Methods and compositions for oxygen electrocatalysis

In one aspect, the disclosure relates to catalysts for electrochemical water splitting, in particular catalysts useful for oxygen evolution at an anode in electrochemical water splitting. The disclosed catalysts compositions comprise a catalyst core component, a shell component, and optionally a catalyst outer component; wherein the catalyst core component comprises a composition having the chemical formula MxPy; where M is a transition metal; wherein x is a number from about 1 to about 20; wherein y is a number from about 1 to about 20; wherein the shell component comprises a conducting polymer; and wherein the catalyst outer component is a transition metal that is not the same as the transition metal M. This abstract is intended as a scanning tool for purposes of searching in the particular art and is not intended to be limiting of the present disclosure.

BACKGROUND

Energy production via hydrogen using electrochemical water splitting is an emerging technology in clean and renewable storage of energy. The anodic oxygen evolution reaction (OER) is of the foremost concern for practical feasibility of alkaline water electrolyzers. Oxygen evolution requires a large overpotential due to the formation of high energy intermediates (Ref. 1). As a result of poor kinetics, conventional Pt/Ru/Ir based catalysts require an overpotential >0.25 V (Ref. 2). In addition, these elements are expensive, and not sufficiently abundant to meet future energy application needs. Recent research has seen development of catalysts utilizing alloys of Pt group metals, transition metal oxides, nitrides, chalcogenides or phosphides. However, many of these catalysts still exhibit low efficiency in terms of overpotential.

Despite advances in research directed to catalysts for efficient electrochemical water splitting to generate hydrogen, there remain a scarcity of catalysts that do not require a large overpotential at the anodic oxygen reaction. These needs and other needs are satisfied by the present disclosure.

DETAILED DESCRIPTION

Definitions

As used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a transition metal,” “a catalyst,” or “a polypyrrole,” including, but not limited to, two or more such transition metals, catalysts, or polypyrroles, and the like.

As used herein, the term “effective amount” refers to an amount that is sufficient to achieve the desired modification of a physical property of the composition or material. For example, an “effective amount” of a transition metal refers to an amount that is sufficient to achieve the desired improvement in the property modulated by the formulation component, e.g. achieving the desired improvement in oxygen overpotential. The specific level in terms of wt %, ppm, or amount of a component, such as a transition metal, in a composition required as an effective amount will depend upon a variety of factors including the amount and type of transition metal, amount and type of shell component, such as polypyrrole, amount, and type of additional transition metal electrodeposited onto the shell component.

Catalyst Compositions

In various aspects, disclosed herein are catalyst compositions comprising a catalyst core component, a shell component, and optionally a catalyst outer component. In some instances, the disclosed catalyst composition comprises a catalyst core component, a shell component, and a catalyst outer component. Collectively, a catalyst composition comprising a catalyst core component, a shell component, and a catalyst outer component can be referred herein throughout as a disclosed hybrid core-shell catalyst composition. A catalyst core, in various aspects, can comprise at least one transition metal phosphide, i.e., at least one transition metal and phosphorus. The shell component can be a conducting polymer, such as a polypyrrole, a polyaniline, a polythiophene, or combinations thereof. In various aspects, the catalyst outer component can be a transition metal. In other instances, the disclosed catalyst composition comprises a catalyst core component and a shell component. Collectively, a catalyst composition comprising a catalyst core component and a shell component can be referred herein throughout as a disclosed core-shell catalyst composition.

A schematic representation of a disclosed composition comprising a catalyst core component and a shell component is shownFIG.1A. As shown therein, in some instances, the shell component comprises a conducting polymer with nitrogen available for binding. The nitrogen in the conducting polymer can bind to the catalyst core which it surrounds. However, not all nitrogen in the conducting polymer are necessarily bound to the catalyst core, thereby providing unbound nitrogen which can bind to the outer catalyst component. A schematic representation is shown inFIG.1Bof a disclosed catalyst composition comprising a catalyst core component and a shell component, with a catalyst outer component immobilized on the shell component. As shown therein, some of the nitrogen in the conducting polymer of the shell component are bound to the catalyst outer component. Without wishing to be bound by a particular theory, it is believed that the catalyst outer component is located on an outer surface of the conducting polymer of the shell component. That is, dispersed between the catalyst outer component and the catalyst core component is the shell component comprising the conducting polymer. It should be noted thatFIG.1Bfurther shows a reaction catalyzed by the disclosed catalyst in which a hydroxide ion is converted to oxygen. Without wishing to be bound by a particular theory, it is believe that the disclosed catalyst composition a catalyst core component, a shell component, and a catalyst outer component provide a synergic effect comprising the transition metal of the core catalyst interacting with a nitrogen in the shell component and the transition metal in the catalyst outer component interacting with another nitrogen in the shell component.

