Surface Electrochemical Reactions Research Articles

Orthogonal Electrochemical Stability of Bulk and Surface in Lead Halide Perovskite Thin Films and Nanocrystals.

Lead halide perovskites have attracted significant attention for their wide-ranging applications in optoelectronic devices. A ubiquitous element in these applications is that charging of the perovskite is involved, which can trigger electrochemical degradation reactions. Understanding the underlying factors governing these degradation processes is crucial for improving the stability of perovskite-based devices. For bulk semiconductors, the electrochemical decomposition potentials depend on the stabilization of atoms in the lattice-a parameter linked to the material's solubility. For perovskite nanocrystals (NCs), electrochemical surface reactions are strongly influenced by the binding equilibrium of passivating ligands. Here, we report a spectro-electrochemical study on CsPbBr3 NCs and bulk thin films in contact with various electrolytes, aimed at understanding the factors that control cathodic degradation. These measurements reveal that the cathodic decomposition of NCs is primarily determined by the solubility of surface ligands, with diminished cathodic degradation for NCs in high-polarity electrolyte solvents where ligand solubilities are lower. However, the solubility of the surface ligands and bulk lattice of NCs are orthogonal, such that no electrolyte could be identified where both the surface and bulk are stabilized against cathodic decomposition. This poses inherent challenges for electrochemical applications: (i) The electrochemical stability window of CsPbBr3 NCs is constrained by the reduction potential of dissolved Pb2+ complexes, and (ii) cathodic decomposition occurs well before the conduction band can be populated with electrons. Our findings provide insights to enhance the electrochemical stability of perovskite thin films and NCs, emphasizing the importance of a combined selection of surface passivation and electrolyte.

Few-layer MoS2 promotes SnO2@C nano-composites for high performance sodium ion batteries.

Due to its abundance, high theoretical capacity, and environmental benefits, tin dioxide (SnO2) shows great potential as an anode material in sodium-ion batteries (SIBs). However, the inadequate electrical conductivity and significant volume fluctuations during the Na+ insertion/extraction process are major limitations to its practical application. Herein, few-layered MoS2@SnO2@C (FMSC) composites with hierarchical nanostructures were prepared through a two-step hydrothermal method. As expected, the electrochemical tests show that the FMSC exhibits superior electrochemical properties, such as an outstanding rate capability of 288.9 mA h g-1 at a current density of 2 A g-1, a high reversible capacity of 415.9 mA h g-1 after 50 cycles at a current density of 0.1 A g-1, and remarkable cycling stability of 158.4 mA h g-1 after 4400 cycles at a current density of 5 A g-1, as an anode material for SIBs. The exceptional performance can be attributed to the presence of a thin layer of MoS2, which enhances surface electrochemical reactions and provides a flexible structure.

Evaluation of the hydrogenation reaction on the electrocatalytic nitrobenzene degradation over (FeCoNiCuZn)xOy high entropy oxides (HEOs)

Environmental pollution is one of the main challenges currently faced by mankind; especially industrial waste-waters treatment and remediation using energy-efficient methodologies. This research focused on the electrochemical degradation of the hazardous nitrobenzene (NB) in aqueous solutions, using novel high-entropy oxide (HEO) electrodes to elucidate the influence of the electrochemical reduction step on the degradation process. The effect of solution pH, dissolved oxygen concentration, anodic applied potential, and cell configurations on NB hydrogenation reaction were evaluated. A synergistic effect between the hydrogenation of nitrobenzene and the degradation was found to drastically enhance the conversion rates. The presence of dissolved oxygen promoted the oxidation reactions, reaching more than 90% nitrobenzene removal at 1.7 V and pH 14. The degradation of the organic by-products was attributed to the formation of the superoxide radical (O2*-) as the main oxidant species. A degradation mechanism was proposed based on the detected intermediates and the decoupling of electrochemical surface reactions observed by EIS at different solution pH.

