Oxygen Electrode Research Articles

In-situ reconstruction of rock-like 3D hierarchical MIL-53(Fe) self-supporting electrode with oxygen vacancy induced ultra-long stable and efficient water oxidation

The exploitation of efficient, stable and cheap electrocatalyst for oxygen evolution reaction (OER) is very significant to the development of energy technology. In this study, Fe-based metal–organic frameworks (MIL-53(Fe)) self-supporting electrode with a 3D hierarchical open structure was developed through a semi-sacrificial strategy. The self-supporting electrode exhibits an excellent OER performance with an overpotential of 328 mV at 100 mA cm−2 in 1 M KOH, which is superior than that of IrO2 catalyst. Importantly, the optimized self-supporting electrode could operate at 100 mA cm−2 for 520 h without visible decrease in activity. It was also found that the structure of MIL-53(Fe) was in-situ self-reconstructed into oxyhydroxides during OER process. However, the 3D hierarchical open structure assembled with nano-microstructures kept well, which ensured the long-term stability of our self-supporting electrode for OER. Furthermore, density functional theory (DFT) calculations reveal that the FeOOH with rich oxygen vacancy transformed from MIL-53(Fe) plays a key role for the OER catalytic activity. And, the uninterrupted formation of oxygen vacancy during OER process ensures the continuous OER catalytic activity, which is the original source for the ultra-long stability of the self-supporting electrode toward OER. This work explores the way for the construction of efficient self-supporting oxygen electrodes based on MOFs.

Electrochemical Biosensors for Express Analysis of the Integral Toxicity of Polymer Materials.

Biosensors based on an oxygen electrode, a mediator electrode, and a mediator microbial biofuel cell (MFC) using the bacteria Gluconobacter oxydans B-1280 were formed and tested to determine the integral toxicity. G. oxydans bacteria exhibited high sensitivity to the toxic effects of phenol, 2,4-dinitrophenol, salicylic and trichloroacetic acid, and a number of heavy metal ions. The system "G. oxydans bacteria-ferrocene-graphite-paste electrode" was superior in sensitivity to biosensors formed using an oxygen electrode and MFC, in particular regarding heavy metal ions (EC50 of Cr3+, Mn2+, and Cd2+ was 0.8 mg/dm3, 0.3 mg/dm3 and 1.6 mg/dm3, respectively). It was determined that the period of stable functioning of electrochemical systems during measurements was reduced by half (from 30 to 15 days) due to changes in the enzyme system of microbial cells when exposed to toxicants. Samples of the products made from polymeric materials were analyzed using developed biosensor systems and standard biotesting methods based on inhibiting the growth of duckweed Lemna minor, reducing the motility of bull sperm, and quenching the luminescence of the commercial test system "Ecolum". The developed bioelectrocatalytic systems were comparable in sensitivity to commercial biosensors, which made it possible to correlate the results and identify, by all methods, a highly toxic sample containing diphenylmethane-4,4'-diisocyanate according to GC-MS data.

Local electrochemical and thermal investigation of a segmented reversible solid oxide fuel cell and electrolyzer

A novel setup has been designed and mounted to investigate the local electrochemical and thermal behavior of a cell from a 4-cell short stack under reversible operation solid oxide fuel cell/solid oxide electrolyzer (SOFC/SOE). It consists in the partition of the cell's oxygen electrode into 20 segments, where each segment is controlled by a specific electronic load. This allows to control the segments independently in galvanostatic or potentiostatic mode. The configuration also enables local electrochemical impedance spectroscopy (EIS) measurements of each segment. 20 thermocouples were added to measure the local temperature of the segments in order to gain a deeper insight into the cell thermal distribution. This helps correlating the local thermal and electrochemical responses to the operating parameters. This specific setup allowed mapping the cell's local behavior based on a multidimensional operating conditions matrix, including current density, temperature, feed gas composition, sweep gas flow and polarization. Moreover, a durability test of more than 1,000 h helped investigating the local long-term degradation in a reversible (day SOE, night SOFC) configuration. These results are of significant importance towards optimal operation of a reversible SOC with enhanced durability.

