Theory-Driven Design of Electrocatalysts for the Two-Electron Oxygen Reduction Reaction Based on Dispersed Metal Phthalocyanines

Open AccessCCS ChemistryRESEARCH ARTICLE1 Feb 2022The Enhancement of Selectivity and Activity for Two-Electron Oxygen Reduction Reaction by Tuned Oxygen Defects on Amorphous Hydroxide Catalysts Junheng Huang†, Changle Fu†, Junxiang Chen, Nangan Senthilkumar, Xinxin Peng and Zhenhai Wen Junheng Huang† CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of the Chinese Academy of Science, Beijing 100049 , Changle Fu† CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of the Chinese Academy of Science, Beijing 100049 , Junxiang Chen CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of the Chinese Academy of Science, Beijing 100049 , Nangan Senthilkumar CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 , Xinxin Peng CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of the Chinese Academy of Science, Beijing 100049 and Zhenhai Wen *Corresponding author: E-mail Address: [email protected] CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of the Chinese Academy of Science, Beijing 100049 https://doi.org/10.31635/ccschem.021.202000750 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Amorphous catalysts, thanks to their uniquely coordinated unsaturated properties and abundance of defect sites, tend to possess higher activity and selectivity than their crystalline counterparts. In this work, we report a facile and general solvent-controlled precipitation method to prepare hybrids of graphene oxide (GO) supporting amorphous metal hydroxide [A-M(OH)x/GO, M = Cu, Co, and Mn], which provides us with tangible materials to study the structure–performance relationship of various amorphous oxides. The systematic investigation of A-Cu(OH)2/GO by coupling ex situ/in situ characteristic techniques with electrochemical studies reveals that electrocatalytic activity and selectivity toward a two-electron oxygen reduction reaction (ORR) is highly dependent on the coordinated Cu catalytic sites and the disordered structure of A-Cu(OH)2. In situ X-ray absorption near-edge structure (XANES) and density functional theory (DFT) calculation verify that the degree of OH* poisoning (ΔG0OH*) tuned by three-OH-coordinated Cu sites in amorphous structures plays a crucial role in selective catalysis of ORR for H2O2 production. The optimized A-Cu(OH)2/GO shows superior activity and high selectivity (~95%) toward H2O2, as demonstrated by a zinc–air battery capable of on-site H2O2 production with a rate as high as 3401.5 mmol h−1 g−1. Download figure Download PowerPoint Introduction As renewable electricity becomes increasingly abundant and one of the most economically competitive energy sources, electrochemical synthesis is expected to be a promising technology for replacing some traditional chemical engineering processes to produce value-added products, leading to a new era of research growth.1,2 Electrocatalysts are of critical importance to the efficient implementation of the associated electrochemical synthesis, and tremendous effort has thus been devoted into the development of high-performance electrocatalysts.3,4 In particular, extensive exploration for electrocatalyst development has confirmed that some amorphous nanomaterials, including metal oxides, layered double hydroxides, and spinel compounds, are highly active toward electrochemical reactions due to the structural characteristics of adjustable composition, homogeneous characters, lattice defects-induced active sites, and unsaturated coordinating active sites.5–8 Hence, amorphous nanomaterials receive intense attention in diverse catalytic fields, including petroleum chemicals, energy conversion and storage, fine chemicals, aspects of environmental maintenance, and electrochemical applications.9 Nevertheless, only a limited amount of research has been reported regarding amorphous nanocatalysts,10–12 mainly because synthetic attempts to prepare amorphous nanostructures in a controllable way have progressed slowly. As such, it still remains challenging to understand the role of surface coordination atoms of amorphous nanostructures in catalytic activity, selectivity, and catalyst lifetime. So far, various synthesis methods have been reported to prepare amorphous electrocatalysts. For instance, feasible electrodeposition13–17 and photochemical metal–organic deposition18–21 methods have been developed for the preparation of amorphous metal oxide film catalysts at low temperature. However, these methods are more likely suited for preparation of thin-film amorphous materials, thus facing challenges to generalize to the other nanostructures, leading to the formidable challenge on broader application. These constraints can be overwhelmed by the solution-processed methods (e.g., coprecipitation, hydrothermal and sol–gel methods) to some extent.4,8,22,23 However, these methods are quite sensitive to the synthetic condition, and there's room for further investigation. It is thus highly desirable to develop tunable and general synthesis strategies to prepare amorphous nanomaterials, which in turn offer us the opportunity to understand the associated electrocatalytic characteristics better. Hydrogen peroxide (H2O2), as one of the most powerful oxidizers that can be converted into hydroxyl radicals with high reactivity, has been widely applied as an oxidizer for bleaching agents and antiseptics. For instance, the conventional wastewater recycling techniques practiced with H2O2 offer a desirable recycling efficiency, owing to the intrinsic characteristics of H2O2, such as low cost, strong oxidative tendency, ability to eradicate disease-causing organisms, and the generation of eco-friendly byproducts of oxygen and water.24–28 Despite these attractive features, the production of H2O2 via anthraquinone oxidation process, which was formalized in 1936 and used almost exclusively today, faces great challenges due to its energy-intensive process and release of hazardous byproducts into the environment.24,29 The electrochemical technique of oxygen reduction reaction (ORR) has recently been recognized as an alternative technique to the anthraquinone oxidation process for the production of H2O2,30,31 in which ORR process is a two-electrons transferred pathway for H2O2 production rather than a four-electrons transferred pathway to produce water. Accordingly, a variety of materials, including precious and nonprecious metals, metal oxides, carbon, and their composites, have been explored as catalysts aiming to achieve high-selectivity and -activity catalysis for selective O2 conversion into H2O2.32–41 Importantly, the activity and selectivity strongly depend on the neutral binding of intermediate OOH*. The amorphous catalysts with abundant defect sites and disordered structure have the potential to optimize the OOH* adsorption energy (ΔG0OOH*) at thermoneutral equilibrium potential. Although great progress has been made in the study of four-electron ORR catalysts,42–46 unfortunately, to the best of our knowledge, reports are rare about the development of amorphous catalysts for selective ORR conversion into H2O2, and explanations of the underlying mechanism remain ambiguous.47–50 We herein report a solvent-controlled precipitation (SCP) method for preparation of graphene oxide (GO)-supported copper hydroxide nanostructures with tunability from amorphous [A-Cu(OH)2/GO] to crystalline [C-Cu(OH)2/GO] structures. This method can be readily extended to a general strategy to prepare GO-supported amorphous transition-metal hydroxide nanostructures. The A-Cu(OH)2/GO exhibits an impressively high catalytic activity and selectivity toward ORR into H2O2. The role of surface segregation has been revealed by coupling various characteristic techniques, systematic electrochemical tests with DFT calculations to demonstrate that the three-OH-coordinated Cu sites with increased reversibility of the redox state in A-Cu(OH)2/GO plays a pivotal role in selective catalysis of ORR to produce H2O2. Experimental Methods Material synthesis The GO was prepared by the developed Hummers method. Briefly, 98 wt % sulfuric acid (100 mL) was slowly added to graphite powder (2 g) under the ice-bath with stirring for 120 min. Then 8 g of potassium permanganate was gradually added into the mixture at a temperature maintained below 10 °C. After stirring for 120 min, the solution was heated to 40 °C and then stirred for another 120 min. Then 400 mL water was added dropwise into the solution, and the temperature was kept at 40 °C with stirring for 60 min. Then 20 mL of 30 wt % H2O2 was slowly added to the solution and stirred for 30 min. The solution turned light yellow. The prepared GO was filtered and washed with 5 wt % hydrochloric acid five times, and then purified by deionized water. Filtered GO cake was dried at 60 °C. The GO powder was redispersed in water by sonication to get a GO aqueous solution of 2 mg mL−1. A one-step coprecipitation method was applied to synthesize the A-Cu(OH)2/GO sample. 0.25 mmol CuCl2·2H2O and 8 mL GO aqueous solution (2 mg mL−1) were dissolved in 32 mL glycol to form a homogeneous solution, and 14 mol L−1 NH4OH was added to the solution with vigorous stirring until the pH value reached 9. After stirring for 10 min, the solution was isolated by centrifugation, washed three times with water, and dried by vacuum freeze dryer. A-X% Cu(OH)2/GO (X = 15, 21, 28, 34, and 40) with different copper contents was synthesized by changing the quality of the CuCl2·2H2O (0.0625, 0.125, 0.5, and 1 mmol). Furthermore, the preparation process of A-Co(OH)x/GO and A-Mn(OH)x/GO was similar to that for A-Cu(OH)2/GO with a change in the metal source with 0.25 mmol CoCl2·6H2O and 0.25 mmol MnCl2·4H2O, respectively. A-M(OH)x/Gly solution (M = Cu, Co, and Mn) was prepared without mixing with GO. The C-Cu(OH)2/GO sample was prepared by a similar method of A-Cu(OH)2/GO with a change in the glycol solvent to 32 mL H2O. Briefly, 0.25 mmol CuCl2·2H2O and 8 mL GO aqueous solution (2 mg mL−1) were dissolved in 32 mL H2O to form a homogeneous solution, and 14 mol L−1 NH4OH was added to the solution with vigorous stirring until the pH value reached 9. After stirring for 10 min, the solution was isolated by centrifugation, washed three times with water, and dried by vacuum freeze dryer. These experimental steps were employed for the preparation of C-Co(OH)2/GO and C-Mn3O4/GO, respectively, but replaced the precursor of CuCl2·2H2O with 0.25 mmol CoCl2·6H2O and 0.25 mmol MnCl2·4H2O. Materials characterization Powder X-ray diffraction (PXRD) patterns were recorded on Miniflex6000 X-ray diffractometer (Rigaku Corp., Japan) at 40 kV and 15 mA using Cu-Kα radiation (λ = 1.54178 Å). The scanning rate was 3° min−1 from 5° to 65° in 2θ. The Raman spectra were measured by LabRAM HR (HORIBA Jobin Yvon Corp., Paris, France) with a 532 nm exaction laser. TEM and high-resolution TEM (HRTEM) were carried out by using Tenai F20 (FEI Corp., Hillsboro, OR) microscope with an acceleration voltage of 200 kV. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image was operated at 80 keV. Spherical aberration-corrected TEM images were carried out by Titan Cubed Themis G2 300 (FEI and Thermo Scientific Corp., United States) operated at 80 keV. The surface roughness was performed on the atomic force microscopy (AFM) in means of Dimension icon Scanning Probe Microscope (SPM) systems and digital instruments software (Version 6.12; Bruker Corp., United States). X-ray photoelectron spectroscopy (XPS) was performed by using ESCALAB™ 250Xi XPS spectrometer (Thermo Fisher Corporation, MA, United States) with Al Kα source. The binding energies obtained in the XPS spectral analysis which were corrected for specimen charging by referencing C 1 s to 284.8 eV. Inductively coupled plasma-optical emission spectrometer (ICP-OES) of Varian 710 [Agilent (VARIAN) Corp., United States] was used to determine the elemental composition of the catalysts. X-ray absorption fine structure measurements and analysis X-ray absorption fine structure (XAFS) data of Cu K-edge were collected at the BL14W1 station in the Shanghai Synchrotron Radiation Facility (SSRF) and the 1W1B station in in the Beijing Synchrotron Radiation Facility (BSRF). The storage rings of SSRF and BSRF were operated at 3.5 GeV with the current of 300 mA and at 2.5 GeV with the current of 250 mA, respectively. The acquired extended XAFS (EXAFS) data were processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages (Matthew Newville, University of Chicago, Chicago, IL). The k3-weighted EXAFS spectra were obtained by subtracting the post-edge background from the overall absorption and then normalizing with respect to the edge-jump step. Subsequently, k3-weighted χ(k) data in the k-space ranging from 2.3 to 14.0 Å−1 were Fourier-transformed to real (R) space using a hanning window function (dk = 1.0 Å−1) to separate the EXAFS contributions from different coordination shells. EXAFS fitting details To obtain the quantitative structural parameters around Cu atoms, least-squares curve parameter fitting was performed using the ARTEMIS module of IFEFFIT.51 Effective scattering amplitudes and phase-shifts for the Cu–O and Cu–Cu pairs were calculated with the ab initio code FEFF8.0.3 (Matthew Newville, University of Chicago). First of all, fits for the EXAFS data at Cu K-edge for bulk counterparts were performed. The coordination numbers of the first to second coordination shells were fixed as the nominal values, while the internal atomic distances R, Debye–Waller factor σ2, and the edge-energy shift E0 were allowed to run freely. The amplitude reduction factor S02 was also treated as an adjustable variable, and the obtained value for the bulk counterpart was fixed in fitting the subsequent Cu edge data for samples. The fit was done on the k3-weighted EXAFS function χ(k) data from 2.3 to 13.6 Å−1 in the R-range of 1.0–2.0 Å. The coordination numbers N, interatomic distances R, Debye–Waller factor σ2, and the edge-energy shift ΔE0 were allowed to run freely. Following the above fitting strategy, we got satisfactory curve-fitting results. In situ X-ray absorption near-edge structure (XANES) of Cu K-edge was measured by a self-built in situ electrochemical cell filled with O2-saturated 0.1 M KOH electrolyte using the catalyst loaded carbon paper (CP) as a working electrode, and saturated calomel electrode (SCE) electrodes and carbon rod were used as the reference and the counter electrodes, respectively. In situ XANES spectra were operated at the potential in the order of 1.0 V → 0.5 V → 1.0 V (vs RHE). XAFS data for each potential were recorded after the electrochemical equilibrium for 5 min, and were collected in fluorescence mode. Photometric peroxide measurement The photometric peroxide measurement was carried out by a cerium sulfate Ce(SO4)2 titration method,52 with a UV–vis spectroscope at 320 nm. In this work, we recorded the UV–vis spectrum curve on a Lambda 950 (PerkinElmer, Waltham, MA) with a wide linear range of the absorbance value from 0.1 to 3.0. The measured results with absorbance value below 3.0 were repeatable, reliable, and accurate. The color of the Ce(SO4)2 solution changed from a yellow solution of Ce4+ into colorless Ce3+ by following reaction: 2 Ce 4 + + H 2 O 2 → 2 Ce 3 + + 2 H + + O 2 (1)The H2O2 concentration exhibits a linear relationship to the absorption value with an adjusted R2 of 0.9993 and a rather low standard deviation, even with an absorbance value of up to 2.4 ( Supporting Information Figure S18). To ensure accuracy, the Ce(SO4)2 concentration should be diluted in the determination of H2O2 concentration if the absorbance value exceeds 2.5. For the H2O2 Faradaic efficiency (FE) measurement, H-cell was carried out by using 1 M KOH as both anolyte and catholyte (15 mL each), and the electrolytes were separated by a nafion 117 membrane. Teflon-treated CP loaded with catalysts (0.1 mg cm−2) was used as a working electrode, and a carbon rod was used as counter electrode. The catalyst-loading area was 1 cm × 0.5 cm, and the rest of the CPs were sealed with insulating sealant ( Supporting Information Figure S7). After an ORR measurement, a small volume of the catholyte was taken and neutralized, then 0.6 mM Ce(SO4)2 [19.9 mg Ce(SO4)2 in 100 mL sulfuric acid solution] was added. Subsequently the peroxide concentrations were determined at a certain potential until a certain amount of charge (3 C) was accumulated. The FE was calculated as follows: FE ( H 2 O 2 ) ( % ) = 2 C V F Q (2)where C is the H2O2 concentration (mol L−1), V is the volume of electrolyte (L), F is the Faraday constant (C mol−1), and Q is the amount of charge passed (C). Electrochemical measurements All electrochemical measurements were conducted by a CHI 760D (CH Instruments, Inc., Shanghai, China) electrochemistry workstation. A SCE electrodes and a carbon rod were used as the reference and the counter electrodes, respectively. A rotating ring disk electrode (RRDE; 0.1256 cm2) was used as the working electrode. To detect the H2O2 produced on the disk electrode, the Pt ring electrode was set to 1.23 V versus RHE at a speed of 1600 rpm. The cyclic voltammetry (CV) measurement at a scan rate of 50 mV s−1 and linear sweep voltammetry (LSV) at a scan rate of 5 mV s−1 were measured in Ar-saturated and O2-saturated 0.1 M KOH electrolyte, respectively. The effect of different loading amounts of A-Cu(OH)2/GO on the electrocatalytic activities and selectivity is also addressed in Supporting Information Figures S10 and S11. The optimal loading amount was fixed as 0.07 mg cm−2. Peroxide reduction reaction (PRR) was characterized in Ar-saturated 0.1 M KOH electrolyte containing 10 mM H2O2. All potentials were converted to reversible hydrogen electrode mg of catalyst was in a water and solution mL) with the of for at 1 to form a homogeneous catalyst the catalysts were loaded on to achieve a of 0.07 mg cm−2. The of ORR was corrected by the current measured in Ar-saturated The selectivity and the of transferred toward H2O2 production were calculated using the following H 2 O 2 selectivity H 2 O 2 ( % ) = 200 + of transferred = 4 + is the measured disk is the ring and = is the efficiency determined by using the reversible The electrochemical surface area was measured by the electrode potential in the ( Supporting Information Figure under the used for ORR The roughness factor of electrochemical surface area is by = C C electrochemical surface area ( cm 2 ) of the electrode is by A = A × current density cm 2 ) is by A = A the is the of the current and scan rate = and the reference ( C ) of a surface electrode surface is to be about 40 as a is the area of the electrode. and are calculated and on Supporting Information for each samples. H2O2 current ( H 2 O 2 ) was by disk current and the H2O2 Faradaic efficiency was obtained from the disk current and the ring current according to the of activity was obtained by the H2O2 production current ( H 2 O 2 ) and catalyst loading FE = The H 2 O 2 production current H 2 O 2 = FE = = activity = H 2 O 2 The below was used to the transferred and 1 = 1 + 1 = 1 + 1 F A 2 3 1 2 1 C is the F is C mol−1), A is the area of electrode is the of O2 × s−1 for 0.1 M is the electrode rate in is the s−1 for 0.1 M is the bulk concentration of O2 × mol L−1 for 0.1 M details All the calculations are implemented by in the DFT calculations were performed with under the using the functional for and the for and The were on a set with a energy of 30 and a of 300 The have been treated by the technique of and using a parameter of The optimized cell of a is a with the lattice parameter of × × on such a a 2 × 2 with a vacuum of is used to the the other we a to We from the 2 × 2 × 2 of crystalline but some OH* to the coordination from 4 to on the EXAFS results. Then we the by temperature at for with the of 1 The temperature is via 5 After the A-Cu(OH)2/GO is Then we one and one by the atoms After the with fixed cell was performed at 300 for another we the associated and one 30 after the equilibrium was reached in the first 100 which a of Then structural the associated adsorption energies of OH* and OOH* were calculated on these As for reaction we that structural the which is the intermediate for was only to with but to the for and that on but only after a is via the oxidation and of coordinated OH* and the associated reactions for on and + H + + → H 2 O + + O 2 + H + + → + H + + → H 2 O 2 + for the reaction on and 14 also with the of as a To the energies of the hydrogen electrode method by is to the energy of by the energy of the the adsorption energies of the associated reaction OOH* and OH* are calculated using the we the energies of the with and without A on catalyst M as and Then we get the adsorption energy of by = calculation should be on Pt = the value of as the we can the value of to the ORR energy of Pt As for the reaction has been in according to at = 1.23 the energy from to should be = = = and + = with the in the the reaction on we M ( ) = M ( ) = M A ( ) A ( ) + ( ) + ( RHE 1.23 V ) M ( ) = M A ( ) Pt ( ) + Pt ( ) + 3 ( RHE 1.23 V ) M ( + H 2 O ) = + 4 ( RHE 1.23 V ) potential such method can to the systematic by the of a different functional different that have reached the For DFT calculations to electrocatalytic we the energy is more than the and The A-Cu(OH)2/GO and C-Cu(OH)2/GO were readily prepared by method in condition, as in Figure CuCl2·2H2O powder was dissolved in glycol solvent with a solution by NH4OH dropwise until the pH of the solution amorphous were produced in the glycol solvent by the hydroxyl with copper and the solution turned but as in Figure and Supporting Information Figure it was to amorphous from glycol solvent even at rpm. the of the A-Cu(OH)2/GO was collected by at that the glycol solvent plays a role in the of and GO as of A-Cu(OH)2. In the were and with water as solvent the of In a of amorphous A-X% Cu(OH)2/GO with different copper were synthesized by only the precursor In the crystalline C-Cu(OH)2/GO was prepared by similar synthesis methods by using water as a solvent of Figure 1 process and structure investigation. to the C-Cu(OH)2/GO and to the A-Cu(OH)2/GO The digital image of the in water solution and amorphous are by the while the solution remain in solution without after The Cu K-edge XANES spectrum and the of EXAFS for

