Phosphorus‐Modulated Generation of Defective Molybdenum Sites as Synergistic Active Centers for Durable Oxygen Evolution

It is well known that electrocatalytic oxygen evolution reaction (OER) activities primarily depend on the active centers of electrocatalysts. In some oxide electrocatalysts, high‐valence metal sites (e.g., molybdenum oxide) are generally not the real active centers for electrocatalytic reactions, which is largely due to their undesired intermediate adsorption behaviors. As a proof‐of‐concept, molybdenum oxide catalysts are selected as a representative model, in which the intrinsic molybdenum sites are not the favorable active sites. Via phosphorus‐modulated defective engineering, the inactive molybdenum sites can be regenerated as synergistic active centers for promoting OER. By virtue of comprehensive comparison , it is revealed that the OER performance of oxide catalysts is highly associated with the phosphorus sites and the molybdenum/oxygen defects. Specifically, the optimal catalyst delivers an overpotential of 287 mV to achieve the current density of 10 mA cm−2, accompanied by only 2% performance decay for continuous operation up to 50 h. It is expected that this work sheds light on the enrichment of metal active sites via activating inert metal sites on oxide catalysts for boosting electrocatalytic properties.


Introduction
Electrocatalysis has been recognized as one of promising strategies in sustainable energy conversion fields, in which the kinetics largely depend on the physicochemical properties of catalysts, such as active centers and surface chemistry. [1,2]s one of oxygen(O)-involving fourelectron (4e − ) reactions, oxygen evolution reaction (OER) can proceed via either an adsorbate evolution mechanism (AEM) or a lattice-oxygen-mediated mechanism (LOM) on the active centers. [3]For the AEM mechanism, a scaling relationship is identified between the adsorption energies of O-containing intermediates (e.g., OH*, O*, and OOH*) on the surfaces of OER catalysts, resulting in an upper limit on the activity. [4]For the latter one, the direct combination of two metal oxo centers is achieved to form the O-O bonding. [5]][10][11][12][13] Particularly, in some metal oxide electrocatalysts, high-valence metal sites are generally not the real active centers for electrocatalytic reactions.[21][22][23][24][25][26] However, the activation on the intrinsically inert metal sites as the real active centers remains challenging until now.29][30][31] As a proof-of-concept, MoO 3 catalysts are selected as a representative model, in which the intrinsic Mo sites are not the favorable active centers for alkaline OER.Via phosphorus (P)modulated defective engineering, which are achieved through phosphorization reactions at elevated temperatures under either O-rich (air) or O-deficient (Ar) environment, the inactive Mo sites could be regenerated as synergistic active centers for promoting OER.By virtue of comprehensive comparison on OER activities and structural examinations on valences and defects, it is revealed that the OER performance of oxide catalysts is highly associated with the P sites and the Mo/O defects, which are also theoretically evidenced on catalytic activity enhancement effect.Specifically, the optimal catalyst, which is obtained at a phosphorization temperature of 750 °C under Ar flow d , delivers an overpotential of 287 mV to achieve the current density of 10 mA cm −2 , accompanied by only 2% performance decay for continuous operation up to 50 h in the alkaline solution.This work thus provides a general perspective on rationally structural modification for activating inert metal sites as synergistic active centers during electrocatalysis.

