Alkaline Hydrogen Evolution Reaction Kinetics Research Articles

VOx-doped CoP catalysts with synergistic dual-active configuration for boosting hydrogen evolution kinetics

Hydrogen evolution reaction (HER), is of great significance to harvest H2 via energy-saving alkaline water electrolysis yet still challenging. Integrating heterogeneous materials is effective in designing favorable electrocatalysts for water dissociation (WD) and H-immediate involved adsorption/desorption behavior and triggering positive interactions to break the kinetics limitation. Herein, a promising configuration of a vanadium-oxide species integrated with cobalt-phosphide plane that grown on the nickel foam (labeled as VOx-CoP/NF), is demonstrated to boost the alkaline HER kinetics comprehensively. The doped vanadium-oxide species preferentially perform the WD process to generate H-intermediates. Meanwhile, interaction between VOx and CoP induce the electronic redistribution that optimizes 3d-2p orbital hybridization state, which balances the adsorption strength of H* to facilitate the hydrogen transfer and enrichment. Eventually, VOx-CoP/NF exhibits supereminent alkaline HER performance with an overpotential of only 65/245 mV at 10/400 mA cm−2, superior to individual CoP and most representative cobalt-phosphide candidates. A flowing anion exchange membrane (AEM) urea-assisted water electrolyzer with VOx-CoP/NF as a bifunctional catalyst, is assembled and achieves a satisfactory performance, verifying its practical feasibility for efficient H2 production. The concept exemplified in this work is that synchronous optimization of elementary reaction step kinetics of HER through reasonable constructing well-defined configuration is instructive for the design of HER electrocatalysts.

Multi-interface engineering of nickel-based electrocatalysts for alkaline hydrogen evolution reaction

High gravimetric energy density and zero carbon emission of hydrogen have motivated hydrogen energy to be an attractive alternative to fossil fuels. Electrochemical water splitting in alkaline medium, driven by green electricity from renewable sources, has been mentioned as a potential solution for sustainable hydrogen production. Hydrogen evolution reaction (HER), as a cathodic half-reaction of water splitting, requires additional overpotential to obtain protons via water adsorption/dissociation, suffering from slow kinetics in alkaline solution. Robust and active nickel (Ni)-based electrocatalyst is a promising candidate for achieving precious-metal comparable performance owing to its platinum-like electronic structures with more efficient electrical power consumption. Various modification strategies have been explored on Ni-based catalysts, among which multi-interface engineering is one of the most effective routines to optimize both the intrinsic activity of Ni-based electrocatalysts and the extrinsic stacked component limitations. Herein, we systematically summarize the recent progress of multi-interface engineering of Ni-based electrocatalysts to improve their alkaline HER catalytic activity. The origin of sluggish alkaline HER kinetics is first discussed. Subsequently, three kinds of interfaces, geometrically and reactively, conductive substrate/electrocatalyst interface, electrocatalyst internal heterointerface, and electrocatalyst/electrolyte interface, were cataloged and discussed on their contribution mechanisms toward alkaline HER. Particular focuses lie on the microstructural and electronic modulation of key intermediates with energetically favorable adsorption/desorption behaviors via rationally designed interfaces. Finally, challenges and perspectives for multi-interface engineering are discussed. We hope that this review will be inspiring and beneficial for the exploration of efficient Ni-based electrocatalysts for alkaline water electrolysis.

An advanced Ru-based alkaline HER electrocatalyst benefiting from Volmer-step promoting 5d and 3d co-catalysts.

In this study, a trimetallic catalyst, NiWRu@NF, is electrodeposited onto nickel foam using chronoamperometry to enhance the hydrogen evolution reaction (HER) in alkaline water electrolysis. The catalyst combines nickel, tungsten, and ruthenium components, strategically designed for efficiency and cost-effectiveness, hydroxyl transfer and water dissociation, and acceleration of hydrogen combination, respectively. Evaluation of NiWRu@NF reveals exceptional performance, with a low overpotential of -50 mV and high current density of -10 mA cm-2, signifying its efficiency in promoting HER. Tafel values further corroborate the catalyst's effectiveness, indicating a rapid reaction rate of hydrogen evolution in such a highly alkaline medium compared to other controls studied along with it. This study underscores the significance of NiWRu@NF in advancing alkaline HER kinetics, paving the way for more efficient electrolysis processes.

