Xiojun Zeng,Hiqi Zhng,Xiofeng Zhng,Qingqing Zhng,Yunxi Chen,＊＊＊,Ronghi Yu,Mrtin Moskovits

a School of Materials Science and Engineering,Jingdezhen Ceramic University,Jingdezhen,333403,China

b Guangdong Academy of Science,Guangdong Institute of New Materials,National Engineering Laboratory for Modern Materials Surface Engineering Technology,The Key Lab of Guangdong for Modern Surface Engineering Technology,Guangzhou,510650,China

c School of Materials Science and Engineering,Beihang University,Beijing,100191,China

d Department of Chemistry and Biochemistry,University of California Santa Barbara,Santa Barbara,CA,93106,United States

Keywords:Multi-size Multi-component Phyllosilicate SiO2/CoxP heterostructure OER

ABSTRACT Rational design of electrocatalysts is important for a sustainable oxygen evolution reaction(OER).It is still a huge challenge to engineer active sites in multi-sizes and multi-components simultaneously.Here,a series of CoxP nanoparticles(NPs)confined in an SiO2 matrix(SiO2/CoxP)is designed and synthesized as OER electrocatalysts.The phosphorization of the hydrolyzed Co-phyllosilicate promotes the formation of ultrasmall and small Co2P and CoP.These are firmly confined in the SiO2 matrix.The coupling of multi-size and multi-component CoxP catalysts can regulate reaction kinetics and electron transfer ability,enrich the active sites,and eventually promote the intrinsic OER activity.The SiO2 matrix provides abundant porous structure and oxygen vacancies,and these facilitate the exposure of active sites and improve conductivity.Because of the synergy and interplay of multisized/component CoxP NPs and the porous SiO2 matrix,the unique SiO2/CoxP heterostructure exhibits low overpotential(293 mV@10 mA cm-2),and robust stability(decay 12 mV after 5000 CV cycles,97.4% of initial current after 100 h chronoamperometric)for the OER process,exceeding many advanced metal phosphide electrocatalysts.This work provides a novel tactic to design low-cost,simple,and highly efficient OER electrocatalysts.

1.Introduction

Extensive dependence on fossil energy(oil,coal,natural gas,etc.)has caused severe environmental pollution and energy crises,urgently promoting the development of sustainable new energy,and making renewable energy one of the preferred ways to address the energy crisis[1–3].Hydrogen,which delivers no carbon footprint and has the merit of high energy density,can be produced from electrochemical water splitting(EWS)and is recognized as an attractive and effective renewable energy source[4,5].However,the slow reaction kinetics of the anodic oxygen evolution reaction(OER)and the existing high-cost of noble-metal OER electrocatalysts(Ru/Ir and its oxides)have greatly hindered the large-scale application of EWS[6,7].Nevertheless,it is desirable to obtain cost-effective and catalytically active non-noble metal OER electrocatalysts.

Among various cost-effective OER electrocatalysts,transition metal phosphides(Fe2P,Co2P,Ni2P,etc.)have been identified as promising because of their metallic properties and efficient catalytic performance[8–10].Generally,a single-composition catalyst exhibits relatively low activity because of the specific affinity for oxygen reaction intermediates(＊OH,＊O,＊OOH)[11].Based on the Sabatier criterion,a high activity catalyst needs neither too weak nor too strong interactions with the reaction intermediates[12].Hence,combining multiple active catalysts with different bonding effects,such as metal alloys or metal phosphides with adjustable element ratios,is a favorable strategy for realizing high activity[13–15],due to the optimized catalyst-reactant interaction.Forexample,Chen at al.prepared CoP/Co2P hybrids[16],which provided multi-components,resulting in improved conductivity and substrate adsorption capability,ultimately showing enhanced OER performance.Similar component engineering optimizations were also discovered in OER electrocatalyst of CoP–Co2P nanocrystals encapsulated by P-doped carbon(PC)and combined with P-doped graphene(PG)[17].Liu et al.designed a Mn-doped CoP that shows excellent alkaline OER activity[18].Importantly,Mn doping can increase the gap state near the Fermi level of the active O site,promoting the deprotonation of＊OH to＊O and reducing the energy barrier of the rate-determining step.Apart from component engineering,reducing the particle size of active species and stabilizing the structural integrity of active species is also an efficient approach to improve the electrocatalytic performance by providing abundant low-coordinated sites and stable sites[19,20].Owing to the construction of rich coupling interfaces and the synergistic effect of rich heterostructures,multi-level particle size can regulate reaction kinetics and electron transfer ability,thereby exhibiting ideal OER activity[21,22].In addition,creating a porous structure and abundant oxygen vacancies in a matrix can effectively expose the active sites and improve conductivity[23,24].For instance,Song et al.reported Ru and Ni co-doped CoP(Ru,Ni–CoP)porous nanofibers with low overpotential at high current density[25].Hierarchical porous structures can present highly exposed active sites and facilitate fast mass transfer.To this end,it is essential to design porous matrix and metal phosphides with rational component and size grading by adjusting the combination of heterostructures,engineering active sites,optimizing energy adsorption,and accelerating the exposure of active sites.

