Electrochemical kinetics:a surface-science supported picture of hydrogen electrochemistry on Ru(0001) and Pt/Ru(0001)

In this short review, we compare the kinetics of hydrogen desorption in vacuum to those involved in the electrochemical hydrogen evolution/oxidation reactions (HER/HOR) at two types of atomically smooth model surfaces: bare Ru(0001), and the same surface covered by a 1.1 atomic layer thick Pt film. Low/high H2 (D2) desorption rates at room temperature in vacuum quantitatively correspond to low/high exchange current densities for the HOR/HER in electrochemistry. In view of the “volcano plot” concept, these represent two surfaces that adsorb hydrogen atoms, Had, too strongly and too weakly, respectively. Atomically smooth, vacuum annealed model surfaces are the closest approximation to the idealised slab geometries typically studied by density functional theory (DFT). A predictive volcano plot based on DFT-based adsorption energies for the Had intermediates agrees well with the experiments if two things are considered: (i) the steady-state coverage of Had intermediates and (ii)local variations in film thickness. The sluggish HER/HOR kinetics of Ru(0001) allows for excellent visibility of cyclic voltammetry (CV) features even in H2 saturated solution. The CV switches between a Had and a OHad/Oad dominated regime, but the presence of H2 in the electrolyte increases the Had dominated potential window by a factor of two. Whereas in plain electrolyte two electrochemical adsorption processes compete in forming adlayers, it is one electrochemical and one chemical one in the case of H2 saturated electrolyte. We demonstrate and quantitatively explain that dissociative H2 adsorption is more important than H+ discharge for Had formation in the low potential regime on Ru(0001)

Ultrahigh vacuum and electrocatalysis:the powers of quantitative surface imaging

We highlight the impact of Ultrahigh Vacuum (UHV)-born surface science on modern electrocatalysis. The microscopic, atomic level picture of surface adsorption and reaction, which was developed in the surface science community in decades of systematic research on single crystals in UHV, has meanwhile become state-of-the-art also in electrochemistry. For the example of CO on Pt(111) single crystals, which has been extensively studied at the solid/gas and the solid/liquid interface using atomic resolution scanning tunnelling microscopy (STM), we highlight how both interfaces may have even more in common than often assumed. We then illustrate how planar model surfaces such as mono- and bimetallic single crystals and surface alloys, prepared and thoroughly analysed in UHV, enabled a systematic search for improved electrocatalysts. Surface alloys, thermodynamically more stable than foreign metal islands, are a particularly important sub-group of model surfaces, which so far have only been fabricated in UHV. We also flag that model surfaces may not always assume the structure anticipated for the respective experiment, regardless of their preparation in UHV or by electrochemical methods. “Accidental” surface alloying may be more common than often assumed, leading to misinterpretations of the structure-property relationships targeted in many model studies. We highlight that controlled surface alloy formation should be a key step in any model study looking at bimetallic systems in order to get an idea what the effect of unintended alloying could possibly be, and to cross-check whether alloyed surfaces may potentially be the better electrocatalysts in the first place

Catalysing surface film formation

The solid electrolyte interphase that forms on graphite anodes plays a vital role in the performance of lithium-ion batteries. Now research shows that the formation of lithium fluoride deposits — one of the main components of the solid electrolyte interphase — is strongly influenced by the electrocatalytic activity of the anode

Lithium-oxygen cells:an analytical model to explain the key features in the discharge voltage profiles

Sodium-oxygen Lithium-oxygen (Li-O2) cells are popular due to their potentially high energy density. A characteristic fingerprint of a given cell is the voltage profile during constant-current discharge. We suggest that the typical initial dip and the following increase of the voltage result from a temporary increase and slow decrease in the concentration of dissolved superoxide, respectively, feeding into the Nernst equation. The steady-state superoxide concentration decreases as the surface area of the solid precipitation product (Li2O2) increases. Importantly, these products bury the electrochemically active carbon surface. Assuming that the electrochemical step only occurs on bare carbon, the Tafel equation provides an expression for the increasing overpotential as a result of the shrinking effective electrode area. This boils the discharge voltage profile down to the sum of two logarithms, grasping all relevant features in recorded discharge voltage profiles

