Three‐Dimensional Metal–Organic Framework Graphene Nanocomposite as a Highly Efficient and Stable Electrocatalyst for the Oxygen Reduction Reaction in Acidic Media

We report on the design of a bio‐inspired composite as a noble‐metal‐free electrocatalyst for the oxygen reduction reaction (ORR). The composite is made from the assembly of pyridine‐functionalized graphene (G‐py) and a 3 D metal–organic framework (MOF) deposited onto a glassy carbon electrode (GCE). The 3 D heme‐like MOF was synthesized from tetrakis(4‐carboxyphenyl)porphyrin iron(III) chloride and Zr6 clusters for the assembly of the stable porous coordination network. G‐py, which possesses an axial ligand to anchor to the centers of porphyrin in the MOF, results in a significant change in the electronic and geometric structure of centers, which enhances the rate of ORR and durability during cycling in acidic media. The occurrence potential of the ORR by the composite is shifted to the positive potential near 100 mV. Our results introduce a new strategy for the rational design of inexpensive and highly stable oxygen reduction electrocatalysts for fuel cells without the requirement of pyrolysis.


Introduction
The reduction of oxygen is not only as ignificant reaction in biological systems, [1] but it is also ac hallenging part of fuel cells and metal air batteries. [2] Undoubtedly,f uel cells are one of the mosti mportant alternatives to fossil fuels if we consider the astonishing increase in the consumption of energy.S uch electrochemical energy conversion systemsh ave become attractive because of their cleanliness and high efficiency. [3] However,t he sluggishk inetics of the oxygen reduction reaction (ORR) at the cathode remains the bottleneckf or the use of fuel cells. [2] Pt [4] and its alloys [5] of various sizes and shapes have been used extensively as cathode catalysts, butt heir widespread use has been severely limited by their high cost, scarcity,a nd low stability, especially in acidic media. Therefore, at remendousm otivation has been created for the synthesis of cheap,h igh-performance, and accessible electrocatalysts to replace Pt. [6][7][8] Cheaper noblem etals, [9] metal oxides, [10] metal chalcogenides, [11] metal macrocycle (M-N-C) catalysts, [12] and carbon-based materials have attracted great attentiona sa lternatives. [13,14] Since Jasinski [15] reported the electroactivity of cobalt phthalocyanine for the ORR, this group of metal macro-cycles (Fe, Co) has been placed at the center of numerousi nvestigations. [16,17] To overcomet he degradation of metal macrocycles,m etal porphyrins or phthalocyanines accompanied by carbon supports involveh igh-temperature pyrolysis, whichr esults in metal nitrogen carbon hybridsw ith enhanced stability and activity. [18] In some cases, pyrolysis was not achieved, and metal macrocyclesw ere connected to the carbon support through covalentb onding or p-p interactions. [19][20][21][22] For example, Youe tal. synthesized am aterial in which cobalt [ 5,15-(p-aminophenyl)-10,20-(pentafluorophenyl) porphyrin]w as attached covalently to graphene oxide. [23] However,m ost of the investigations on syntheticM -N-C catalysts focused on four-coordinate systems (M-N 4 -C) and few attempts have been made to alter the coordination environment of the metal center. [16,21] With inspiration from cytochrome co xidase, Cao and co-workers designed an ew ORR electrocatalyst that uses pyridinefunctionalized carbon nanotubes (CNTs) to anchor iron phthalocyanine molecules and provide the axial ligand as the fifth ligand forthe Fe center. [24] Recently,anew approachtodevelop high-performance electrocatalysts using the template-free pyrolysiso fm etal-porphyrin-based conjugated mesoporous polymer frameworks has been proposed. [25,26] Accordingly,W ue tal. prepared as elf-sup-portedO RR electrocatalyst from the pyrolysis of cobalt-porphyrin-based conjugated mesoporousp olymer (CoP-CMP) frameworks. The resulting materials showedc atalytic activity and stabilityi nb oth alkaline and acidic media. [27] Metal-organic frameworks (MOFs),w hich consist of metal ions or clusters connected by organic linkers, constitute one of the most substantial progresses in the field of nanoporous materials [28,29] and have many practical applications,s uch as gas We report on the design of ab io-inspired composite as an oble-metal-free electrocatalyst for the oxygen reduction reaction (ORR).The composite is made from the assembly of pyridine-functionalized graphene (G-py) and