Replacement of Ca by Ni in a Perovskite Titanate to Yield a Novel Perovskite Exsolution Architecture for Oxygen‐Evolution Reactions

For efficient catalysis and electrocatalysis well‐designed, high‐surface‐area support architectures covered with highly dispersed metal nanoparticles with good catalyst‐support interactions are required. In situ grown Ni nanoparticles on perovskites have been recently reported to enhance catalytic activities in high‐temperature systems such as solid oxide cells (SOCs). However, the micrometer‐scale primary particles prepared by conventional solid‐state reactions have limited surface area and tend to retain much of the active catalytic element within the bulk, limiting efficacy of such exsolution processes in low‐temperature systems. Here, a new, highly efficient, solvothermal route is demonstrated to exsolution from smaller scale primary particles. Furthermore, unlike previous reports of B‐site exsolution, it seems that the metal nanoparticles are exsolved from the A‐site of these perovskites. The catalysts show large active site areas and strong metal‐support interaction (SMSI), leading to ≈26% higher geometric activity (25 times higher mass activity with 1.4 V of E_on‐set) and stability for oxygen‐evolution reaction (OER) with only 0.72 µg base metal contents compared to typical 20 wt% Ni/C and even commercial 20 wt% Ir/C. The findings obtained here demonstrate the potential design and development of heterogeneous catalysts in various low‐temperature electrochemical systems including alkaline fuel cells and metal–air batteries.     

Zn0.35Co0.65O – A Stable and Highly Active Oxygen Evolution Catalyst Formed by Zinc Leaching and Tetrahedral Coordinated Cobalt in Wurtzite Structure

To arrive to sustainable hydrogen‐based energy solutions, the understanding of water‐splitting catalysts plays the most crucial role. Herein, state‐of‐the‐art hypotheses are combined on electrocatalytic active metal sites toward the oxygen evolution reaction (OER) to develop a highly efficient catalyst based on Earth‐abundant cobalt and zinc oxides. The precursor catalyst Zn_0.35Co_0.65O is synthesized via a fast microwave‐assisted approach at low temperatures. Subsequently, it transforms in situ from the wurtzite structure to the layered γ‐Co(O)OH, while most of its zinc leaches out. This material shows outstanding catalytic performance and stability toward the OER in 1 m KOH (overpotential at 10 mA cm^?2 η_initial = 306 mV, η_{98 h} = 318 mV). By comparing the electrochemical results and ex situ analyses to today's literature, clear structure‐activity correlations are able to be identified. The findings suggest that coordinately unsaturated cobalt octahedra on the surface are indeed the active centers for the OER.     

Highly Active and Stable Graphene Tubes Decorated with FeCoNi Alloy Nanoparticles via a Template‐Free Graphitization for Bifunctional Oxygen Reduction and Evolution

Development of highly active and stable bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts from earth‐abundant elements remains a grand challenge for highly demanded reversible fuel cells and metal–air batteries. Carbon catalysts have many advantages over others due to their low cost, excellent electrical conductivity, high surface area, and easy functionalization. However, they typically cannot withstand the highly oxidative OER environment. Here, a new class of bifunctional electrocatalyst is reported, consisting of ultralarge sized nitrogen doped graphene tubes (N‐GTs) (>500 nm) decorated with FeCoNi alloy particles. These tubes are prepared from an inexpensive precursor, dicyandiamide, via a template‐free graphitization process. The ORR/OER activity and the stability of these graphene tube catalysts depend strongly on the transition metal precursors. The best performing FeCoNi‐derived N‐GT catalyst exhibits excellent ORR and OER activity along with adequate electrochemical durability over a wide potential window (0–1.9 V) in alkaline media. The measured OER current is solely due to desirable O₂ evolution, rather than carbon oxidation. Extensive electrochemical and physical characterization indicated that high graphitization degree, thicker tube walls, proper nitrogen doping, and presence of FeCoNi alloy particles are vital for high bifunctional activity and electrochemical durability of tubular carbon catalysts.     