Inspired by the high porosity and confinement effects of phyllosilicate,we transformed Co-phyllosilicate and inserted it into SiO2/CoxP heterostructures,in which both ultrasmall and small Co2P/CoP are tightly confined in the porous SiO2matrix with abundant oxygen vacancies.Benefiting from porous structure and the confinement effect of SiO2matrix,multi-size/component CoxP active sites are realized.In terms of the synergy and interplay of multi-sized/component CoxP and porous SiO2,the SiO2/CoxP electrocatalyst exhibits robust activity and excellent long-term stability toward OER.

2.Experiments

2.1.Synthesis of Co-phyllosilicate

To synthesize Co-phyllosilicate,first,8 mmol of tetraethyl orthosilicate(TEOS)and 8 mmol of cobalt nitrate hexahydrate(Co(NO3)2?6H2O)were dissolved in a mixture of 30 mL of deionized water and 10 mL of ethanol.After stirring for 15 min,4 mL of concentrated ammonia hydroxide(NH4OH,25%~28%)was added slowly and continually stirred at room temperature for 8 h.The resulting mixture was collected by centrifugation and washed several times with deionized water and ethanol.The obtained product was dried in a vacuum oven overnight at 90°C for 24 h.For comparison,we optimized the additional amount of cobalt nitrate hexahydrates(4 mmol,16 mmol)to further analyze the appropriate ratio of the catalyst and ensure that other parameters remain unchanged.Phyllosilicate was prepared by a similar method except for the addition of Co(NO3)2?6H2O.

2.2.Synthesis of SiO2/Co3O4 catalyst

0.4 g of the above Co-phyllosilicate product was placed in a porcelain boat and heat-treated in a muffle furnace.The sample was calcined at 700°C for 2 h under air atmosphere,and the heating rate was 5°C min-1.For comparison,an SiO2sample was prepared by calcining phyllosilicate under the same conditions.

2.3.Synthesis of SiO2/CoxP catalyst

The above Co-phyllosilicate(0.3 g)and sodium hypophosphite monohydrate(3 g,NaH2PO2?H2O)were put at two separate positions in a porcelain boat,with NaH2PO2?H2O at the upstream side of the tube furnace.After 30 min of nitrogen gas flow,the tube furnace was heated to 500°C at a heating rate of 2°C min-1,and maintained at this temperature for 2 h under a nitrogen atmosphere.To achieve the higher activity catalyst,we have also optimized the phosphorization temperature,such as phosphating Co-phyllosilicate at 300°C,700°C,and 900°C.

2.4.Alkaline etching of SiO2/CoxP catalyst

To investigate the role of the matrix SiO2,we performed alkali etching on the SiO2/CoxP catalyst.Briefly,a certain amount of SiO2/CoxP catalyst phosphate at 500°C was dispersed in NaOH aqueous solution(0.5 M,100 mL),and stirred at 90°C for 6 h.The mixture was collected by centrifugation and washed twice with deionized water.The obtained product was dried in a vacuum oven at 80°C for 24 h.