a3Dm etal-organic framework (MOF)d eposited onto ag lassy carbon electrode (GCE). The 3D heme-like MOF was synthesized from tetrakis(4carboxyphenyl)porphyrin iron(III) chloride and Zr 6 clusters for the assembly of the stable porous coordination network. G-py, which possesses an axial ligand to anchor to the centers of porphyrin in the MOF,r esults in as ignificant change in the electronic and geometric structure of centers, which enhances the rate of ORR and durability during cycling in acidic media. The occurrence potentialo ft he ORR by the composite is shifted to the positive potential near 100 mV.O ur results introduce an ew strategy for ther ational design of inexpensive and highly stable oxygen reduction electrocatalysts for fuel cells without the requirement of pyrolysis. storage, gas separation, catalytic activity,d rug delivery,a nd sensors. [30,31] As MOFs possess ah igh density of metal ion sites and ah igh surface area, these porous frameworks are interesting as sacrificial materials to be pyrolyzed in the synthesis of nonprecious metal catalysts. [32][33][34][35] Although some MOFs have been used as as acrificial precursor to prepareO RR catalysts, only af ew examples of preserved MOF structures (no pyrolysis step) have been reported. [36][37][38] For the first time, in 2012 Jahan et al. [36] reported on ac omposite graphene/2 DF ep orphyrin MOF structure (G-dye-FeP)n with enhanced electrocatalytic activity towards the ORR in an alkaline medium. In addition, Mao et al. [37] demonstrated that Cu-btc (btc = 1,3,5-benzenetricarboxcylica cid) MOF modified with bipyridine as an auxiliary ligand exhibited as table electrocatalytic activity towards the ORR. Another example is ap orousM OF that contains Cu centers andf ully activated nanocages as an oble-metal-free electrocatalyst for the ORR. [38] Hence, with ar ational choice of buildingb locks, porous materials with delicate functions that provide many appropriate interactions for catalytic purposes can be constructed. To the best of our knowledge, some MOFs are made up of porphyrins as buildingb locks, [39,40] but only af ew porphyrin-based MOFs exist with Fe and Co centers. [41,42] Feng andc o-workers [43] employed tetrakis(4-carboxyphenyl)porphyrin-Fe III Cl (FeTCPPCl) as ah eme-like ligand and highly stable Zr 6 clustersa sn odes for the synthesis of a3 DZ r-MOF with superb features such as ultra high stability, accessible mesoporous channels for substrate molecules, and ah igh surface area. Given the successful role of PCN-222 (porousc oordination network) in peroxidase mimic applicationsa nd stabilityi na cidic media, we concluded that this MOF was one of the best candidates for our goal. For postsynthetic modification, stability improvement, and to mimic cytochrome co xidase, [1] we have designedanew composite of PCN-222 (3 DM OF) with pyridine-functionalizedg raphene (G-py), whiche xhibits ah ighly efficient catalytic performance for the ORR and stability in acidic media. G-py possesses an axial ligand to anchor to the Fe centers of porphyrin buildingb locks in PCN-222, which changes the coordination environment of the metal centers. Moreover,g raphene nanosheets provide an easy way for fast electron transfer to the active sites through the axial ligands. In other words, pyridine functions as ac onnector to pass the charget ot he Fe center and increases the electrocatalytic activity of PCN-222 to facilitate the ORR.

Synthesis and description of catalysts
Graphenen anosheets were decorated with the pyridinium moiety through ad iazonium reaction( Scheme1). Thep resence of pyridine on the graphene nanosheets was proved by using CHN analysisand FTIR spectroscopy.The structure, stability,a nd morphology of G-py werei nvestigated by using UV/Vis spectroscopy,S EM, XRD, and thermogravimetric analysis (TGA).
The latter step comprises the combination of G-py andP CN-222. The PCN-222-G-py composite was synthesized,a nd the pyridine function of G-py was coordinated to the center of the porphyrin (Scheme 3). More detail about the location of graphene and PCN-222 are shown in Scheme 3( inset). The final composite has al ayered structure in which nanosheets of graphene are placed on the faces of nanocrystals.X -ray photoelectron spectroscopy (XPS) indicates ag ood agreement with our predicted design, in which the coordination of Fe centers is changed through the connection of pyridine. Thes tructure of the composite was studied more extensivelyb yu sing FTIR spectroscopy,TGA, SEM, and XRD.