Highly Active Trimetallic NiFeCr Layered Double Hydroxide Electrocatalysts for Oxygen Evolution Reaction

The development of efficient and robust earth‐abundant electrocatalysts for the oxygen evolution reaction (OER) is an ongoing challenge. Here, a novel and stable trimetallic NiFeCr layered double hydroxide (LDH) electrocatalyst for improving OER kinetics is rationally designed and synthesized. Electrochemical testing of a series of trimetallic NiFeCr LDH materials at similar catalyst loading and electrochemical surface area shows that the molar ratio Ni:Fe:Cr = 6:2:1 exhibits the best intrinsic OER catalytic activity compared to other NiFeCr LDH compositions. Furthermore, these nanostructures are directly grown on conductive carbon paper for a high surface area 3D electrode that can achieve a catalytic current density of 25 mA cm^?2 at an overpotential as low as 225 mV and a small Tafel slope of 69 mV dec^?1 in alkaline electrolyte. The optimized NiFeCr catalyst is stable under OER conditions and X‐ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy, and elemental analysis confirm the stability of trimetallic NiFeCr LDH after electrochemical testing. Due to the synergistic interactions among the metal centers, trimetallic NiFeCr LDH is significantly more active than NiFe LDH and among the most active OER catalysts to date. This work also presents general strategies to design more efficient metal oxide/hydroxide OER electrocatalysts.     

Co–Ni‐Based Nanotubes/Nanosheets as Efficient Water Splitting Electrocatalysts

One promising approach to hydrogen energy utilization from full water splitting relies on the successful development of earth‐abundant, efficient, and stable electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, homologous Co–Ni‐based nanotube/nanosheet structures with tunable Co/Ni ratios, including hydroxides and nitrides, are grown on conductive substrates by a cation‐exchanging method to grow hydroxides, followed by anion exchanging to obtain corresponding nitrides. These hydroxide OER catalysts and nitride HER catalysts exhibit low overpotentials, small Tafel slopes, and high current densities, which are attributed to their large electrochemically reactive surface, 1D morphologies for charge conduction, and octahedral coordination states of metal ions for efficient catalytic activities. The homologous Co–Ni‐based nanotube hydroxides and nitrides suggest promising electrocatalysts for full water splitting with high efficiency, good stability, convenient fabrication, and low cost.     

Controlling the Active Sites of Sulfur‐Doped Carbon Nanotube–Graphene Nanolobes for Highly Efficient Oxygen Evolution and Reduction Catalysis

Controlling active sites of metal‐free catalysts is an important strategy to enhance activity of the oxygen evolution reaction (OER). Many attempts have been made to develop metal‐free catalysts, but the lack of understanding of active‐sites at the atomic‐level has slowed the design of highly active and stable metal‐free catalysts. A sequential two‐step strategy to dope sulfur into carbon nanotube–graphene nanolobes is developed. This bidoping strategy introduces stable sulfur–carbon active‐sites. Fluorescence emission of the sulfur K‐edge by X‐ray absorption near edge spectroscopy (XANES) and scanning transmission electron microscopy electron energy loss spectroscopy (STEM‐EELS) mapping and spectra confirm that increasing the incorporation of heterocyclic sulfur into the carbon ring of CNTs not only enhances OER activity with an overpotential of 350 mV at a current density of 10 mA cm^?2, but also retains 100% of stability after 75 h. The bidoped sulfur carbon nanotube–graphene nanolobes behave like the state‐of‐the‐art catalysts for OER but outperform those systems in terms of turnover frequency (TOF) which is two orders of magnitude greater than (20% Ir/C) at 400 mV overpotential with very high mass activity 1000 mA cm^?2 at 570 mV. Moreover, the sulfur bidoping strategy shows high catalytic activity for the oxygen reduction reaction (ORR). Stable bifunctional (ORR and OER) catalysts are low cost, and light‐weight bidoped sulfur carbon nanotubes are potential candidates for next‐generation metal‐free regenerative fuel cells.     

