Oxygen Reduction In Alkaline Electrolyte Research Articles

PtM/CNT (M = Mo, Ni, CoCr) Electrocatalysts with Reduced Platinum Content for Anodic Hydrogen Oxidation and Cathodic Oxygen Reduction in Alkaline Electrolytes

Bimetallic catalysts containing platinum and transition metals (PtM, M = Mo, Ni, CoCr) were synthesized on carbon nanotubes (CNTs) functionalized in an alkaline medium. Their platinum content is 10–15% by mass. PtM/CNTNaOH are active in both the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) in alkaline electrolytes. Although catalysts based on a single transition metal are inactive in the HOR, their activity in the cathode process of ORR increases relative to CNTNaOH. When using the rotating ring-disk electrode method for ORR, PtM/CNT showed a high selectivity in reducing oxygen directly to water. In HOR, the PtM/CNT catalyst had an activity comparable to that of a commercial monoplatinum catalyst. The results obtained show that it is possible to use the PtM/CNT catalyst in an alkaline fuel cell both as an anode and as a cathode.

Effect of the Platinum Mass Content in a Catalyst and the State of the Support Surface on the Path of the Oxygen Reduction Reaction in Alkaline Electrolyte

The effect of the support nature and the mass of platinum on the stability, electrochemical characteristics of monoplatinum catalysts, and the reaction path of electrochemical oxygen reduction in alkaline electrolyte is studied. Catalysts with the Pt mass content of 10, 20, 40, 60 wt % are synthesized by the polyol method on carbon nanotubes functionalized in NaOH and doped with nitrogen. The activity, the percentage of hydrogen peroxide formed, and the number of electrons participating in the oxygen reduction reaction are determined from the data obtained by the rotating ring-disk electrode method. For catalysts synthesized on the nitrogen-doped carbon nanotubes, the highest selectivity in the reaction of oxygen reduction to water is observed; the higher Pt surface area at the electrode, the greater is the selectivity, because the contribution of the support surface to the total oxygen reduction reaction decreased. Both the presence of hydrogen peroxide and a decrease in stability result from the decrease in the platinum content in the catalyst.

Investigating Perovskite Oxide Catalysts As Bifunctional Oxygen Electrodes Using Operando XAS