Energy production via hydrogen using electrochemical water splitting is an emerging technology in clean and renewable storage of energy. The anodic oxygen evolution reaction (OER) is of the foremost concern for practical feasibility of alkaline water electrolyzers. Oxygen evolution requires a large overpotential due to the formation of high energy intermediates Ref. 1). As a result of poor kinetics, even the state of art Pt/Ru/Ir based catalysts require an overpotential >0.25 V (Ref. 2). In addition, these elements are expensive, and not sufficiently abundant to meet future energy application needs. Recent research has seen development of robust catalysts like alloys of Pt group metals, transition metal oxides, nitrides, chalcogenides or phosphides. However, many of these catalysts still exhibit low proficiency in terms of overpotential. The disclosed catalyst compositions comprising abundant transition metals catalysts overcome the high overpotential and provide new catalysts for electrochemical water splitting.

Although, heretofore, transition metal (TM) catalysts, especially Mn, Co and Ni based, have shown promising results outpacing the finest commercial catalysts (Ref. 3). In particular, TM phosphides (TMPs) have received considerable attention towards OER owing to their stability in electrolysis potential range. Among conventional TMPs, nickel phosphide is an attractive catalyst that is reasonably stable and available in multiple stoichiometries, e.g., Ni3P (Ref. 4), Ni12P5, Ni2P (Ref. 5), and Ni5P4(Ref. 6), which are active in water splitting reactions. Recently, Menezes et al reported that Ni12P5is an effective catalyst for OER due to excess stoichiometric Ni, resulting in higher density of active sites (Ref. 7). Research on these catalysts typically focuses on understanding the kinetics and stability. The disclosed catalysts herein provide an improved catalyst composition with enhanced electrochemical performance obtained via modifying the surface functionalities using a conducting polymer.

Conducting polymers (CPs) such as polypyrrole (PPy), polyaniline (PANI) and polythiophenes (PTh) have been previously described (Ref. 8). Among these, PPy is frequently studied as a conducting polymer due to its conductivity, ease of synthesis, biocompatibility, chemical stability as well as its porous structure and high surface area (Ref. 9). Moreover, PPy has nitrogen as a heteroatom, which can form M-N bonds with different materials resulting in enhanced electrocatalytic activity (Ref. 10). For example, integrating an electronegative N to a cationic Ni site can affect the electronic state of Ni resulting in stable higher oxidation states. Cao et al. (Ref. 11) modified the surface of MnCo2O4with PPy by in situ chemical polymerization of excess pyrrole with H2O2(10 wt. %) and found that the PPy layer on MnCo2O4surface provided a conductive network for fast electron transfer resulting in enhanced electrocatalytic activities for both ORR and OER. Jia et al. (Ref. 12) demonstrated that a ternary CoNiMn-LDH/PPy/RGO composite exhibited bifunctional electrocatalysts for OER and ORR with enhanced catalytic activities because of the synergic effect among the three different constituents, thus, highlighting the charge transfer properties of conjugated PPy in overall electrocatalytic performance of the composites.

The disclosed catalysts herein provide a class of hybrid catalysts comprised of core-shell structure with a catalyst outer component, e.g., Co@Ni12P5/PPy. The M-N catalyst core interacting with a nitrogen of the shell component acts as an active site for OER due to accelerated charge transfer rate between the layers. It is expected that this synergic interaction (M-N) modulates the electron density around the metal atom by inducing charge redistribution resulting in altered binding energies of hydroxide or intermediate ions at the active site. Conventional TMPs exhibit lower activity compared to bimetallic phosphides due to change in energy for chemisorption of hydroxide at the active site (M-O; see Ref. 13). Recently, Mendoza-Garcia et al (Ref. 14) reported a bimetallic Co(2-x)FexP catalyst that showed enhanced electrochemical performance compared to Co2P or Fe2P due to alloying effect. Li et al (Ref. 15) have reported using 3D NiCoP on Ni foam. The catalyst compositions disclosed herein tailor the electronic properties of two different metals by functionalizing with a conducting polymer. As a result, a dual synergic effect is obtained, e.g., Co—N and N—Ni, with Co anchored at the N in PPy shell. Without wishing to be bound by a particular theory, it is believed that simultaneous integration of conducting properties with high density of catalytic active sites (M-N) is associated with the highly enhanced electrochemical OER activity realized with the disclosed catalysts.