Characterization of G-py
The CHN elemental analysiso fG -py indicates the presence of 6.47 wt %o fn itrogen, which is evident from the connection of pyridine ligand to the graphene sheets. [24,36] The UV/Vis spectrum of graphene oxide shows l max = 228 nm, which corresponds to p-p*t ransitions of aromatic C= Cb onds, and as houlder peak at l % 308 nm caused by n-p* transitions of the carbonyl groups (Figure 1a). For G-py, l max is redshifted to 257 nm because of the restorationo ft he p-conjugated network in reduced graphene oxide (r-GO). [44,45] The stabilityo ft he graphene nanosheets was investigated by using TGA (Supporting Information).

Characterization of PCN-222a nd PCN-222-G-py
It is necessary to know which changes in the MOF structure and its properties are created by the presence of graphene. Therefore, optical properties, vibrational bands, and the structure of the MOF and its composites were studied.
The absorption spectra of TPPCOOMe,[ TPP-COOMe]Fe III Cl, PCN-222, and PCN-222-G-py (25 wt %) are shown in Figure 1b. For TPPCOOMe,t he Soret band (transitionf rom p to LUMO) at l = 419 nm and Qb ands (HOMO!LUMO) at l = 514, 549, 589, and 646 nm can be observed. If Fe is inserted into the porphyrin to form [TPP-COOMe]Fe III Cl, the number of Qb ands is reduced from four to two at l = 570 and 609 nm and aS oret band appearsw ith ab lueshift at l = 414 nm. [46] The UV/Vis spectrumo fP CN-222 shows part of the Soret band along with broadening.I nt he spectrum of PCN-222-G-py,t he Soret band appearsw ith ab lueshift compared to that of PCN-222. No clear shift can be monitored for ap hysical mixture of PCN-222 and G-py ( Figure S2). This shows that PCN-222-G-py possesses ad ifferent electronic structure to PCN-222. [23,36] Furthermore, the presence of r-GO in PCN-222-G-py creates an ew band at l = 244 nm, which corresponds to G-py.
FTIR spectra of GO, G-py,P CN-222, and PCN-222-G-py (25 wt %) are shown in Figure 2. In GO, the C=Os tretching vibrations of carboxyl and carbonyl groups are observed at ñ = 1727 cm À1 .T he OÀHs tretching vibration at ñ = 3412 cm À1 and the CÀOs tretching vibration at ñ = 1058 cm À1 can be observed. The peaks at ñ = 1622, 1382, and 1235 cm À1 are assigned to C=Cf rom the unoxidized sp 2 CÀCb ond, the stretching vibration of CÀOo fc arboxylic acid, and CÀOH stretching, respectively. [23,45,47] In the spectrumo fG -py,t he intensities of the peaks associated with the oxygen groups decrease dramatically and someo ft hem vanish. [45] The presence of pyridine on the r-GO sheets can be observed by peaks at ñ = 802, 1515-1652, and 1339 cm À1 ,w hich correspond to CÀHp yridine (outof-plane bending), C=Cs tretching, and C=Np yridine stretching, respectively.M oreover,t he C=Ov ibration of COOH appears at ñ = 1741 cm À1 in G-py. [23,48,49] The spectrumo fP CN-222 showst wo peaks at ñ = 1691 and 1417 cm À1 assigned to the COO (asymmetric) and COO (symmetric) stretching vibrations, respectively.T he peaks at ñ = 2920-3090, 1557-1603, and 1326 cm À1 are attributed to the CÀHb ond of the benzene and pyrrole ring, C=C( phenyl and pyrrole) stretching functional groups, and pyrrole deformation, respectively.C ÀH( out-ofplane bending of the phenylr ings)i so bserved at ñ = 712 and 804 cm À1 . [50] For the composites, because of the anchoring of the pyridine functiont ot he Fe center,t he intensity of the FeÀ Np eak located at ñ = 1000 cm À1 increases. Some of the peaks of PCN-222 appear in the spectra of the composites. Furthermore, the C=Cs tretching vibration mode covers the ñ % 1600 cm À1 zone.