Highly Dispersive Metal Atoms Anchored on Carbon Matrix Obtained by Direct Rapid Pyrolysis of Metal Complexes

Two-Dimensional Metal–Organic Frameworks with Unique Oriented Layers for Oxygen Reduction Reaction: Tailoring the Activity through Exposed Crystal Facets

Carbon nanotube (CNT) possesses unique characteristics with extremely high mechanical resistance. This conducting material is suitable to fabricate electrode materials. Also it is expected to utilize CNT as an ideal supporting material because of its large surface area-t-volume ratio. However, it is quite difficult to equip functional materials including catalysts with CNT having surface inertness. To immobilize materials on CNT, some modification of the CNT surfaces, such as partial oxidation, is required to generate functional groups, to which functional materials are anchored. In our previous studies, we found that metal sputtering onto room temperature ionic liquid (RTIL) under reduced pressure produces metal nanoparticles suspended in the ionic liquid.1-3 By this way, several kinds of nanoparticles including alloy4 and hollow nanoparticles5 have been synthesized, so far. In addition, putting Au nanoparticles suspended IL onto the inert surface of a highly oriented pyrolytic graphite (HOPG) followed by heating was found to immobilize firmly the nanoparticles on the HOPG surface.6 In the present research, attempts have been made to preparation of Pt nanoparticle-adsorbed carbon nanotubes (Pt-CNT) by the aforementioned method and electrocatalytic activities of the prepared Pt-CNT toward O2reduction was examined. RTILs used for the preparation of Pt nanoparticles were dried in vacuum prior to use. A glass plate (2.5 ×2.5 cm), on which RTIL (0.4 mL) was spread, was set in a Cressington 108 auto SE sputter coater. A Pt foil target (Φ5.7 cm, 99.98%) was placed on 4.5 cm above the glass plate. Sputter deposition onto RTIL was conducted with 40 mA of sputter current in dry Ar (99.999 %) atmosphere whose pressure did not exceed 7 ± 1 Pa. The sputtering was conducted at room temperature (298 ± 2 K). Preparation of Pt-CNT was attempted by agitating the Pt sputtered RTIL (0.4 mL) with untreated CNTs (1 mg) at 573 K for 5 h. The resultant mixture was rinsed by dry acetonitrile several times to remove RTIL, followed by drying in vacuo. A transmission electron microscope (TEM) image of the resulting CNT is given in Figure 1, which shows adsorption of numerous Pt nanoparticles on the surface of CNT. Chemical analysis of Pt-CNT revealed that RTIL worked as a paste to stick Pt nanoparticles on the CNT surface. The mean particle size of Pt nanoparticles on SWCNT was ca. 3.5 nm and the amount of Pt nanoparticle on the Pt-SWCNT composite was 32.9 wt%. We prepared three Pt-SWCNT composites whose amounts of Pt on SWCNT were 7.1, 16.0, 24.1 and 32.9 wt% in order to examine their electrocatalytic activities toward oxygen reduction. A rotating ring-disk electrode (RRDE) with glassy carbon disk and Pt ring electrodes was used. Pt-CNT (1 mg) was dispersed in 0.2 mL iso-propanol and 5 μL of the dispersion liquid was applied to the disk electrode and fixed by putting 5 μL of a Nafion iso-propanol solution. The prepared electrode was pretreated with multiple potential scans between 0.5 and 1.25 V vs. RHE in 0.1 M HClO4 aqueous solution under N2. The electrochemical surface areas (ECSAs) were measured from cyclic voltammograms, giving 34.4, 30.3, 40.2, and 48.7 m2g-1of ECSAs for Pt-CNTs with 7.1, 16.0, 24.1 and 32.9 wt% Pt. Figure 2 shows voltammograms of disk electrodes modified with four kinds Pt-CNTs and the corresponding voltammograms for the ring electrodes. The onset potential of oxygen reduction became positive with increment of the Pt loading amount. The ring electrode detects H2O2produced in oxygen reduction at the disk electrode, however its amount was quite low; generation rate was below 5 % for Pt-CNT with 24.1 wt% and 32.9 wt% Pt even at potentials negative than 0.4 V vs. RHE, and also it was close to 0% for all cases at potentials between 0.7 to 0.85 V. These results showed that Pt-CNT has a favorable electrocatalytic activities for oxygen reduction.