Results and Discussion
To address this issue on intrinsically inert metal sites, in this work, the synergistic generation of Mo active centers in the MoO 3 catalyst is proposed to be achieved via the anionic P-induced defective engineering.During phosphorization at elevated temperatures, the P species can be easily implanted into crystal lattices and meanwhile cationic/anionic defects can be created for generating defective sites, which are crucial for optimizing surface microenvironment and promoting OER activities. [32]To verify this hypothesis, theoretical calculation techniques were used to examine the possible varieties on the electronic structures and to identify the real active centers by comparing the intermediate adsorption behaviors at Mo, P, and O sites.As demonstrated in Figure 1a, to simplify the calculation and stabilize structures, two modified MoO 3 structures, the P-site/Mo-defect (P-MoO 3 -Mo v ) and the P-site/O-defect (P-MoO 3 -O v ), were selected as target models, in comparison to the pristine MoO 3 and the solely P-site MoO 3 (P-MoO 3 ) as reference models.As illustrated in Figure 1b, the pristine MoO 3 is a typical semiconductor with a bandgap of ≈2 eV, in which the conduction and valence bands are primarily contributed by the Mo_d and O_p orbits, respectively. [33]After embedded with the solely P-site to produce the defect-free P-MoO 3 model, the semiconductor-like behavior was maintained but a reduced bandgap was identified compared to the pristine MoO 3 model.In the presence of cationic Mo-defect, the bandgap of the P-MoO 3 -Mo v model narrows into ≈1.5 eV and the valence bands are primarily contributed by both P_p and O_p orbits.If the O-defect is formed, the P-MoO 3 -O v model is changed into a conductor-like state.Figure 1c shows the calculated free energy profiles of MoO  However, the overpotentials of the O r -PMCC-X catalysts stabilize at approximate levels of ≈320 and 350 mV at 10 and 30 mA cm −2 , respectively.Also, compared with these P-site-free MoO 3 catalysts treated under O-rich or O-deficient condition (O r -MCC-X, or O d -MCC-X, X = 350, 550, 750, or 950 °C), the optimal phosphorization temperature is 750 °C under the Ar flow (Figure S1 and Table S2, Supporting Information).For example, the overpotential of the O d -MCC-750 catalyst at 10 mA cm −2 is 330 mV, which is much smaller than these obtained under the other temperatures (Figure S2, Supporting Information).Besides, Tafel plots confirm that most of catalysts equipped by P-site manifest good reaction kinetics, verified by a similar slope at ≈50 mV dec −1 (Figure S3 and Table S4, Supporting Information).
Then, to confirm the possible contribution of Mo sites for OER, the Mo-free catalyst (PCC-750) treated at 750 °C shows much inferior catalytic activities than the O d -PMCC-750 catalyst (Figure S5-S7, Supporting Information).Considering the effect originated from the conductive substrate, the nickel (Ni) foam was used to replace the CC substrate for producing the PM-Ni-750 reference catalyst, and the performance was less satisfactory in comparison to the O d -PMCC-750 catalyst obtained under the same conditions (Figure 2c,d; Figure S8 and Table S3, Supporting Information).Besides, the contribution of P sites is evidenced by the smaller charge transfer resistance (R ct ) in the electrochemical impedance spectroscopy As another crucial parameter for catalyst evaluation, the stability of the O d -PMCC-750 catalyst was assessed by using the chronopotentiometry method, as shown in Figure 2f.It can be concluded that the potential can be well maintained without obvious decrease after continuous operation for 50 h at 10 mA cm −2 .Further comparison with these reported electrocatalysts, the O d -PMCC-750 catalyst is comparable to these Mocontaining alkaline OER catalysts (Figure 2g; Table S4, Supporting Information). [19,25,34]s discussed above, the O d -PMCC-750 catalyst delivers the best OER performance.To illustrate the underlying reasons, the structural merits associated with phases, valences and defects are comprehensively examined and analyzed.Scanning electron microscopy (SEM) techniques were first applied to examine the distribution states of MoO 3 particles, in which the MoO 3 particles aggregately formed on the CC surfaces.It can be clearly observed that the particle distribution densities decrease as phosphorization temperature increases to 550 °C or above, particularly under the air flow (Figure S12 and S13, Supporting Information), which is primarily caused by the metal evaporation and irreversible loss of Mo atoms.Under the Ar flow, compared to the MCC-X catalysts (Figure S14, Supporting Information), the morphology of MoO 3 particles is changed into semi-spherical shapes with smooth surfaces for the O d -PMCC-X catalysts (Figure S15, Supporting Information).Figure 3a shows the uniform distribution of Mo sites for the O d -PMCC-750 catalyst.If an overhigh temperature of 950 °C was used, the undesired etching on CC surfaces occurred, resulting in porous structures and metal loss for the O d -PMCC-950 catalyst.