The thermals tability of PCN-222 (activated) and the composites (10, 25, and 50 wt %) were investigated by using TGA. In the PCN-222 sample, weight is lost gradually at % 350-800 8C, and no weightl oss occurs up to 350 8C( Figure S4). [43] For composites, the amount of remnant liberated after thermal pyrolysis diminishes with the increasingc ontent of G-py in the composite from 10 to 50 wt %( Figure 3). The proportion of residue for the composites (10, 25, and5 0wt%)a re 32.69, 19.37, and 6.8 %, respectively. [36] To determine the morphologyo ft he samples, SEM images were recorded ( Figure 4). The SEM images of the functionalized graphene demonstrate that the 2D sheet was well maintained duringt he functionalization reaction (Figure 4a and b). [44] PCN-222 has needle-shaped crystals of 2 mmi nw idth (Figure 4c), whereas the dimensions of the composites are decreased and the width of the needle-shaped crystalsi s% 500 nm (Figure 4e). In comparison to that shown in Figure 4d,t he image of the crystalsp resented in Figure 4f show tracks on their surface, probablyb ecause of the presence of graphene nanosheets.
XPS was used to explore the local chemical environments and chemical compositions of the electrocatalysts. The XPS spectra of PCN-222 and PCN-222-G-py( 25 wt %) display peaks for C1s, N1s, O1s, Fe 2p, Zr 3p, and Zr 3d ( Figure S5). The highresolution XPS spectrum of the Fe 2p 3/2 level indicates one www.chemcatchem.org peak at ab inding energy (BE) of 711.5 eV for PCN-222,w hich demonstrates the presence of Fe + +3 inside the porphyrin core, [51] whereas the binding energy of the Fe 2p 3/2 in the spectrum of PCN-222-G-py (25wt%)increases to 712.29 eV because of the anchoringo ft he pyridine functiont ot he Fe center ( Figure 5a). The coordination of pyridinea sa na xial ligand to the Fe centerd ecreases the electron density from the core because pyridine is a p-acceptor ligand accordingt ot he Mçssbauer spectra reported previously,w hich is in good agreement with our results. [52] Therefore, the Fe centersi nP CN-222-G-py (25 wt %) have more positive charge than those in PCN-222. The presence of the more positive centers in PCN-222-G-py (25 wt %) is desirable for the adsorption of O 2 moleculesi nt he first step of the ORR. [53] In the N1sr egion, as ingle peak at BE = 398.9 eV relatedt o the FeÀNb ond in the porphyrin core of PCN-222 accompanied with as atellite can be observed (Figure 5b). In PCN-222-G-py (25 wt %), the N1sp eak splits into two bands located at BE = 398.9 and 400.7 eV,w hich correspond to the pyridinic No fGpy and the "pyrrole-type" No ft he porphyrin core, respectively. [51,54,55] XRD patterns of PCN-222 and the composites are presented in Figure 6. For the MOF,m ost peaks are located in the smallangle region because of the mesoporousn ature of PCN-222. This is in agreement with that reported previously for PCN-222. [43] In the composites (10 and 25 wt %), the main diffraction pattern of PCN-222 is preserved and the increaseo ft he amountso fG -py,a sa ni mpurity,r educes the clarity of the peaks related to PCN-222, which suggestst hat the MOF structure is mainly retained and the graphene layersi nduce as light distortion. PCN-222-G-py( 50 wt %) has ac ompletely amorphousp hase because of the high amount of G-py in the material.
Investigation of the presenceo fgraphene nanosheets inside the framework PCN-222-G-py (25 wt %) was dissolved in 2 m NaOH solution and washed with water ande thanolt or emove impuritiesf rom  www.chemcatchem.org the G-py.T he precipitate was dried and then dispersed in DMF with sonication for 30 min. The presence of r-GO sheets was revealed by using SEM (Figure 7a) and Ramans pectroscopy. The Ramans pectrum presented in Figure 7b shows the D band (ñ = 1320 cm À1 )a nd Gb and (ñ = 1590 cm À1 )o ftheg raphene sheet. [56] Electrochemical characterizationofthe modified electrode The electroactivity of the constructed modified electrodes was evaluatedb ys tudying the redox reactions of Fe(CN) 6 3À/4À (Figure 8a)b yu sing cyclic voltammetry (CV). As observed, the peak currents increased after the introduction of modifiers on the electrode surfacec ompared to those obtained on the bare glassy carbon electrode (GCE). Moreover,ad ecrease of the peak potential difference (DE p = E p,a ÀE p,c )o ft he ferricyanide redox reaction on the surface of the modified electrodes led to the enhancement of the charge transfer kinetics. These results confirm the significant improvement of the electrochemical activity of GCE as ac onsequence of the electrode surface modification and, therefore, the effectiveness of the synthesized modifiers.