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

Open AccessCCS ChemistryRESEARCH ARTICLES22 Oct 2022Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N5 Site for Enhanced Electrocatalytic Reaction Bingyu Huang†, Senhe Huang†, Chenbao Lu, Longbin Li, Judan Chen, Ting Hu, Dirk Lützenkirchen-Hecht, Kai Yuan, Xiaodong Zhuang and Yiwang Chen Bingyu Huang† Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Senhe Huang† Themeso-Entropy Matter Lab, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 , Chenbao Lu Themeso-Entropy Matter Lab, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 , Longbin Li Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Judan Chen Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Ting Hu School of Materials Science and Engineering, Nanchang University, Nanchang 330031 , Dirk Lützenkirchen-Hecht Faculty of Mathematics and Natural Sciences-Physics Department, Bergische Universität Wuppertal, Wuppertal D-42119 , Kai Yuan *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Xiaodong Zhuang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Themeso-Entropy Matter Lab, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 and Yiwang Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 Institute of Advanced Scientific Research (iASR), Key Lab of Fluorine and Silicon for Energy Materials and Chemistry of Ministry of Education, Jiangxi Normal University, Nanchang 330022 https://doi.org/10.31635/ccschem.022.202202241 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Metal porphyrins are star molecules that possess well-defined coordination metal centers for versatile catalytic reactions. However, most previous work has focused on the correlations between in-plane symmetric configuration of metal-N4 sites and their catalytic performance. Addressing the catalytic contribution of additional axial coordination to such symmetric configuration remains a challenge. Theoretical calculations revealed that axially anchoring an extra pyridine on the tetra-coordinated cobalt porphyrin (Co-N4) to construct penta-coordinated cobalt porphyrin (Co-N5) renders cobalt a higher electron density, thereby favoring the rate-determining O2 adsorption/activation and reducing the oxygen electroreduction barrier. Therefore, a well-defined Co-N5 site is rationally introduced into the azo-linked polymer framework for a fundamental structure–catalytic performance correlation study. As-prepared Co-N5 catalyst exhibits a 26 mV positive shift in half-wave potential compared with the pyridine-free Co-N4 counterpart, discloses a markedly higher power density (141.4 mW cm−2), and possesses better long-term durability (over 160 h cycles) in a Zn-air battery. Moreover, such a Co-N5 catalyst also showcases potential applications for CO2 reduction with high CO2-to-CO conversion faradic efficiency and better selectivity than the Co-N4 counterpart because coordination of the fifth pyridine evokes electronic localization that suppresses a competitive side reaction. This work proves the positive electrocatalytic contribution of axial penta-coordination on well-defined metal-porphyrin-based catalysts and offers atomic understanding of the structure–performance correlation on single atom catalysts for future catalyst design. Download figure Download PowerPoint Introduction Currently, many advanced electrocatalysts have been developed to facilitate the sluggish kinetics of the multiple-electron transfer process in energy conversion reactions, such as the oxygen reduction reaction (ORR) and CO2 reduction reaction (CO2RR).1–4 Due to maximized atom utilization, mainstream research has focused on the isolated transition metal/nitrogen coordinated (M-N-C) single-atom catalysts (SACs) with high catalytic activity and selectivity.5–9 Despite extensive investigations, the rational design and controllable and precise synthesis of M-N-C catalysts continues to be the main obstacle to multiple-electron transfer processes.10–12 The preparation of M-N-C catalysts normally needs high-temperature pyrolysis to increase the graphitization degree for better conductivity.13–17 Unfortunately, the pyrolysis process inevitably evokes metal aggregation due to the thermal decomposition of metal precursors and high surface energy of single metal atoms, posing challenges for maintaining atomic-metal isolation.13,18,19 The inhomogeneity and indistinction of the catalytic environment of carbonaceous materials give rise to tremendous difficulties in simultaneously enhancing the catalysts' activity and selectivity. In addition, the inherently less-defined active sites formed after pyrolysis leaves an ambiguous structure–performance relationship and seriously precludes us from exploring the in-depth mechanism for different electrocatalytic reactions.20–24 Therefore, the above-mentioned challenges stimulate the vigorous search for developing cost-effective and high-performance pyrolysis-free electrocatalysts with well-defined active sites. Transition-metal macrocycles, such as cobalt porphyrins, which possess the distinct tetra-coordinated cobalt porphyrin (Co-N4) site, have been heavily studied.25–28 Benefitting from the production of fewer radical oxygen species during the electrocatalytic process, the Co-N4 structure is more robust and advantageous for electrocatalysis of ORR.16,20,29–31 However, the representative plane-symmetric electron configuration of Co-N4 is not the optimal structure for the chemisorption and activation of reactants.32–34 Breaking the structure symmetry with penta-coordination to regulate the charge redistribution of Co-N4 sites could promote the electrocatalytic process, but the accurate synthesis of this structural motif is still a noteworthy challenge. Besides, the role of penta-coordination and the exact local microenvironment of such active sites have not yet been ascertained.35,36 In view of this, offering a model catalyst as an ideal platform is urgently required to elucidate the contribution of axial penta-coordination on catalytic activity, selectivity, and durability of Co-N4 sites. In this work, we probed the positive effect of axial pyridinic penta-coordinated cobalt porphyrin (Co-N5) to achieve optimized electronic localization on the Co-N5 site for boosting ORR through density functional theory (DFT) calculations as well as electrochemical analysis. DFT calculations indicate Co-N5 with axial pyridinic coordination possesses obviously higher electronic density on the Co center in comparison with Co-N4. Taking advantage of the electronic-push effect of penta-coordination, the ORR rate-determining step of O2 adsorption/activation can be significantly promoted, ensuring better ORR performance of Co-N5 than the Co-N4 model. Hence, we innovatively designed and synthesized an azo-linked polymer framework with atomically dispersed electron-rich Co-N5 sites for ORR through a pyrolysis-free axial-pyridinic coordination strategy. The Co-N5 catalyst displays impressive ORR performance with a more positive half-wave potential of 0.811 V and lower Tafel slope of 39 mV dec−1 than the Co-N4 counterpart (0.785 V and 50 mV dec−1), which support the theoretical predictions. Encouragingly, such a penta-coordination-induced electronic localization strategy also promises the potential for improving the CO2-to-CO conversion faradic efficiency and selectivity of electrocatalytic CO2 reduction. This work provides new design strategies toward well-defined single-atom electrocatalysts with axial coordination and offers new models for fundamentally understanding the catalytic mechanism of asymmetric coordination systems. Experimental Methods Synthesis of [email protected] catalysts First, 20 mg (27.3 μmol) of CoTAPP and 80 mg of G-py were redispersed in 20 mL of dimethyl sulfoxide (DMSO). Then, the mixture was stirred for 15 h to ensure CoTAPP was fully coordinated to G-py. Immediately after the coordination, 35.2 mg (0.11 mmol, 4 equiv to CoTAPP) of PhI(OAc)2 was added to the mixed solution, and the mixture was stirred for 24 h. The resultant precipitate was filtered and washed with DMSO, methanol, deionized water, and ethanol for three times, and dried at 60 °C overnight. [email protected] was harvested as a black powder (86.8 mg, 87% yield). [email protected] and [email protected] were prepared with a procedure similar to [email protected] Experimental details, materials characterization methods, and synthesis of other samples