2.5.Characterization

The microstructures and elemental content of the catalysts were observed by scanning electron microscope(SEM,JSM-6700F,JEOL,Japan)and energy dispersive X-ray spectroscopy(EDS,Oxford,Xplore).Transmission electron microscopy(TEM)images and EDS mapping were recorded on a transmission electron microscope(JEM-2100F,JEOL).The crystal phases of the catalysts were analyzed with X-ray diffraction(XRD,D8-Advance,Bruker,Germany).The chemical composition was investigated by X-ray photoelectron spectroscopy(XPS,Thermo escalab 250Xi).N2adsorption-desorption isotherms were performed on a Micromeritics surface area analyzer(BRT,ASAP2020 M,Micromeritics,America)to evaluate the Brunauer-Emmett-Teller(BET)specific surface area and pore size of the catalysts.

2.6.Electrochemical measurements

The electrochemical performance of the catalysts was analyzed in a three-electrode battery controlled by a CHI760E electrochemical workstation equipped with a rotation apparatus(RRDE-3A,ALS Inc.,Tokyo,Japan).The electrolyte,counter electrode,and reference electrode were KOH solution(1 M),platinum wire,and saturated silver/silver chloride electrode,respectively.The work electrode was prepared by dropping the catalyst ink on a glassy carbon electrode(3 mm in diameter).To prepare the work electrode,4 mg of catalyst was added to 1 mL of Nafion solution(5 wt%),and the catalyst ink was obtained by ultrasonic treatment for 2 h.Then the catalyst ink was dropped onto the glassy carbon electrode with a loading of 0.2 mg cm-2and dried naturally in the air.Cyclic voltammetry(CV),linear sweep voltammetry(LSV),and electrochemical impedance spectroscopy(EIS)curves were used to evaluate the electrochemical performance of the catalysts.CV curves were recorded at the scan rates of 20,40,60,80,and 100 mV s-1.LSV curves were operated at a scan rate of 5 mV s-1.The chronoamperometry was measured under a constant potential.EIS measurements were tested in the frequency range of 0.01 Hz–100 kHz at an AC voltage of 5 mV.According to the Nernst equation,the measured potentials versus Ag/AgCl were converted to a reversible hydrogen electrode(RHE)scale(ERHE=EAg/AgCl+0.0591＊pH+0.197).

3.Results and discussion

The synthesizing process of Co-phyllosilicate is presented in Fig.1.Briefly,To construct the Co-phyllosilicate,TEOS was hydrolyzed in a basic liquor containing Co(NO3)2[26],and then the product was calcined or phosphated at 500°C to obtain the target catalyst.The synthesis of Co-phyllosilicate is based on the St¨ober method,in which ethanol is added to reduce solvent polarity,and the presence of NH4OH is to provide a sufficient concentration of NH4+ions[23].Therefore,ion pairs are important for their nucleation during the precipitation ofpoly(silicic acid),where long poly(silicic acid)chains nucleate before short chains(Fig.1a)[27].Specifically,the diffusion of OH-ions can catalyze the condensation reaction,leading to the cross-linking of poly(silicic acid)chains.The long polyelectrolytes preferentially nucleate first and capture numerous NH4+/Co2+ions together(Fig.1b).Then,the monomers and short poly(silicic acid)nucleate on the surface of the nanoparticles(NPs)to form a more cross-linked shell.However,the captured large amount of“base”will reverse the cross-linking reaction and thus the ion pairs will inhibit the cross-linking of poly(silicic acid)chains.Therefore,after phosphorization,the Co ions confined in SiO2

will be converted into CoxP(Fig.1c).The metal-support interaction will be formed between CoxP and SiO2support[28].Due to the confinement effect and the metal-support interaction,the formed CoxP NPs will be very small and stably anchored in the matrix.Meanwhile,the removal of NH4+ions and some surface groups will facilitate the formation of porous SiO2.The SiO2/Co3O4catalysts can be obtained by calcining the Co-phyllosilicate(Fig.1d).

Fig.1.Schematic illustration of the formation for Co-phyllosilicate(a,b),SiO2/CoxP(c),and SiO2/Co3O4(d).