As confirmed previously,t he presence of PCN-222 and G-py (as PCN-222-G-py (25 wt %)) on the surfaceo ft he electrode increases its effective surface area. To evaluate the effective surface area of the various modified electrodes, the CVs of 0.1 m KCl solution that contained 1mm K 3 Fe(CN) 6 as ar edox probe were recorded at different scan rates. The electroactive surface area was estimated according to the Randles-Sevcik equation [Eq. (1)]: [57] i p,c ¼ 2:69 Â 10 5 n 1:5 AC o D 0:5 u 0:5 ð1Þ    in which,f or K 3 Fe(CN) 6 , i p,c is the cathodic peak currenta nd n, A, D, u,a nd C o refer to the electron transfer number (n = 1), the electroactive surface area of the electrode [cm 2 ], the diffusion coefficient (D = 7.6 10 À6 cm 2 s À1 ), the potential sweep rate, and the concentrationo fK 3 Fe(CN) 6 (1 mm), respectively. From the slope of the plot of i p,c versus u 1/2 ,t he microscopic areas can be calculated. Here, the electroactive surfacea rea of G-py,P CN-222-GCE, and PCN-222-G-py (25 wt %)-GCE were 5.3, 6.9, and1 1.0 times greater than that of bare GCE, respectively. These results indicate clearlyt hat the microscopica rea of the bare electrode increased after the incorporation of the modifiers onto the electrode surface.

Electrocatalytic behavior of modified electrodes towards the ORR
The electrochemical reduction of oxygen was investigated on the modified and bare GCE surfaces to clarify the differences between their electrocatalytic activities. The CVs recorded in 0.5 m H 2 SO 4 solution saturated with O 2 at GCE,P CN-222-GCE, PCN-222-G-py (10 wt %)-GCE, PCN-222-G-py( 25 wt %)-GCE, and PCN-222-G-py (50 wt %)-GCE are given in Figure 8b.I nN 2 -satu-rated solution, no clear cathodic peak appeared within the potentialr ange of + +0.5 to À0.5 V. However, in the O 2 -saturated solution,areduction peak was observed, which reveals the occurrenceo ft he ORR process. Oxygen shows ab road and weak reduction peak, with ap eak current of approximately À1.45 mAa tÀ0.395 Vo nt he bare GCE (curve a), whereas for PCN-222-GCE, PCN-222-G-py (10 wt %)-GCE, PCN-222-G-py (25 wt %)-GCE, and PCN-222-G-py (50 wt %) the cathodic peak currents of the ORR are enhanced significantly,a nd well-defined irreversible reduction peaks appeara tE p,c = À0.031, 0.018, 0.082, and 0.080, respectively.T he thin layer of PCN-222, PCN-222-G-py (10 wt %), PCN-222-G-py (25 wt %), or PCN-222-G-py (50 wt %) coated on the GCE surfacei ncreased the peak current of oxygen reduction by 15.1, 16.2, 43.1, and 61.6 times, respectively,c ompared to bare GCE (curves b-e). As shown, the reduction peak current of oxygen was furthere nhanced by increasing the amount of G-py in the PCN-222-G-py composite, which reveals the effectiveness of the prepared composites. These results confirm the high electrocatalytic activities of the employed modifiers, which lead to the significant enhancement of the ORR kinetics.
CVs of GO-GCE, G-py-GCE, andP CN-222-G-py (25 wt %)-GCE recorded in 0.5 m H 2 SO 4 solution saturated with O 2 with ap otential sweep rate of 50 mV s À1 are shown in Figure 9a.A tG O-GCE, av ery small peak with ah igh overvoltage was observed at À0.4 V. The deposition of r-GO directly on the GCE (G-py-GCE) results in the appearance of ab road peak with ah igh background current for the ORR at À0.165 V. Interestingly,t he observation of aw ell-shaped reductionp eak with ah igher peak current at more positive potentials on the surface of PCN-222-G-py (25 wt %)-GCE reveals the enhanced electron transfer kinetics of the related process. Notably,t he background current of the bare electrode increased significantly after its surface modification by G-py.I nc ontrast, the use of GO or PCN-222-G-py as am odifier leads to al ower increase in the background current, which is more reliable for quantification purposes as the Faraday current can be detected sensitively.T he observed improvement in the electrocatalytic activity may be causedb yt he synergistic effects of framework porosity and the electrocatalytic activity of PCN-222 as well as the high surface area and good electrical conductivity of r-GO in the structureo ft he PCN-222-G-pyc omposite.