WITHDRAWN: Detection and impact of hypoxic regions in multicellular tumor spheroid cultures formed by head and neck squamous cell carcinoma cells lines

In solid tumors like head and neck cancer (HNC), chronic and acute hypoxia have serious adverse clinical consequences including poorer overall patient prognosis, enhanced metastasis, increased genomic instability, and resistance to radiation-, chemo-, or immuno-therapies. However, cells in the two-dimensional monolayer cultures typically used for cancer drug discovery experience 20%-21% O2 levels (normoxic) which are 4-fold higher than O2 levels in normal tissues and ≥10-fold higher than in the hypoxic regions of solid tumors. The oxygen electrodes, exogenous bio-reductive markers, and increased expression of endogenous hypoxia-regulated proteins like HIF-1α generally used to mark hypoxic regions in solid tumors are impractical in large sample numbers and longitudinal studies. We used a novel homogeneous live-cell permeant HypoxiTRAK™ (HPTK) molecular probe compatible with high content imaging detection, analysis, and throughput to identify and quantify hypoxia levels in live HNC multicellular tumor spheroid (MCTS) cultures over time. Accumulation of fluorescence HPTK metabolite in live normoxic HNC MCTS cultures correlated with hypoxia detection by both pimonidazole and HIF-1α staining. In HNC MCTSs, hypoxic cytotoxicity ratios for the hypoxia activated prodrugs (HAP) evofosfamide and tirapazamine were much smaller than have been reported for uniformly hypoxic 2D monolayers in gas chambers, and many viable cells remained after HAP exposure. Cells in solid tumors and MCTSs experience three distinct O2 microenvironments dictated by their distances from blood vessels or MCTS surfaces, respectively; oxic, hypoxic, or intermediate levels of hypoxia. These studies support the application of more physiologically relevant in vitro 3D models that recapitulate the heterogeneous microenvironments of solid tumors for preclinical cancer drug discovery.

Correlating Oxygen Electrode Degradation to Cr Vaporization from Metallic Interconnects in Solid Oxide Cell Stacks

Symmetrical cells consisting of La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) oxygen electrodes screen printed on both sides of a Ce0.9Gd0.1O2-δ (CGO) electrolyte were tested at 800 °C while being held close to a piece of Crofer 22 APU alloy. The alloy was either just pre-oxidized or coated with MnCo2O4 and heat treated prior to the exposure test to elucidate the effects of different Cr vaporization rates. Degradation of the symmetrical cells was monitored by electrochemical impedance spectroscopy, and TOF-SIMS, SEM and EDX analysis were used to examine Cr deposition on the electrodes after the exposure. The results show that the degradation rate of the symmetrical cell is directly proportional to the concentration of gaseous Cr(VI)-species, which had been assessed in a previous experiment. The Cr vaporization rate from Crofer 22 APU with a dense MnCo2O4 coating was measured in moisturized air up with up to 40% H2O and found to be invariant with respect to the steam activity. The degradation rate of symmetrical cells was accelerated by humidity in the air, but, noteworthy, this was found also in the absence of a Cr source.

Highly dispersed TaC nanowire cathodes via template-assisted electrolysis for enhanced lithium–oxygen batteries

High efficiency oxygen electrode is essential to achieve long cycle life of lithium–oxygen battery. In this study, we present a novel approach involving the fabrication of porous, interwoven tantalum carbide (TaC) nanowires via molten salt electrolysis, with multi-walled carbon nanotubes (MWCNTs) serving both as a carbon source and as a structural template. In contrast, using carbon nanoparticles as carbon source results in severe agglomeration of TaC particles. These TaC nanowires offer sufficient storage space for discharge products, establish efficient ions and O2 diffusion pathways, and expose numerous catalytic active sites to effectively mitigating the formation of carbonate byproducts. The lithium–oxygen battery with TaC nanowires oxygen electrode demonstrates a high specific discharge capacity of 11,856.69 mAh g−1, a stable cycle life of 120 cycles at 500 mA g−1 with the limited capacity of 1000 mAh g−1, and a discharge/charge overvoltage gap of 1.13 V. This study introduces a promising approach for the development of new catalysts based on transition metal carbides for lithium–oxygen batteries.