Ultrathin High Surface Area Nickel Boride (NixB) Nanosheets as Highly Efficient Electrocatalyst for Oxygen Evolution

The overriding obstacle to mass production of hydrogen from water as the premium fuel for powering our planet is the frustratingly slow kinetics of the oxygen evolution reaction (OER). Additionally, inadequate understanding of the key barriers of the OER is a hindrance to insightful design of advanced OER catalysts. This study presents ultrathin amorphous high‐surface area nickel boride (Ni_x B) nanosheets as a low‐cost, very efficient and stable catalyst for the OER for electrochemical water splitting. The catalyst affords 10 mA cm^?2 at 0.38 V overpotential during OER in 1.0 m KOH, reducing to only 0.28 V at 20 mA cm^?2 when supported on nickel foam, which ranks it among the best reported nonprecious catalysts for oxygen evolution. Operando X‐ray absorption fine‐structure spectroscopy measurements reveal prevalence of NiOOH, as well as Ni‐B under OER conditions, owing to a Ni‐B core@nickel oxyhydroxide shell (Ni‐B@NiO_x H) structure, and increase in disorder of the NiO_x H layer, thus revealing important insight into the transient states of the catalyst during oxygen evolution.     

Monolithic Photoassisted Water Splitting Device Using Anodized Ni‐Fe Oxygen Evolution Catalytic Substrate

Large‐scale industrial application of solar‐driven water splitting has called for the development of oxygen evolution reaction (OER) catalysts that deliver high catalytic activity and stability. Here it is shown that an efficient OER catalytic substrate can be developed by roll‐to‐roll fabrication of electrodeposited Ni‐Fe foils, followed by anodization. An amorphous oxyhydroxide layer directly formed on Ni‐Fe foils exhibits high catalytic activity toward water oxidation in 1 m KOH solution, which requires an overpotential of 0.251 V to reach current density of 10 mA cm^–2. The developed catalytic electrode shows the best OER activity among catalysts with film structure. The catalyst also shows prolonged stability at vigorous gas evolution condition for 36 h. To demonstrate the monolithic photoassisted water splitting device, an amorphous silicon solar cell is fabricated on Ni‐Fe catalytic substrate, resulting in lowering OER overpotential under light illumination. This monolithic device is the first demonstration that the OER catalytic substrates and the solar cells are integrated and can be easily applied for industrial scale solar‐driven water electrolysis.     

Nitrogen and Sulfur Codoped Graphite Foam as a Self‐Supported Metal‐Free Electrocatalytic Electrode for Water Oxidation

The oxidation of water to produce oxygen gas is related to a variety of energy storage systems. Thus, the development of efficient, cheap, durable, and scalable electrocatalysts for oxygen evolution reaction (OER) is of great importance. Here, a high‐performance OER catalyst, nitrogen and sulfur codoped graphite foam (NSGF) is reported. This NSGF is prepared from commercial graphite foil and directly applied as an electrocatalytic electrode without using a current collector and a polymeric binder. It exhibits an extremely low overpotential of 0.380 V to reach a current density of 10 mA cm^?2 and shows fast kinetics with a small Tafel slope of 96 mV dec^?1 in 0.1 m KOH. This electrocatalytic performance is superior or comparable to those of previously reported metal‐free OER catalysts.     

Single Atoms and Clusters Based Nanomaterials for Hydrogen Evolution,Oxygen Evolution Reactions,and Full Water Splitting

The sustainable and scalable production of hydrogen through hydrogen evolution reaction (HER) and oxygen through oxygen evolution reaction (OER) in water splitting demands efficient and robust electrocatalysts. Currently, state‐of‐the‐art electrocatalysts of Pt and IrO₂/RuO₂ exhibit the benchmark catalytic activity toward HER and OER, respectively. However, expanding their practical application is hindered by their exorbitant price and scarcity. Therefore, the development of alternative effective electrocatalysts for water splitting is crucial. In the last few decades, substantial effort has been devoted to the development of alternative HER/OER and water splitting catalysts based on various transition metals (including Fe, Co, Ni, Mo, and atomic Pt) which show promising catalytic activities and durability. In this review, after a brief introduction and basic mechanism of HER/OER, the authors systematically discuss the recent progress in design, synthesis, and application of single atom and cluster‐based HER/OER and water splitting catalysts. Moreover, the crucial factors that can tune the activity of catalysts toward HER/OER and water splitting such as morphology, crystal defects, hybridization of metals with nonmetals, heteroatom doping, alloying, and formation of metals inside graphitic layered materials are discussed. Finally, the existing challenges and future perspectives for improving the performance of electrocatalysts for water splitting are addressed.     