Emissions: Step on the natural gas for German cars

The decision by Germany's Federal Council to phase out petrol and diesel vehicles by 2030 is at odds with the government's investment in renewable energy, which is not enough to produce the extra power that electric cars will need. We show how natural gas could plug the gap. Replacing internal-combustion vehicles with electric cars would reduce Germany's primary energy needs by 60%, from about 570 terawatt-hours (TWh) to about 230 TWh (detailed calculations available from the authors). However, the government's brake on renewables, mainly to protect stability of the electricity grid, means that only 63 TWh will come online by 2030 (see also Nature 534, 152; 2016). Making up the deficit with electricity generated by burning natural gas would create 131 million tonnes of carbon dioxide, which would still save 30 million tonnes on 2014 road-transport emissions. To decarbonize its transport sector entirely — and to meet the shortfall under its plan to phase out nuclear energy by 2030 — Germany will need to step up production of renewable energy and develop smart storage grids

Influence of carbon microstructure on the Li–O2 battery first-discharge kinetics

Defects in the carbon microstructure have been reported to enhance the discharge performance of Li–O2 battery. However, systematic studies correlating the presence of defects with the discharge kinetics have not addressed the variation of carbon electrode surface areas. In this work, carbon blacks and carbon nanofibers with different defect densities were investigated for their discharge properties. The electrolyte-accessible areas of the carbon electrodes were obtained from Cyclic voltammetry measurements. The microstructure and surface areas of the carbons were characterized by Raman spectroscopy, electron microscopy, and N2 isotherm. Linear sweep voltammetry and galvanostatic discharge experiments consistently demonstrated that graphitic carbons have more negative onset potentials and more negative discharge potentials at the same current density than defective carbons. The linear sweep voltammetry data were normalized to the carbon masses, Brunauer–Emmet–Teller surface areas, and double layer capacitance-derived areas for comparison. Plot of inverse charge transfer resistance and double layer capacitance from electrochemical impedance spectroscopy measurements were used to extract current density values without knowledge of electrode areas. The current densities from impedance measurements exhibited good agreement with the data from linear sweep experiments. The electrochemical experiments conclusively showed that defects on the graphitic microstructure increase the discharge kinetics of the Li–O2 battery

On battery recovery effect in wireless sensor nodes

With the perennial demand for longer runtime of battery-powered Wireless Sensor Nodes (WSNs), several techniques have been proposed to increase the battery runtime. One such class of techniques exploiting the battery recovery effect phenomenon claims that performing an intermittent discharge instead of a continuous discharge will increase the usable battery capacity. Several works in the areas of embedded systems and wireless sensor networks have assumed the existence of this recovery effect and proposed different power management techniques in the form of power supply architectures (multiple battery setup) and communication protocols (burst mode transmission) in order to exploit it. However, until now, a systematic experimental evaluation of the recovery effect has not been performed with real battery cells, using high accuracy battery testers to confirm the existence of this recovery phenomenon. In this paper, a systematic evaluation procedure is developed to verify the existence of this battery recovery effect. Using our evaluation procedure we investigated Alkaline, Nickel-Metal Hydride (NiMH) and Lithium-Ion (Li-Ion) battery chemistries, which are commonly used as power supplies for WSN applications. Our experimental results do not show any evidence of the aforementioned recovery effect in these battery chemistries. In particular, our results show a significant deviation from the stochastic battery models, which were used by many power management techniques. Therefore, the existing power management approaches that rely on this recovery effect do not hold in practice. Instead of a battery recovery effect, our experimental results show the existence of the rate capacity effect, which is the reduction of usable battery capacity with higher discharge power, to be the dominant electrochemical phenomenon that should be considered for maximizing the runtime of WSN applications. We outline power management techniques that minimize the rate capacity effect in order to obtain a higher energy output from the battery

Solid Electrolyte Interphase:Can faster formation at lower potentials yield better performance?