Statistical in situ scanning electron microscopy investigation on the failure of oxide scales

AbstractOxide scales play a pivotal role in obstructing surface chemical and electrochemical reactions, hence hindering chemo‐mechanical effects such as liquid metal embrittlement of steels. Therefore, the critical conditions and failure mechanism of the oxide film are of major interest in the safe service of steels. Though in situ microscopic methods may directly visualize the failure mechanism, they are often challenged by the lack of statistically reliable evaluation of the critical conditions. Here, by combining in situ scanning electron microscopy with a tapered specimen tensile test in a single experiment, we uniquely achieve a mechanistic study with statistically reliable quantification of the critical strains for each step of the dynamic process of film rupture. This is demonstrated with the oxide films formed on a ferrite–martensite steel in liquid lead–bismuth eutectic alloy at elevated temperatures, with in situ results falling right into the predictions of the statistical analysis. Explicitly, the integrated experimental methodology may facilitate the materials genome engineering of steels with superior service performance.

Cathode Surface Segregating Modification for Boosting Oxygen Reduction Reactions: Coupling Theory and Experiment in Proton Conducting Solid Oxide Fuel Cells (H-SOFCs)

The surface electrochemical catalysis reaction is complex due to the unpredictable surface morphology and severe reaction environment, especially for cathodes in proton conducting solid oxide fuel cells (H-SOFCs) where water vapor is generated and evaporated at a high operating temperature. The first-generation cathode materials (LaxSr1-xMO3-δ, M=Fe, Co, Mn) tend to have A-site cation segregation, and this segregation layer reduces the performance. Surface segregation phenomena tends to be ignored by computer simulations because surface termination is described as a highly crystallized layer, misleading the development of material computing science. Therefore, a combination of theory and surface investigation is essential for future work in this community. In this work, we use density functional theory (DFT) for designing a higher-performing cathode material, La0.35Bi0.15Sr0.5FeO3-δ (LBSF), for improvement of both bulk and surface electrochemical character in an intrinsic manner, predicting a candidate material with good performance that can be used in solid oxide proton conducting fuel cells (H-SOFCs). By combining a sensitive surface analysis technique, low energy ion scattering (LEIS), surface segregation is finely considered and determined that the particles are fully covered with a Sr segregation layer on the outer most surface. Aberration-Corrected transmission electron microscopy (ACTEM) is also applied and shows an amorphous structure at the particle edge. With the determination of this segregation layer, a reality-closed model is given, and related surface reactions are modelled based on this layer instead of a conventional cleaved, well-crystallized surface. Also, as bismuth diffuses towards the surface due to the lone pair effect, the segregation becomes an oxygen reduction reaction preferred layer instead of a blocking layer. After computational prediction and surface analysis, LBSF is successfully applied into H-SOFCs and shows excellent performance with a peak power density of 1630 mW cm−2 at 700 oC. In addition, long term stability tests were conducted to ensure stability in future commercial use, and the experimental results show that LBSF works stable under extremely high temperature operating environments. In addition, a full cell was stabilized for about 300 hours, indicating a potential commercial application in the future.

Investigating electrochemical corrosion at Mg alloy-steel joint interface using scanning electrochemical cell impedance microscopy (SECCIM)

Developing strategies to prevent corrosion at the interface of dissimilar metal alloys is challenging because of the presence of heterogenous distribution of galvanic couples and microstructural features that significantly change the corrosion rate. Devising strategies to mitigate this interfacial corrosion requires quantitative and correlative understanding of its surface electrochemical reaction. In this work, scanning electrochemical cell impedance microscopy (SECCIM) was employed to study location-specific corrosion in the interfacial region of dissimilar alloys, such as AZ31 (magnesium alloy) and DP590 (steel) welded using the Friction-stir Assisted Scribe Technique (FAST) processes. Herein, SECCM and SECCIM were used to perform correlative mapping of the local electrochemical impedance spectroscopic and potentiodynamic polarization to measure the effect of electronic and microstructural changes in the welded interfacial region on corrosion kinetics. Microstructural characterization including scanning electron microscopy and electron backscatter diffraction was performed to correlate changes in microstructural features and chemistry with the corresponding electronic properties that affect corrosion behavior. The variations in corrosion potential, corrosion current density, and electrochemical impedance spectroscopy behavior across the interface provide deeper insights on the interfacial region—which is chemically and microstructurally distinct from both bare AZ31 and DP590 that can help prevent corrosion in dissimilar metal structures.

