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Fenglin Wang, Zhicheng Zheng, Dan Wu, Hao Wan, Gen Chen, Ning Zhang, Xiaohe Liu, [Renzhi Ma](https://orcid.org/0000-0001-7126-2006)

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[Tunable Pt–NiO interaction-induced efficient electrocatalytic water oxidation and methanol oxidation](https://mdr.nims.go.jp/datasets/da3f6c53-c59d-42e3-839d-f9b4685690f6)

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Tunable Pt–NiO interaction-induced efficient electrocatalytic water oxidation and methanol oxidationChemicalScienceEDGE ARTICLEOpen Access Article. Published on 27 May 2024. Downloaded on 9/1/2024 3:24:05 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal  | View IssueTunable Pt–NiOaZhongyuan Critical Metals Laboratory, ZheR. China. E-mail: wanhao@zzu.edu.cn; liuxbSchool of Materials Science and Engineerinand Advanced Functional Materials of HuChangsha, Hunan 410083, P. R. ChinacResearch Center for Materials NanoarchitMaterials Science (NIMS), Tsukuba, Ibaraknims.go.jp† Electronic supplementary informatinformation, DFT calculations, XRD andpattern, HRTEM image, mass-normalizehttps://doi.org/10.1039/d4sc00454jCite this: Chem. Sci., 2024, 15, 10172All publication charges for this articlehave been paid for by the Royal Societyof ChemistryReceived 19th January 2024Accepted 26th May 2024DOI: 10.1039/d4sc00454jrsc.li/chemical-science10172 | Chem. Sci., 2024, 15, 10172–10interaction-induced efficientelectrocatalytic water oxidation and methanoloxidation†Fenglin Wang,ab Zhicheng Zheng,b Dan Wu,b Hao Wan, *a Gen Chen,bNing Zhang, b Xiaohe Liu *ab and Renzhi Ma *cMetal–support interaction engineering is considered an efficient strategy for optimizing the catalyticactivity. Nevertheless, the fine regulation of metal–support interactions as well as understanding thecorresponding catalytic mechanisms (particularly those of non-carbon support-based counterparts)remains challenging. Herein, a controllable adsorption–impregnation strategy was proposed for thepreparation of a porous nonlayered 2D NiO nanoflake support anchored with different forms of Ptnanoarchitectures, i.e. single atoms, clusters and nanoparticles. Benefiting from the unique porousarchitecture of NiO nanosheets, abundant active defect sites facilitated the immobilization of Pt singleatoms onto the NiO crystal, resulting in NiO lattice distortion and thus changing the valence state of Pt,chemical bonding, and the coordination environment of the metal center. The synergy of the porousNiO support and the unexpected Pt single atom–NiO interactions effectively accelerated mass transferand reduced the reaction kinetic barriers, contributing to a significantly enhanced mass activity of 5.59 AmgPt−1 at an overpotential of 0.274 V toward the electrocatalytic oxygen evolution reaction (OER) while0.42 A mgPt−1 at a potential of 0.7 V vs. RHE for the methanol oxidation reaction (MOR) in an alkalinesystem, respectively. This work may offer fundamental guidance for developing metal–loaded/dispersedsupport nanomaterials toward electrocatalysis through the fine regulation of metal–support interactions.IntroductionTremendous energy conversion technologies have beendirected towards renewable energy sources and carbonneutralization, including water electrolysis and direct methanolfuel cells (DMFCs).1 Hydrogen energy, the most eco-friendlyrenewable energy source, can be sustainably generated by con-ducting water electrolysis. However, the anodic multi-electronwater oxidation (i.e. oxygen evolution reaction, OER) possessessluggish kinetics manifested in a high energy barrier andoverpotential,2 which signicantly affects the efficiency of theoverall water splitting. Compared to hydrogen energy, renew-able methanol serves as another commonly utilized fuel thatngzhou University, Zhengzhou 450001, P.h@csu.edu.cng, Key Laboratory of Electronic Packagingnan Province, Central South University,ectonics (MANA), National Institute fori 305-0044, Japan. E-mail: MA.Renzhi@ion (ESI) available: ExperimentalSEM data, HAADF-STEM image, SAEDd CV curves for the MOR. See DOI:181can be easily stored and transported with existing infrastructureof petrol.3 DMFCs represent a kind of remarkable energyconversion device for the high-efficiency utilization of repro-ducible methanol through the oxidization of fuel, realizing thedirect conversion of chemical energy into electric energy.4,5Nevertheless, the development of effective anodic electro-catalysts for the sluggish methanol oxidation reaction (MOR,CH3OH + H2O / CO2 + 6H+ + 6e−) remains the bottleneck ofDMFCs.6,7 In addition, the MOR process involves a complex 