The sluggish kinetics of the alkaline hydrogen electrode have been attributed to the need to adsorb both H and OH optimally. In this work, single-crystal voltammetry and microkinetic modeling show ...that an OH-mediated mechanism is not viable on Pt(110). Only a direct Volmer step can explain observed kinetic trends with OH adsorption strength in KOH and LiOH electrolytes. Instead, OH behaves as a rapidly equilibrated spectator species that decreases available surface sites and slows hydrogen kinetics. These results identify kinetic barriers from interfacial water structure, not adsorption energies, as key to explaining changes in hydrogen kinetics with pH.
The pH-dependent kinetics of the hydrogen oxidation and evolution reactions (HERs and HORs) remain a fundamental conundrum in modern electrochemistry. Recent efforts have focused on the impact of the ...interfacial water network on the reaction kinetics. In this work, we quantify the importance of interfacial water dynamics on the overall hydrogen reaction kinetics with kinetic isotope effect (KIE) voltammetry experiments on single-crystal Pt(111) and Pt(110). Our results find a surface-sensitive KIE for both the HER and the HOR that is measurable in base but not in acid. Remarkably, the HOR in KOD on Pt(111) yields a KIE of up to 3.4 at moderate overpotentials, greater than any expected secondary KIE values, yet the HOR in DClO4 yields no measurable KIE. These results provide direct evidence that solvent dynamics play a crucial role in the alkaline but not in the acidic hydrogen reactions, thus reinforcing the importance of “beyond adsorption” phenomena in modern electrocatalysis.
The slow kinetics of the hydrogen oxidation and hydrogen evolution reactions (HER and HOR) in alkaline compared to acidic media remain a fundamental conundrum in modern electrocatalysis. Recent ...efforts have proposed that OH, as well as H, must bind optimally for improved kinetics, but the exact role of adsorbed OH is not yet known. In this work, we combine steady-state single-crystal voltammetry and microkinetic modeling to determine the roles of adsorbed hydroxide and the so-called bifunctional mechanism in alkaline HER and HOR kinetics. We consider both a direct Volmer mechanism, in which H and OH compete for sites on Pt (110), and an OH-mediated mechanism, in which Pt (111) adsorbs H while transition metal clusters adsorb OH. Our experimental and computational results show that on a thermodynamic coverage basis, increasing OH adsorption strength cannot promote faster HER/HOR kinetics. Only changes to the kinetic rate constants can explain experimental observations. We speculate that adequate electrocatalyst design in alkaline media additionally requires manipulation of interfacial water structure to lower energetic barriers for HER and HOR.
Although electrochemical hydrogen evolution and oxidation are arguably the best-understood reactions in electrocatalysis, the anomalous effect of pH on hydrogen reaction kinetics has defied simple ...explanation for decades. This longstanding puzzle exposes gaps in the fundamental understanding of electrocatalysis by showing that singular adsorption descriptors (e.g., the hydrogen binding energy) cannot describe kinetic effects across electrolytes. In this Perspective, we discuss the strengths and shortcomings of binding energies as HER/HOR activity descriptors across different electrolytes and catalyst surfaces, with a special emphasis on the bifunctional mechanism, and identify several “beyond adsorption” descriptors for chemical dynamics in the double layer, including the potential of zero (free/total) charge, the binding energy of coadsorbed spectator species, transition state barrier heights, and the solvation strength of electrolyte cations. Recent evidence for and against the importance of these phenomena is assessed in the context of hydrogen electrocatalysis to determine their feasibility to accurately predict catalyst behavior. Finally, we propose paths forward for improving the mechanistic understanding of how specific interactions between the surface and species in solution affect macroscopic rates, which include combining single-crystal voltammetry, electroanalytical chemistry, in-operando spectroscopy, atomic-scale DFT calculations, and molecular “double-layer dopants".