With the increase in renewable energy usage comes the need for energy storage systems due to intermittency issues. Hydrogen storage systems have been identified as one solution. Unitized regenerative fuel cells (URFC) combine electrolyzers and fuel cells in one device, allowing electricity to be stored and used easily. However, the oxygen electrodes are still affected by high overpotentials and slow kinetics. Perovskite oxides have been identified as a class of materials, which are low-cost, tunable, and active for the oxygen reduction (ORR) and evolution (OER) reactions. Here, we investigate perovskites as bifunctional catalysts for ORR and OER in alkaline solution. We examine and compare two strategies for bifunctional catalysts: using one catalyst, which is able to perform OER and ORR vs. a combination of two catalysts, one active for ORR and one active for OER. Frequently, the catalysts’ performances for these two reactions are measured separately.1,2,3 Here, we investigate how these bifunctional catalysts respond to cycling between the OER and ORR regions.Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) is known to be a promising OER catalyst.4,5,6 However, without carbon, it lacks ORR activity.4 La(1-x)SrxMnO3 (LSM) is a promising ORR catalyst.3,7 However, without modification, it has been shown to have limited OER activity.3 Separately, these catalysts lack high performance for both reactions. Here, we combine the two catalysts into a BSCF/LSM/Carbon composite electrode and compare to electrodes prepared from the constituent single material components. In addition, we have synthesized single material perovskites containing both Co and Mn that to the best of our knowledge have never been tested as electrodes for ORR/OER. In order to understand the catalysts’ behaviors under OER and ORR conditions, X-ray adsorption spectroscopy (XAS) was measured continuously while performing cyclic voltammetry. We were able to monitor the continuous changes of the Co, Mn, and Fe oxidation states and local environment during OER and ORR with remarkably high time/applied potential resolution. Our findings illustrate the reversible and irreversible changes that can occur during OER and ORR and provide strategies for future bifunctional catalyst design.References Kirsanova, M. A.; Okatenko, V. D.; Aksyonov, D. A.; Forslund, R. P.; Mefford, J. T.; Stevenson, K. J.; Abakumov, A. M. Bifunctional OER/ORR Catalytic Activity in the Tetrahedral YBaCo 4 O 7.3 Oxide. Mater. Chem. A 2019, 7 (1), 330–341.Elumeeva, K.; Masa, J.; Sierau, J.; Tietz, F.; Muhler, M.; Schuhmann, W. Perovskite-Based Bifunctional Electrocatalysts for Oxygen Evolution and Oxygen Reduction in Alkaline Electrolytes. Acta 2016, 208, 25–32.Xu, W.; Apodaca, N.; Wang, H.; Yan, L.; Chen, G.; Zhou, M.; Ding, D.; Choudhury, P.; Luo, H. A-Site Excessive (La0.8Sr0.2)1+ XMnO3 Perovskite Oxides for Bifunctional Oxygen Catalyst in Alkaline Media. ACS Catal. 2019, 9 (6), 5074–5083.Fabbri, E.; Nachtegaal, M.; Cheng, X.; Schmidt, T. J. Superior Bifunctional Electrocatalytic Activity of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ /Carbon Composite Electrodes: Insight into the Local Electronic Structure. Energy Mater. 2015, 5 (17), 1402033.Fabbri, E.; Nachtegaal, M.; Binninger, T.; Cheng, X.; Kim, B.-J.; Durst, J.; Bozza, F.; Graule, T.; Schäublin, R.; Wiles, L.; Pertoso, M.; Danilovic, N.; Ayers, K. E.; Schmidt, T. J. Dynamic Surface Self-Reconstruction Is the Key of Highly Active Perovskite Nano-Electrocatalysts for Water Splitting. Mater. 2017, 16 (9), 925–931.Kim, B. J.; Fabbri, E.; Abbott, D. F.; Cheng, X.; Clark, A. H.; Nachtegaal, M.; Borlaf, M.; Castelli, I. E.; Graule, T.; Schmidt, T. J. Functional Role of Fe-Doping in Co-Based Perovskite Oxide Catalysts for Oxygen Evolution Reaction. Am. Chem. Soc. 2019, 141 (13), 5231–5240.Tulloch, J.; Donne, S. W. Activity of Perovskite La1−xSrxMnO3 Catalysts towards Oxygen Reduction in Alkaline Electrolytes. Power Sources 2009, 188 (2), 359–366.

Synthesis of PdV/C nanoparticles using phase transfer method for oxygen reduction in alkaline electrolytes

Bimetallic PdV/C nanoparticles have been synthesized by phase-transfer method for catalyzing the oxygen reduction reaction (ORR) in alkaline electrolytes. PdV/C nanoparticles with different V contents are spherical with a mean diameter of 3–4 nm, and the addition of V expands the lattice parameter of Pd as shown by XRD. XPS analysis indicated the binding energy of Pd0 3d peak increased by ca. 1 eV. Density functional theory (DFT) calculations results shows d-band center of Pd down-shift after V-doping. Based on the electrocatalytic results, introducing V can improve the catalytic activity for ORR, methanol crossover tolerance and stability. Especially, Pd4V/C shows higher initial potential (Eonset = 1.027 V), excellent methanol crossover tolerance (98.03% retained) and long-term stability (81.30% retained), which are comparable with Pt/CJM. This work provides a new method of synthesizing PdV/C nanoparticles, which have the potential to be used as the cathode electrocatalysts for fuel cells.

Anion‐Regulated Hydroxysulfide Monoliths as OER/ORR/HER Electrocatalysts and their Applications in Self‐Powered Electrochemical Water Splitting

AbstractTrifunctional electrocatalysis for the oxygen evolution reaction, oxygen reduction reaction, and hydrogen evolution reaction constitutes the bottleneck of various sustainable energy devices and systems including rechargeable metal–air batteries and overall water splitting. Emerging macroscopically nanostructured catalysts are strongly requested for superior electrocatalytic activities and practical applications. Here, a 3D hydroxysulfide monolith with anion‐regulated NiFe hydroxysulfide nanosheets is rationally proposed. With abundant active sites and unique structure, the as‐obtained hydroxysulfide monolith exhibits superior electrocatalytic performance in oxygen evolution, hydrogen evolution, and oxygen reduction in alkaline electrolyte. When directly serving as electrodes, a small charge/discharge voltage gap of 0.76 V at 2.0 mA cm−2 and a high peak power density of 248 mW cm−2 are achieved for the liquid Zn−air batteries, and a low cell voltage of 1.62 V at 10 mA cm−2 is detected for water splitting units. When two Zn−air batteries are serially connected to power the water splitting unit, bulky O2 and H2 bubbles are continuously generated on both the composite electrodes. This work demonstrates an effective strategy for controllable anion regulation and rational design of the 3D self‐supporting hydroxysulfides, which is also enlightening for other advanced energy materials and diverse applications.