Electrocatalytic behavior of optimized PCN-222-G-py towards the ORR
In another attempt, the nature of the electroreduction of oxygen was investigated by recording CVs of PCN-222-G-py-GCE in 0.5 m H 2 SO 4 saturated with O 2 at differents weep rates (25-700 mV s À1 ;F igure 9b). The results showedagood linear relationship between the peak current and scan rate with acorrelationc oefficient (R 2 )o f0 .997 (Figure 9c). This indicates that the mass transfer and the electrochemical reduction of oxygen are adsorption-controlled processes.A ni ncrease of the potential sweep rate leads to as light shift of the peak potential to more negative values, which reveals the irreversible nature of the electrochemical processf or oxygen reduction. in which R 2 = 0.994, E p,c is in mV and u is in mV s À1 .According to the Laviron equation[Eq. (3)] for an adsorption-controlled irreversible process [58] a value of 0.59 was obtained for the term an.
Here, E 0 , a, n,a nd k 0 are the formal potential, transferc oefficient, number of electrons involved in the rate-determining step of the electrode process, and the electrochemical rate constant,r espectively,a nd the other symbolsh ave their usual meanings. If we consider that a = 0.5, the value of n was calculated to be 1, which reveals the involvement of one electron in the rate-determining step of the electroreduction of O 2 .
The durability of PCN-222-G-py (25 wt %) as an ORR catalyst for the cathode was evaluated.T he test was performed by recording successive CVs in 0.5 m H 2 SO 4 saturated withO 2 at as weep rate of 50 mV S À1 .T he resultse xhibited av ery slow current attenuation over 100 scans, which suggests the good stabilitya nd acceptable performance of the modified electrode.

Role of the pyridine connection
To explore the key role of the pyridine connectiont ot he Fe centers,w ee xamined another sample for the ORR in which the nanosheets of r-GO are not functionalized with pyridine. This sample (PCN-222/r-GO (25 wt %)) was prepared according to the following procedure.
First, PCN-222(Fe) (1.5 mg) and r-GO (0.5 mg;2 5wt%,b ased on the mass of PCN-222) were dispersed in 2mLo fw ater by sonication for 30 min. The prepared PCN-222/r-GO suspension (4 mL) was drop-casted onto the GCE surface. The electrode was allowed to dry at room temperature for 30 min. Again, the PCN-222/r-GO suspension (4 mL) was loadedo nto the GCE and allowed to dry for 30 min.
The  www.chemcatchem.org tive potentials than that of PCN-222 because of the good electrical conductivity of r-GO. However,i fp yridine ligandsw ere connected to the iron porphyrins and apath for electron transfer was constructed in the composite (PCN-222-G-py), the peak current increased significantly and was located at am ore positive potential. Generally,t he creation of ac hemical pathway to pass the electrons is the main reasont ou se pyridine, which leads to the significant amplification of the electrochemical oxygen reduction kinetics.
Stability and repeated use of PCN-222-G-py(25 wt %) As somep rotone xchange membrane (PEM) fuel cells use acidic media, the stabilityo ft he electrocatalyst in an acidic mediumi sn ecessary.T he stabilityo ft he GCE modifiedb yt he prepared composite was evaluated by recording 100 successive CVs in 0.5 m H 2 SO 4 solution saturated with O 2 at as can rate of 50 mV s À1 (Figure S7 a). Small changes in the peak potential (5 mV after 100 cycles)a nd ad ecrease of the peak current with the increasing number of CV cycles were observed because of the decline of O 2 concentration. In the next step, O 2 was purged into the acidic solution anda nother 100 successive CVs were recorded.N otably,t he same results were obtained in both steps, which reveal the good stability of the prepared modified electrode. Similar resultsw ere obtained for PCN-222 (Figure S7 b).