To accurately track phase transition upon annealing temperatures under Ar flowing, X-ray diffraction (XRD) measurements on Mo-based samples were conducted (Figure S16, Supporting Information).It is clearly evidenced that the primary phase is MoO 3 (PDF 05-0508) after annealed and a trace of Mo 4 O 11 (PDF 05-0337) metaphase also appears when the temperature was between 550 and 750 °C.The phosphorization process was conducted by using CC-based samples at the temperature of 350-950 °C.After modulated by P at the elevated temperature of 750 °C, the sample was denoted as O d -PMCC-750, in which the solely MoO 3 phase was identified.Transmission electron microscopy (TEM) images of the O d -PMCC-750 catalyst (Figure S17, Supporting Information) clearly show the presence of small and crystallized MoO 3 particles in the presence of both the crystallized and amorphous MoO 3 phases coexist (Figure 3b).Hence, contributed by the partial evaporation of Mo atoms and the reduction of P sites at 750 °C, the crystal structure of MoO 3 is changed into a defective state, which offers an opportunity for activating the Mo sites as the synergistic centers towards OER.
Subsequently, to elucidate the varieties on the Mo-O bonding states after changed into a defective state from a pristine one, crystal orbital Hamilton population (COHP) analysis was conducted. [35,36]As shown in Figure 3c,d Finally, in order to reveal the OER activity enhancement mechanism for the O d -PMCC-750 catalyst, structural characterizations and comparisons were conducted before and after OER.To exclude the influence from the CC substrate, the changes on the CC were tracked by potential-dependent Raman spectra operated from 1.0 to 1.7 V and then for CV cycles up to 2500, and no obvious change is identified from ≈1.3 V, indicating a stable CC substrate for supporting Mo-O centers (Figure 3e).Then, energy dispersive spectroscopic (EDS) mapping patterns of the O d -PMCC-950 catalyst before (Figure S18, Supporting Information), and after OER (Figure S19, Supporting Information) verify the loss of Mo and P during OER, which favors for maintaining the defective states to sustain a stable and long-term OER cycle life.
Besides, the chemical environment of the O d -PMCC-750 catalyst after OER was explored by using X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (NEXAFS) techniques.The comparison on the Mo 3d XPS spectra can draw some conclusions that are summarized as follows: i) as the temperature increases, the chemical bonding states of Mo in all samples move toward high oxidization valences under both air (Figure S20, Supporting Information) and Ar flow (Figure S21, Supporting Information), [37] and the overhigh temperatures lead to the loss of Mo sites accompanied by the formation of defects; ii) chemical bonding changes of Mo in the presence of P sites are accompanied by the amorphization of Mo species, endowing the O d -PMCC-X and O r -PMCC-X catalysts with Mo-P bonding states; [38,39] iii) compared to the O r -PMCC-X catalysts treated under the air flow, the chemical states are abundant for the O d -PMCC-X catalysts treated under the Ar flow.With respect to P 2p spectra, in the O d -PMCC-X catalysts (Figure S22, Supporting Information), the main bonding states of P are formed by coupled with O (P-O) and carbon (C-P). [32]At relatively low binding energy, the Mo-P bonding in the O d -PMCC-750 catalyst is identified (Figure 3f). [40]As for NEXAFS on the Mo L-edge of the O d -PMCC-750 catalyst (Figure 3g), before OER, the splitted and low-intensity Mo L-edge peaks (≈2529 eV) indicate the complex environment of Mo coordinated by both P and O sites, however, a dominate and broad peak appears at ≈2530 eV after OER, suggesting the dynamic structural disorder induced by continuous oxidization of Mo and P during OER.This can be further evidenced by the trace P K-edge signals (Figure 3h) and the changed O K-edge spectra (Figure 3i) of the O d -PMCC-750 catalyst after OER, the oxidation and the loss of P sites is more evident during OER.These results suggest that the P sites are more reactive during OER, and the removal of P sites could induce more defective sites, which is much beneficial to intermediate adsorption behaviors and thus catalytic activity enhancement.
To verify the practicability of the O d -PMCC-750 catalyst for overall water splitting, a two-electrode system consisting of the O d -PMCC-750 anode and the commercial Pt/C cathode was assembled in 1.0 M KOH.As compared in Figure 4a, only 1.58 and 1.69 V were required to achieve the current densities of 10 and 50 mA cm −2 for the Pt/C||O d -PMCC-750 system, outperforming the commercial Pt/C||RuO 2 two-electrode system (1.62 V for 10 mA cm −2 and 1.81 V for 50 mA cm −2 ).Also, the water splitting system based on the O d -PMCC-750 catalyst is superior to many reported noble metal-based systems (Figure 4b; Table S5, Supporting Information), [41] such as NiFe LDH/Ni foam (1.7 V) [42] and CoFeZr oxides/Ni foam (1.63 V). [43] Furthermore, the O d -PMCC-750 catalyst-based system could support continuous operation up to 52 h at 10 mA cm −2 (Figure 4c), better than the RuO 2 -based system (Pt/C||RuO 2 ) illustrating the superior stability.Even at a high density of 50 mA cm −2 for 50 h (Figure S23, Supporting Information), the advantage of the O d -PMCC-750 catalyst-based system was more obvious, far exceeding the Pt/C||RuO 2 system.