In various aspects, a catalyst core comprises a composition having a chemical formula of MxPy, where M is a transition metal, P is phosphorus, x is a number from about 1 to about 20, and y is a number from about 1 to about 20. In some instances, x and y are integer numbers. In a further aspect, M is a transition metal selected from a metal from Group 3 to Group 12 of the periodic table that is a d-block metal. In a still further aspect, M is a transition metal selected from Mn, Co, and Ni. In a yet further aspect, M is Ni. In a further aspect, x is a number selected from 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60; any range encompassed by the foregoing values; and any combination of the foregoing values. In a further aspect, y is a number selected from 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20; any range encompassed by the foregoing values; and any combination of the foregoing values. In a further aspect, M is Ni. In a still further aspect, the catalyst core comprises Ni3P, Ni12P5, Ni2P, or Ni5P4. In a particular aspect, the catalyst core comprises Ni12P5.

In some instances, the catalyst core has a three-dimensional shape that is spherical, pseudo-spherical, ellipsoidal, and the like. In some instances, the catalyst core has a three-dimensional shape that is a distribution of irregularly shaped particles. The catalyst core has a size such that the longest axis of the catalyst core three-dimensional shape is from about 100 nm to about 2 μm.

Methods of Preparing the Catalyst Compositions

The disclosed catalyst compositions are prepared via a sequential series of synthesis steps. First, the core catalyst is prepared, e.g., using a method similar to that described by Ni, et al (Ref. 16). Second, the shell component is polymerized onto the surface of the core catalyst in a suitable solvent, e.g., water, with a suitable monomer, e.g., pyrrole. The core catalyst is present as a suspension in the solvent at a suitable concentration, e.g., about 1 mg to about 1000 mg of the catalyst core per milliliter of suspension. Sonication or another suitable method can be used to suspend the core catalyst in the solvent. The monomer can be used a suitable concentration similar to that described in the Examples herein below. A suitable oxidant, e.g., ferric chloride (FeCl3), can be used in a manner as described herein below in the Examples. Third, the catalyst outer component can be immobilized on the core-shell catalyst prepared as described in the foregoing, e.g., by electrodeposition, as described herein below.

Methods of Using the Catalyst Compositions

The disclosed catalyst compositions can be used as a catalyst for oxygen electrocatalysis as shown for the anodic reaction in the following:
Cathodic: 4H++4e−→2H2
Anodic: 2H2O→O2+4H++4e−

REFERENCES

EXAMPLES

Synthesis of Ni12P5Catalyst Core. Ni12P5catalyst core, in the form of microspheres, was synthesized using a procedure developed by Ni et al (Ref. 16). In a typical synthesis, NiCl2.6H2O and CoCl2.6H2O were taken in 1:2 molar ratio. NaH2PO2.H2O was added as the phosphorous source in 20 mL DI water. The pH was adjusted using NaHCO3. All contents were transferred to a Teflon autoclave and heated at 170° C. for 10 h. The resulting precipitate was washed several times with DI water and ethanol. Finally, the microspheres were dried at 60° C. overnight.

Synthesis of Ni12P5/PPy composite. Ni12P5/PPy composite was prepared via oxidative chemical polymerization. Typically, 60 mg of Ni12P5and 30 μL of pyrrole were dispersed in 6 mL DW and sonicated for 30 min to obtain a good dispersion. In a separate flask, 70.2 mg FeCl3was dissolved in 6 mL DW. Then, the FeCl3solution was added to the monomer solution slowly under continuous stirring. The final solution was refluxed at room temperature overnight. The product was obtained by centrifugation and then washed several times with DW. Finally, the obtained Ni12P5/PPy composite was dried at 60° C. for 24 h.

