine complex. The rates of partial reactions of Cu(II) reduction and Co(II) oxidation were studied as a function of the electrode potential (−0.3 V to 0.05 V vs. SHE) under stopped-flow and wall jet conditions. The increase in the rate of partial reactions and the overall process under wall jet conditions suggests diffusion limitations occur in the transport of reacting species (Cu(II) and Co(II)) to the electrode and/or inhibiting products of reaction (Co(III) and ethylenediamine) to the bulk of the solution. Anodic dissolution of copper starts at 0.0 V.

The extensive investigation via classical Molecular Dynamics (MD) simulations of the halide-free “water-in-salt” electrolyte (WiSE) consisting of sodium acetate (8 m) and potassium acetate (32 m), unveils the interactions between cations, anions and water molecules. The WiSE's application as electrolyte in symmetric aqueous sodium-ion batteries, featuring NASICON-type Na2VTi(PO4)3/C (NVTP/C) as active material at both the positive and the negative electrode, is also reported. In situ X-ray diffraction (XRD) measurements resolve the structural evolution of NVTP/C during the highly reversible sodium de/intercalation. Differential Electrochemical Mass Spectrometry (DEMS) confirms the remarkable stability of the highly concentrated electrolyte. Symmetric cells employing two NVTP/C electrodes and a green 32K8N electrolyte show an average discharge voltage of 1.13 V with stable cycling performance and a coulombic efficiency above 99.1% at 1C and 99.9% at 10C over 500 cycles. Compared to the fluorinated 9.2 m NaOTF, the 32K8N electrolyte has substantially lower cost, environmental impact and superior coulombic efficiency in symmetric cells.

Formation of oxidized surface films at a gold electrode in sulfuric acid medium during anodization was studied using the electro-chemical quartz crystal microbalance (EQCM) technique. The oxidation process consists of two single-electron steps. Forming a surface species from adsorbed water is the first step. Generating a surface species from the species is the second step. The second step is accompanied by a concerted place exchange reaction involving the surface species and gold atoms below the surface, and rehydration of the gold surface. The reduction of the surface retraces the oxidation processes. During the faradaic processes, sulfate species are adsorbing on (or desorbing from) the gold surface, but produce no detectible mass change. They have symmetry on the surface in the relevant potential region and compete for five surface gold atoms with five adsorbed water, and/or moieties. In the double-layer region, mass changes caused by the adsorption/desorption of sulfate species. Changing frequency-potential responses accompany cyclic potential scans because of roughening of the gold electrode's surface caused by sulfate-included gold dissolution and redeposition processes. (c)1998 The Electrochemical Society

In order to learn more about the effect of H2fuel starvation on the degradation of Pt anode catalysts and in particular on the role of the support therein, we have investigated the impact of a simulated fuel starvation procedure on three different polymer electrolyte fuel cell anode catalysts, Pt/C, Pt/TiO2and Pt/TiO2+C, by differential electrochemical mass spectrometry (DEMS) measurements. Monitoring the potential as well as the mass spectrometric signal transients of the possible reaction products CO2(carbon oxidation reaction – COR) and O2(oxygen evolution reaction – OER) upon switching from H2-saturated to N2-saturated electrolyte under galvanostatic oxidation conditions, we find a rapid increase of the potential to values at about 1.75 V upon electrolyte exchange, together with the onset of CO2formation and O2evolution, where the latter dominates at high potentials. The ratio of O2evolution and carbon support oxidation differs significantly for different catalyst supports, with the COR contributing more for the carbon-containing catalysts in the initial stage of the fuel starvation period, and the OER prevailing for the Pt/TiO2catalyst over the whole starvation time. The complex interplay between different catalyst degradation processes during fuel starvation is discussed.

The electrooxidation of formic acid on a Au film electrode in perchloric acid solution was investigated under controlled and continuous mass transport conditions, employing rapid scan in situATR-FTIR spectroscopy measurements with 25 ms time resolution. Comparison of the dynamics and the potential dependence of adsorbed spectator species (perchlorate) and of adsorbed bridge-bonded formates and evaluation of kinetic isotope effects on the reaction characteristics provides detailed information on the reaction mechanism.

The pKa values of methanediol deprotonation were found to decrease with increase in solution ionic strength as estimated by a 13C NMR titration method.

Combination of electron impact ionization (EI-MS) and electrospray ionization mass spectrometry (ESI-MS) allowed us for simultaneous quantitative online detection of both volatile (carbon dioxide) and non-volatile (formic acid and formaldehyde) reaction products during continuous methanol electrooxidation. The overall reaction rate was monitored via the Faradaic current, carbon dioxide formation was followed mass spectrometrically (electron impact mass spectrometry) via a membrane inlet system in a thin-layer flow-cell, while formaldehyde and formic acid were detected by ESI-MS after the online derivatization-extraction-separation procedure described previously (W. Zhao et al., Anal. Chem. 82, 2472 (2010)).

Transport effects may not only influence the rate of electrocatalytic reactions, but also the product distribution. The underlying effects were studied under well-defined reaction and transport conditions, using nanostructured Pt/glassy carbon electrodes, which consist of ordered arrays of electrocatalytically active Pt nanostructures on a planar glassy carbon substrate. The effect of varying the density of the Pt nanostructures or the electrolyte flow rate on the reaction characteristics and product distribution was investigated for apparently simple electrocatalytic reactions such as CO oxidation, methanol oxidation and oxygen reduction. It is shown that reducing the Pt coverage leads to an increase of the relative amount of the reaction intermediates. Similar effects are obtained for increasing the electrolyte flow rate. The results are discussed on a molecular scale in terms of the 'desorption - re-adsorption - reaction' model which we had introduced recently (Y.E. Seidel et al., Faraday Discuss. 140, 167 (2008)).

The electrooxidation of small organic molecules proceeds via a complex reaction network, involving a number of stable, adsorbed or desorbed (volatile) reaction intermediates and side products in addition to the main product. These reaction networks will be discussed for a number of C1-C3 molecules, focusing on the electrooxidation of ethanol. The discussion is based on ATR-FTIRS/DEMS flow cell measurements, where in-situ IR spectroscopy in an ATR configuration is employed for the detection of adsorbed reaction intermediates and the product distribution is quantitatively determined via on-line DEMS. The implications on the mechanism of the electrooxidation of small organic molecules will be addressed.

The electrooxidation of methanol, ethanol and ethylene glycol on a Pt/Vulcan catalyst was investigated over a wide temperature range [20 to 100oC] at 3 bar overpressure and under continuous electrolyte flow by differential electrochemical mass spectrometry [DEMS]. Oxidation of methanol, ethanol and ethylene glycol at 0.6 V at ambient temperatures yields CO2 current efficiencies of ~40, 5 and 4%, which increase to 100, 24 and 33% at 100oC, respectively. The apparent activation energies for the overall reaction and for oxidation to CO2 were found to be 58 and 64 kJ mol-1 for methanol oxidation, for ethanol / ethylene glycol they were 41 / 32 kJ mol-1 and 73 / 53 kJ mol-1, respectively. The mechanistic and kinetic implications of these data and the importance of model studies under realistic reaction conditions will be discussed.