6-electron transfer mechanism and multiple pathways, such asthe CO-containing pathway or others, which severely depend oncatalysts.6,7 In this regard, it is challenging and of scienticsignicance to design electrocatalysts with high activitiestoward the OER and MOR.8,9Precious platinum (Pt) has attracted great interest in theoxidation reaction eld due to its intriguing physicochemicalproperties and unprecedented catalytic efficiency.10,11 Strategiessuch as constructing ingenious geometrical nanostructures andmodulating the Pt valence state are aimed at enhancing Ptutilization and electrocatalytic activity, but still remain chal-lenging. Single-atom catalysts (SACs) have emerged as novelnanostructures with unique advantages in precisely controllingactive sites, which is attributed to the exposure of numeroussurface atoms, the tunable electronic structure and the coor-dinative environment.12 The typical synthetic methods such as© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://crossmark.crossref.org/dialog/?doi=10.1039/d4sc00454j&domain=pdf&date_stamp=2024-06-28http://orcid.org/0000-0002-4487-3538http://orcid.org/0000-0002-3033-0276http://orcid.org/0000-0003-1297-9597http://orcid.org/0000-0001-7126-2006https://doi.org/10.1039/d4sc00454jhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4sc00454jhttps://pubs.rsc.org/en/journals/journal/SChttps://pubs.rsc.org/en/journals/journal/SC?issueid=SC015026Fig. 1 Compositional and structural characterization of the porousNiO substrate and NiO–PtSA, NiO–Ptcluster, NiO–Ptparticle. (A) Sche-matic illustration of the fabrication procedure for porous NiO–PtSA,NiO–Ptcluster and NiO–Ptparticle catalysts. (B) XRD patterns of (I) a-Ni(OH)2-DS, (II) porous NiO (calcined at 700 °C), (III) NiO–PtSA, (IV)NiO–Ptcluster, and (V) NiO–Ptparticle. (C) SEM image of a-Ni(OH)2. (D)TEM image, (E) SAED pattern and (F) HRTEM image of NiO. (G) BETisotherm of porous NiO. (H) BJH-plot of pore volume vs. pore diam-eter, the inset is a HAADF image of porous NiO. (I) UV-visibleEdge Article Chemical ScienceOpen Access Article. Published on 27 May 2024. Downloaded on 9/1/2024 3:24:05 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineimpregnation, co-precipitation, sputtering and alloying havebeen employed for the doping of single atoms, showcasing theversatility and potential of SACs in catalysis.13–16Notably, the catalytic activity of an entire SAC is closelydependent on the anchored substrate. Besides paying attentionto single-atom center species, it is necessary to regulate thesurrounding environment to achieve the optimal electronicstructure and coordination by tuning the chemical bondsbetween single atoms and the substrate.17–21 For example, Ptsingle atoms supported by carbon black or carbon nanotubesexhibit inert catalytic activity, whereas Pt single atoms immo-bilized on RuO2 show superior catalytic mass activity andstability.12,22 Thereby, the selection of substrates for SACsimplies a complex and multi-perspective challenge in boostingthe SAC activity toward the OER and MOR, such as alloys,23MoS2,15 MXene nanosheets,24,25 metal nanoparticles16,26 andtransition metal oxides.13 Notably, NiO is a promising substratedue to its facile phase transformation into highly active NiOOHwith a layered double hydroxide structure during the catalyticoxidation process,27 which is conducive to providing a migra-tion channel for OER intermediates and thus facilitating theoxidative removal of COads on the activity sites in the MOR.Recently, metal–support interactions, mainly those betweenmetals and carbon substrates, have been massively studied forenhancing the electrocatalytic performance. For instance, Ptsingle atoms loaded on thiolatedmultiwalled carbon nanotubesare barely signicant to the MOR catalytic activity, while Ptclusters possess a small MOR overpotential,28 attributed to therequirement of at least three Pt atoms to form a combinedactive site for the MOR.29 The dispersion and size of Pt are vitalfactors in determining the metal–support interactions and thusoptimizing the electrocatalytic activity. Considering that singleatoms predominantly anchor on the surface of materials, thecharge state and distribution of SACs on the support directlyinuence Gibbs free energies of the crucial intermediatesinvolved in the catalytic process, thereby tuning the rate-determining steps involved in the OER and MOR. Hence,regulating the noble metal–NiO support interactions bycontrolling the metal size and nanoarchitecture is an effectiveapproach to modulate the electronic structure and bondingstate around single atom-based catalysts.Herein, a facile adsorption–impregnation strategy