Electrodeposition of Co on Ni12P5/PPy. Deposition of cobalt on Ni12P5@PPy composite was performed in aqueous solution containing 0.05 M CoCl2.6H2O with 0.1 M LiClO4as supporting electrolyte. The deposition was carried out through chronoamperometric conditions at a potential of 1.0 V vs. Ag/AgCl for 30 seconds. After deposition of Co, the electrode was rinsed DI water and dried in air. Additionally, Co was also deposited on bare PPy and Ni12P5for control experiments.

Characterization of materials. The phase and structure of Ni12P5and Ni12P5/PPy were characterized by X-ray diffraction (Bruker/D8 Advance) using Cu anode target (λ=1.5418 Å). Fourier transform infrared (FTIR) spectroscopy analysis was performed in a FTIR spectrophotometer (Thermo Scientific). Analysis of morphology and composition was done by SEM-EDS using Hitachi S-800 Electron Microscope with an accelerating voltage of 25 kV at 15 mm working distance. TEM analysis was done using a Tecnai F20 transmission electron microscope with a point resolution of 0.24 nm. Surface analysis was done on catalysts using Thermo Scientific K-Alpha XPS system.

Electrochemical analysis. A Gamry electrochemical workstation was used to study the electrocatalytic OER activity of all catalysts using a rotating disk electrode (RDE). Pt wire as counter and saturated calomel (SCE) reference electrodes were employed. After every measurement, SCE was kept in distilled water and thoroughly cleaned before storing in 4M KCl. Electrode inks were prepared by sonicating 2.0 mg sample in 500 μL isopropanol, 500 μL distilled water and Nafion™ (1 wt %; 10 μL) for 30 min. 10 μL was drop casted onto the glassy carbon electrode (GCE) and dried in air. Prior to any electrochemical measurement, working electrode was cleaned thoroughly with 0.05 μm alumina and 1M KOH electrolyte was purged for 20 mins with N2. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were done at 50 mV/s and 5 mV/s, respectively over a potential range (1.2-1.86 V vs RHE). EIS measurements were taken at a potential of 1.66V vs RHE. Each catalysts were at least tested three times under same experimental conditions.

Characterization and Analysis of Disclosed Catalysts. A catalyst core comprising Ni12P5microspheres was synthesized via a scalable template based hydrothermal method. The microspheres were then modified to form a disclosed core shell catalyst, i.e., Ni12P5/PPy composite comprising Ni12P5catalyst core and a shell comprising polypyrrole, PPy. The PPy was prepared by chemical polymerization of pyrrole using FeCl3as an oxidant. The shell comprising PPy provided a doped polypyrrole (PPy) shell comprising random N+Cl−centers. Introduction of PPy shell around the Ni12P5catalyst core resulted in new vibrational peaks at 1559 and 1476 cm−1, indicating the asymmetric and symmetric ring stretching modes of PPy as shown inFIG.2A(for comparison see Ref. 17). The peaks at 1298 and 1174 cm−1correspond to the stretching of the C—N and C—H deformation vibrations of PPy (for reference see Ref. 18). Also, a characteristic peak at 1080 cm−1, in both Ni12P5as well as Ni12P5/PPy, corresponding to Ni—Pstrconfirmed the formation of a metal phosphide.[19]The phase and structure of the Ni12P5/PPy core-shell catalyst determined using PXRD presented a tetragonal space group −I4/m as shown inFIG.2B. The intense broad peaks indicate the presence of small crystallite size (1.17 μm obtained by calculation using the Scherrer equation). A single wide peak near 41-42.5° corresponds to three lattice planes (202), (321) and (400) of Ni12P5. There is no crystalline peak arising from PPy.

Polymerization of PPy around the catalyst core was done using FeCl3as an oxidant. Varying the oxidants can result in different electrical conductivity, specific surface area, porosity, etc. For example, Sha et al. showed that the type of oxidant used in the preparation of PPy-based catalysts affected the catalytic activity of the resulting catalyst (see Ref. 21). The catalyst prepared using FeCl3as an oxidant showed better electrocatalytic performance compared to using other oxidants including ammonium persulphate (APS) and hydrogen peroxide (H2O2; see Ref. 21). While employing FeCl3can provide effective doping of the conducting polymer, its strong oxidizing nature may potentially affect the morphology and structure of the shell. Interestingly, the tetragonal phase as well as the spherical morphology remained intact in the composite confirming its robust nature.