are available in the Supporting Information. Results and Discussion Theoretical calculations and catalytic mechanism Based on cobalt porphyrin, two models with different coordination environments, axial pyridinic Co-N5 model and Co-N4 model were designed (Figure 1a and Supporting Information Figure S1). The elementary steps and corresponding ORR adsorption configurations on the two models are presented in Supporting Information Figure S2. The Gibbs free energies at different potentials for all elementary steps were evaluated to illustrate how the penta-coordination affects intrinsic ORR activity. As given in Figure 1b, all elementary steps are distinctly downhill at U = 0 V, thus the reaction is exothermic and able to proceed spontaneously. When the potential rises to 1.23 V, the rate-determining step of Co-N4 is the first step (* + O2 + H2O + e− → *OOH + OH−) with a high energy barrier of 0.34 eV. In contrast, the final release step (*OH + e− → OH− + *) of the Co-N5 model is the rate-determining step, and its free energy can be distinctly reduced to only 0.26 eV. The thermodynamic limiting potentials, which represent the maximum potential to ensure all steps downhill are 0.97 and 0.89 V for Co-N5 and Co-N4 models, respectively (Figure 1c and Supporting Information Figure S3), revealed the Co-N5 model requires the minimum overpotential to drive the oxygen reduction. This result implies that the axial pyridine coordination plays a crucial role in regulating the ORR intermediates adsorption strength and decreasing the reaction barrier of the rate-determining step. Similarly, axial pyridine coordination can also alter the Fe electronic microenvironment, thereby enhancing the catalytic activity of the Fe-N5 model ( Supporting Information Figures S4–S7), further validating the universality of additional axial coordination. Figure 1 | (a) DFT calculation models of Co-N5 and Co-N4 for the electrochemically catalyzed ORR. (b) Free energy diagrams at U = 0 V and U = 1.23 V, and (c) free energy diagrams for the thermodynamic limiting potentials of Co-N5 model and Co-N4 model. (d) PDOS of Co atom for Co-N5 model (top) and Co-N4 model (bottom); the d-band center is denoted by the dashed gray line. Differential charge density distribution after O2 absorption on (e) Co-N5 model and (f) Co-N4 model. Download figure Download PowerPoint To further study axial coordination induced changes in the Co center's electronic configuration and interaction with oxygen-containing intermediates, projected density of states (PDOS) was conducted. The axial coordination obviously tunes the Co 3d orbital according to the PDOS in Figure 1d. The d-band center of the Co-N5 model at −1.59 eV is closer to the Fermi level than that of Co-N4 at −1.76 eV, thereby leading to an increase in O-containing intermediates' adsorption.37 The enhanced adsorption ensures subsequent ORR steps proceed through a more efficient four-electron pathway. Hence, the thermodynamic onset potentials improvement can be ascribed to fine-tuned adsorption strength of the ORR intermediates, which directly determines the activity and selectivity of catalysts. In addition, the higher PDOS near the Fermi level represents more abundant charge carriers and better electronic conductivity for the Co-N5 model. During the ORR process, the activated Co d orbitals and their hybridization with O p orbitals co-determine the adsorption strength for oxygen-containing adsorbates.38–42 After O2 adsorption and *OH formation, the increased overlapping degree of the strong σ-bond that originates from the Co dz2 orbital and O p orbital, along with the decreased O2 antibonding orbital filling degree that appears above the Fermi level for Co-N5 model, can be observed. It theoretically suggests that the extra fifth pyridine coordination assures a tighter connection between Co centers and O2 ( Supporting Information Figures S8 and S9), thus enabling higher ORR selectivity towards the four-electron pathway.43 As verified by charge density differences ( Supporting Information Figure S10), obvious asymmetrical charge distribution caused by axial coordination can be found for the Co-N5 model in comparison with the symmetric Co-N4 model. An apparent charge accumulation on the Co center is found to form the electron-rich Co-N5 site due to the electronic-push effect of axial pyridine. As expected, the O2 only absorbs on the individual Co center (Figure 1e,f), which can provide superior ORR electrocatalytic sites. Furthermore, the axial pyridine can construct an electronic pathway that renders adequate charge transfer to oxygen molecules from the conductive graphene layer. In general, the electron-rich Co-N5 site promises stable chemisorption and activation of O2, which facilitates O–O bond cleavage, thus offering better selectivity for the four-electron reduction pathway. Therefore, the axial-pyridine coordination-induced electronic localization strategy is viable to efficaciously enhance the ORR kinetics. Synthesis and structural characterization To experimentally confirm the calculation results and demonstrate the significance of penta-coordination architecture in ORR, an azo-linked penta-coordinated cobalt porphyrin-based polymer (CoTAPP-Azo) anchored on pyridine functionalized graphene (G-py) ([email protected]) was synthesized. The synthesis route and structure of [email protected] and the counterpart [email protected] (directly grown CoTAPP-Azo on pristine graphene) are schematically revealed in Figure 2a. Tetrakis(4-aminophenyl) porphyrin (TAPP) was first chelated with cobalt cation to obtain CoTAPP ( Supporting Information Figure S11). TAPP coordinating with Fe (FeTAPP) and Ni (NiTAPP) were also successfully obtained using the same procedure. The formation of CoTAPP, FeTAPP, and NiTAPP were confirmed by mass spectrometry ( Supporting Information Figure S12) and Fourier-transform infrared (FTIR) spectroscopy ( Supporting Information Figure S13). The successful pyridine functionalization in G-py was confirmed by thermogravimetric analysis ( Supporting Information Figure S14), Raman spectroscopy ( Supporting Information Figure S15), and X-ray photoelectron spectroscopy (XPS) ( Supporting Information Figure S16). Subsequently, CoTAPP was anchored on G-py through axial pyridine coordination; following an azo-coupling reaction, [email protected] was finally obtained (see details in Supporting Information Scheme S1). For comparison, FeTAPP-based polymer hybridized with G-py ([email protected]) and graphene ([email protected]) and NiTAPP-based polymer hybridized with G-py ([email protected]), were synthesized by similar routes. Figure 2 | (a) Synthetic route of CoTA[email protected]y and [email protected] (b) HAADF-STEM image of [email protected] (c) High-resolution Co 2p XPS spectra of [email protected] and [email protected] Download figure Download PowerPoint The morphology of [email protected] was first identified by scanning electron microscopy (SEM). Compared with the thin-layered sheet structure of G-py, a thicker layer was observed, indicating G-py was covered by CoTAPP-Azo ( Supporting Information Figures S17 and 18). The transmission electron microscopy (TEM) image of [email protected] also presented representative lamellar plate morphology ( Supporting Information Figure S18c). To further discern the structural features at the atomic level, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was carried out. HAADF-STEM images and the corresponding energy dispersive spectroscopy (EDS) of [email protected] indicated homogeneous spatial distribution of C, N, Co elements ( Supporting Information Figure S19). The abundant bright points (highlighted by red circles) offer direct evidence for the uniform distribution of single Co atoms (Figure 2b). Similarly, [email protected], [email protected], and [email protected] all exhibited similar lamella morphology with homogeneously distributed elements ( Supporting Information Figures S20–S22). The chemical structures of [email protected] were preliminarily examined by FTIR spectroscopy ( Supporting Information Figure S23). Compared with CoTAPP, the corresponding FTIR spectrum of [email protected] displayed a significant intensity increase of N=N stretching signals (1570 cm−1, 1215 cm−1) along with an obviously weakened N–H peak at 3400 cm−1, proving the successful polymerization of CoTAPP. The increased ID/IG ratio of 0.94 for [email protected] to 1.07 