The repeatability of PCN-222-G-py-GCE and PCN-222-GCE was investigated by immersing PCN-222-G-py-GCE and PCN-222-GCE in O 2 -saturated 0.5 m H 2 SO 4 solution and recording CV,a nd the electrodes werel eft in solution for 24 h. Then, CVs were recorded in the same O 2 -saturated solution. Similar results were obtained, which reveals the good repeatability of the prepared modified electrodes (the peak potential change was less than 10 mV). These resultsa re in good agreement with that reported previously for the stabilityo fP CN-222 in acidic mediabyu sing XRD. [43] Methanol tolerance test One of the disadvantages of direct methanol fuel cells is that the Pt/C commercial catalyst is prone to methanol poisoning. As ag ood electrocatalyst is inert to methanol oxidation, we have investigatedt he methanol crossover effect for PCN-222-G-Py.N oc learr esponse for PCN-222-G-Py-GCE is detectedi n O 2 -saturated 0.5 m H 2 SO 4 solution with 3 m methanol ( Figure S8). Thus the composite has as electivity for the ORR accompanied by at olerance of methanol crossover effects. According to the previous report of Jahan et al.,t he Pt/C catalyst shows ad istinct oxidation peak caused by the methanol oxidation reaction. [59] Advantages of the 3D structure of the designedelectrocatalyst To exploret he advantagesoft he 3D structureo fthe electrocatalyst, other samples such as Fe-TCPP/r-GO (1.5 mg Fe-TCPP and 0.5 mg r-GO in 2mLwater) wereexamined (Supporting In-formation). The results indicate the necessity to use a3Dstructure such as aM OF for the preparation of the electrocatalyst. If the iron porphyrins that are responsible for the ORR are located in a3 Ds tructure, they have better operation because of the specific properties of MOFs such as good stabilityu nder harsh conditions and an appropriate dispersion in solvent. Furthermore, the 3D structure of the MOF prevents the aggregation of iron porphyrins. Therefore, the access of O 2 to the active centeroft he electrocatalyst is easier.
The demetalation of porphyrinsd uring the ORR is an unavoidablep rocess. [24] Notably, the Fe centers in PCN-222-G-py (25 wt %) have an axial ligand. Probably,t he demetalation of porphyrins in the building blocks of PCN-222 takes place slowly because of the extra coordination bond.
Comparisonb etween the activity and stability of PCN-222-G-py (25 wt %) and Pt/C electrodes According to ar eport published previously, [59] the cathodic peak for the ORR at aG CE loaded with Pt (20 wt %/C) is at À0.2 Vw ith ac urrent density of % 2mAcm À2 in O 2 -saturated 0.5 m H 2 SO 4 solution. Our PCN-222-G-py (25 wt %)-GCE shows the potentialp eak at 0.082 Vw ith ac urrent density of 3.55 mA cm À2 under the same conditions (Figure S10 a). These resultsd emonstratet hat the use of PCN-222-G-py (25 wt %) as aG CE modifier leads to the occurrence of the ORR at am ore positive potentialw ith ac urrent density of approximately 1.8 times of that of Pt/C.

Conclusions
We have designed an ew composite from a3 Dp orphyrinbased metal-organic framework (MOF) and graphene nanosheetst hat has ah ighly stabilitya nd good performance as an electrocatalyst for the oxygen reduction reaction in acidic media. Our results revealed that the presence of graphene and the MOF enhancet he electrocatalytic activity of the final composite synergistically because of the pyridine functional groups that connectt he graphene sheets and the MOF to provide an easy transfer of electrons, which facilitates oxygen reduction. The use of ag lassy carbon electrode (GCE) modified with the prepared PCN-222-G-pyc omposite shiftedt he oxygen reduction reaction onset potential positively (477 mV) compared to ab are GCE andw ith an increased peak current that is 43.1 times higherthan that of the bare GCE. Apparatus SEM images were recorded by using aL EO 1455VP operated at an acceleration voltage of 10 kV.F TIR spectra were recorded at RT by using aB ruker (Tensor 27) spectrometer.T he samples were ground with KBr and pressed into disks. UV/Vis spectra were recorded by using aU V/Vis PerkinElmer Lambda 35 spectrophotometer.P owder XRD was performed by using aP hillips PW 1800 X-ray diffractometer with aCuK a line (1 = 1.54060 A o )a st he incident beam. TGA was performed by using aM ettler To ledo TGA/SDTA8 51 instrument under N 2 at ah eating rate of 10 8Cmin À1 .X PS was performed by using aG amma data-scienta ESCA200 hemispherical analyzer equipped with an AlK a X-ray source (1486.6 eV) with am onochromator.T he binding energies were calibrated relative to the C1s peak at 285 eV.V oltammetric experiments were performed by using an Autolab PGSTAT30 digital potentiostat/galvanostat. All the measurements were recorded at RT by using at hree-electrode system with aGCE as the working electrode, Pt wire as the counter electrode, and Ag/AgCl (1 m KCl) as the reference electrode. Typically,C Vs were performed at as can rate of 50 mV S À1 .A ll potentials were measured and reported using aA g/AgCl reference electrode. The CV experiments were conducted in N 2 -a nd O 2 -saturated 0.5 m H 2 SO 4 for the ORR.