SEM images inFIGS.3A-3Cdisplay Ni12P5microspheres of approximately 1 μm diameter corroborating the results obtained using the Scherrer equation. A core shell structure of Ni12P5/PPy was confirmed inFIGS.3G-3I, showing a continuous envelope of the polymer on the Ni12P5catalyst core. EDS color mapping of the catalysts indicates the uniform distribution of Ni and P in the core, as shown inFIGS.3E-3F and3J-3L. The selected mapping area is shown inFIG.3D and3J, respectively. For precise morphological remarks, TEM images of the Ni12P5catalyst care are shown inFIGS.4A-4C. A highly crystalline SAED pattern is shown in the inset corresponding to Ni12P5catalyst core inFIG.4A. HRTEM analysis revealed two exposed crystallographic planes corresponding to (240) and (112) of the I4/m phase. The TEM images of the Ni12P5/PPy core shell structure are shown inFIGS.4E-4G. The images show different stages of polymerization. Initially, an inconsistent thin layer of polymerization occurs at the surface of the Ni12P5microspheres (seeFIG.4E). Polymerization occurs selectively around the catalyst core. Without wishing to be bound by a particular theory, this is likely due to electrostatic interaction of Py monomer at the Ni12P5surface prior to oxidation. The images show that the polymer chain spreads along the catalyst core completely covering the surface (seeFIG.4F). Through the process of aging, polypyrrole forms a uniform shell of approximately 200 nm thickness (seeFIG.4G). Despite this thick shell, HRTEM exposes (112) lattice plane of Ni12P5, suggesting a porous amorphous nature of the PPy shell. It is possible that the amorphous nature of the PPy shell possibly prevented the crystalline SAED spots. It is believed, without wishing to be bound by a particular theory, that a highly porous PPy shell can allow efficient diffusion of reactant molecules to the Ni-PPy interface.

Insights into understanding the role of the disclosed PPy shell can be obtained from change in chemical states of each element.FIGS.5A-5Eshow XPS profile data. These data suggest the possible interaction of PPy with the Ni12P5core. Metal phosphides have a strong tendency for surface oxidation leading to phosphates (P 2p-133 eV). Therefore, it is common for Ni to exist in mixed oxidation states i.e. +2 (nickel phosphate) and a small positive +δ(0<δ<2) corresponding to 855.9 eV and 852.4 eV, respectively (FIG.5A; for reference see Ref.22). The typical peak at 129.4 eV confirms the formation of metal phosphide with Pδ−(FIG.5B). Furthermore, the adsorbed OH−and PO43−seen in O 1s (FIG.5C) corroborates the highly oxidative surface. There are no extra peaks observed in the O 1s profile, most likely due to polymerization on the catalyst core. Appearance of characteristic C═N, C—OH or C═O peaks in C 1s profile (FIG.5E) of the composites confirms the successful introduction of the conjugated polymer shell. The C—C peak at 284.8 eV is from the carbon paper used as substrate for the analysis. The XPS profile of N shows no peak for Ni12P5, as expected. However, N exists in three different coordination environment in the composites (FIG.5D) i.e. ═N—, undoped N—H center and doped+N—H center (for comparison, see Ref. 23). The ═N— peak points out the highly conjugated nature of the polymer and unlocalized electrons contributing to aromaticity of the monomer. The peak at 399.6 eV likely arises due to the uneven doping by FeCl3, as mentioned earlier, forming an electropositive N center with higher binding energy. The most intense peak likely arises due to a large number of undoped N centers at slightly lower binding energy (399.2 eV). These undoped N centers in PPy can act as probable binding sites for Ni.

To confirm this Ni—N interaction, relative intensities and peak position of Niδ++in both the catalyst cores and composites were evaluated. There is no change in the intensity of Ni2+due to PPy incorporation. However, the relative intensity of Niδ+in Ni12P5/PPy and Ni12P5increases drastically along with a shift of 0.5 eV towards higher binding energy. Due to the intervention of electronegative N, the binding energy of Niδ+shifts to generate a slightly more positive Niδ++. The electronegative N in PPy also promotes selective diffusion of Niδ+to the surface via electrostatic interaction. This Ni enriched surface increases the probability of binding strongly with undoped N of the polymer. The resulting surface enrichment of Ni is confirmed by increase in % atomic concentration in the composites by a factor of 2, obtained from XPS results (FIG.5F).