Fig.2.(a)XRD patterns of Co-phyllosilicate,SiO2/Co3O4,and SiO2/CoxP catalysts.XPS spectra of(b)survey(insert of elemental contents for Co,P,Si,O,C),(c)Co 2p,(d)P 2p,(e)Si 2p,and(f)O 1s of SiO2/CoxP catalysts.

The crystal phases of Co-phyllosilicate,SiO2/Co3O4,and SiO2/CoxP catalysts were investigated by XRD.Two obvious peaks at around 34°and 60°are observed in the XRD pattern of the Co-phyllosilicate sample(Fig.2a).These belong to the Co3Si2O5(OH)4(JCPDS No.21–0872)phase,as reported in previous work[29,30].After calcination of Co-phyllosilicate in the air,the wrapped Co2+will be converted into cobalt oxide.The obtained cobalt oxide has a phase consistent with the Co3O4 phase(JCPDS No.42–1467)[31].Additionally,after phosphorization,the wrapped Co2+will be converted into cobalt phosphate.The diffraction peaks at 31.1°,40.8°,44.9°,48.4°,52.9°,and 55.5°can be ascribed to the(110),(111),(021),(120),(002),and(030)planes of Co2P(JCPDS:54–0413)[32],indicating the presence of high-purity Co2P.On the other hand,a broad peak appears near 28°.This peak is attributed to the conversion of the phyllosilicate to the SiO2phase during the phosphorization process.XPS was performed to analyze the electronic states of the SiO2/CoxP catalyst.The XPS survey in Fig.2b shows that the molar ratio of Co in the SiO2/CoxP catalyst is about 10.9%,which is 3.5 times that of P.It is observed that Co 2p spectra display two main fitted peaks at 781.2 eV and 797.1 eV and two satellite peaks at 786.5 eV and 803.3 eV(Fig.2c).This is consistent with reported values of Co2P,CoP,and surface oxidized species[33,34].For P 2p spectra(Fig.2d),a peak located at 133.7 eV is attributed to the P–O bond,and a peak at 130.0 eV is assigned to the metal-P bond,further suggesting the existence of CoxP.Furthermore,the Si 2p spectra(Fig.2e)were resolved into three peaks at 101.8 eV,103.1 eV,and 103.8 eV,corresponding to SiO,Si2O3,and SiO2[35],demonstrating some oxygen in the SiO2matrix.It is noted that four peaks can be separated in the O 1s spectra(Fig.2f).The XPS peaks at 530.3 eV,531.3 eV,532.0 eV,and 532.9 eV are attributed to Metal-O,oxygen vacancy,Si–O,and P–O bands[24,30].This further reveals the formation of oxygen vacancy in the matrix,which is preferable to electrocatalytic activity.

The morphology of the as-fabricated Co-phyllosilicate,SiO2/Co3O4,and SiO2/CoxP catalysts are shown in Fig.3 as observed by the SEM.Fig.3a and S1 illustrate the irregular NPs with a size of 80–100 nm and wrinkled surface for the Co-phyllosilicate catalyst.The Co-phyllosilicate delivers a thick flaky-like structure on its surface.After calcination in air,some thinner flaky structures appear on the surface of NPs(Fig.3b and S2).These are caused by further shrinkage derived from the volatilization of the water and part of the functional groups in the Co-phyllosilicate.Notably,after phosphorization,the wrinkled NPs become porous NPs(Fig.3c and S3).This facilitates the exposure of active sites and promotes electrolyte contact.Fig.3d shows the N2adsorption/desorption isotherm and pore size distribution of the SiO2/CoxP catalyst.The BET specific surface area of SiO2/CoxP was determined to be 40.2 m2g-1.The pore size distributions shown in the inset of Fig.3d shows that SiO2/CoxP contains a large number of mesopores(2.6 nm,48.4 nm),which further suggests a porous structure for the SiO2/CoxP catalyst.This unique mesoporous structure can expose more active sites,thereby promoting electron and mass transfer in the catalytic process.