Mechanistic Insights into Electrodeposition in Seawater at Variable Electrochemical Potentials

AbstractThe classical approach to manufacturing cement and concrete involves considerable and energy‐intensive exploitation of minerals from the environment, calling for alternative and sustainable methods to obtain such resources. Electrodeposition resulting from local pH changes near an oxygen or water reduction electrode can be used to grow valuable minerals in seawater for use in the cement and concrete industries, with the potential to use clean electricity without relying on the mining of resources. Limited knowledge is available about the electrochemical reaction networks that yield mineral precipitates via electrodeposition in seawater, hindering a complete assessment of the promise of such an approach to serving the construction industry. This work presents an investigation of the electrodeposition in seawater as a function of the applied potential. The work identifies multiple electrochemical potential regimes that yield distinct polymorphs, quantities, and production rates of calcium and magnesium‐based minerals due to the varying rates of oxygen reduction and water‐splitting reactions. The quality of the produced minerals is comparable to that of traditionally mined aggregates in terms of morphology and composition, supporting that seawater represents a vast source of raw materials for use in the construction industry upon an enhancement in the efficiency of hydroxide‐producing reactions and the reactor design.

A kinetic study on oxygen redox reaction of a double-perovskite reversible oxygen electrode— Part II: Modelling analysis

Mixed ionic and electronic conductor double perovskites are very promising oxygen electrode materials for solid oxide cell technology. However, understanding their specific kinetic mechanism is a fundamental preliminary step towards detecting the best reachable performance, optimising the operation conditions and the electrode architecture. Indeed, the contributions of different rate-determining steps can vary as a function of the working point. In this framework, after a detailed experimental campaign devoted to the study of SmBa0.8Ca0.2Co2O5+δ (SBCCO) oxygen electrode behaviour, the authors propose a theoretical analysis of oxygen reduction and oxygen evolution reaction paths that couples a preliminary study through equivalent circuit analysis with a physics-based model to predict the operation of SBCCO as a reversible oxygen electrode. Following a semi-empirical approach, the kinetics formulation was derived from thermodynamics and electrochemistry fundamental principles and was tuned on electrochemical impedance spectroscopy (EIS) spectra in order to retrieve the unknown kinetic parameters. The successful cross-checking of the simulated results with the experimental data obtained by direct current measurements validated the proposed model, here applicable in further works on full cells to simulate the SBCCO oxygen reversible electrode performance.

A kinetic study on oxygen redox reaction of a double-perovskite reversible oxygen electrode—Part I: Experimental analysis

The carbon-free energy transition requires the spread of advanced technologies based on high-performing materials. In this framework and particularly referring to electrochemical energy converting systems, double perovskites are arousing more and more interest as mixed ionic electronic conductors with flexible manufacturing, appropriate tailoring for many tasks and high chemical stability. Among their possible applications, they form excellent oxygen electrodes in solid oxide cell technology used as fuel cells, steam/CO2 electrolysis cells and electrochemical air separation units. In view of the encouraging results shown by SmBa1−x Ca x Co2O5+δ co-doped double perovskite, this research work aims at a detailed analysis of SmBa0.8Ca0.2Co2O5+δ performance and the identification of kinetic paths for oxygen reduction and oxidation reactions. The electrochemical characterization was performed over a wide range of operation conditions to evaluate the electrode reversible behaviour and the interplay of the recognized phenomena governing the overall electrode kinetics.

Evaluating the Oxygen Electrode Reactions of La Single-Atom Catalysts with the N/C Coordination Effect.

There is a growing demand for bifunctional electrocatalysts for oxygen electrodes in rechargeable metal-air batteries. This article investigates the bifunctional activity of La single-atom catalysts with N/C coordination (LaNxC6-x@Gra) using density functional theory (DFT). The augmentation of N coordination will result in enhanced synthetic stability. The coordination between nitrogen and carbon (N/C) has a significant influence on the working stability of the system under consideration. In the context of active atoms, the coordination between nitrogen and carbon (N/C coordination) has a significant impact on the electronic structure. This, in turn, influences the adsorption performance and catalytic activity of the catalysts. In the case of stable coordination environments, a correlation exists between the f-orbital center (εf) and the overpotential (η) via the adsorption free energy of intermediates (ΔG*ads). This correlation serves as a useful tool for predicting catalytic performance. The LaNxC6-x@Gra exhibits remarkable bifunctional activity due to its complementary performance, with an overpotential for the oxygen reduction reaction (ηORR) of 0.66 V and an overpotential for the oxygen evolution reaction (ηOER) of 0.43 V. This makes it a promising candidate for use as a bifunctional electrocatalyst in oxygen electrodes.