Copper nanoparticles encapsulated in reduced graphene oxide electrode (CuxrGO1-x) for the electrochemical quantification of metformin in water

Electrochemical oxidation of anti-diabetic metformin (MTF) drug was studied on glassy carbon electrode (GCE) prepared by electroplating of copper nanoparticles (Cu NPs) on reduced graphene oxide (rGO). Results showed that the crystallite size and dispersity of Cu NPs on rGO sheets controlled the redox characteristics of CuxrGO1-x. Voltammetry revealed faradaic Cu(II)/Cu(I) redox current at specific anodic potentials as a function of MTF concentration due to biguanide Cu(II)-MTF complexation. rGO, an electron promoter, improved the current density while lowering the overpotential of MTF oxidation. Both the electrochemical surface area (ECSA) and reaction sensitivity were optimal at the Cu to rGO mass ratio of 7:3 for MTF detection. The electrode also exhibited high degree of reproducibility, selectivity, and recovery in 0.1 M of phosphate buffer solution and pH 12. There was a high degree of linearity between anodic current and MTF concentration in the range of 2 to 70 µM at a slope of 1.375 µA/µM and detection limit of 0.09 µM by differential pulse voltammetry (DPV) technique.

Iron redox behavior and oxygen reduction activity of Fe-N-C electrocatalysts in different electrolytes

The iron redox behavior and oxygen reduction reaction (ORR) activity of Fe-N-C ORR electrocatalysts synthesized by a variety of techniques were investigated as a function of the identity of the electrolyte anion (bisulfate/sulfate or perchlorate) at a constant pH. In situ X-ray absorption spectroscopy data support the assignment of the redox peaks in the voltammograms to the Fe3+/Fe2+ redox couple. It was found that for a given Fe-N-C catalyst, there is a correlation between the Fe redox couple peak potential and the ORR activity in perchloric acid electrolyte, but not in sulfuric acid electrolyte. While a higher Fe redox couple potential (≥ 110 mV higher) was observed in perchloric acid electrolyte, a higher ORR activity was obtained in sulfuric acid electrolyte. The higher ORR activity observed in sulfuric acid than perchloric acid was correlated with the higher peak current and larger faradaic charge for the Fe redox couple. A study of the Fe redox behavior using a cavity microelectrode, eliminating the impact of ionomer, showed that the interaction of H2SO4 with Fe-N-C is stronger than that of HClO4 and that Fe redox in both electrolytes is a reversible surface electrochemical reaction.

Operando monitoring of a room temperature nanocomposite methanol sensor.

The sensing of volatile organic compounds by composites containing metal oxide semiconductors is typically explained via adsorption-desorption and surface electrochemical reactions changing the sensor's resistance. The analysis of molecular processes on chemiresistive gas sensors is often based on indirect evidence, whereas in situ or operando studies monitoring the gas/surface interactions enable a direct insight. Here we report a cross-disciplinary approach employing spectroscopy of working sensors to investigate room temperature methanol detection, contrasting well-characterized nanocomposite (TiO2@rGO-NC) and reduced-graphene oxide (rGO) sensors. Methanol interactions with the sensors were examined by (quasi) operando-DRIFTS and in situ-ATR-FTIR spectroscopy, the first paralleled by simultaneous measurements of resistance. The sensing mechanism was also studied by mass spectroscopy (MS), revealing the surface electrochemical reactions. The operando and in situ spectroscopy techniques demonstrated that the sensing mechanism on the nanocomposite relies on the combined effect of methanol reversible physisorption and irreversible chemisorption, sensor modification over time, and electron/O2 depletion-restoration due to a surface electrochemical reaction forming CO2 and H2O.