Atomically Dispersed Mo Supported on Metallic Co9S8 Nanoflakes as an Advanced Noble‐Metal‐Free Bifunctional Water Splitting Catalyst Working in Universal pH Conditions

Water splitting requires development of cost‐effective multifunctional materials that can catalyze both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) efficiently. Currently, the OER relies on the noble‐metal catalysts; since with other catalysts, its operation environment is greatly limited in alkaline conditions. Herein, an advanced water oxidation catalyst based on metallic Co₉S₈ decorated with single‐atomic Mo (0.99 wt%) is synthesized (Mo‐Co₉S₈@C). It exhibits pronounced water oxidization activity in acid, alkali, and neutral media by showing positive onset potentials of 200, 90, and 290 mV, respectively, which manifests the best Co₉S₈‐based single‐atom Mo catalyst till now. Moreover, it also demonstrates excellent HER performance over a wide pH range. Consequently, the catalyst even outperforms noble metal Pt/IrO₂‐based catalysts for overall water splitting (only requiring 1.68 V in acid, and 1.56 V in alkaline). Impressively, it works under a current density of 10 mA cm^?2 with no obvious decay during a 24 h (0.5 m H₂SO₄) and 72 h (1.0 m KOH) durability experiment. Density functional theory (DFT) simulations reveal that the synergistic effects of atomically dispersed Mo with Co‐containing substrates can efficiently alter the binding energies of adsorbed intermediate species and decrease the overpotentials of the water splitting.     

3D Synergistically Active Carbon Nanofibers for Improved Oxygen Evolution

Developing earth‐abundant and active electrocatalysts for the oxygen evolution reaction (OER) as replacements for conventional noble metal catalysts is of scientific and technological importance for achieving cost‐effective and efficient conversion and storage of renewable energy. However, most of the promising candidates thus far are exclusively metal‐based catalysts, which are disadvantaged by relatively restricted electron mobility, corrosion susceptibility, and detrimental environmental influences. Herein, hierarchically porous nitrogen (N) and phosphorus (P) codoped carbon nanofibers directly grown on conductive carbon paper are prepared through an electrochemically induced polymerization process in the presence of aniline monomer and phosphonic acid. The resultant material exhibits robust stability (little activity attenuation after 12 h continuous operation) and high activity with low overpotential (310 mV at 10 mA cm^?2) toward electrocatalytic oxygen production, with performance comparable to that of the precious iridium oxide (IrO₂) benchmark. Experimental measurements reveal that dual doping of N and P can result in an increased active surface area and abundant active sites in comparison with the single doped and pristine carbon counterparts, and density functional theory calculations indicate that N and P dopants can coactivate the adjacent C atoms, inducing synergistically enhanced activity toward OER.     

Eutectic‐Derived Mesoporous Ni‐Fe‐O Nanowire Network Catalyzing Oxygen Evolution and Overall Water Splitting

The development of efficient and abundant water oxidation catalysts is essential for the large‐scale storage of renewable energy in the form of hydrogen fuel via electrolytic water splitting, but still remains challenging. Based upon eutectic reaction and dealloying inheritance effect, herein, novel Ni‐Fe‐O‐based composite with a unique mesoporous nanowire network structure is designed and synthesized. The composite exhibits exceptionally low overpotential (10 mA cm^?2 at an overpotential of 244 mV), low Tafel slope (39 mV dec^?1), and superior long‐term stability (remains 10 mA cm^?2 for over 60 h without degradation) toward oxygen evolution reaction (OER) in 1 m KOH. Moreover, an alkaline water electrolyzer is constructed with the Ni‐Fe‐O composite as catalyst for both anode and cathode. This electrolyzer displays superior electrolysis performance (affording 10 mA cm^?2 at 1.64 V) and long‐term durability. The remarkable features of the catalyst lie in its unique mesoporous nanowire network architecture and the synergistic effect of the metal core and the active metal oxide, giving rise to the strikingly enhanced active surface area, accelerated electron/ion transport, and further promoted reaction kinetics of OER.     