Conclusions
In summary, an effective strategy was proposed for activating the intrinsically inert Mo sites in the oxide electrocatalyst as the synergistic active centers towards OER.The resultant MoO 3 catalyst with P sites and Mo/O defects (O d -PMCC-750) obtained at a phosphorization temperature of 750 °C under the O-deficient Ar environment manifested a low overpotential of 287 mV at 10 mA cm −2 with only 2% performance decay for continuous operation up to 50 h in the alkaline solution and reached an amperelevel current density at 1.7 V.When assembled as an anode in the Pt/C||O d -PMCC-750 system for overall water splitting, only 1.58 and 1.69 V were needed to reach 10 and 50 mA cm −2 , respectively.The electrochemical performance demonstrated that

Experimental Section
Synthesis of O r -MCC and O d -MCC Catalysts: 200.0 mg (NH 4 ) 6 Mo 7 O 24 and 1.0 g water were dissolved into 13.0 g ethanol containing 0.2 g P123.Then, 13.0 mL ethylene glycol were added in the mixture under the continuous stirring for 24 h at the room temperature.After a piece of pre-cleaned CC (W0S1009, CeTech) soaked into the mixture, all were transferred into a stainless-steel autoclave (45 mL), and then heated to 170 °C and maintained for 3 h.When the reaction was finished, the CC was washed by water and ethanol and dried at 60 °C for 12 h, which was denoted as MCC.Finally, the MCC was further treated by annealing for 1 h under air (labeled as O r -MCC-X, X represents temperature, X = 350 °C, 550 °C, 750 °C, or 950 °C) or Ar (labeled as O d -MCC-X, X represents temperature, X = 350 °C, 550 °C, 750 °C, or 950 °C) atmosphere.

Synthesis of O r -PMCC and O d -PMCC Catalysts:
The as-prepared MCC were soaked in 1.0 M NaH 2 PO 2 solution for 12 h, and then dried at 60 °C, which was denoted as PMCC.Subsequently, the PMCC was further treated by annealing for 1 h under air (labeled as O r -PMCC-X, X represents temperature, X = 350 °C, 550 °C, 750 °C, or 950 °C) or Ar (labeled as O d -PMCC-X, X represents temperature, X = 350 °C, 550 °C, 750 °C, or 950 °C) atmosphere.
Synthesis of CC-750, PCC-750 and PM-Ni-750 Catalysts: For comparison, the reference samples were prepared by using the cleaned CC at an annealing temperature of 750 °C under Ar atmosphere, which was denoted as CC-750.When the CC was pre-treated in 1.0 M NaH 2 PO 2 solution, the corresponding samples were labeled as PCC-750, after annealing under the same conditions.Besides, considering the possible effect from CC, the PM-Ni-750 sample was prepared by using Ni foam as the substrate to replace the CC under the same synthetic conditions for O d -PMCC-750.

Figure 1 .
Figure 1.a) Schematic illustration of the catalyst models with the Mo-vacancy (P-MoO 3 -Mo v ) or O-vacancy (P-MoO 3 -O v ).b) Density of state (DOS) of MoO 3 , P-MoO 3 , P-MoO 3 -Mo v , and P-MoO 3 -O v models.c) Calculated free energy profiles of MoO 3 , P-MoO 3 , P-MoO 3 -Mo v , and P-MoO 3 -O v catalysts for OER at the U = 0 V.
3 , P-MoO 3 , P-MoO 3 -Mo v , and P-MoO 3 -O v models for OER.By comparison on the intermediate adsorption behaviors at the Mo, P, and O sites, three main conclusions can be summarized as follows.i) O sites: the Gibbs free energy at the first step (*OH) decrease from 3.9 eV in the pristine MoO 3 model to 2.2 eV in the P-MoO 3 -Mo v and P-MoO 3 -O v models; ii) P sites: the negative energy values for *OH in all models indicate the strong adsorption behaviors, and the energy barrier required at the rate-determining steps (RDSs) (*OOH) in the P-MoO 3 -Mo v and P-MoO 3 -O v models are similar (2.6-2.8 eV), much lower than that of the pristine MoO 3 model (4.2 eV); iii) Mo sites: the Mo2