Micrometer-scale single crystalline particles of niobium titanium oxide enabling an Ah-level pouch cell with superior fast-charging capability.

Wadsley-Roth phase niobium titanium oxide (TiNb2O7) is widely regarded as a promising anode candidate for fast-charging lithium-ion batteries due to its safe working potential and doubled capacity in comparison to the commercial fast-charging anode material (lithium titanium oxide, Li4Ti5O12). Although good fast charge/discharge performance was shown for nanostructured TiNb2O7, the small size would cause the low electrode compensation density and energy density of batteries, as well as parasitic reactions. Fundamental understanding of the electrochemical lithium insertion/extraction process and the structural evolution for the micrometer-scale single crystalline TiNb2O7 (MSC-TiNb2O7) could provide insights to understand its inherent properties and possibility for fast-charging application. Here, we revealed the highly reversible structural evolution of the MSC-TiNb2O7 during the lithiation/delithiation processes. Interestingly, an ion-conductive lithium niobate interphase was in situ formed on the MSC-TiNb2O7 surface during the formation cycle, which could facilitate fast ion diffusion on the material surface and support fast electrochemical reaction kinetics. Experimentally, the MSC-TiNb2O7 delivered a high reversible capacity of 291.9 mA h g-1 at 0.5C with a high initial Coulombic efficiency (>95%), and showed superb rate capability with a reasonable capacity of 55.6 mA h g-1 under a high current density of 40C. An Ah-level pouch cell with a lithium cobalt oxide (LiCoO2) cathode exhibited 91.5% capacity retention at 3C charging rate, which revealed the significant role of high crystallinity and in situ formation of an ion conductive nano-interphase in realizing fast charging capability of practical TiNb2O7-based lithium-ion batteries.

Asymmetric rectified electric and concentration fields in multicomponent electrolytes with surface reactions.

Recent experimental studies have utilized AC electric fields and electrochemical reactions in multicomponent electrolyte solutions to control colloidal assembly. However, theoretical investigations have thus far been limited to binary electrolytes and have overlooked the impact of electrochemical reactions. In this study, we address these limitations by analyzing a system with multicomponent electrolytes, while also relaxing the assumption of ideally blocking electrodes to capture the effect of surface electrochemical reactions. Through a regular perturbation analysis in the low-applied-potential regime, we solve the Poisson-Nernst-Planck equations and obtain effective equations for electrical potential and ion concentrations. By employing a combination of numerical and analytical calculations, our analysis reveals a significant finding: electrochemical reactions alone can generate asymmetric rectified electric fields (AREFs), i.e., time-averaged, long-range electric fields, even when the diffusivities of the ionic species are equal. This finding expands our understanding beyond the conventional notion that AREFs arise solely from diffusivity contrast. Furthermore, we demonstrate that AREFs induced by electrochemical reactions can be stronger than those resulting from asymmetric diffusivities. Additionally, we report the emergence of asymmetric rectified concentration fields (ARCFs), i.e., time-averaged, long-range concentration fields, which supports the electrodiffusiophoresis mechanism of colloidal assembly observed in experiments. We also derive analytical expressions for AREFs and ARCFs, emphasizing the role of imbalances in ionic strength and charge density, respectively, as the driving forces behind their formation. The results presented in this article advance the field of colloidal assembly and also have implications for improved understanding of electrolyte transport in electrochemical devices.

Enhancing catalytic activity of CO2 electrolysis via B-site cation doped perovskite cathode in solid oxide electrolysis cell