A metal–organic framework derived 3D hierarchical Co/N-doped carbon nanotube/nanoparticle composite as an active electrocatalyst for oxygen reduction in alkaline electrolyte

A 3D hierarchical Co/N-doped carbon nanotube/nanoparticle composite was successfully prepared as a high-performance ORR electrocatalyst, via pyrolysis of bimetallic Co, Zn-ZIF crystals coated on the surface of silica templates.

One-pot synthesis of Pd@PtNi core-shell nanoflowers supported on the multi-walled carbon nanotubes with boosting activity toward oxygen reduction in alkaline electrolyte

Pt-based nanocrystals with controlled morphologies and structures are one of most promising electrocatalysts for oxygen reduction reaction (ORR). Herein, a facile one-pot wet-chemical method is developed to synthesize Pd@PtNi core-shell nanoflowers (CSNFs) supported on the multi-walled carbon nanotubes (MWNCTs). Brij 58 is demonstrated as a structure-directing agent to generate the nanoflower and ascorbic acid acts as a reductant to form a core-shell structure. By tuning the molar ratio of Pd and Pt, Pd@PtNi/MWCNTs CSNFs show obviously improved ORR activity and durability in alkaline electrolyte compared with PtNi/MWCNTs nanoflowers and commercial Pt/C. The results illustrate that the core-shell structure and porous feature of nanoflower are both beneficial to the enhancement of the catalytic properties.

Perovskite-based bifunctional electrocatalysts for oxygen evolution and oxygen reduction in alkaline electrolytes

Due to the high cost of precious metal-based electrocatalysts for oxygen reduction and oxygen evolution, the development of alternative low cost and efficient catalysts is of high importance for energy storage and conversion technologies. Although non-precious catalysts that can efficiently catalyze oxygen reduction and oxygen evolution have been developed, electrocatalysts with high bifunctional activity for both oxygen evolution and reduction are needed. Perovskites based on modified lanthanum cobaltite possess significant activity for the oxygen evolution reaction. We describe the synthesis of a bifunctional oxygen electrode with simultaneous activity for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline media by direct growth of nitrogen-doped carbon nanotubes on the surface of a perovskite containing Co and Fe by means of chemical vapor deposition. The difference in the overvoltage between ORR (at 1mA/cm2) and OER (at 10mA/cm2) was below 880mV in 0.1M KOH. The formation of H2O2 during the ORR was reduced by at least three fold when using the bifunctional catalyst as compared to the non-modified perovskite. Long-term durability tests indicate stable performance for at least 37h during the OER and 23h during the ORR.

Enhanced Catalytic Activities of NiPt Truncated Octahedral Nanoparticles toward Ethylene Glycol Oxidation and Oxygen Reduction in Alkaline Electrolyte

The high cost and poor durability of Pt nanoparticles (NPs) are great limits for the proton exchange membrane fuel cells (PEMFCs) from being scaled-up for commercial applications. Pt-based bimetallic NPs together with a uniform distribution can effectively reduce the usage of expensive Pt while increasing poison resistance of intermediates. In this work, a simple one-pot method was used to successfully synthesize ultrafine (about 7.5 nm) uniform NiPt truncated octahedral nanoparticles (TONPs) in dimethylformamid (DMF) without any seeds or templates. The as-prepared NiPt TONPs with Pt-rich surfaces exhibit greatly improved catalytic activities together with good tolerance and better stability for ethylene glycol oxidation reaction (EGOR) and oxygen reduction reaction (ORR) in comparison with NiPt NPs and commercial Pt/C catalysts in alkaline electrolyte. For example, the value of mass and specific activities for EGOR are 23.2 and 17.6 times higher comparing with those of commercial Pt/C, respectively. Our results demonstrate that the dramatic enhancement is mainly attributed to Pt-rich surface, larger specific surface area, together with coupling between Ni and Pt atoms. This developed method provides a promising pathway for simple preparation of highly efficient electrocatalysts for PEMFCs in the near future.