Interfacial engineering of POM-stabilized Ni quantum dots on porous titanium mesh for high-rate and stable alkaline hydrogen production.

The development of low-cost, high-efficiency, and stable electrocatalysts for the alkaline hydrogen evolution reaction (HER) is a key challenge because the alkaline HER kinetics is slowed by an additional water dissociation step. Herein, we report an interfacial engineering strategy for polyoxometalate (POM)-stabilized nickel (Ni) quantum dots decorated on the surface of porous titanium mesh (POMs-Ni@PTM) for high-rate and stable alkaline hydrogen production. Benefiting from the strong interfacial interactions among POMs, Ni atoms, and PTM substrates, as well as unique POM-Ni quantum dot structures, the optimized POMs-Ni@PTM electrocatalyst exhibits a remarkable alkaline HER performance with an overpotential (η10) of 30.1 mV to reach a current density of 10 mA cm-2, which is much better than those of bare Ni decorated porous titanium mesh (Ni@PTM) (η10 = 171.1 mV) and POM decorated porous titanium mesh (POMs@PTM) electrocatalysts (η10 = 493.6 mV), comparable to that of the commercial 20 wt% platinum/carbon (20% Pt/C) electrocatalyst (η10 = 20 mV). Moreover, the optimized POMs-Ni@PTM electrocatalyst demonstrates excellent stability under continuous alkaline water-splitting at a current density of ∼100 mA cm-2 for 100 h, demonstrating great potential for its practical application.

Constructing Interfacial Oxygen Vacancy and Ruthenium Lewis Acid–Base Pairs to Boost the Alkaline Hydrogen Evolution Reaction Kinetics

AbstractSimultaneous optimization of the energy level of water dissociation, hydrogen and hydroxide desorption is the key to achieving fast kinetics for the alkaline hydrogen evolution reaction (HER). Herein, the well‐dispersed Ru clusters on the surface of amorphous/crystalline CeO2‐δ (Ru/ac‐CeO2‐δ) is demonstrated to be an excellent electrocatalyst for significantly boosting the alkaline HER kinetics owing to the presence of unique oxygen vacancy (VO) and Ru Lewis acid–base pairs (LABPs). The representative Ru/ac‐CeO2‐δ exhibits an outstanding mass activity of 7180 mA mgRu−1 that is approximately 9 times higher than that of commercial Pt/C at the potential of −0.1 V (V vs RHE) and an extremely low overpotential of 21.2 mV at a geometric current density of 10 mA cm−2. Experimental and theoretical studies reveal that the VO as Lewis acid sites facilitate the adsorption of H2O and cleavage of H‐OH bonds, meanwhile, the weak Lewis basic Ru clusters favor for the hydrogen desorption. Importantly, the desorption of OH from VO sites is accelerated via a water‐assisted proton exchange pathway, and thus boost the kinetics of alkaline HER. This study sheds new light on the design of high‐efficiency electrocatalysts with LABPs for the enhanced alkaline HER.

Constructing Interfacial Oxygen Vacancy and Ruthenium Lewis Acid-Base Pairs to Boost the Alkaline Hydrogen Evolution Reaction Kinetics.

Simultaneous optimization of the energy level of water dissociation, hydrogen and hydroxide desorption is the key to achieving fast kinetics for the alkaline hydrogen evolution reaction (HER). Herein, the well-dispersed Ru clusters on the surface of amorphous/crystalline CeO2-δ (Ru/ac-CeO2-δ ) is demonstrated to be an excellent electrocatalyst for significantly boosting the alkaline HER kinetics owing to the presence of unique oxygen vacancy (VO ) and Ru Lewis acid-base pairs (LABPs). The representative Ru/ac-CeO2-δ exhibits an outstanding mass activity of 7180 mA mgRu -1 that is approximately 9 times higher than that of commercial Pt/C at the potential of -0.1 V (V vs RHE) and an extremely low overpotential of 21.2 mV at a geometric current density of 10 mA cm-2 . Experimental and theoretical studies reveal that the VO as Lewis acid sites facilitate the adsorption of H2 O and cleavage of H-OH bonds, meanwhile, the weak Lewis basic Ru clusters favor for the hydrogen desorption. Importantly, the desorption of OH from VO sites is accelerated via a water-assisted proton exchange pathway, and thus boost the kinetics of alkaline HER. This study sheds new light on the design of high-efficiency electrocatalysts with LABPs for the enhanced alkaline HER.