To make a Lithium Ion Battery (LIB) reliably rechargeable over many cycles, its graphite-based negative electrode requires the solid electrolyte interphase (SEI) as a protection layer. The SEI is formed through chemical and particularly electrochemical side reactions of electrolyte components in the first charging cycle(s) after manufacturing of a LIB. The SEI ideally serves two purposes: (i) act as a sieve permeable to Li ions but not to other electrolyte components and (ii) passivate the electrode against further electrolyte decomposition. Core element of conventional SEI formation is a lengthy, low-current galvanostatic charging step, which due to its time consumption contributes heavily to cell manufacturing costs. Here, we report on some non-conventional SEI formation protocols for composite carbon electrodes, inspired by recent experimental findings at smooth model electrodes. Acknowledging that the SEI forms in two main steps, taking place in a high-potential and a low-potential region, respectively, we demonstrate that less time spent in the high-potential region not only makes the process faster but even yields SEIs with superior kinetic properties. We tentatively explain this via basic rules of thin film growth and the role of grain boundaries for ion transport. We also report on the positive influence of multi-frequency potential modulations applied between high-potential and low-potential formation. Given that any new cell chemistry in principle requires its own tailor-made formation process, technologic success of future LIB cells will benefit from a systematic, well-understood toolbox of formation protocols. This paper is meant as a first step, highlighting potentially low-hanging fruits, but also flagging the demand for further systematic studies on model systems and on commercially manufactured cells

Current density distribution in cylindrical Li-Ion cells during impedance measurements

In this work, modified commercial cylindrical lithium-ion cells with multiple separate current tabs are used to analyze the influence of tab pattern, frequency and temperature on electrochemical impedance spectroscopy. In a first step, the effect of different current tab arrangements on the impedance spectra is analyzed and possible electrochemical causes are discussed. In a second step, one terminal is used to apply a sinusoidal current while the other terminals are used to monitor the local potential distribution at different positions along the electrodes of the cell. It is observed that the characteristic decay of the voltage amplitude along the electrode changes non-linearly with frequency, where high-frequent currents experience a stronger attenuation along the current collector than low-frequent currents. In further experiments, the decay characteristic is controlled by the cell temperature, driven by the increasing resistance of the current collector and the enhanced kinetic and transport properties of the active material and electrolyte. Measurements indicate that the ac current distribution depends strongly on the frequency and the temperature. In this context, the challenges for electrochemical impedance spectroscopy as cell diagnostic technique for commercial cells are discussed

Formation of the Solid Electrolyte Interphase at Constant Potentials:a Model Study on Highly Oriented Pyrolytic Graphite

The solid electrolyte interphase (SEI) on graphite anodes is a key enabler for rechargeable lithium ion batteries (LIBs). It ensures that only Li+ ions and no damaging electrolyte components enter the anode and hinders electrolyte decomposition. Its growth should be confined to the initial SEI formation process and stop once the battery is in operation to avoid capacity/power loss. In technical LIB cells, the SEI is formed at constant current, with the potential of the graphite anode slowly drifting from higher to lower voltages. SEI formation rate, composition, and structure depend on the potential and on the chemical properties of the anode surface. Here, we characterize SEIs formed at constant potentials on the chemically inactive basal plane of highly oriented pyrolytic graphite (HOPG). X‐ray photoemission spectroscopy (XPS) detects carbonate‐species only at lower formation potentials. Cyclic voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) with Fc/Fc+ as an electrochemical probe demonstrate how the formation potential influences ion transport and electrochemical kinetics to and at the anode surface, respectively. Breaking the EIS data down to a Distribution of Relaxation Times (DRT) reveals distinct kinetics and transport related peaks with varying Arrhenius‐type temperature dependencies. We discuss our findings in the context of previous electrochemical studies and existing SEI models and of SEI formation protocols suitable for industry