wasemployed to adjust the light condition and reaction time,facilitating the immobilization of different Pt nanostructures(single atoms, clusters, and nanoparticles) on the porous NiOnanoake substrate, and thus achieving NiO–Pt single atoms(NiO–PtSA), NiO–Pt clusters (NiO–Ptcluster) and NiO–Pt particles(NiO–Ptparticle) respectively. The porous architecture of 2Dnonlayered NiO offered abundant defects and accelerated masstransfer. The metal–NiO interactions and their effects on thecatalytic performance were systematically studied. Remarkably,the immobilized Pt single atoms effectively reduced the reactionkinetic barrier and enhanced the catalytic activity in an alkalinesystem. As a result, the as-constructed NiO–PtSA catalyst dis-played high OER performance with an enhanced mass activityof 5.59 A mgPt−1 at a current density of 10 mA cm−2, and an© 2024 The Author(s). Published by the Royal Society of Chemistryenhanced MOR peak mass activity of 0.42 A mgPt−1 at 0.7 Vvs. RHE.Results and discussionDiverse Pt nanostructures (i.e. single atoms, clusters or nano-particles) immobilized on a porous NiO nanosheet substratewere synthesized by a facile adsorption–impregnation method,as illustrated in Fig. 1A. First, a-type layered nickel hydroxideintercalated with anionic surfactant dodecyl sulfate (denoted asa-Ni(OH)2-DS) was prepared via an oil bath process. The basalreection series of the product well matched with a typical a-Ni(OH)2-DS layered structure with an interlayer spacing of2.6 nm (Fig. 1B(I)). A substantial quantity of uniform nanoakeswas observed in the representative scanning electron micros-copy (SEM) image (Fig. 1C). Second, the inuence of calcinationtemperature on the morphology and structure of the calcinedproduct was investigated in detail. Interlayer anionic DS− couldnot be completely removed at 600 °C (Fig. S1 and S2†), whilepores would form and lead to morphological defects ata calcined temperature of 800 °C (Fig. S3 and S4†). Therefore,the optimal NiO with standard crystal (JCPDS No. 65-2901,Fig. 1B(II)) and porous nanosheet morphology (Fig. S5†) wasobtained through calcination at an ambient temperature of 700°C. The porous feature of the NiO nanoakes was furtherconrmed by transmission electron microscopy (TEM)(Fig. 1D). A high-resolution TEM image (HRTEM) indicatedabsorption spectra of NiO and the three NiO/Pt samples.Chem. Sci., 2024, 15, 10172–10181 | 10173http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4sc00454jFig. 2 Atomic structure characterization of the as-prepared NiO–PtSA,NiO–Ptcluster, and NiO–Ptparticle. (A and B) AC-HAADF-STEM images ofNiO–PtSA. (C) Schematic illustration and ICP-OES of NiO–PtSA. (D andE) AC-HAADF-STEM images of NiO–Ptcluster. (F) Schematic illustrationand ICP-OES of NiO–Ptcluster. (G and H) AC-HAADF-STEM images ofNiO–Ptparticle. (I) Schematic illustration and ICP-OES of NiO–Ptparticle.Chemical Science Edge ArticleOpen Access Article. Published on 27 May 2024. Downloaded on 9/1/2024 3:24:05 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineclear lattice fringes with a lattice space of 0.24 nm, corre-sponding to the (111) planes of NiO (Fig. 1F). The selected areaelectron diffraction (SAED) pattern, as shown in Fig. 1E,revealed the polycrystalline nature of NiO nanoakes.The surface properties of NiO were investigated by Bru-nauer–Emmett–Teller (BET) characterization. The isotherms ofNiO showed a hysteresis loop with high relative pressure (P/P0)in the range of 0 to 1 according to the N2 adsorption–desorptionisotherms (Fig. 1G), and the BET surface area for NiO was thusestimated as 36.99 m2 g−1. Based on the related pore sizedistribution plot by the Barrett–Joyner–Halenda (BJH) method,the pore size of NiO nanoplates was predominantly distributedbetween 1 and 20 nm. The average pore diameter for adsorptionand desorption was 10.24 nm and 11.24 nm, respectively, asshown in Fig. 1H. The porous feature morphology of NiO maybe convenient to supply pores, defects and abundant activesites, making it an outstanding substrate. In the nal step, theNiO–PtSA, NiO–Ptcluster, and NiO–Ptparticle were prepared byimmersing porous NiO into the deionized water containingchloroplatinic acid (H2PtCl6) solution for spontaneous adsorp-tion. The evolution in the morphology and structure of thedifferent Pt nanostructure decorated catalysts was created bytuning the synthetic reaction time and light condition. Noobvious differences and crystal diffraction peaks of Pt for thethree catalysts were observed in the XRD patterns, whichmay bedue to the low Pt content (Fig. 1B(III)–(V)). However, the Ptpeaks show a gradually increased tendency according to the