Fundamental understanding of how processing affects composite battery electrode structure and performance is still lacking, especially for industry-relevant electrodes with low fractions of inactive ...material. This work combines rheology, electronic conductivity measurements, and battery rate capability tests to prove that short-range electronic contacts are more important to cathode rate capability than either ion transport or long-range electronic conductivity. LiNi0.33Mn0.33Co0.33O2, carbon black, and polyvinylidene difluoride in 1-methyl-2-pyrrolidinone represent a typical commercial electrode with <5.5 wt% inactive material. Dry-mixing carbon black with active material decreases the relative fraction of bulk (free) carbon, as shown by small angle oscillatory shear and microscopy. More free carbon leads to a stronger gel network (more long-range particle contacts) and higher electronic conductivity of the dried films. Improvements in battery rate capability at constant electrode porosity do not correlate to electronic conductivity, but rather show an optimum fraction of free carbon. Simple comparison of rate capability in electrodes with increased total carbon loading (3 wt%) shows improvement for all fractions of free carbon. These results clearly indicate that ion transport cannot be limiting and highlight the critical importance of short-range electronic contacts for controlling battery performance.
•Investigate impact of dry-mixing on industrially relevant secondary electrodes.•Trends in slurry rheology predict trends in electronic conductivity.•Electronic conductivity and rheology do not predict rate capability.•Improving short-range electron contacts has the greatest impact on rate capability.
Three-electrode configurations allow targeted studies of reaction mechanisms, including charge storage in and interphase formation on electrode materials for emerging sodium-ion batteries. However, ...using sodium metal as a reference electrode results in spontaneous formation of electroactive soluble decomposition products in an ester-based electrolyte. These electrolyte decomposition products undergo oxidation at carbon electrodes at potentials that can be mistaken for reversible sodium (de)intercalation, thus obscuring true measurements of material properties.
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•Electrolyte decomposes at Na counter and reference electrodes at open circuit.•Soluble decomposition products oxidized <1 V vs Na/Na+.•Oxidation mimics desirable intercalation currents, creates experimental artifacts.
The ionic conductivity,
σ, of mixtures of poly(ethylene oxide) (PEO) and lithium bis(trifluoromethanesulfone)imide (LiTFSI) was measured as a function of molecular weight of the PEO chains,
M, over ...the range 0.2–5000
kg/mol. Our data are consistent with an expression
σ
=
σ
0
+
K/
M proposed by Shi and Vincent Solid State Ionics 60 (1993) where
σ
0 and
K are exponential and linear functions of inverse temperature respectively. Explicit expressions for
σ
0 and
K are provided.
► PEO/LiTFSI electrolyte conductivity measured over wide molecular weight range ► Conductivity contributions arise from both segmental motion and chain diffusion. ► Explicit expressions provided for molecular weight dependence of conductivity
The solid–electrolyte interphase (SEI) is well-known to provide critical protection between the strongly reducing negative electrode and the organic electrolytes of nonaqueous batteries. Batteries ...with a poorly passivating SEI will suffer from rapid capacity fade and short lifetimes. Despite its importance and extensive study of its structure and composition, the mechanism of SEI passivation remains poorly understood. In this work, we demonstrate using electrochemical collector-generator measurements that the SEI is chemically selective in its passivation and propose a model based on catalytic active sites to explain its performance. Electrochemically interrogating the SEI with functionalized ferrocene mediators shows that the through-film mediator reduction is much more sensitive to mediator functional group than size, indicating preferential partitioning into the organic SEI layer. Additional experiments with controlled electrode crosstalk show that incorporation of dissolved transition metals increases both the density and the activity of active sites within the SEI. We conclude that the inner, inorganic layer is responsible for preventing charge transfer through the SEI while the outer, organic layer is minimally important. Our model reconciles contradictory observations from the literature and identifies the most important components of a functional battery interface.
This past decade has seen extensive research in lithium-sulfur batteries with exemplary works mitigating the notorious polysulfide shuttling. However, these works utilize ether electrolytes that are ...highly volatile severely hindering their practicality. Here, we stabilize a rare monoclinic γ-sulfur phase within carbon nanofibers that enables successful operation of Lithium-Sulfur (Li-S) batteries in carbonate electrolyte for 4000 cycles. Carbonates are known to adversely react with the intermediate polysulfides and shut down Li-S batteries in first discharge. Through electrochemical characterization and post-mortem spectroscopy/ microscopy studies on cycled cells, we demonstrate an altered redox mechanism in our cells that reversibly converts monoclinic sulfur to Li
S without the formation of intermediate polysulfides for the entire range of 4000 cycles. To the best of our knowledge, this is the first study to report the synthesis of stable γ-sulfur and its application in Li-S batteries. We hope that this striking discovery of solid-to-solid reaction will trigger new fundamental and applied research in carbonate electrolyte Li-S batteries.
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