Toward feasible single atom-based hydrogen evolution electrocatalysts via artificial ensemble sites for anion exchange membrane water electrolyzer

Approaching an efficient anion exchange membrane water electrolyzer (AEMWE) with satisfactorily high kinetics in the alkaline hydrogen evolution reaction (HER) is desired. We design an advanced platinum (Pt) single atom (SA)-based electrocatalyst by incorporating the Ni nanoparticle as an artificial ensemble site adjacent to Pt SA. The designed Pt SA electrocatalyst achieves higher areal current density (500 mA cm−2 at 1.8 V) in the single cell of the AEMWE and better cell voltage stability than the Pt/C electrocatalyst. The Ni nanoparticle assists in separating the binding sites of H* and OH*, in which Ni atoms provide adsorption sites for H*, while OH* adsorbs on the Pt SA. This separation effect drastically accelerates the energy barrier required for the water dissociation reaction in the Volmer step and simultaneously optimizes the H* and OH* binding energy, which extremely enhances the alkaline HER kinetics, thereby demonstrating the feasibility of Pt SA electrocatalysts for AEMWE.

Optimizing the reaction pathway of nitride electrode by co-doping strategy for boosting alkaline hydrogen evolution reaction kinetics

Optimizing the reaction pathway of nitride electrode by co-doping strategy for boosting alkaline hydrogen evolution reaction kinetics

Interfacial coupling and dopant-induced electronic modulation enabling Mn–CoP/MXene heterostructures to reduce alkaline HER energy barriers

Electronic structure modulation is a promising strategy to reduce energy barriers of hydrogen evolution reaction (HER). We have constructed 3D monodispersed Mn doped CoP nanoflowers supported on MXene sheets (Mn–CoP/MXene) to boost the alkaline HER kinetics, and the enhanced mechanisms were clarified by photoelectron spectroscopies tests and density functional calculations. The Mn doping and MXene coupling resulted in the modulated electronic structure and, thus, improved active sites for balancing the free energy of H2O dissociation and *H intermediates. The HER activity has been strengthened by the synergy of Mn doping and MXene coupling with an overpotential of only 95 mV to 10 mA cm−2, a ultra-small Tafel slope of 54.1 mV dec−1, and outstanding stability in 1.0 M KOH electrolyte. This work highlights the potential of the synthetic strategy of heteroatom doping and 2D materials coupling to be extended to prepare advanced and cost-effective TMPs-based electrocatalysts.

Boosting alkaline hydrogen evolution reaction kinetics by a local electric field created by polarization of CeO2(100)

In this study, a simple and versatile strategy is demonstrated to create acid-like local environments (ALLE) to boost alkaline hydrogen evolution reaction (HER) kinetics of Pt/C, Pt/CoS2, and other electrocatalysts by introducing a local electric field (LEF) stemming from the polar-faceted cubic CeO2 (C–CeO2). The surface polarity of the C–CeO2 facets is confirmed by trimethylphosphine oxide-31P nuclear magnetic resonance. Operando Raman scattering provides evidence that H3O+ intermediates produce the ALLE. This ALLE shows great enhancement in alkaline HER with activities of the C–CeO2 modified Pt/C and Pt/CoS2 promoted by 2.5 and 5.5 times compared to the counterparts, respectively. The LEF changes the mechanism from slow Volmer–Heyrovsky to fast Volmer–Tafel process. As a result, Pt/CoS2 + C–CeO2 shows a small Tafel slop of 23.8 mV dec−1 and low overpotentials of 20 and 59 mV at current densities of 10 and 100 mA cm−2, respectively. Importantly, the LEF produces similar effects in other electrocatalysts.