Electrocatalytic Oxygen Evolution Reaction in Acidic Environments – Reaction Mechanisms and Catalysts

The low efficiency of the electrocatalytic oxidation of water to O₂ (oxygen evolution reaction‐OER) is considered as one of the major roadblocks for the storage of electricity from renewable sources in form of molecular fuels like H₂ or hydrocarbons. Especially in acidic environments, compatible with the powerful proton exchange membrane (PEM), an earth‐abundant OER catalyst that combines high activity and high stability is still unknown. Current PEM‐compatible OER catalysts still rely mostly on Ir and/or Ru as active components, which are both very scarce elements of the platinum group. Hence, the Ir and/or Ru amount in OER catalysts has to be strictly minimized. Unfortunately, the OER mechanism, which is the most powerful tool for OER catalyst optimization, still remains unclear. In this review, we first summarize the current state of our understanding of the OER mechanism on PEM‐compatible heterogeneous electrocatalysts, before we compare and contrast that to the OER mechanism on homogenous catalysts. Thereafter, an overview over monometallic OER catalysts is provided to obtain insights into structure‐function relations followed by a review of current material optimization concepts and support materials. Moreover, missing links required to complete the mechanistic picture as well as the most promising material optimization concepts are pointed out.     

High‐Performance Trifunctional Electrocatalysts Based on FeCo/Co2P Hybrid Nanoparticles for Zinc–Air Battery and Self‐Powered Overall Water Splitting

Currently, it is still a significant challenge to simultaneously boost various reactions by one electrocatalyst with high activity, excellent durability, as well as low cost. Herein, hybrid trifunctional electrocatalysts are explored via a facile one‐pot strategy toward an efficient oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The catalysts are rationally designed to be composed by FeCo nanoparticles encapsuled in graphitic carbon films, Co₂P nanoparticles, and N,P‐codoped carbon nanofiber networks. The FeCo nanoparticles and the synergistic effect from Co₂P and FeCo nanoparticles make the dominant contributions to the ORR, OER, and HER activities, respectively. Their bifunctional activity parameter (?E) for ORR and OER is low to 0.77 V, which is much smaller than those of most nonprecious metal catalysts ever reported, and comparable with state‐of‐the‐art Pt/C and RuO₂ (0.78 V). Accordingly, the as‐assembled Zn–air battery exhibits a high power density of 154 mW cm^?2 with a low charge–discharge voltage gap of 0.83 V (at 10 mA cm^?2) and excellent stability. The as‐constructed overall water‐splitting cell achieves a current density of 10 mA cm^?2 (at 1.68 V), which is comparable to the best reported trifunctional catalysts.     

Enabling Iron‐Based Highly Effective Electrochemical Water‐Splitting and Selective Oxygenation of Organic Substrates through In Situ Surface Modification of Intermetallic Iron Stannide Precatalyst

A strategy to overcome the unsatisfying catalytic performance and the durability of monometallic iron‐based materials for the electrochemical oxygen evolution reaction (OER) is provided by heterobimetallic iron–metal systems. Monometallic Fe catalysts show limited performance mostly due to poor conductivity and stability. Here, by taking advantage of the structurally ordered and highly conducting FeSn₂ nanostructure, for the first time, an intermetallic iron material is employed as an efficient anode for the alkaline OER, overall water‐splitting, and also for selective oxygenation of organic substrates. The electrophoretically deposited FeSn₂ on nickel foam (NF) and fluorine‐doped tin oxide (FTO) electrodes displays remarkable OER activity and durability with substantially low overpotentials of 197 and 273 mV at 10 mA cm^?2, respectively, which outperform most of the benchmarking NiFe‐based catalysts. The resulting superior activity is attributed to the in situ generation of α‐FeO(OH)@FeSn₂ where α‐FeO(OH) acts as the active site while FeSn₂ remains the conductive core. When the FeSn₂ anode is coupled with a Pt cathode for overall alkaline water‐splitting, a reduced cell potential (1.53 V) is attained outperforming that of noble metal‐based catalysts. FeSn₂ is further applied as an anode to produce value‐added products through selective oxygenation reactions of organic substrates.     