for [email protected] in the Raman spectra ( Supporting Information Figure S24) verifies surface lattice interference caused by penta-coordination. X-ray diffraction (XRD) confirmed that G-py was coated by CoTAPP-Azo, and the absence of crystalline cobalt species diffraction peaks demonstrated the highly dispersed state of Co atoms ( Supporting Information Figure S25), agreeing well with the HAADF-STEM observations. [email protected] and [email protected] both show similar XRD patterns ( Supporting Information Figures S26 and S27). Furthermore, XPS was extracted to unravel the nature of the chemical bonding of [email protected] and [email protected] ( Supporting Information Figures S28–S31). Compared with [email protected], the N 1s XPS spectrum for [email protected] displayed an obvious pyridinic N peak (398.3 eV, Supporting Information Figure S29). The C 1s XPS spectrum of [email protected] showed that the C–N peak shifts to higher binding energy (BE) than that of [email protected] (ΔBE = 0.4 eV), revealing the decreased electron density of C atom. In the Co 2p spectra (Figure 2c), [email protected] underwent a shift to lower BE for Co 2p3/2 (780.3 eV) and Co 2p1/2 (795.6 eV) peaks, relative to that of [email protected] (780.8 and 796.0 eV for Co 2p3/2 and Co 2p1/2, respectively), providing evidence for the electronic localization on the Co with penta-coordination.44,45 This result confirms that axial pyridine ligands act as channels between G-py and CoTAPP-Azo and boost the charge transfer from graphene to Co centers, which is consistent with the electron-rich Co-N5 model in theoretical calculations. Importantly, similar phenomena were found in [email protected] and [email protected], suggesting the universality of such an electron localization approach. The UV–vis spectrum of [email protected] revealed a clear porphyrin Soret band at 448 nm ( Supporting Information Figure S32). Interestingly, an appreciable peak change to 438 nm in [email protected] was observed due to the penta-coordination-induced electron transfer. Meanwhile, photoluminescence spectroscopy was conducted to explore the charge separation behaviors ( Supporting Information Figure S33). Significantly increased quenching occurred in [email protected] compared with [email protected], further demonstrating that axial pyridine can enhance the charge transfer between G-py and CoTAPP-Azo. The work function, which represents the minimum energy needed to draw one inner electron from the nucleus, was obtained by ultraviolet photoelectron spectroscopy. [email protected] showed a 0.51 eV shift to higher BE than [email protected] in the second electron cut-off edge ( Supporting Information Figure S34). Hence, with respect to 4.49 eV for [email protected], the smaller work function for [email protected] (3.98 eV) indicated that the electrons are more likely to be activated and transferred outward.46,47 We assessed the porosity of the samples by nitrogen physisorption isotherms ( Supporting Information Figure S35). [email protected] and [email protected] both displayed the typical type-IV isotherms with an evident hysteresis loop, indicative of the coexistence of micropores and mesopores. The Brunauer–Emmett–Teller specific surface areas for [email protected] and [email protected] were calculated to be 506.6 and 440.3 m²/g, respectively. Pore size distribution demonstrated by nonlocalized DFT showed the pore widths centered at 1.9 and 4.8 nm for [email protected] ( Supporting Information Figure S36). These results demonstrate the improved surface area and hierarchical porous structure of [email protected], which are conducive to increase the quantity of accessible active sites and enhance the mass transport. To investigate the local coordination environment of the Co center, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses were examined. The stronger white-line peak intensity of [email protected] (7727 eV) compared with [email protected] was assigned to the larger coordination number of the Co center for [email protected] than [email protected] (Figure 3a).48 The quantitative fits ( Supporting Information Table S1) showed a Co atom in [email protected] was straightforwardly connected by N atoms with a coordination number of 5.1 and the average bond length of 1.98 Å. From the Co K-edge Fourier-transformed EXAFS spectra (Figure 3b), the first shell peak for [email protected] around 1.43 Å corresponded to the Co–N scattering path, accompanying the absent Co–Co peak at 2.18 Å, jointly signifying the isolated Co atoms' configuration. [email protected] displayed an obviously positive shift for the Co–N first shell in than [email protected], suggesting an Co–N bond length caused by the to the EXAFS results (Figure and Supporting Information Figures and in to an of for [email protected], the first shell intensity for [email protected] was enhanced with an to revealing the larger number of N coordination The was also for the coordination In Figure the intensity maximum at for [email protected] was assigned to Co–N Co–Co and corresponding coordination was demonstrating the of the Co The shift of [email protected] maximum compared with that of CoTAPP is caused by the in [email protected] Hence, on and EXAFS as well as the XPS we that [email protected] by Co-N5 sites was successfully synthesized. Figure | (a) Co K-edge spectra and (b) the of Co K-edge EXAFS spectra of [email protected] with Co and CoTAPP as (c) The corresponding EXAFS of [email protected] model with Co N and C (d) of [email protected] in comparison with Co and CoTAPP. Download figure Download PowerPoint Electrocatalytic ORR performance To how axial penta-coordination affects ORR [email protected] and [email protected] were first evaluated with a In ( Supporting Information Figure significant peaks in for [email protected], but a peak was observed in The (Figure and Supporting Information Figure that [email protected] the ORR performance with the most positive half-wave potential of 0.811 V which is 26 mV more positive than that of [email protected] (0.785 V These results indicate that [email protected] possesses superior catalytic performance that from the electron localization caused by pyridine penta-coordination. in comparison with [email protected], the density for [email protected] was enhanced because the axial pyridine as between G-py and CoTAPP-Azo, thus boosting the charge transfer from graphene to CoTAPP layer. Figure 4 | (a) ORR of [email protected] and [email protected] catalysts at (b) Tafel (c) density by electrochemically active surface and (d) comparison of Tafel and for [email protected] and [email protected] catalysts. (e) The of [email protected] and [email protected] and after (f) and power density specific for Zn-air at and long-term performance of Zn-air at a density of using [email protected], [email protected], and as catalysts. Download figure Download PowerPoint To further into the reaction kinetics of [email protected], the at from to were ( Supporting Information Figure The ( Supporting Information Figure which were from at different potentials, revealed the toward the of Based on the the number for [email protected] was which with the This proves that the Co-N5 site a four-electron pathway towards ORR, [email protected] has a smaller of ( Supporting Information Figure We the enhanced ORR selectivity to the Co-N5 configuration that can be with because of the effect between Co and pyridinic the resultant O–O bond with stronger stretching is thus providing higher selectivity for four-electron ORR. [email protected] has a more positive than [email protected] ( Supporting Information Figures that ORR activity can be improved by microenvironment through extra axial penta-coordination on different metal Moreover, the Tafel slope of 39 mV dec−1 the process of [email protected] for oxygen reduction (Figure and Supporting Information Figure by the of electrochemical spectroscopy ( Supporting Information Figure the decreased in the for [email protected], indicating the optimized and electron toward ORR kinetics. In addition, [email protected] exhibited the electrochemically active surface area ( Supporting Information Figure which is conducive to more active sites. density with an in the ORR can be found (Figure the intrinsic activity for [email protected] For further comparison, the performance density Tafel density and for different electrocatalysts are (Figure and