TEM is used to further inspect the internal structure and material composition of SiO2/CoxP catalysts.Fig.4a and S4 reveal that some small CoxP NPs(with darker contrast)with a size of 20–50 nm are anchored on the SiO2matrix.Moreover,the SiO2matrix shows many mesopores,asmarked in the red circle in Fig.4a.Fig.S5 further confirms the porosity of the SiO2matrix,and this is in good agreement with the above SEM and BET results.These porous structures can facilitate the penetration of electrolytes and the transmission of ions.Most importantly,there are many ultrasmall CoxP NPs with a size~2.5 nm that are also confined in the SiO2matrix,as marked in the blue circle in Fig.4b.The diffusion of the Co species is physically restricted during the heat treatment process because of the confinement effect of silicate.The HRTEM image in Fig.4c reveals the strong heterointerface existing between small CoxP NPs and the SiO2matrix.The lattice fringes of 0.22 nm,0.32 nm,and 0.35 nm(marked as a pink line)match well with the corresponding(121),(101),(001)planes respectively of Co2P[33,36,37].The lattice fringes of 0.2 nm and 0.37 nm(marked as an orange line)match well with the corresponding(112)and(101)planes respectively of CoP[34,38].Selected area electron diffraction(SAED)in Fig.4d(region A of Fig.4c)and e(region B of Fig.4c)also confirmed the coexistence and their poly-crystalline structure of Co2P and CoP.The existence of CoxP of different components can enrich the active site species.Importantly,the confined ultrasmall NPs are also assigned to the Co2P(0.18 nm of(002)plane,0.25 nm of(210)plane)and CoP species[33,37].Furthermore,the lattice fringes of 0.31 nm,0.21 nm,and 0.19 nm(marked as a yellow line)in Fig.4c and e are very close to those of Co2P and CoP,and are caused by the changes in the ratio of Co and P elements.The composition information of the as-designed SiO2/CoxP catalysts is shown in Fig.4g.The elements of Co,P,Si,and O are distributed throughout the entire catalyst uniformly.The as-prepared SiO2/CoxP catalysts enable a stable catalyst-support structure,a porous structure,multi-sized catalyst active sites,and abundant active site species.

Fig.3.SEM images of(a)Co-phyllosilicate,(b)SiO2/Co3O4,and(c)SiO2/CoxP catalysts.(d)N2 adsorption-desorption isotherms(insert of pore size)of SiO2/CoxP catalysts.

It is expected that the SiO2/CoxP catalyst with stable catalyst-support structure,porous structure,multi-sized active sites,and abundant active species will deliver robust catalytic activity.Hence,the OER performance of the SiO2/CoxP catalyst was investigated.As shown in Fig.5a,the assynthesized SiO2/CoxP catalyst only needs 1.523 V(vs.RHE)to reach the current density of 10 mA cm-2.This is lower than that of commercial RuO2(1.540 V vs.RHE).The Co-phyllosilicate and SiO2show very poor OER activity because of the absence of sufficient active sites(Fig.5a and S6).The small overpotential of 293 mV(@10 mA cm-2)of the SiO2/CoxP catalyst is better than many advanced OER catalysts,such as the recent advanced catalyst listed in Table S1.However,Co-phyllosilicate shows low OER activity(390 mV@10 mA cm-2),and the SiO2/Co3O4catalyst has almost no OER activity,suggesting the success of the phosphorization process.Clearly,too much or too little Co in the SiO2/CoxP catalyst is not conducive to catalyzing the OER process(Fig.S7).Similarly,the OER activity of SiO2/CoxP calcined at 700°C is higher than that of SiO2/CoxP calcined at 300°C,500°C,and 900°C(Fig.S8).Most importantly,after alkaline etching,the SiO2/CoxP deteriorated(320 mV@10 mA cm-2,Fig.S9),demonstrating the protection and synergy effects of porous SiO2matrix with oxygen vacancies.To explore the OER kinetics of the catalysts,the Tafel slope was calculated,as observed in Fig.5b.The SiO2/CoxP catalyst displays the lowest Tafel slope of 120 mV dec-1,indicating that the OER electrocatalytic kinetics on SiO2/CoxP are faster.The presence of multi-size catalysts can regulate reaction kinetics and electron transfer ability.