Figure 2 .
Figure 2. a) iR-corrected linear sweep voltammetry (LSV) curves and b) overpotentials at different densities (10 and 30 mA cm −2 ) of the catalysts annealed at different temperatures in the O-rich (O r -PMCC-X) or O-deficient (O d -PMCC-X) environment for alkaline OER in a 1.0 M KOH solution at a scan rate of 5 mV s −1 .c) LSV curves of the O d -PMCC-750 and PM-Ni-750 catalysts for OER in 1.0 M KOH solution at the scan rate of 5 mV s −1 .d) Comparison on the overpotential of the O d -PMCC-750 and PM-Ni-750 catalysts at different current densities of 30, 100, and 300 mA cm −2 .e) EIS spectra of the O d -MCC-750 and O d -PMCC-750 catalysts for alkaline OER.f) Chronoamperometric curves of the O d -PMCC-750 catalyst for 50 h in the alkaline solution.g) Comparison of the O d -PMCC-750 catalyst with previously reported Mo-containing OER catalysts.
(EIS) of the O d -PMCC-750 catalyst (≈5 Ω) than that of the P-free O d -MCC-750 catalyst (≈16 Ω) (Figure 2e).Moreover, the O d -PMCC-750 catalyst possesses a large electrochemical surface area (ECSA), as indicated by the electrochemical double-layer capacitance (C dl ), in comparison to the O d -MCC-750 catalyst (Figure S9-11, Supporting Information).These results confirm that the enhanced OER activity of the O d -PMCC-750 catalyst is largely associated with the Mo and P sites.
, the positive and negative values of COHP indicate the bonding and anti-bonding states, respectively.After structural modifications with P sites and Mo/O defects, the occupied anti-bonding states for the Mo-O bond interactions under Fermi level obviously appear.As a quantified indicator for assessing the Mo-O bond strength, the absolute values of the integrated COHP (ICOHP) for MoO 3 , P-MoO 3 , P-MoO 3 -Mo v , and P-MoO 3 -O v models were calculated.Compared to the pristine MoO 3 with the largest one (2.55),all other models in the presence of P sites and/or Mo/O defects possess the reduced values, particularly for the P-MoO 3 -O v model (2.37).The lowest value suggests that the co-existence of P sites and O-defect could induce the weakest Mo-O bond, which can possibly facilitate electron diffusion and promote catalytic activities.

Figure 3 .
Figure 3. a) Scanning electron microscopy (SEM) and b) high-resolution transmission electron microscopy (HRTEM) images of the O d -PMCC-750 catalyst showing the co-existence of crystallized and amorphous phases.c,d) Crystal orbital Hamilton population (COHP) and integrated COHP (ICOHP) analysis of Mo-O bonding near the active sites of MoO 3 , P-MoO 3 , P-MoO 3 -Mo v , and P-MoO 3 -O v models.e) Potential-dependent Raman spectra of the O d -PMCC-750 catalysts during 1.0-1.7 V and 500-2500 CV cycles.f) High-resolution X-ray photoelectron spectroscopy (XPS) P 2p spectra in the O d -PMCC-750 catalyst.g-i) NEXAFS spectra at g) Mo L-edge, h) P K-edge, i) O K-edge in the O d -PMCC-750 catalyst before and after OER.

Figure 4 .
Figure 4. Electrocatalytic water splitting properties by using the optimal O d -PMCC-750 catalyst with regenerated Mo active-sites.a) LSV curves of the Pt/C||O d -PMCC-750 and Pt/C||RuO 2 for overall water splitting in 1.0 M KOH solution at a scan rate of 5 mV s −1 .b) Comparison on the operation voltage with these reported water splitting systems.c) Chronoamperometric curves of the Pt/C||O d -PMCC-750 and Pt/C||RuO 2 systems at 10 mA cm −2 for 52 h.

MoO 3 -
based catalysts outperformed most of the reported transition metal-based OER catalysts.This work thus sheds new light on the catalytic enhancement mechanisms and proposes a new proposal for designing oxide nanocatalysts by creating the synergistic active centers.