Single-atom catalysts (SACs) exhibit intriguing performance in electrocatalysis owing to their maximized atom utilizations and unique electronic structures, but effective anchoring metal atoms with defined coordination structure on hierarchical integrated electrode remain a challenge. Herein, a fast and facial flame-assisted strategy is developed to construct oxygen-coordinated SACs on integrated carbon nanotube (CNT) arrays with promising applications in electrocatalysis. Density functional theory calculations show that oxygen in carbon substrate imparts homogeneous sites for the efficient anchoring of metal atoms, thereby enabling SACs to disperse uniformly and firmly and thus bringing optimized activities. Moreover, the integrated CNT array with abundant oxygen-containing groups is constructed and has been used as an efficient matrix for anchoring metal atoms (CNT-O@M) via a flame-assisted method. The as-prepared CNT-O@M (M = Co and Pt as typical examples) shows excellent activities in electrocatalytic oxygen reduction reaction and hydrogen evolution reaction with utilization of active site as high as 75.7%, which is superior to the reported SACs. Particularly, the performance of CNT-O@M can maintain stably under various harsh conditions, showing a promising prospect in the long-time applications. The methodology and concept proposed in this work could be extended to the synthesis of a variety of integrated SACs for efficient electrocatalysis.

Electrocatalysts for the oxygen reduction reaction (ORR) are extremely crucial for advanced energy conversion technologies, such as fuel cell batteries. A promising ORR catalyst usually should have low overpotentials, rich catalytic sites and low cost. In the past decade, single-atom catalyst (SAC) TM-N4 (TM = Fe, Co, etc.) embedded graphene matrixes have been widely studied for their promising performance and low cost for ORR catalysis, but the effect of coordination on the ORR activity is not fully understood. In this work, we will employ density functional theory (DFT) calculations to systematically investigate the ORR activity of 40 different 3d transition metal single-atom catalysts (SACs) supported on nitrogen-doped graphene supports, ranging from vanadium to zinc. Five different nitrogen coordination configurations (TM-NxC4-x with x = 0, 1, 2, 3, and 4) were studied to reveal how C/N substitution affects the ORR activity. By looking at the stability, free energy diagram, overpotential, and scaling relationship, our calculation showed that partial C substitution can effectively improve the ORR performance of Mn, Co, Ni, and Zn-based SACs. The volcano plot obtained from the scaling relationship indicated that the substitution of N by C could distinctively affect the potential-limiting step in the ORR, which leads to the enhanced or weakened ORR performance. Density of states and d-band center analysis suggested that this coordination-tuned ORR activity can be explained by the shift of the d-band center due to the coordination effect. Finally, four candidates with optimal ORR activity and dynamic stability were proposed from the pool: NiC4, CoNC3, CrN4, and ZnN3C. Our work provides a feasible designing strategy to improve the ORR activity of graphene-based TM-N4 SACs by tuning the coordination environment, which may have potential implication in the high-performance fuel cell development.

Single Pd atoms anchored graphitic carbon nitride for highly selective and stable photocatalysis of nitric oxide

In the context of catalytic CO2 reduction (CO2RR), the interference of the inherent hydrogen evolution reaction (HER) and the possible selectivity towards CO have posed a significant challenge to the generation of formic acid. To address this hurdle, in this work, we have investigated the impact of different single-atom metal catalysts on tuning selectivity by employing density functional theory (DFT) calculations to scrutinize the reaction pathways. Single-atom catalysts supported on carbon-based systems have proven to be pivotal in altering both the activity and selectivity of the CO2RR. In this study, a series of single-atom-metal-loaded g-C3N4 monolayers (MCN, M = Ni, Cu, Zn, Ga, Cd, In, Sn, Pb, Ag, Au, Bi, Pd and Pt) were systematically examined. Through detailed DFT calculations, we explored their influence on reaction selectivity between the *COOH and *OCHO intermediates. Notably, NiCN favors the reaction via the *OCHO route, with a significantly lower rate-determining potential of 0.36 eV, which is approximately 73.5% lower than that of the CN system (1.36 eV). Most importantly, the Ni single-atom catalyst with lower coordination significantly enhances CO2 adsorption, promoting CO2RR over HER. Overall, this study, guided by DFT calculations, provides a theoretical prediction of how the selection of single-atom metal catalysts can effectively modulate the reaction pathway, thereby offering a potential solution for achieving high product selectivity in CO2RR.