Electrochemical impedance spectroscopy(EIS)was performed to understand the electron-transfer kinetics of OER.Fig.S10 shows that the SiO2/CoxP catalyst enables a smaller charge-transfer resistance(9.6 Ω,Rct),revealing favorable reaction kinetics for SiO2/CoxP[39].The good electrical conductivity of the metal phosphide CoxP and its multi-component(CoP,Co2P)together contribute to the low Rctvalue[16].The contact resistance(Rs)for SiO2/CoxP catalyst is 11.8 Ω,confirming its small resistance and good ion response[40].At the same time,we carried out an electrochemical double layer capacitance(Cdl)test to gain insight into the intrinsic catalytic activity and estimate the electrocatalytic active surface area(ECSA)of the electrocatalysts[41].Notably,the SiO2/CoxP catalyst shows the largest Cdlvalue of 15.5 mF cm-2compared to SiO2/Co3O4(1.0 mF cm-2),and commercial RuO2(7.4 mF cm-2),as displayed in Fig.5c.These results indicate that the SiO2/CoxP catalyst can bring larger ECSA and provide more exposed active sites toward OER,consistent with the Tafel slope and EIS results.ECSA is calculated using the following equation:ECSA=Cdl/Cs,whereCsis the specific capacitance of a flat surface of 0.04 mF cm-2.As shown in Fig.S11a,SiO2/CoxP enables the highest ECSA value of 387.5 mF cm-2.The ECSA-corrected LSV curve in Fig.S11b shows that SiO2/CoxP exhibits the lowest onset potential and a higher current density at a given potential,indicating its higher intrinsic OER activity.

Fig.4.(a,b)TEM images,(c,f)HRTEM images,(d,e)SAED,(g)elemental mapping images of SiO2/CoxP catalysts.

Fig.5.(a)LSV curves,(b)Tafel slopes,and(c)Cdl of different catalysts toward OER.(d)CV cycling and(e)long-term cycling performance of SiO2/CoxP catalyst.(f)TEM images of SiO2/CoxP after long-term cycling.

Stability is an important indicator for the practical application of electrocatalysts.The LSV polarization curve of SiO2/CoxP before and after 5000 successive CV cycles clearly displays that the overpotential at 10 mA cm-2attenuates by only 12 mV,suggesting excellent long-term stability for SiO2/CoxP(Fig.5d).In addition,we further evaluated the stability of SiO2/CoxP under a continuous OER process at a constant potential of 1.523 V(vs.RHE)(Fig.5e).Interestingly,the current density remains at 97.4%over a testing period of 100 h,further indicating the superior stability of the SiO2/CoxP catalyst.It is noted that the structure of SiO2/CoxP maintains the integrity of ultrasmall and small CoxP NPs that are tightly confined in the SiO2matrix(Fig.5f).HRTEM images of SiO2/CoxP after the stability test show that the CoxP still maintains Co2P and CoP species(Fig.S12),further identifying the stable SiO2/CoxP electrocatalyst[42].

4.Conclusions

In summary,the SiO2/CoxP electrocatalyst,where multi-sized and multi-component CoxP NPs are tightly confined in porous SiO2matrix with abundant oxygen vacancies,has been successfully prepared as an OER catalyst by hydrolysis and phosphorization processes.The designed SiO2/CoxP delivers robust electrocatalytic OER performance with a small overpotential of 293 mV at 10 mA cm-2and excellent long-term stability(12 mV attenuation after 5000 CV cycles).The good OER performance can be attributed to the synergy and interplay of multi-sized/component CoxP NPs and the porous SiO2matrix with abundant oxygen vacancies.The porous structures can facilitate the penetration of electrolytes and the transmission of ions.At the same time,the presence of multi-size and multi-component CoxP catalysts can regulate reaction kinetics and electron transfer ability,enrich the active sites,and eventually promote intrinsic OER activity.

Declaration of competing interest

None.

Acknowledgments

This work was supported by the Training Program for Academic and Technical Leaders of Major Disciplines in Jiangxi Province(No.20212BCJ23020),the Science and Technology Project of Jiangxi Provincial Department of Education(No.GJJ211305),the National Natural Science Foundation of China(No.51671010),and the National University Students Innovation and Entrepreneurship Training Program(No.202110408005).

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.nanoms.2022.03.002.