UV-visible absorption spectra (Fig. 1I), indicating that the Ptcontent increased in the sequence of NiO–PtSA, NiO–Ptcluster,and NiO–Ptparticle.No signicant differences were observed in the lamellarmorphologies of NiO–PtSA and NiO–Ptcluster catalysts based onthe visualized SEM images (Fig. S6A and S6C†), while thesurface of NiO–Ptparticle nanoakes was modied by some smallnanoparticles (Fig. S6E†). The high-angle annular dark-eldscanning transmission electron microscopy (HAADF-STEM)images (Fig. S6B and S6D†) further conrmed the porousnature of NiO–PtSA and NiO–Ptcluster nanoakes. The whitehighlights recorded in the HAADF-STEM observation of NiO–Ptparticle might be ascribed to Pt nanoparticles (Fig. S6F†). Thecorresponding STEM-EDX maps demonstrated uniformdispersion of Ni and Pt elements throughout NiO nanoakes.To further conrm the component of light spots, SAED andHRTEM of the three catalysts were conducted, as displayed inFig. S7–S9,† respectively. The lattice fringes and SAED patternsof NiO–PtSA and NiO–Ptcluster catalysts matched well with NiOcrystals, verifying the substrate as NiO. In contrast, the latticefringes of 0.22 nm were assigned to the Pt (111) plane for thenanoparticles on the NiO surface, affirming the bright spots asPt nanoparticles. Additionally, for comparison, pure Pt nano-particles were fabricated under the same conditions as those forNiO–Ptparticle without the addition of NiO (Fig. S10–S12†).To intuitively investigate the dispersion state of Pt,aberration-corrected high-angle annular darkled-scanningtransmission electron microscopy (AC HAADF-STEM) wasused to unambiguously contrast the sub-nanometer metalparticles. Fig. 2A and B display the AC-HAADF-STEM images of10174 | Chem. Sci., 2024, 15, 10172–10181NiO–PtSA, showing brighter spots (i.e. Pt atoms, marked by thegreen circles) that were atomically dispersed. Meanwhile, nobright Pt atoms were observed in between any two adjacentlattice fringes, revealing the absence of Pt atoms in the inter-stitial sites of the lattices. It was thus speculated that Pt atomssubstituted Ni sites, i.e., achieved the doping behavior in NiOlattices. The Pt single atom loading was further revealed by ICP-OES as 0.73 wt%, which may change the electronic structureand coordination of the center atoms due to the substitutioninto the crystal lattices, as illustrated in Fig. 2C. When the Ptloading increased up to 1.22 wt%, not only highly dispersed Ptsingle atoms but also some bright clusters were found in the AC-HAADF-STEM images of NiO–Ptcluster (Fig. 2D and E), indicatingthat Pt atoms could be gradually nucleated on the surface ofNiO nanoakes and grew into Pt clusters at such an increased Ptcontent, as shown in Fig. 2F. In contrast, aggregated spots of∼3 nm were observed in the AC-HAADF-STEM images of NiO–Ptparticle with a high Pt loading of 3.68 wt%, corresponding to Ptnanoparticles (Fig. 2G and H). The above results show that Ptmay be nucleated from single atoms to nanoparticles throughclusters on the surface of NiO nanoakes, as displayed inFig. 2I.The electronic state and chemical composition of the Ptatoms in different NiO/Pt catalysts were investigated by X-rayphotoelectron spectroscopy (XPS), as shown in Fig. 3A–C. InPt 4f spectra, three characteristic states of Pt were deconvoluted,which were ascribed to metallic Pt0 (70.9 eV, 74.2 eV), Pt2+(72.9 eV, 76.4 eV) and Pt4+ (78.35 eV), respectively. In particular,Pt atoms in NiO–PtSA mainly existed in the valence state of Pt2+,indicating that Pt was coordinated with O in the NiO crystal.Besides the Pt2+ coordinations, the signal of metallic Pt0 wasalso detected for NiO–Ptcluster, indicating that Pt was nucleated© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4sc00454jFig. 3 Electronic state and coordination characterization. XPS spectra of (A) Pt 4f, (B) Ni 2p and (C) O 1s for NiO–PtSA, NiO–Ptcluster, and NiO–Ptparticle. (D) XANES spectra, (E) the corresponding FT-EXAFS curves and (F) EXAFS wavelet transform plots of NiO–PtSA, Pt foil and PtO2.Edge Article Chemical ScienceOpen Access Article. Published on 27 May 2024. Downloaded on 9/1/2024 3:24:05 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineand grew into clusters. In contrast, the Pt nanoparticles in theNiO–Ptparticle catalyst were formed as Pt0, Pt2+ and Pt4+ species;herein Pt4+ might be derived from the adsorbed PtCl62− ions onthe surface of the samples.30,31 The XPS results of Pt wereconsistent with the AC-HAADF-STEM observations. Fig. 3Bdepicts the high-resolution Ni 2p spectrum, in which the peaksat 855.8 eV and 873.3 eV along with two weak satellite peaks at861.0 eV and 879.4 eV corresponded to the Ni 2p3/2 and