Exploring highly active and cost-efficient single-atom catalysts (SACs) for oxygen reduction reaction (ORR) is critical for the large-scale application of Zn-air battery. Herein, density functional theory (DFT) calculations predict that the intrinsic ORR activity of the active metal of SACs follows the trend of Co > Fe > Ni ≈ Cu, in which Co SACs possess the best ORR activity due to its optimized spin density. Guided by DFT calculations, four kinds of transition metal single atoms embedded in 3D porous nitrogen-doped carbon nanosheets (MSAs@PNCN, M = Co, Ni, Fe, Cu) are synthesized via a facile NaCl-template assisted strategy. The resulting MSAs@PNCN displays ORR activity trend in lines with the theoretical predictions, and the Co SAs@PNCN exhibits the best ORR activity (E1/2 = 0.851V), being comparable to that of Pt/C under alkaline conditions. X-ray absorption fine structure (XAFS) spectra verify the atomically dispersed Co-N4 sites are the catalytically active sites. The highly active CoN4 sites and the unique 3D porous structure contribute to the outstanding ORR performance of Co SAs@PNCN. Furthermore, the Co SAs@PNCN catalyst is employed as cathode in Zn-air battery, which can deliver a large power density of 220 mW cm-2 and maintain robust cycling stability over 530 cycles.

Recently, while studying the oxygen reduction reaction (ORR) in alkaline medium on Mn oxides with various crystal structures, we documented [1] an unparalleled activity of Mn2O3 oxide with bixbyite structure. Then we also observed a clear correlation of the electrocatalytic activity with the formal potential of Mn(IV/III) interfacial redox transition and suggested the ORR mechanism involving this transition as a mediator step. To extend our understanding of this complex mechanism, we also compared the behavior of various oxides in peroxide oxidation/reduction reactions [2] and in respect to the peroxide yield during the ORR. All materials were studied in the form of composites with Sibunit carbon (weight ratio 1:1). This carbon binder provides a high utilization of the oxide surface in electrocatalysis [3]. The voltammetry in deairated 1M NaOH solution and rotating disc electrode (RDE) measurements in the same but O2-saturated solution were used to determine the total charge and the ORR kinetic currents. Rotating ring disc electrode (RRDE) was applied to compare the peroxide yields on various Mn oxides. All the values were compared after normalizing them to the BET surface area. The number of electrons involved in the oxygen reduction was close to 4 and 3 for more and less active oxides, respectively. Less active oxides demonstrated significantly higher peroxide yields and slower peroxide reduction, with a convection-independent limiting current which documents a slow chemical step in a wide potential region. The independent experimental facts which can be considered for comparison with simulations are as follows: surface concentration of the Mn active centers (estimated from the total charge in Mn(IV/III) transition region);specific activity of pure carbon generating additional peroxide, which further reacts on the oxide component in carbon-oxide composites (see in Ref.[4] about the dual role of carbon);ORR polarization curves;polarization curves for peroxide reactions;peroxide yield during the ORR. This wide set of data provides a valuable input for multi-parametric simulations, which significantly decreases the number of possible solutions. The modeling involved the rate constants for five reaction steps as well as the parameter for interactions of the adsorbed oxygen species involved in the Mn(IV/III) transition (in terms of Frumkin isotherm). We found that the complete set of data can be interpreted only assuming the structural dependence of at least two rate constants: one at the initial stage (peroxide formation) and another at the final stage (peroxide reduction). Another intriguing possibility to explain the very high Mn2O3 activity in the ORR is to assume a direct ORR pathway for this oxide, and to consider a series pathway for all other oxides. To elucidate whether the latter hypothesis is reasonable, we addressed the O-O bond break on Mn2O3 and MnOOH (typical examples of high and low activities, respectively) in the framework of a quantum chemical approach resting on the cluster model of the oxide surface and the DFT level of theory. The computational results were used to map the effective reaction path. We found that for both model surfaces the direct bond cleavage at the oxide surface is hardly feasable both for the O2 molecule and most likely for peroxide intermediates. This finding makes it possible to exclude the direct pathway and to focus on molecular reasons of the structural effects on the rate constant for other steps such as the reduction of the adsorbed peroxide intermediate. The work is supported by EraNetRus program, project (#270 NANO-Morf). [1] A.S. Ryabova et al, Electrochimica Acta 187 (2016) 161–172. [2] A.S. Ryabova et al, ChemElectroChem 3 (2016) 1667 – 1677. [3] A.S. Ryabova et al, Electrochimica Acta 246 (2017) 643–653. [4] T. Poux et al, Catalysis Today 189 (2012) 83– 92.

Feasibility study to probe the growth of chromate based conversion on Al by means of rotating ring disc electrode (RRDE) measurements

In-situ synthesized TiC@CNT as high-performance catalysts for oxygen reduction reaction

Polymer electrolyte membrane fuel cells (PEMFC) are electrochemical devices which can directly convert the chemical energy of a fuel (such as hydrogen or a low-molecular weight alcohol) and an oxidant (i.e. oxygen) into electrical energy with high efficiency. Moreover, due their low operating temperature, they are suitable for automotive or portable applications. However, the slow kinetics of oxygen reduction reaction (ORR) requires the use of costly Pt-based catalysts at the cathode in order to obtain the desired power density values. Nevertheless, the cathode is still responsible for the main voltage loss in the cell. The overall objective of the research carried out in this Ph.D. thesis was the development of Pt-free ORR catalysts starting from different carbon, nitrogen and transition metals precursors. Different synthesis approaches were used in order to obtain an improvement of the activity, and to understand the influence of the synthesis process variables. In particular, the influence of carbon supports (commercial and synthesized in the lab), nitrogen and transition metals precursors, templating agents, number and temperature of pyrolysis were examined. The catalysts produced were characterized by means of several instrumental techniques such as N2 physisorption, XRD, XPS, EDX, SEM, FESEM, TEM, Raman and FTIR. The effect of the presence of different transition metals on the pyrolysis process was investigated by TGA coupled with a mass spectroscopy analysis, in order to have an insight on their influence in the formation of ORR active sites. The activity toward ORR was assessed by RDE-RRDE (rotating disk electrode - rotating ring disk electrode) analysis and by gas-diffusion electrode in a 3-electrodes electrochemical cell configuration. The electrochemical techniques used were cyclic voltammetry (CV), linear sweep voltammetry (LSV), staircase voltammetry (SV), chronoamperometry and electrochemical impedance spectroscopy (EIS). These electrochemical tests were performed in both acid and alkaline conditions, with reference to the potential applications in both H+ and OH- conducing polymer electrolyte membrane fuel cells. This first part of research was carried out in the laboratories of the Gre.En2 (Green Energy and Engineering) Group in the Department of Applied Science and Technology (DISAT) at Politecnico di Torino. Then, in the second part, some of the most promising electrocatalysts in terms of ORR activity were in different types of single PEMFC. In particular, using acidic electrolyte membrane, the tests were performed using H2 or methanol as fuels. In the case of direct methanol fuel cell (DMFC) tests, short-term durability tests were done in order to compare the durability performance of our catalysts with a standard Pt-based catalysts. The tests with alkaline electrolyte membrane were performed using ethanol as fuel. This second part of research was carried out at the Universidad Autonoma de Madrid in the laboratories of the Department of Applied Physical-Chemistry. Here the structure of the thesis: Chapter 1 is a general introduction about the PEMFC fuel cell technology, particularly focusing on the non-noble metal catalysts for ORR as potential alternative to Pt. Chapter 2 is focused on the use of different types of reduced graphene oxide as support for the synthesis of Fe-N/C catalysts. In Chapter 3, a complex between Co ions and a N-containing ligand molecule is impregnated on multi walled carbon nanotubes and pyrolyzed one or two times for producing a Co-N-C catalyst, and the influence of the second pyrolysis on the activity improvement was investigated. Chapter 4 deals the optimization of the synthesis process of a Fe-N-C catalyst using polypyrrole as N source and mesoporous carbon a C-support. In Chapter 5 the study of the influence of different silica templates on the morphology on the ORR activity of a Fe-N-C catalyst synthesized using Fe-ph