The Enhancement of Selectivity and Activity for Two-Electron Oxygen Reduction Reaction by Tuned Oxygen Defects on Amorphous Hydroxide Catalysts

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 in Ar-saturated 0.1 M KOH electrolyte 10 mM H2O2. All were converted to 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 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 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