Harmonious Heterointerfaces Formed on 2D‐Pt Nanodendrites by Facet‐Respective Stepwise Metal Deposition for Enhanced Hydrogen Evolution Reaction

AbstractThe performance of nanocrystal (NC) catalysts could be maximized by introducing rationally designed heterointerfaces formed by the facet‐ and spatio‐specific modification with other materials of desired size and thickness. However, such heterointerfaces are limited in scope and synthetically challenging. Herein, we applied a wet chemistry method to tunably deposit Pd and Ni on the available surfaces of porous 2D−Pt nanodendrites (NDs). Using 2D silica nanoreactors to house the 2D‐PtND, an 0.5‐nm‐thick epitaxial Pd or Ni layer (e‐Pd or e‐Ni) was exclusively formed on the flat {110} surface of 2D−Pt, while a non‐epitaxial Pd or Ni layer (n‐Pd or n‐Ni) was typically deposited at the {111/100} edge in absence of nanoreactor. Notably, these differently located Pd/Pt and Ni/Pt heterointerfaces experienced distinct electronic effect to influence unequally in electrocatalytic synergy for hydrogen evolution reaction (HER). For instance, an enhanced H2 generation on the Pt{110} facet with 2D‐2D interfaced e‐Pd deposition and faster water dissociation on the edge‐located n‐Ni overpowered their facet‐located counterparts in respective HER catalysis. Therefore, a feasible assembling of the valuable heterointerfaces in the optimal 2D n‐Ni/e‐Pd/Pt catalyst overcame the sluggish alkaline HER kinetics, with a catalytic activity 7.9 times higher than that of commercial Pt/C.

Harmonious Heterointerfaces Formed on 2D-Pt Nanodendrites by Facet-Respective Stepwise Metal Deposition for Enhanced Hydrogen Evolution Reaction.

The performance of nanocrystal (NC) catalysts could be maximized by introducing rationally designed heterointerfaces formed by the facet- and spatio-specific modification with other materials of desired size and thickness. However, such heterointerfaces are limited in scope and synthetically challenging. Herein, we applied a wet chemistry method to tunably deposit Pd and Ni on the available surfaces of porous 2D-Pt nanodendrites (NDs). Using 2D silica nanoreactors to house the 2D-PtND, an 0.5-nm-thick epitaxial Pd or Ni layer (e-Pd ore-Ni) was exclusively formed on the flat {110} surface of 2D-Pt, while a non-epitaxial Pd or Ni layer (n-Pd or n-Ni) was typically deposited at the {111/100} edge in absence of nanoreactor. Notably, these differently located Pd/Pt and Ni/Pt heterointerfaces experienced distinct electronic effect to influence unevenly in electrocatalytic synergy for hydrogen evolution reaction (HER). For instance, an enhanced H2 generation on the Pt{110} facet with 2D-2D interfacede-Pddeposition and faster water dissociation on the edge-locatedn-Ni overpowered their facet-located counterparts in respective HER catalysis. Therefore, a feasible assembling of the valuable heterointerfaces in the optimal 2D n-Ni/e-Pd/Pt catalyst overcame the sluggish alkaline HER kinetics, with a catalytic activity 7.9 times higher than that of commercial Pt/C.

High Alkaline Electrochemical Hydrogen Evolution on a Pt/GaN Heterostructure

AbstractThe construction of suitable metal/substrate interfaces that can synergistically optimize the water dissociation, hydrogen transfer and hydrogen adsorption processes has proved to be a promising approach to boost the alkaline electrochemical hydrogen evolution reaction (HER) but remains challenging. Herein, a Pt/GaN heterostructure electrocatalyst is constructed using the electrochemical deposition technique and used for alkaline electrochemical HER. The as‐synthesized Pt/GaN catalyst exhibits competitive HER performance, including low overpotentials of 24 and 441 mV to reach current densities of 10 and 1000 mA cm−2, respectively, and a superior mass activity (7.34 A mgPt−1 at the overpotential of 100 mV), which is 25.7 and 5.0 times higher than commercial Pt/C and as‐synthesized Pt/CP, respectively. First‐principle calculations suggest that the Pt/GaN heterostructure can greatly reduce the energy barrier for H2O dissociation, create a local acid‐like microenvironment through a H spillover process and optimize the value of ΔGH, leading to a remarkable acceleration of the alkaline HER kinetics. This discovery provides new insights for the development of alkaline HER catalysts.

Modulating Water Splitting Kinetics via Charge Transfer and Interfacial Hydrogen Spillover Effect for Robust Hydrogen Evolution Catalysis in Alkaline Media.