MOF Template‐Directed Fabrication of Hierarchically Structured Electrocatalysts for Efficient Oxygen Evolution Reaction

The ever‐increasing demand for clean and renewable power sources has sparked intensive research on water splitting to produce hydrogen, in which the exploration of electrocatalysts is the central issue. Herein, a new strategy, metal–organic framework template‐directed fabrication of hierarchically structured Co₃O₄@X (X = Co₃O₄, CoS, C, and CoP) electrocatalysts for efficient oxygen evolution reaction (OER) is developed, where Co₃O₄@X are derived from cobalt carbonatehydroxide@zeolitic‐imidazolate‐framework‐67 (CCH@ZIF‐67). Unique hierarchical structure and synergistic effect of resulting catalysts endow abundant exposed active sites, facile ion diffusion path, and improved conductivity, being favorable for improving catalytic activity of them. Consequently, these derivatives Co₃O₄@X reveal highly efficient electrocatalytic performance with long‐term durability for the OER, much superior to previously reported cobalt‐based catalysts as well as the Ir/C catalyst. Particularly, Co₃O₄@CoP exhibits the highest electrocatalytic capability with the lower overpotential of 238 mV at the current density of 10 mA cm^?2. Furthermore, Co₃O₄@X can also efficiently catalyze other small molecules through electro‐oxidation reaction (e.g., glycerol, methanol, or ethanol). It is expected that the strategy presented here can be extended to the fabrication of other composite electrode materials with hierarchical structures for more efficient water splitting.     

Ultrafine Pt Nanoparticle‐Decorated Pyrite‐Type CoS2 Nanosheet Arrays Coated on Carbon Cloth as a Bifunctional Electrode for Overall Water Splitting

To improve the utilization efficiency of precious metals, metal‐supported materials provide a direction for fabricating highly active and stable heterogeneous catalysts. Herein, carbon cloth (CC)‐supported Earth‐abundant CoS₂ nanosheet arrays (CoS₂/CC) are presented as ideal substrates for ultrafine Pt deposition (Pt‐CoS₂/CC) to achieve remarkable performance toward the hydrogen and oxygen evolution reactions (HER/OER) in alkaline solutions. Notably, the Pt‐CoS₂/CC hybrid delivers an overpotential of 24 mV at 10 mA cm^?2 and a mass activity of 3.89 A Pt_mg^?1, which is 4.7 times higher than that of commercial Pt/C, at an overpotential of 130 mV for catalyzing the HER. An alkali‐electrolyzer using Pt‐CoS₂/CC as a bifunctional electrode enables a water‐splitting current density of 10 mA cm^?2 at a low voltage of 1.55 V and can sustain for more than 20 h, which is superior to that of the state‐of‐the‐art Pt/C+RuO₂ catalyst. Further experimental and theoretical simulation studies demonstrate that strong electronic interaction between Pt and CoS₂ synergistically optimize hydrogen adsorption/desorption behaviors and facilitate the in situ generation of OER active species, enhancing the overall water‐splitting performance. This work highlights the regulation of interfacial and electronic synergy in pursuit of highly efficient and durable supported catalysts for hydrogen and oxygen electrocatalytic applications.     

Bimetallic Zeolitic Imidazolite Framework Derived Carbon Nanotubes Embedded with Co Nanoparticles for Efficient Bifunctional Oxygen Electrocatalyst

Bifunctional oxygen catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with high activities and low‐cost are of prime importance and challenging in the development of fuel cells and rechargeable metal–air batteries. This study reports a porous carbon nanomaterial loaded with cobalt nanoparticles (Co@NC‐x/y) derived from pyrolysis of a Co/Zn bimetallic zeolitic imidazolite framework, which exhibits incredibly high activity as bifunctional oxygen catalysts. For instance, the optimal catalyst of Co@NC‐3/1 has the interconnected framework structure between porous carbon and embedded carbon nanotubes, which shows the superb ORR activity with onset potential of ≈1.15 V and half‐wave potential of ≈0.93 V. Moreover, it presents high OER activity that can be further enhanced to over commercial RuO₂ by P‐doped with overpotentials of 1.57 V versus reversible hydrogen electrode at 10 mA cm^?2 and long‐term stability for 2000 circles and a Tafel slope of 85 mV dec^?1. Significantly, the nanomaterial demonstrates better catalytic performance and durability than Pt/C for ORR and commercial RuO₂ and IrO₂ for OER. These findings suggest the importance of a synergistic effect of graphitic carbon, nanotubes, exposed Co–N_x active sites, and interconnected framework structure of various carbons for bifunctional oxygen electrocatalysts.