Fabrication of the pyrolyzing carbon-supported cobalt–dicyandiamide electrocatalysts and study on the active sites and mechanism for oxygen reduction in alkaline electrolyte

In this work, carbon-supported cobalt–dicyandiamide (Co–DCD) was synthesized by a simple chemical method followed heat-treated at 600,700, 800, 900, and 1000 °C to acquire electrocatalysts with excellent activity for the oxygen reduction reaction (ORR). The resulting catalysts Co–N/C–T pyrolyzed at 600–1000 °C all showed substantial activities to ORR, and the catalyst heat-treated at 800 °C exhibited the best ORR activity. The catalytic performance of Co y –N x /C 0.25–T catalysts synthesized with different amounts of DCD as nitrogen source and Co(OAc)2⋅4H2O as metal precursor were measured by cyclic voltammetry in alkaline electrolyte. The onset potential for oxygen reduction on the optimum catalyst was approximately 0.938 V (vs. RHE) in 0.1 M NaOH solution, higher by 83 mV than that on the commercial catalyst of 20 % JM Pt/C. Fourier transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were used to characterize the catalyst in terms of the structure and composition in order to sketch the contours of the catalytic active sites of the catalysts. The characterization studies indicate that pyridinic N–C was the most important of the catalytic active sites and responsible for the ORR catalytic activity of Co–N/C–T in alkaline electrolyte. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) was also used to obtain the overall ORR electron transfer number and electron transfer coefficiency. The overall electron transfer number for ORR catalyzed by the optimum catalyst Co0.2–N2.35/C0.25–800 was determined to be 3.47 by CV and 3.89–3.96 by LSV,respectively, suggesting that the ORR was a mixture of two- and four-electron transfer pathways, but dominated by a four-electron transfer process. Based on these measurements and other references, an ORR mechanism was proposed to facilitate further investigation. The results also show that this novel catalyst with ORR excellent activity would have outstanding methanol tolerance and potential application as a kind of nonprecious metal cathodic ORR catalyst for direct methanol fuel cell.

Metal‐Free Electrocatalyst for Oxygen Reduction: Synthesis‐Controlled Density of Catalytic Centers and Impact on ORR

AbstractThis work demonstrates a rapid and scalable route for the preparation of N‐doped carbon spheres of 80–120 nm via pyrolysis of polypyrrole as the only carbon and nitrogen source. The resulting porous catalyst has a nitrogen doping level of 6–8 at%. Electrochemical studies show that N‐doped C is very active toward oxygen reduction in alkaline electrolyte and the mechanism of ORR process is controlled by the surface concentration of catalytic active sites that promote either a direct four‐electron or two‐electron process. An interesting observation is that we can generate precursors for the N‐doped carbon with desirable particle size, shape and with the preferential structure (linear polypyrrole from the αα coupling during slow polymerization or cross‐linked polypyrrole from αβ coupling during fast polymerization) that promotes the formation of favorable catalytic sites for O2 reduction. The XPS analysis in conjunction with RDE voltammetry highlights the effect of polymer precursor synthesis on the chemical structure and a resulting electrochemical activity of the N‐doped carbon materials.

Metal-free catalysts for oxygen reduction in alkaline electrolytes: Influence of the presence of Co, Fe, Mn and Ni inclusions

Metal-free nitrogen modified carbon catalysts (NC) are very closely related to MNC catalysts which contain a transition metal(s) (M), usually Fe or Co as an essential constituent. We investigated the influence of metal inclusions on the activity of nitrogen-doped carbon black in the electrocatalysis of the oxygen reduction reaction (ORR). A reference metal-free NC catalyst was prepared by pyrolysis of a polypyrrole/Vulcan XC72 composite at 800°C for 2h under helium. Controlled amounts of Co, Fe, Mn and Ni in low concentrations were then introduced into NC by impregnating it with the corresponding meso-tetra(4-pyridyl) porphyrin metal complex followed by further pyrolysis at 650°C for 2h under helium. The resulting catalysts were investigated for ORR using rotating disk electrode and rotating-ring disk electrode voltammetry in 0.1M KOH. Additionally, the rate of decomposition of hydrogen peroxide by the different catalysts was determined in order to probe the influence of the metal inclusions on the mechanism and selectivity of the ORR. The results show that Fe, Co and Mn inclusions cause a substantial decrease of the overpotential of the reaction and enhance the catalytic current, whereas the presence of Ni has a poisoning effect on ORR. In the presence of Fe, the catalysts apparently reduce oxygen selectively to OH− in a direct four electron transfer process as opposed to the two-step, two electron pathway involving hydrogen peroxide as an intermediate for the case of the NC catalyst.