Solid oxide electrolysis cell (SOEC) is considered as a high energy-efficient CO2 conversion method and a promising technology to achieve carbon neutralization. Thus, in this work, the reduced Co or Ni doped Pr0.4Sr0.6Fe0.9Mo0.1O3 perovskite oxides are used as the cathode of SOEC. After reduction, Co-Fe alloy nanoparticles are in situ exsolved from Pr0.4Sr0.6Co0.2Fe0.7Mo0.1O3 and Pr0.4Sr0.6Co0.45Fe0.45Mo0.1O3, and NiFe10.8 alloy nanoparticles are attached on the Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 matrix. The reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 exhibits the highest oxygen vacancy concentration and CO2 adsorption ability, which further promote oxygen surface exchange and electrochemical surface exchange reaction. Besides, the reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 exhibits the highest surface reaction rate constant (kchem) of 3.54 × 10-4 cm s-1 and highest oxygen chemical bulk diffusion coefficient (Dchem) of 27.53 × 10-5 cm2 s-1, suggesting that reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 shows the best catalytic activity for CO2 reduction reaction. The symmetrical cell with reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 shows the lowest polarization resistance (Rp) value, and at 800 °C, the electrolysis cell with reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 cathode also exhibits the best performance of 0.81 A cm-2 in 50%CO-50%CO2 and 1.05 A cm-2 in pure CO2 at 1.5 V, indicating that reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 is a promising cathode for CO2 electrolysis.

NiFe2V2O8@N-Doped carbon Yolk-Double shell spheres for efficient lithium storage

Polymetallic oxide nanostructures with intricate architectures have become the promising subject of much recent investigations into high-efficiency electrode materials for lithium-ion batteries (LIBs). However, their smooth implementation is still plagued by the fabrication of multi-component structures as well as the delivered inadequate specific capacities. Here, complex NiFe2V2O8@N-doped carbon nanostructures with hierarchically porous yolk-double shell spherical architectures via a facile template-free approach are developed and evaluated as the high-efficiency anode materials for LIBs. Benefited from the yolk-double shell constructions, impressively, the electrode harvests a high reversible capacity of 1347.6 mA h g−1 over 200 cycles without visible deterioration at 0.2 A g−1, a robust rate capability (726.1 mA h g−1 at 5 A g−1), and a desirable cycle stability (75.6% capacity retention over 300 cycles at 1 A g−1). Such excellent electrochemical Li-ion storage capability is believed to result from the fast ion-diffusion kinetics enabled by the synergetic cation and oxygen redox because of the introduction of vanadium redox couples, which can be further boosted by capacitive-dominated surface electrochemical reaction. This work provides enlightening insights into the elaborate design and fabrication of complex porous yolk-shell-like nanostructures with intricate compositions for high-efficiency lithium storage.

(Invited) In Situ Electrochemical Raman Spectroscopy for Nanoscale to Macroscale Electrochemical Processes

Raman spectroscopy has been increasingly applied in studying the electrochemical processes, including adsorption and reaction, under in situ or operando condition, because Raman spectroscopy can conveniently work in aqueous solution and provide fingerprint molecular information. In this talk, I will present the recent work in my group on developing various electrochemical Raman spectroscopy for in-situ and operando studies of electrochemical systems at the limits of time and spatial resolution and sensitivityWith the help of the plasmonic enhancement at the tip of a plasmonic material, electrochemical tip-enhanced Raman spectroscopy (EC-TERS) offers itself as the only nanospectroscopy for investigating the electrochemical interface with unprecedent spatial resolution and explicit chemical fingerprint information. We developed the electrochemical TERS both based on STM and AFM using the water immersion objective and homemade optical instrument. EC-TERS was applied to obtain the molecular fingerprint information of surface adsorption and electrochemical reaction on single crystal surfaces and monitor the plasmon-driven reaction at the nanoscale.Aiming at addressing the challenge of monitoring the highly dynamic changing electrochemical interfaces under potential control and presence of the electrolyte, a transient electrochemical surface-enhanced Raman spectroscopy (TEC-SERS) was developed to monitor the structural evolution of surface species at a time resolution that equals the transient electrochemical methods (e.g. cyclic voltammetry and chronoamperometry), allowing the precise correlation of Raman signal with the molecular signature information and the electrochemical current signal. He observed an interesting two-step reaction processes that are impossible to observe by conventional steady state SERS methods. A wide-field Raman imaging system was developed to offer a high imaging speed to monitor the spatial and temporal change at every point over the whole electrode surface simultaneously. The heterogeneity of electrocatalytic reaction on the electrode surface was observed. This technique offers the possibility to spatially correlate electrochemical behavior and the particular surface sites where the reaction happens.