Designing and synthesizing advanced electrocatalysts with superior intrinsic activity toward hydrogen evolution reaction (HER) in alkaline media is critical for the hydrogen economy. Herein, a novel Ir@Rhene heterojunction electrocatalyst is synthesized via epitaxially confining ultrasmall and low-coordinate Ir nanoclusters on the ultrathin Rh metallene accompanying the formation of Ir/IrO2 Janus nanoparticles. The as-prepared heterojunctions display outstanding alkaline HER activity, with an overpotential of only 17mV at 10mA cm-2 and an ultralow Tafel slope of 14.7mV dec-1 . Both structural characterizations and theoretical calculations demonstrate that the Ir@Rhene heterointerfaces induce charge density redistribution, resulting in the increment of the electron density around the O atoms in the IrO2 site and thus delivering much lower water dissociation energy. In addition, the dual-site synergetic effects between IrO2 and Ir/Rh interface trigger and improve the interfacial hydrogen spillover, thereby subtly avoiding the steric blocking of the active site and eventually accelerating the alkaline HER kinetics.

O-vacancy-rich and heterostructured Cu/Cu2O/NiO@NiCu foam self-supported advanced electrocatalyst towards hydrogen evolution: An experimental and DFT study

Defect-engineering and heterojunction-creating on a catalyst have been endorsed as effective approaches to enhance the hydrogen evolution reaction (HER) behavior. By skillfully employing the transformation of Cu2S/Ni3S2 to NiO/Cu2O and the electroreduction of partial Cu2O to Cu/Cu2O for the O-vacancy (Ov) production and in-situ reactions for the heterostructure construction, we fabricated a self-supported heterostructured hybrid with Ov-rich Cu/Cu2O coupled with few NiO grown in-situ on the NiCu bimetallic foam (named as Cu/NiO/Cu2O@NCF). Cu/NiO/Cu2O@NCF records a low overpotential of 79 mV at 10 mA cm−2 with a small Tafel slope of 60 mV dec−1 and sustained the activity without distinct decay for 100 h towards catalyzing HER in 1.0 M KOH. More profoundly, density functional theory computations reveal that Ov-Cu2O holds a much lower energy barrier for the rate-determining step of the H2O dissociation and a more electron density with respect to Ov-NiO, thereby accelerating the alkaline HER kinetics.

Boron-doping induced electron redistribution of carbon-coated nickel nanoparticles towards efficient alkaline hydrogen evolution

Metal nickel (Ni) has been developed as hydrogen evolution reaction (HER) catalyst for water-alkali electrolyzers for almost a century. However, the catalytic efficiency and durability of metallic Ni is still far from satisfactory due to its inappropriate electronic structure and non-corrosion-resistant property. Herein, we report a facile strategy to fabricate the carbon-coated boron-doping nickel nanoparticles (Ni-B@C) via a convenient boronization and a subsequent carbonization treatment of the Ni-based metal organic framework (Ni2(OH)2(C8H4O4)). Such process also ensured the catalyst with high electrochemically active surface area. Particularly, the B-doping would induce the electron redistribution of Ni, resulting in optimized H2O dissociation and H* association, thereby significantly improving the alkaline HER kinetics. Benefiting from the contribution of these favourable properties, the as-prepared Ni-B@C catalyst exhibits remarkable alkaline HER activity with an overpotential of only 26 mV to reach a current density of 10 mA cm−2 and, more impressively, with superior durability in the constant-current test of 50 h at the different current densities of 10, 50 and 100 mA cm–2. The combination of these excellent characteristics clearly makes Ni-B@C catalyst stand out from existing competitors towards alkaline HER.

Tiny Ni3S2 boosting MoS2 hydrogen evolution in alkali by enlarging coupling boundaries and stimulating basal plane

The relatively slow reaction kinetics of the hydrogen evolution reaction (HER) by water electrolysis in alkali hinder its large-scale industrial production. To improve the HER activity in alkaline media, a novel Ni3S2/MoS2/CC catalytic electrode was synthesized by a simple two-step hydrothermal method in this work. The modification of MoS2 by Ni3S2 could facilitate the adsorption and dissociation of water, thus accelerating the alkaline HER kinetics. Moreover, the unique morphology of small Ni3S2 nanoparticles grown on MoS2 nanosheets not only increased the interface coupling boundaries, which acted as the most efficient active sites for the Volmer step in alkaline medium, but also sufficiently activated the MoS2 basal plane, thus providing more active sites. Consequently, Ni3S2/MoS2/CC only needed overpotentials of 189.4 and 240 mV to drive current densities of 100 and 300 mA·cm−2, respectively. More importantly, its catalytic performance of Ni3S2/MoS2/CC even exceeded that of Pt/C at a high current density after 261.7 mA·cm−2 in 1.0 M KOH.