Ni 2p1/2signals of NiO, respectively. This was assigned to the valencestate of Ni2+ by forming the Ni–O bond in the NiO crystal. The O1s spectra of the three catalysts were divided into two charac-teristic peaks of lattice oxygen (Olat, 529.3 eV) and oxygenvacancies (Ovan, 532.3 eV), in which the lattice oxygen wasformed by M–O (M = Ni and/or Pt) and the oxygen vacanciesoriginate from the porous interfacial defects in the NiOnanoplates.32For the in-depth study of the coordination environment andelectronic conguration of Pt single atoms in the NiO–PtSA, X-ray absorption ne structure (XAFS) spectroscopy was carriedout. The Pt L3-edge normalized X-ray absorption near-edgestructure (XANES) spectrum of NiO–PtSA is signicantly distin-guished from those of Pt foil and PtO2 (Fig. 3D), in which theintensity of white-line peaks at 11 568 eV corresponded to the© 2024 The Author(s). Published by the Royal Society of Chemistrytransfer of the Pt 2p3/2 core electron to 5d states, and thus wasused as an indicator of Pt 5d band occupancy.33,34 The decreasedwhite-line intensity corresponded to the increased Pt 5d occu-pancy. Thus, lower 5d occupancy indicated the less charge lossof the Pt single atoms aer coordinating with the substrate incomparison with Pt foil. This implied that Pt in NiO–PtSA waspositively charged due to the electron transfer from Pt to thesurrounding O atoms in the NiO substrate.24,35 The charge stateof Pt in NiO–PtSA was closely affected by the coordination withthe substrate. The atomic coordination conguration of Pt wasfurther revealed by the extended XAFS (EXAFS) at the Pt L3-edge,as shown in Fig. 3E. The absence of the typical Pt–Pt peak at 2.65Å for NiO–PtSA strongly indicated that Pt is in the atomicallydispersed single atom state, matching well with the AC-HAADF-STEM observations. Specically, the dominant peak of Pt–Ocoordination for NiO–PtSA slightly shied to 1.83 Å in compar-ison with 1.62 Å of PtO2 in the R-space spectrum, indicating theformed Pt–O as Pt2+ instead of Pt4+, which is consistent with theXPS results. As shown in Fig. 3F, the wavelet-transform plots ofNiO–PtSA showed a maximum of about 10.5 Å which was lowerthan 11.4 Å of Pt foil but higher than 7.4 Å of PtO2, furtherconrming the coordination conditions of Pt–O for Pt singleatoms in the NiO–PtSA sample.Chem. Sci., 2024, 15, 10172–10181 | 10175http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4sc00454jChemical Science Edge ArticleOpen Access Article. Published on 27 May 2024. Downloaded on 9/1/2024 3:24:05 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineThe OER electrocatalytic activities of as-fabricated NiO/Ptcatalysts were measured in 1.0 M KOH electrolyte, as dis-played in Fig. 4. The NiO–PtSA catalyst possessed the highestOER activity with a rather low overpotential of 0.274 V delivering10 mA cm−2 in comparison with the other catalysts (Fig. 4A).Furthermore, the NiO–PtSA catalyst exhibited a smaller Tafelslope of 83 mV dec−1 and an enhanced double-layer capacitance(Cdl) of 58 mF cm−2 compared to the hydroxide precursor andthe other two NiO/Pt catalysts featuring different Pt nano-structures (Fig. 4B and C), suggesting that atomically dispersedPt single atoms on the NiO substrate could supply moreaccessible active sites for the OER. Moreover, when normalizedwith the Pt loading mass, NiO–PtSA delivered a higher current of5.59 A mgPt−1 at an overpotential of 0.274 V, which was 2.64 and9.63 times that of NiO–Ptcluster (2.11 A mgPt−1) and NiO–PtparticleFig. 4 Electrocatalytic alkaline OER performance of the catalysts in 1.0kinds of NiO/Pt catalysts, the inset shows the overpotentials of different cEstimation of Cdl by fitting a linear regression at different scan rates. (D) TO2. (G) Chronoamperometric curves of NiO–PtSA. (H) Nyquist plots of thNiO–PtSA at 5 mV s−1. (J) Nyquist plots of NiO–PtSA at different potentialsshowing the relationship between surface species, interfaces and reactio10176 | Chem. Sci., 2024, 15, 10172–10181(0.58 A mgPt−1), respectively (Fig. 4D), indicating a much highermass activity for NiO–PtSA. The above results highlight that theincorporation of NiO with a minimal amount of Pt single atomscan extremely maximize the mass activity toward alkaline OER,signicantly reducing the Pt cost while increasing its utiliza-tion. Moreover, the NiO–PtSA possessed a higher turnoverfrequency (TOF) and O2 production rate compared with theother as-prepared catalysts according to the calculations andanalyses (Fig. 4E and F). Furthermore, the NiO–PtSA catalystdemonstrated high durability in continuous electrocatalysis,with negligible potential uctuation detected over 20 h duringthe OER at current densities of both 10 mA cm−2 and 50 mAcm−2 (Fig. 4G). The structural and morphological character-ization of the NiO–PtSA catalyst aer the stability test at 10 mAcm−2 suggested negligible changes aer a long-term alkalineM KOH electrolyte. (A) OER polarization curves of a-Ni(OH)2, NiO andatalysts delivering a current density of 10 mA cm−2. (B) Tafel slopes. (C)he mass activity of NiO/Pt catalysts. (E) TOFs and (F) production rate ofe catalysts, the inset shows the fitting equivalent circuit. (I) CV curve of. (K) Bode plots for NiO–PtSA during the OER. (L) Schematic illustrationns for NiO–PtSA during the OER.© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4sc00454jEdge Article Chemical ScienceOpen Access Article. Published on 27 May 2024. Downloaded on 9/1/2024 3:24:05 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineOER process (Fig. S13 and S14†). Meanwhile, the charge trans-fer resistance (Rct) of NiO–PtSA was lower than those of precur-sors and Pt-based catalysts (Fig. 4H and Table S1†), which wasmainly due to the optimized electronic structure and coordi-nation environment originating from the atomically Pt dopantin the NiO crystal. Fig. 4I shows the cyclic voltammetry (CV)curve of NiO–PtSA, in which a pair of redox peaks were observedbefore the OER region, revealing that the catalyst was electro-oxidized before the OER process. To probe the interface elec-tron transfer reactions, operando EIS was conducted to obtainthe Nyquist (Fig. 4J) and Bode plots (Fig. 4K) during the OER forNiO–PtSA. The charge transfer resistance signicantly decreasedwith the increase of potential, accompanied by distinct electro–oxidation reaction and OER. The equivalent circuit diagram andmodel, as presented in Fig. 4L, represent the situations beforeand/or aer electro–oxidation reaction.36,37 Based on theintrinsic consistency of the equivalent resistance with thereaction and current density of the NiO–PtSA catalyst, it wasspeculated that the electro-oxidation reaction (in the potentialwindows higher than 1.295 V) took place at the high-frequencyinterface while the OER (in the potential region higher than1.470 V) occurred at the low-frequency interface.To deeply understand the structural transition during theOER, in situ Raman electrochemical investigation was con-ducted on NiO–PtSA during the overpotential sweep from 0 to400 mV (Fig. 5A). Five characteristic peaks appeared at 539, 735,909, 1091 and 1507 cm−1 under no bias condition, which matchwell with the rst-order longitudinal optical (LO), transverseoptical (2TO), LO+TO, 2LO and 2 M modes of NiO, respec-tively.38,39 At an overpotential of 0 V (1.23 V vs. RHE), additionalpeaks slightly appeared at 477 and 558 cm−1, corresponding tothe g-NiOOH phase.40,41 With increased overpotentials, theFig. 5 Mechanism and theoretical calculations on the OER. (A) The in sitThe survey XPS of NiO–PtSA after the OER stability test. The inset is the astability test, obtained from the XPS. (C) Free energy diagrams. ReactionPtSA and (F) Pt site in NiO–Ptcluster.© 2024 The Author(s). Published by the Royal Society of Chemistrysignal intensity of g-NiOOH intensied, while that for NiOdecreased, verifying that introducing Pt single atoms facilitatesa structural transition at a lower potential and plays the key rolein boosting subsequent OER kinetics.XPS was engaged to analyze the surface chemical states andelement components of NiO–PtSA aer the OER stability test(Fig. 5B and S15†). The survey XPS spectrum identied Ni, Pt, Cand O with 11.18, 0.03, 43.00 and 45.8 at%, respectively (theinset of Fig. 5B). Accordingly, the proportions of Pt and Ni were0.35 at% and 99.65 at% (Table S2†), respectively, indicating thatthe Pt content did not show signicant loss aer OERmeasurement compared to that in the initial catalyst (0.73 wt%,0.34 at%). The O 1s spectrum still exhibited two peaks for theoxygen lattice and vacancies, respectively. The Ni 2p spectrumalso deconvoluted to two sets of Ni 2p3/2 and Ni 2p1/2 assigned toNi2+, indicating that the structural transformation of theNiOOH phase into initial NiO is highly reversible. The Pt 4fspectrum was assigned to Pt2+ without any other peaks, indi-cating that Pt single atoms are relatively robust and noagglomeration occurs due to the absence of metallic Pt peaksaer the OER.The energy barrier is a key descriptor to evaluate the rate-determining step for the OER. Density functional theory (DFT)calculations were introduced to calculate the Gibbs free energyon different active sites for pure NiO, NiO–PtSA, and NiO–Ptcluster.Fig. S16† exhibits the intermediate transformation pathway(OH*, O* and OOH*) in the OER process for