With enhanced conducting properties of Ni12P5/PPy and an electropositive Niδ++center, the disclosed core-shell catalysts are believed to have higher potential in electrocatalytic activity for oxygen evolution reaction (OER). Alkaline medium OER was performed on the core-shell catalysts using a rotating ring disk electrode (RDE) at 1600 rpm in 1M KOH. Electrochemical OER performance of Ni12P5/PPy showed improvement in total current density as well as the Tafel slopes (FIGS.5(a) and (b)), in comparison to Ni12P5. The decrease in Tafel slope from 60 mV/dec to 50 mV/dec for the composites emphasize the role of PPy and improved charge transfer properties. Hydroxide ions from the electrolyte can diffuse through the highly porous PPy shell to the extremely active N—Ni interface.

Instead of a metal phosphide surface, the active site now comprises highly polarized N—Ni. As evidenced from XPS results, the polymer composite has higher amounts of Ni at the interface, potentially increasing the number of active sites. The Ni—N bond extends the ability of electron transfer properties at the core shell interface by inducing a more positive nickel center. These sites can exhibit high affinity for chemisorption of negatively charged hydroxyl molecules. Therefore, a layer of PPy covering the Ni12P5catalyst core assists in: (a) a conductive pathway for fast electron transfer due to highly polarized nickel and nitrogen; (b) longer electron diffusion paths due to conjugation in the polymer; and (c) increase in active sites due to surface enrichment of Ni. Due to strong electron coupling interactions, synergic M-N sites can be created at the interface of the catalyst core and polymer shell.

As discussed above, the undoped N centers near the Ni12P5core can bind to positively charged Ni. However, the undoped N centers at the outer edge are still available for additional binding. Due to high affinity for N, transition metals are perfect candidates for anchoring at this site, forming a hybrid core-shell catalyst. In this example, Co was introduced at this site to not only enhance the density of active sites, but to also alter the overpotential of OER due to mixed M-N (Co—N and N—Ni) sites. To incorporate Co onto Ni12P5/PPy layer, an electrodeposition method was employed. This hybrid core-shell catalyst is denoted as Co@Ni12P5/PPy indicating the multiple layers.

Electrochemical diagnostic techniques such as cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were employed to characterize the properties of Co@Ni12P5/PPy. During electrodeposition, N atoms in pyrrole units may act as anchoring sites for immobilization of Co, introducing a new Co—N interface. A PPy shell with two different metals anchored to its edges may provide dual M-N sites with extended electron conjugation. The hybrid Co@Ni12P5/PPy drastically increases the current to 140 mA/cm2at 1.86 V (vs RHE). A low η10of 1.66 V arises due to dual synergic effect, Co—N and N—Ni. Incorporation of multiple M-N bonds in a conjugated PPy matrix alters the electron cloud density of the metals affecting the hydroxyl adsorption for OER.[24]To elucidate the role of PPy a control study was performed where Co was electrodeposited directly on a Ni12P5catalyst core, without the PPy shell as shown inFIG.7B(for comparison see Ref. 25). This worsened the overpotential required for OER for three possible reasons: (a) the absence of N—C electron donor character at the surface of the catalyst core decreases the anchoring sites for Co; (b) the absence of surface enrichment of Ni triggered by PPy can decrease the overall active sites for oxygen binding; and (c) Co introduction can alter the overpotential in Ni—Co bimetallic catalysts, depending on the extent of doping. However, the binding energies of intermediates are modified resulting in a decreased Tafel slope of 54 mV/dec.

For further understanding of the significant improvement in activity of the disclosed hybrid core-shell catalysts, cyclic voltammetric curves were studied for all catalysts. Ni based catalysts demonstrate a typical redox peak pertaining to Ni+2+(OH)2/Ni+3OOH as shown inFIG.7A. This reversible conversion reveals a conductive Ni+3OOH phase resulting in enhanced conducting properties (e.g., see Ref. 26 for comparison). There is an evident negative shift in the Ni2+/Ni3+oxidation peak potential via surface modification with PPy and Co. Modification of the surface with Co and PPy results in cathodic peak shift from 1.45 V to 1.42 V and 1.40 V in Co@Ni12P5and Co@Ni12P5/PPy, respectively. Without wishing to be bound by a particular theory, these data suggest a strong interaction between different layers of the hybrid core-shell catalyst exhibiting higher valence states of metal at a low potential. Metals in higher valence states are better active sites for OER (e.g., for comparison see Ref. 26). However, the effect of bimetallic active sites in Co@Ni12P5on OH−binding energies resulted in a high OER overpotential as shown inFIG.7B.