Synthesis of graphene oxide (GO)
GO was prepared using am odified Hummers and Offeman's method. [60] Typically,g raphite (500 mg), NaNO 3 (5.9 mmol, 500 mg), and concentrated H 2 SO 4 (23 mL) were stirred in an ice bath for 15 min. Next, KMnO 4 (25.3 mmol, 4000 mg) was added slowly.A fter the addition of KMnO 4 ,t he reaction was continued at 35 8Cf or 2h under stirring to form at hick green paste. Excess water (50 mL) was added very slowly followed by stirring for 1h,and the temperature was increased to (90 AE 5) 8C. Finally,w ater (100 mL) was added, followed by the slow addition of H 2 O 2 (30 %, 3mL). The color of the mixture changed to yellow brown during the dropwise addition of H 2 O 2 .T he product was collected by filtration and washed with HCl solution (5 %) and then repeatedly washed with water.T he final product was dried under reduced pressure.

Reduction of GO
As olution of GO (400 mg) in water (320 mL) was sonicated for 1h to disperse the GO sheets completely in water.T hen, 5% sodium carbonate solution (50 mL) was added, and the mixture was stirred in ar ound-bottom flask at (90 AE 5) 8Cf or 9h.A fterwards, sodium borohydride (85 mmol, 3200 mg) in water (80 mL) was added to the GO dispersion, and the pH was adjusted to 10. The reaction was continued at 80 8Cf or 3hunder stirring. The suspension color changed from light brown to black after reduction accompanied by outgassing. The resulting r-GO was collected by filtration and washed with plenty of water. [60] Synthesis of pyridine-functionalized graphene (G-py) The functionalization of graphene was achieved with pyridine groups using the diazonium reaction. [24] Briefly,N aNO 2 (70 mmol, 4900 mg) was dissolved in H 2 O( 7mL) and then cooled on an ice bath. Further,4 -aminopyridine (4-AP;7 0mmol, 6580 mg) was dissolved in 4 m HCl (5 mL) and then cooled in an ice bath. The solution of NaNO 2 was added dropwise to the 4-AP solution. The resulting yellow solution was kept at 0 8Ci na ni ce bath under stirring for 30 min. At the same time, r-GO (150 mg) was dispersed in 1wt% aqueous SDBS surfactant in water (150 mL) by using ab ath sonicator for 1h.T he r-GO solution was cooled to 0 8Ci na ni ce bath, and the yellow solution of NaNO 2 and 4-AP was added dropwise, and the temperature was maintained at 0 8Ct op revent the decomposition of diazonium salts. The mixture was maintained in an ice bath at 0 8Cf or around 4h.N ext, the reaction mixture was stirred at RT for another 4h.F inally,t he solution was filtered and washed several times with water,ethanol, DMF,a nd acetone.
2) [5,10,15,20-Tetrakis(4-methoxycarbonylphenyl)porphyrinato]iron(III)c hloride ([TPP-COOMe]Fe III Cl) TPP-COOMe (1 mmol, 854 mg) and FeCl 2 ·4 H 2 O( 12.8 mmol, 2500 mg) were added to DMF ((100 mL), and the mixture was heated to reflux for 6h.A fter cooling to RT,H 2 O( 150 mL) was introduced. The resulting precipitate was collected by filtration and washed with H 2 O( 50 mL) twice. The obtained solid was dissolved in CHCl 3 and washed three times with 1 m HCl and twice with water.T he organic layer was then dried over anhydrous magnesium sulfate and evaporated to give aq uantitative yield of dark brown crystals.
3) [5,10,15,20-Tetrakis(4-carboxyphenyl)porphyrinato]iron(III) chloride (FeTCPPCl) The obtained ester (750 mg) was stirred in THF (25 mL) and MeOH (25 mL), and as olution of KOH (46 mmol, 2630 mg) in H 2 O( 25 mL) was added. The mixture was heated to reflux for 12 h. After cooling to RT,T HF and MeOH were evaporated. Water was then added to the resulting water phase, and the mixture was heated until the solid was dissolved fully.A fterwards, the homogeneous solution