Ni sites of the pureNiO catalyst, through which the rate-determining step of pureNiO was determined as the deprotonation of OH*. Moreover, thereaction dynamics on the Ni and Pt active sites of NiO–PtSA werefurther investigated (Fig. 5C–E). The rate-determining step wasascribed to the adsorption of OH* to form OOH* with an energyu Raman spectra of NiO–PtSA under various applied overpotentials. (B)tomic content of Ni, Pt, C and O in the NiO–PtSA catalyst after the OERpathways for the OER on the (D) Ni site in NiO–PtSA, (E) Pt site in NiO–Chem. Sci., 2024, 15, 10172–10181 | 10177http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4sc00454jChemical Science Edge ArticleOpen Access Article. Published on 27 May 2024. Downloaded on 9/1/2024 3:24:05 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinebarrier of 1.62 eV on the Pt sites of NiO–PtSA for the OER, whichwas lower than 1.75 eV for the deprotonation of OH* on Ni sites,indicating that Pt single atoms in NiO–PtSA tended to be theactive sites for the OER and were more favorable for O2 genera-tion. When the Pt atoms aggregated into a cluster state, the rate-determining step of the NiO–Ptcluster on Pt sites changed back tothe deprotonation of OH* with an energy barrier of 1.95 eV(Fig. 5C and F), which was higher than the Pt single atom state,suggesting that the Pt clusters might be less conducive to theOER process. Therefore, it could be concluded that tunablemetal–support interactions originating from the size anddispersion of Pt directly affected the OER process. In particular,Pt single atoms inNiO nanoakes reduced the energy barrier andthus promoted the OER process.The MOR performance of the as-prepared catalysts wastested in a mixed 1.0 M KOH + 1.0 M CH3OH aqueous electro-lyte. The catalytic activities were evaluated from CV curves ata scan rate of 50 mV s−1, as recorded in Fig. 6A and S17.† PureNiO exhibited no obvious current response toward the MOR,while all three NiO/Pt catalysts showed a signicant currentincrease when using NiO as the substrate. The NiO–PtSA catalystdisplayed the lowest onset oxidation potential (0.519 V vs. RHE)compared with NiO–Ptcluster (0.541 V vs. RHE) and NiO–Ptparticle(0.615 V vs. RHE), indicating that Pt single atoms enabled NiO–PtSA to be an efficient MOR electrocatalyst compared to theother catalysts. The lowest onset oxidation potential of NiO–PtSArevealed the formation of weakly adsorbed COads on Ni and/orPt active sites, which affected the key step of the co-adsorption of *CO + *OH in the MOR to avoid self-poisoningand deactivation of the catalyst.42,43 In addition, the ratios ofFig. 6 MOR performance in a mixed electrolyte of 1.0 M KOH and 1.0showingmass activities at peak potentials for as-prepared catalysts at 50NiO–PtSA with the typical direction and reverse direction (blue line: typicainset shows the simulated circuit. (E) Chronoamperometry curves for th10178 | Chem. Sci., 2024, 15, 10172–10181the forward peak current density (If) to backward peak currentdensity (Ib), i.e. If/Ib, were estimated to be 5.13, 4.30, 3.52, and1.93 for NiO–PtSA, NiO–Ptcluster, NiO–Ptparticle and pure Pt cata-lysts, respectively. The prominent If/Ib of NiO–PtSA indicateda more efficient MOR process with less residual intermediatespecies which were generated as a result of the incompleteelectrooxidation of CH3OH. Moreover, as presented in Fig. 6B,the mass activity of NiO–PtSA (0.422 A mgPt−1) was 2.62 and18.35 times that of NiO–Ptcluster (0.161 A mgPt−1) and NiO–Ptparticle (0.023 AmgPt−1), respectively. Higher peakmass activityand smaller peak potential of NiO–PtSA indicated a lower acti-vation barrier. Such a prominent mass activity was also superiorto those of previously reported Pt-based catalysts.44–49 The effectof Ib on methanol oxidation was assessed by comparing thetypical forward direction with the reverse direction (Fig. 6C).Two curves were highly overlapped, which revealed that back-ward oxidation was almost not inuenced by the forward orreverse reactions. The EIS data, as shown in Fig. 6D and TableS3,† indicated that NiO–PtSA presented a smaller semicircleradius of impedance than those of NiO–Ptcluster and NiO–Ptparticle, which implied that electron transfer was signicantlyfacilitated because of the Pt single atoms–NiO support inter-action. Chronoamperometry was carried out at 0.70 V vs. RHE tofurther evaluate the durability toward the MOR (Fig. 6E). Itshould be noted that the nal current density of NiO–PtSA aer10 000 s was remarkedly higher than those of NiO–Ptcluster andNiO–Ptparticle. In addition, NiO–PtSA maintained a superiorcapacity retention of 95% aer 500 cycles for the MOR(Fig. S18†). The outstanding electrocatalytic stability andM CH3OH. (A) The mass-normalized CV curves and (B) a comparisonmV s−1 and the previously reported Pt-based catalysts. (C) CV results ofl direction, yellow dashed line: reverse direction). (D) Nyquist plots, thee NiO/Pt catalysts at 0.70 V for 10 000 s.© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4sc00454jEdge Article Chemical ScienceOpen Access Article. Published on 27 May 2024. Downloaded on 9/1/2024 3:24:05 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinedurability of the NiO–PtSA further veried its excellent anti-poisoning properties.ConclusionsA series of NiO/Pt catalysts with different Pt nanostructures(single atoms, clusters, and nanoparticles) were prepared usinga simple adsorption–impregnation strategy by adjusting thelight condition and reaction prolongation, and further appliedto the electrocatalytic OER andMOR. According to experimentalanalyses and theoretical calculations, Pt single atoms in the Pt2+valence state are mainly coordinated with the surrounding O,optimizing the electronic structure and coordination of themetal centers, i.e. the metal–support interactions. This strategycombined the advantages of porous nanoake structures of theNiO substrate with the Pt dopant to optimize the noble metal–NiO support interactions. As a result, the NiO–PtSA catalystshowed the merits of low Pt loading for high Pt utilization, andimproved transmission of active species with a remarkablyenhanced catalytic activity of 5.59 AmgPt−1 for the OER and 0.42A mgPt−1 for the MOR, which signicantly surpassed the NiO–Ptcluster and NiO–Ptparticle. This work will offer an approach ofcarefully modulating metal–non-carbon support interactions todevelop highly active hybrid catalysts for energy-relatedtechnologies.Data availabilityThe theoretical calculations and experimental procedure detailshave been provided within the manuscript and ESI.† The datathat support the ndings of this study are available from thecorresponding authors upon reasonable request.Author contributionsF. W. and H. W. conceived and coordinated the project. H. W.,G. C., N. Z., X. L., and R. M. supervised the project. F. W. per-formed the experiments, analyzed the data, and wrote theoriginal dra. Z. Z. assisted with the DFT calculations andanalysis. D. W. helped with the OER measurements and dataanalyses. The manuscript was nished through the contribu-tions of all authors. All authors have approved the nal versionof the manuscript.Conflicts of interestThe authors declare no competing nancial interest.AcknowledgementsThis work is nancially supported by the National NaturalScience Foundation of China (U20A20123 and 51874357). H. W.acknowledges the support from the Project of Zhongyuan Crit-ical Metals Laboratory (GJJSGFYQ202336). This work was sup-ported in part by the High-Performance Computing Center ofCentral South University.© 2024 The Author(s). Published by the Royal Society of ChemistryNotes and references1 Z. X. Xia, X. M. Zhang, H. Sun, S. L. Wang and G. Q. Sun,Recent advances in multi-scale design and construction ofmaterials for direct methanol fuel cells, Nano Energy, 2019,65, 104048.2 H. Chen, J. Chen, P. Ning, X. Chen, J. Liang, X. Yao, D. Chen,L. Qin, Y. Huang and Z. 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Catal., 2019, 375,267–278.Chem. Sci., 2024, 15, 10172–10181 | 10181http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4sc00454j Tunable Pttnqh_x2013NiO interaction-induced efficient electrocatalytic water oxidation and methanol oxidationElectronic supplementary information (ESI... Tunable Pttnqh_x2013NiO interaction-induced efficient electrocatalytic water oxidation and methanol oxidationElectronic supplementary information (ESI... Tunable Pttnqh_x2013NiO interaction-induced efficient electrocatalytic water oxidation and methanol oxidationElectronic supplementary information (ESI... Tunable Pttnqh_x2013NiO interaction-induced efficient electrocatalytic water oxidation and methanol oxidationElectronic supplementary information (ESI... Tunable Pttnqh_x2013NiO interaction-induced efficient electrocatalytic water oxidation and methanol oxidationElectronic supplementary information (ESI... Tunable Pttnqh_x2013NiO interaction-induced efficient electrocatalytic water oxidation and methanol oxidationElectronic supplementary information (ESI... Tunable Pttnqh_x2013NiO interaction-induced efficient electrocatalytic water oxidation and methanol oxidationElectronic supplementary information (ESI... Tunable Pttnqh_x2013NiO interaction-induced efficient electrocatalytic water oxidation and methanol oxidationElectronic supplementary information (ESI...