Synaptic Current Response of a Liquid Ga Electrode via a Surface Electrochemical Redox Reaction in a NaOH Solution.

An ionic device using a liquid Ga electrode in a 1 M NaOH solution is proposed to generate artificial neural spike signals. The oxidation and reduction at the liquid Ga surface were investigated for different bias voltages at 50 °C. When the positive sweep voltage from the starting voltage (VS) of 1 V was applied to the Ga electrode, the oxidation current flowed immediately and decreased exponentially with time. The spike and decay current behavior resembled the polarization and depolarization at the influx and extrusion of Ca2+ in biological synapses. Different average decay times of ∼81 and ∼310 ms were implemented for VS of −2 and −5 V, respectively, to mimic the synaptic responses to short- and long-term plasticity; these decay states can be exploited for application in binary electrochemical memory devices. The oxidation mechanism of liquid Ga was studied. The differences in Ga ion concentration due to VS led to differences in oxidation behavior. Our device is beneficial for the organ cell–machine interface system because liquid Ga is biocompatible and flexible; thus, it can be applied in biocompatible and flexible neuromorphic device development for neuroprosthetics, human cell–machine interface formation, and personal health care monitoring.

Oxygen vacancies-rich NiCo2O4-4x nanowires assembled on porous carbon derived from cigarette ash: A competitive candidate for hydrogen evolution reaction and supercapacitor

NiCo2O4 has aroused intense interest for hydrogen evolution reaction (HER) and supercapacitor (SCs). Nevertheless, the poor electric conductivity and insufficient electroactive sites hinder its commercial application. In this work, oxygen vacancies-rich NiCo2O4-4x nanowires assembled on porous carbon derived from discarded cigarette ash (named as Ov-NCO/PC-Y) are achieved via hydrothermal treatment combined with partial reduction process. The rich oxygen vacancies could offer remarkable conductivity, rapid ions transport rates, and rich electroactive sites. The introduction of phosphate ions is favorable for weakening the activation energy of their redox reactions, and therefore greatly promoting their surface reactivity and electrochemical reaction reversibility. Moreover, the interconnected 3D porous network structure of porous carbon (PC) could offer large surface area and short transport channel of ions/electrons. In virtue of intrinsic characteristic and structural advantage, Ov-NCO/PC-0.1 can be regarded as an effective HER catalyst with a low overpotential of 135.9 mV to reach 10 mA/cm2. Furthermore, Ov-NCO/PC-0.1 demonstrates superior specific capacitance of 1002.2 F/g, and Ov-NCO/PC-0.1//AC-based aqueous asymmetrical supercapacitor delivers the energy density of 32.7 Wh/kg at the power density of 160.2 W/kg. Moreover, the specific capacitances of Ov-NCO/PC-0.1//AC-based aqueous asymmetrical supercapacitor nearly keep unchanged after 3500 cycles. Overall, these findings shed fundamental insight on constructing high-performance bifunctional transition metal oxides for optimizing electrochemical energy conversion and storage.

Effect of Ligands and Solvents on the Stability of Electron Charged CdSe Colloidal Quantum Dots.

Many colloidal quantum dot (QD)-based devices involve charging of the QD, either via intentional electronic doping or via electrical charge injection or photoexcitation. Previous research has shown that this charging can give rise to undesirable electrochemical surface reactions, leading to the formation of localized in-gap states. However, little is known about the factors that influence the stability of charged QDs against surface oxidation or reduction. Here, we use density functional theory to investigate the effect of various ligands and solvents on the reduction of surface Cd in negatively charged CdSe QDs. We find that X-type ligands can lead to significant shifts in the energy of the band edges but that the in-gap state related to reduced surface Cd is shifted in the same direction. As a result, shifting the band edges to higher energies does not necessarily lead to less stable electron charging. However, subtle changes in the local electrostatic environment lead to a clear correlation between the position of the in-gap state in the bandgap and the energy gained upon surface reduction. Binding ligands directly to the Cd sites most prone to reduction was found to greatly enhance the stability of the electron charged QDs. We find that ligands bind much more weakly after reduction of the Cd site, leading to a loss in binding energy that makes charge localization no longer energetically favorable. Lastly, we show that increasing the polarity of the solvent also increases the stability of QDs charged with electrons. These results highlight the complexity of surface reduction reactions in QDs and provide valuable strategies for improving the stability of charged QDs.