Other control experiments were performed to estimate the activity of PPy and Co@PPy (FIG.7B). The polymer by itself has no appreciable OER activity and requires a metal active site to produce a substantial amount of current. Though anchoring Co over PPy does improve the activity of PPy, the high overpotential due to Co is still not overcome. Consequently, in the disclosed hybrid core-shell catalyst, a dual synergic effect was exhibited by Co and Ni with PPy providing electronegative N sites. Increased number of M-N active sites significantly improved the OER performance along with a low Tafel slope of 48 mV/dec. The disclosed hybrid core-shell catalyst outperformed the traditional RuO2both in terms of onset potential and current density (FIG.7B).

Though many catalysts exhibit excellent potential in OER, repeated cycling results in metal dissolution leading to decreased performance. Subsequently, stability tests were performed on Co@Ni12P5/PPy as shown inFIG.7C. The data show that there is an enhancement in the total current density with a 10 mV decrease in the onset potential after 500 cycles of CV and 10000 s of chronoamperometry. Considering its excellent stability in alkaline conditions, it is significant to understand the charge transfer kinetics of the hybrid Co@Ni12P5/PPy catalyst.

Electrochemical impedance analysis was performed at OER operating potential on these catalysts as shown inFIG.7D. The experimental impedance data were fitted for each catalyst as per the equivalent electrical circuit is given in the inset ofFIG.7A. All catalysts exhibited a solution resistance (Rs) in the range of 6.8-7.1Ω. The trend in charge transfer resistance (Rct) obtained from the Nyquist plot is given as: Co@Ni12P5/PPy (2.93Ω)<Co@NiP (4.91Ω)<Ni12P5/PPy (8.51Ω)<Ni12P5(10.19Ω). The charge transfer resistance is greatly reduced in the hybrid Co@Ni12P5/PPy, which validates the low Tafel slope. The incorporation of higher amount of M-N bonds in the catalyst effectively improves the rate of charge transfer and electron diffusion in the catalyst. In addition to this, the calculated capacitance values obtained using Gamry analyst software indicates a 50-fold increase for the hybrid Co@Ni12P5/PPy in comparison to Ni12P5as shown in Table 1 below.

Since the unitless fitting parameter n is >0.78 for all catalysts, capacitance due to CPE can be considered to be quasi-equivalent to pure capacitance. Data in Table 1 shows an increase in capacitance from 0.15 mF/cm2(Ni12P5) to 3.24 mF/cm2(Ni12P5/PPy). Without wishing to be bound by a particular, it is believed that this trend can be attributed to the electronic interaction between the core-shell morphology. With the introduction of dual synergic effect via Co electrodeposition, there is an enhanced potential for charge storage in Co@Ni12P5/PPy with a capacitance of 8.20 mF/cm2. Without wishing to be bound by a particular theory, it is believed that the trend in CPE values can be directly extrapolated to electrochemically active surface area (ECSA) of the catalysts. From higher capacitive behavior of the hybrid core-shell catalyst, it can be concluded that it possess the highest surface area accessible for oxygen evolution. Intrinsic conjugation in the conducting polymer provides good electrical contact at the metal polymer interface on both edges. Without wishing to be bound by a particular theory, it is believed that this extended electron transfer property may be responsible for the enhanced conducting properties in the hybrid catalyst.

In the present example, a hybrid core-shell catalyst with a precise choice of individual components leading to excellent OER activity in alkaline medium. This hybrid Co@Ni12P5/PPy catalyst features excellent current density at low overpotential and long-term stability. Enhanced activity is primarily due to dual synergic charge transfer effects between the M-N sites. Electronegative N in PPy effectively induces surface enrichment of Niδ++at the Ni12P5/PPy interface. This strong electron coupling between different layers decreases the charge transfer resistance to 2.93Ω with a low Tafel slope achieving faster kinetics. Utilizing only earth-abundant elements and materials, this catalyst outperforms the state of art RuO2. This work offers the possibility of employing PPy shell as an excellent constituent for creating multiple M-N active sites, introducing a new class of polypyrrole analogues for enhancing the activity of metal-based catalysts.