On the benefits of structured composite electrodes in solid oxide cells

Functional composites of electronic and ionic conductors are the backbone of oxygen and fuel electrodes in solid oxide cells. A typical oxygen electrode is fabricated from a lanthanum strontium manganite and yttria-stabilized zirconia (LSM-YSZ) composite, while the fuel electrode is based on a Ni-YSZ cermet. While the composite materials exhibit enhanced kinetic capabilities, the electron-conductive phases, i.e., LSM and Ni, exhibit high electrical conductivity. The goal of this study was to develop an electrode layout with enhanced performance by tailoring the benefits of both, the composites and the single-phase electron conductors, within individual electrodes. The electrodes were fabricated in a two-layer layout with different thicknesses by screen-printing: composite material layer, electron-conductive material. The results suggested that the presence of YSZ is essential in the entire electrode body of a fuel electrode for enhanced performance as it increases the triple phase boundary length, as well as mechanical stability by forming a scaffold for the Ni network. Hence, for the optimal performance a thick layer of Ni-YSZ composite is required. On the contrary, the optimal oxygen electrode requires only very thin layer of the composite, otherwise the presence of YSZ is retarding the electrode’s performance.Graphical abstract

Cobalt-Free BaFe0.6Zr0.1Y0.3O3−δ Oxygen Electrode for Reversible Protonic Ceramic Electrochemical Cells

Reversible protonic ceramic electrochemical cells (R-PCECs) are ideal, high-efficiency devices that are environmentally friendly and have a modular design. This paper studies BaFe0.6Zr0.1Y0.3O3−δ (BFZY3) as a cobalt-free perovskite oxygen electrode for high-performance R-PCECs where Y ions doping can increase the concentration of oxygen vacancies with a remarkable increase in catalytic performance. The cell with configuration of Ni-BZCYYb/BZCYYb/BFZY3 demonstrated promising performance in dual modes of fuel cells (FCs) and electrolysis cells (ECs) at 650 °C with low polarization resistance of 0.13 Ω cm2, peak power density of 546.59 mW/cm2 in FC mode, and current density of − 1.03 A/cm2 at 1.3 V in EC mode. The alternative operation between FC and EC modes for up to eight cycles with a total of 80 h suggests that the cell with BFZY3 is exceptionally stable and reversible over the long term. The results indicated that BFZY3 has considerable potential as an air electrode material for R-PCECs, permitting efficient oxygen reduction and water splitting.

Spinel oxide Ni1-xZnxCo2O4 (x = 0.2–0.8) synergized with Co/N-CNT as electrocatalyst for oxygen electrode

The spinel oxide Ni1-xZnxCo2O4 (x = 0.2–0.8) was synthesized. When x = 0.4, the catalytic activity for oxygen reactions was found to be the most optimal. To enhance its performance, a composites consisting of Co/N-CNT and Ni0.6Zn0.4Co2O4 was prepared. The oxygen electrode catalyzed by 10 % Co/N-CNT exhibited the highest catalytic activity. The current densities of 382.8 mA·cm−2 and 343.7 mA·cm−2 were achieved at the cut-off voltages of 1 V and −0.6 V, respectively. The results of electrochemical impedance test indicated that the charge transfer rate of the oxygen electrode catalyzed by 10 % Co/N-CNT was the fastest, providing further confirmation of its superior catalytic performance.

Synergized N and P co-doped Ti3C2Tx mxene enabling high-performance Li-air batteries

Lithium-oxygen batteries (LOBs) with a theoretical energy density of up to 3500 Wh kg−1 hold a promise for the next-generation high-energy-density batteries. However, the slow oxygen reduction/evolution kinetics at the cathode limits the performance of Li-air batteries. The rational design of efficient catalysts is essential for the improvement of oxygen electrode reaction kinetics. Herein, we report a facile strategy to co-dope N and P atoms simultaneously into Ti3C2Tx (NP-Ti3C2Tx) MXene via an electrostatic self-assembly approach. The co-doped NP-Ti3C2Tx layers expose abundant active sites, providing more space for accommodating the formed Li2O2. Moreover, the N and P co-doping facilitates efficient electron transport in Ti3C2Tx MXene. The LOB with NP-Ti3C2TX catalyst delivers a high discharge capacity of 24,940 mAh/g at 1000 mA g−1. At a cut-off capacity of 1000 mAh/g, this battery runs continuously for 159, 276, 185, and 229 cycles at current densities of 1000, 2000, 3000, and 5000 mA g−1, respectively. Theoretical calculations unveil that N and P co-doping enables lower ηORR and ηOER of only 0.26 V and 0.13 V on Ti3C2Tx MXene, respectively. This work offers a feasible approach for constructing efficient MXene electrocatalysts for Li–air batteries.