N, H Dual‐Doped Black Anatase TiO2 Thin Films toward Significant Self‐Activation in Electrocatalytic Hydrogen Evolution Reaction in Alkaline Media

Titanium dioxide (TiO2) is an important application‐worthy oxide semiconductor. Herein, pulsed laser deposited N, H dual‐doped black anatase TiO2 (N–H:TiO2) films grown under NH3 are examined for the electrochemical water splitting application. This case is compared with only nitrogen or oxygen vacancy doped films. These dual‐doped films are highly conducting (98.77 μΩ cm @ 300 K) with holes as majority carriers. Electrostatic force microscopy also reveals significant differences in the work functions for the cases compared. Interestingly, the doped films not only exhibit impressive hydrogen evolution reaction (HER) activity with an initial overpotential of ≈0.6 V (vs reversible hydrogen electrode [RHE]), but the same reduces significantly down to ≈0.42 V at 10 mA cm−2 after 5 h chronoamperometry. Concurrently, there is a remarkable increase in current density (from ≈10 to 30 mA cm−2) at the overpotential of 0.6 V after 2000 cycles with excellent durability over 15 h. This peculiar self‐activation and performance enhancement of the catalysis is examined through X‐ray photoelectron spectroscopy (XPS) analysis which reveals the increased percentage of oxygen vacancies and incorporation of a new phase TiOxNy on the film surface via electrochemical surface reactions. Density functional theory (DFT) calculations confirm that the N–H:TiO2 thin films have appropriate hydrogen adsorption Gibbs free energies commensurate with observed high HER activity.

Rational Construction of Ruthenium‐Cobalt Oxides Heterostructure in ZIFs‐Derived Double‐Shelled Hollow Polyhedrons for Efficient Hydrogen Evolution Reaction

Transition metal oxides (TMOs) and their heterostructure hybrids have emerged as promising candidates for hydrogen evolution reaction (HER) electrocatalysts based on the recent technological breakthroughs and significant advances. Herein, Ru-Co oxides/Co3 O4 double-shelled hollow polyhedrons (RCO/Co3 O4 -350 DSHPs) with Ru-Co oxides as an outer shell and Co3 O4 as an inner shell by pyrolysis of core-shelled structured RuCo(OH)x @zeolitic-imidazolate-framework-67 derivate at 350 °C are constructed. The unique double-shelled hollow structure provides the large active surface area with rich exposure spaces for the penetration/diffusion of active species and the heterogeneous interface in Ru-Co oxides benefits the electron transfer, simultaneously accelerating the surface electrochemical reactions during HER process. The theory computation further indicates that the existence of heterointerface in RCO/Co3 O4 -350 DSHPs optimize the electronic configuration and further weaken the energy barrier in the HER process, promoting the catalytic activity. As a result, the obtained RCO/Co3 O4 -350 DSHPs exhibit outstanding HER performance with a low overpotential of 21mV at 10mA cm-2 , small Tafel slope of 67mV dec-1 , and robust stability in 1.0 m KOH. This strategy opens new avenues for designing TMOs with the special structure in electrochemical applications.

Bubbles: The Good, the Bad, and the Ugly

Bubbles are bad news for electrolysis due to undesired overpotentials, though their impact may be even further reaching than previously understood. In a recent report in Energy & Environmental Science, Kempler and researchers control the microstructure of electrodes to minimize overpotentials caused by bubbles in a photoelectrochemical cell.