Novel ternary metals-based telluride electrocatalyst with synergistic effects of high valence non-3d metal and oxophilic Te for pH-universal hydrogen evolution reaction

Electrocatalyst designs based on oxophilic foreign atoms are considered a promising approach for developing efficient pH-universal hydrogen evolution reaction (HER) electrocatalysts by overcoming the sluggish alkaline HER kinetics. Here, we design ternary transition metals-based nickel telluride (MoWNiTe) catalysts consisting of high valence non-3d Mo and W metals and oxophilic Te as a first demonstration of non-precious heterogeneous electrocatalysts following the bifunctional mechanism. The MoWNiTe showed excellent HER catalytic performance with overpotentials of 72, 125, and 182 mV to reach the current densities of 10, 100, and 1000 mA cm−2, respectively, and the corresponding Tafel slope of 47, 52, and 58 mV dec−1 in alkaline media, which is much superior to commercial Pt/C. Additionally, the HER performance of MoWNiTe is well maintained up to 3000 h at the current density of 100 mA cm−2. It is further demonstrated that the MoWNiTe exhibits remarkable HER activities with an overpotential of 45 mV (31 mV) and Tafel slope of 60 mV dec−1 (34 mV dec−1) at 10 mA cm−2 in neutral (acid) media. The superior HER performance of MoWNiTe is attributed to the electronic structure modulation, inducing highly active low valence states by the incorporation of high valence non-3d transition metals. It is also attributed to the oxophilic effect of Te, accelerating water dissociation kinetics through a bifunctional catalytic mechanism in alkaline media. Density functional theory calculations further reveal that such synergistic effects lead to reduced free energy for an efficient water dissociation process, resulting in remarkable HER catalytic performances within universal pH environments.

Advanced Pt-based electrocatalysts for the hydrogen evolution reaction in alkaline medium.

Water electro-splitting driven by renewable energy is significant in energy conversion for the development of hydrogen energy sources. The hydrogen evolution reaction (HER) directly generating hydrogen products occurs in cathode catalysis. Over the years, significant progress has been made to boost the HER efficiency by exploratively designing highly active and economical Pt-based electrocatalysts. However, there are still some urgent problems to be solved for Pt-based HER catalysts in more economical alkaline electrolytes, such as the slow kinetics caused by additional hydrolysis dissociation steps, which greatly hinders the practical application. This review systematically summarizes several strategies for optimizing alkaline HER kinetics and provides direct guidelines for the design of highly active Pt-based electrocatalysts. Specifically, the intrinsic HER activity in alkaline water electrolysis can be boosted by accelerating the water dissociation, optimizing the hydrogen binding energy or modulating the spatial dimensions of the electrocatalyst based on the HER mechanism. Finally, we prospect the challenges for the alkaline HER on novel Pt-based electrocatalysts, including the active site study, the HER mechanism exploration and the extensible catalyst preparation technologies.

1D@2D Hierarchical Structures of Co(OH)x Nanosheets on NiMoOx Nanorods Can Mediate Alkaline Hydrogen Evolution with Industry-Level Current Density and Stability.

Developing efficient electrocatalysts at ampere-scale current densities is of paramount importance to advance industrial applications of alkaline water electrolysis. Herein, a hierarchical nanostructured electrocatalyst with two-dimensional Co(OH)x nanosheets grown on one-dimensional NiMoOx nanorods over three-dimensional porous Ni foam substrate is designed. The resulting catalyst delivers ultrahigh hydrogen evolution reaction (HER) activity in the alkaline solution, which only requires overpotentials of 185 and 332mV to achieve the current densities of -500 and -1000mAcm-2 in 1.0m KOH, respectively, and shows robust stability at -1000mAcm-2 for 11 days. The unique 1D @ 2D hierarchical structures with abundant hetero-interfaces can not only expose sufficient active sites but also boost alkaline HER kinetics with fast water dissociation ability. This present work may pave a new insight to design efficient electrocatalysts with hierarchical structures for alkaline HER with industry-level current density and stability.