Electrolyte-electrode interface: A key factor for advanced protonic ceramic electrochemical cells

Protonic ceramic fuel/electrolysis cells (PCFCs/PCECs) show great potential for the efficient and reversible conversion of chemical and electrical energy. The electrolyte and oxygen electrode interface in PCFCs/PCECs is particularly crucial for achieving optimal performance. In this work, Ba evaporation and Y enrichment phenomena are investigated on the surface of a BaCe0.7Zr0.1Y0.2O3-δ electrolyte during high-temperature sintering. Furthermore, an interfacial engineering strategy is proposed by removing the non-stoichiometric perovskite surface layer via a facile polishing process to enable a comparative analysis of the detrimental effects of the undesired layer on the cell performance. Polishing the electrolyte leads to a remarkable ∼50 % enhancement in PCFC performance and ∼120 % enhancement in PCEC performance at 650 °C. Electrochemical impedance spectroscopy coupled with distribution of relaxation time analysis reveals that the improved performance of the polished cell can be attributed to reductions in both the ohmic and polarization losses, with the latter primarily influenced by the facilitated oxygen electrode reactions. This interfacial engineering approach restores the high proton conductivity at the electrolyte surface and strengthens the physical and chemical adhesion between the electrolyte and the oxygen electrode. Our findings shed light on the existing challenges at the electrolyte-electrode interface and underscore the significance of interfacial engineering in enhancing PCFC/PCEC performance.

Regulating d-orbital electronic configuration of Ni-based chalcogenides to enhance the oxygen electrode reactions in Li-O2 batteries

Transition metal chalcogenides based electrocatalysts with regulable d-orbital electronic configuration by various chalcogen anions can exhibit excellent oxygen electrode reaction kinetics in Li-O2 batteries (LOBs). In this work, an electron rearrangement strategy to improve the catalytic activity of hexagonal NiS microsheets for LOBs has been developed via introducing Se anion as a substitution of S. With the as-prepared NiS0.67Se0.33 as cathode catalyst, LOB could achieve evidently lower overpotentials, larger discharge capacity, and ultralong cycling life than that using NiS. With the help of physicochemical techniques and theoretical calculations, the performance enhancement of NiS0.67Se0.33 could be elucidated with its suitable d-orbital electronic configuration. Specifically, Se anion substitution can modulate metal–chalcogen interactions with increased charge density and positively shifted d-band center of Ni metal cation, thereby promoting the d-p orbitals hybridization with increased binding strength to LiO2 intermediates and promoting oxygen reduction/evolution reaction kinetics. Moreover, small amount of Ni dissolution from NiS0.67Se0.33 to the electrolyte because of the rearranged electronic configuration also contributes to the enhanced oxygen electrochemistry and improved LOB performance by serving as a redox mediate catalyst. This work provides a facile anion doping engineering to facilitate the oxygen electrode reactions in LOBs by modulating d-orbital electronic configuration.

A novel family of W-doped Bi0.5Sr0.5FeO3-δ perovskite as oxygen electrode catalyst for CO2 electrolysis in solid oxide electrolysis cells

Co-free Bi0.5Sr0.5FeO3-δ perovskite as a novel oxygen electrode shows good electrochemical performance for solid oxide electrolysis cells (SOECs). In this work, W-doped Bi0.5Sr0.5Fe1-xWxO3-δ (BSFWx, x = 0, 0.05, 0.1, and 0.15) perovskite oxides were prepared and evaluated for CO2 electrolysis in SOECs to further promote its oxygen catalytic activity. All the samples maintain a single-phase structure, and the BSFW0.1 sample presents the lowest polarization resistance value of about 0.067 Ω cm2, which is 47 % smaller than that of the BSF sample. Moreover, the BSFW0.1 half-cell exhibits good stability for 84 h under 0.5 A cm−2 anodizing polarization at 800 °C. The fuel electrode-supported BSFW0.1 single cell reaches a current density of 0.63 A cm−2, which is an improvement of approximately 33.8 % compared with of BSF under 1.5 V when the temperature and feed gas are 800 °C and 50 % CO2–50 % CO, respectively. This prominent performance is ascribed to creating more oxygen vacancies concentration by W doping, which promotes the transport of oxygen species. These outstanding electrochemical properties demonstrate that BSFW0.1 oxide is a promising potential oxygen electrode electrocatalyst for CO2 electrolysis in SOECs.

Promotion of Ca3Co4O9+δ oxygen electrode by in-situ exsolution of nanoparticles for solid oxide cells

Exsolution of active nanoparticles in oxide materials is an effective and feasible way to promote the catalytic activities of electrodes for solid oxide cells (SOCs). In this study, Ca-site deficient CaxCo4O9+δ (CCO-x, x = 1.5, 2, 2.5) materials are facilely prepared and studied as oxygen electrodes of SOCs. Co3O4 nanoparticles are in-situ exsolved and well embedded on the Ca3Co4O9+δ (CCO-3.0) surface by high-temperature annealing at 850 °C in ambient air. Results proves an effectively promotion of the ORR and OER kinetics with Ca deficiency. The polarization resistance of CCO-1.5 oxygen electrode reduces by 60% as compared with that of the CCO-3.0 electrode at 800 °C, from 0.23 to 0.09 Ω cm2. Both the peak power density and the electrolysis current density of the anode-supported SOC with CCO-1.5 oxygen electrode are more than 2 times that of the CCO-3.0 cell. The improved electrochemical kinetics is attributed to the exsolved Co3O4 nanoparticles with high redox reactivity and the enhanced oxygen vacancy concentration. Results make clear reliability of Ca-site deficient CaxCo4O9+δ as a promisingly oxygen electrodes for SOCs.

Improvement of oxygen reduction activity and stability on a perovskite oxide surface by electrochemical potential

The instability of the surface chemistry in transition metal oxide perovskites is the main factor hindering the long-term durability of oxygen electrodes in solid oxide electrochemical cells. The instability of surface chemistry is mainly due to the segregation of A-site dopants from the lattice to the surface. Here we report that cathodic potential can remarkably improve the stability in oxygen reduction reaction and electrochemical activity, by decomposing the near-surface region of the perovskite phase in a porous electrode made of La1-xSrxCo1-xFexO3 mixed with Sm0.2Ce0.8O1.9. Our approach combines X-ray photoelectron spectroscopy and secondary ion mass spectrometry for surface and sub-surface analysis. Formation of Ruddlesden-Popper phase is accompanied by suppression of the A-site dopant segregation, and exsolution of catalytically active Co particles onto the surface. These findings reveal the chemical and structural elements that maintain an active surface for oxygen reduction, and the cathodic potential is one way to generate these desirable chemistries.

Surface engineering of 2D layered MnO2 with co-doping of Ni and Fe for rechargeable zinc-air battery

Surface engineering by doping has always been one of the well-applied effective strategies to improve the performance of transition metal oxides towards their application in energy conversion and storage devices. However, fewer studies have been conducted to understand the effect of co-doping of two or more transition metals in layered transition metal oxides. It is interesting to note how multiple metal ion doping could create a synergistic environment that modulates the electrocatalytic ability of the host lattice. In this work, we have sequentially introduced Fe, followed by Ni, into the δ-MnO2 lattice (NiFeMn) to occupy either interplanar spaces or to substitute the host lattice. Spectroscopic and high resolution microscopic characterizations suggested that the dopants have entered the in-planar voids, as well as in the inter-layer region. The electrochemical characterizations revealed the positive synergistic interaction of Fe and Ni in the MnO2 lattice improved overall bifunctional oxygen electrode activity parameters. In the case of a practical Zn-air battery, the electrode NiFeMn shows 2.4 times improved output power density, cyclic stability, and durability than FeMn, along with a round- trip efficiency of 51 %, and specific capacity of 589.96 mAh g−1 corresponding to energy density of 470.67 mWh g−1. Our studies reveal that both Fe and Ni have played a major role as dopants and their synergistic interaction has significantly boosted the overall electrocatalytic properties of δ-MnO2.