Activity of perovskite La 1− xSr xMnO 3 catalysts towards oxygen reduction in alkaline electrolytes

The behaviour of the perovskite-based series of compounds La 1− x Sr x MnO 3 (where x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) towards oxygen reduction in an ambient temperature alkaline 1 M KOH electrolyte is presented. Within this series, the intermediate compound La 0.4Sr 0.6MnO 3 exhibits the greatest catalytic activity, approaching that of the considerably more expensive fuel cell grade Pt-black examined under the same conditions. The origin of this activity is discussed in terms of material structure and morphology, which exists in the structural transition region between cubic LaMnO 3 and hexagonal SrMnO 3. The small crystallite size and relatively large BET surface area of this material reflect this high level of structural disorder. Furthermore, these features enable this compound to exhibit the greatest proportion of direct four-electron oxygen reduction (preferred) compared to the less efficient two-electron reduction to peroxide.

Electrocatalytic activity of dispersed platinum and silver alloys and manganese oxides for the oxygen reduction in alkaline electrolyte

This work reviews the studies conducted in this laboratory of the oxygen reduction reaction (ORR) on electrocatalysts formed by Pt-M/C (M = V, Cr, Co) and Ag-Pt/C alloys and on different Mn oxides (MnO/C, Mn3O4/C, MnO2/C) in KOH electrolyte. The physical and electronic properties of the materials are investigated by in situ XAS (x-ray absorption spectroscopy) in the XANES (x-ray absorption near edge structure) region. The electrocatalytic activity for the ORR on the different catalysts is compared through mass-transport-corrected Tafel plots. The XANES results for the Pt-M/C and Ag-Pt/C composites at high electrode potentials show lower vacancy of the Pt 5d band compared to pure Pt/C, while for the results indicate a chance of the Mn oxidation state as a function of the electrode potential. The electrochemical measurements evidence increased electrocatalytic activity of the Pt alloys compared to pure Pt and this is attributed to a lowering of the adsorption strength of adsorbed oxygen species caused by the reduced Pt reactivity. An activity enhancement of the Ag atoms on the Ag-Pt/C alloys compared to pure Ag is ascribed to an electronic effect induced by the presence of Pt, increasing the Ag-O adsorption strength. In the case of the MnyOx/C materials, the electrochemical results show low activity for MnO/C and higher activity for MnO2/C and Mn3O4/C. This is explained based on the activation for the ORR, which is higher for the material with higher MnO2 contents and the occurrence of a mediation processes involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen.

Electrocatalytic Activities of LiMPO4 (M = Mn, Co, Ni) for Oxygen Reduction in Alkaline Electrolytes

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Physical Characterization and Electrochemical Activity of Bimetallic Platinum-Silver Particles for Oxygen Reduction in Alkaline Electrolyte

Bimetallic Pt-Ag particles dispersed on a carbon powder with 1:1, 1:2, 1:3, and 1:10 (atomic ratio) Pt:Ag were investigated as electrocatalysts for the oxygen reduction reaction (ORR) in alkaline media. These materials were characterized by X-ray diffraction, in situ X-ray absorption near-edge structure, and extended X-ray absorption fine structure. The electrochemical activity for the ORR was evaluated from steady-state polarization measurements in a rotating ring-disk system and in gas diffusion electrodes (EDG). The results showed that the Pt-Ag particles are formed by a core composed by true Pt-Ag alloys, with the surface covered by a Ag-rich layer. An increase of the particle size and of the fcc unit cell lattice parameter with the increase of Ag content was observed. The electrochemical measurements indicated lower activity for the particles compared to , but higher activity compared to . A number of electrons close to 4 was observed for the ORR in the materials. A discussion of the catalytic activity of the Pt-Ag materials was made in terms of the electronic properties of the Pt and Ag atoms and the morphological structure of the particles.

Nano-porous manganese oxide formed by self-assembled agglomeration of nanocrystallites

We synthesized specific nano-porous dendritic structures similar to cedar leaves of manganese oxides using pulsed laser ablation (PLA) process, with the potential for use as highly active nano-catalysts. The nano-porous structures were formed by self-assembled agglomeration of nanocrystallites without templates. Furthermore, the dendritic nano-porous manganese oxide as an electrocatalyst showed a significant decrease in overpotential on oxygen reduction in alkaline electrolyte.

Effects of carbon pretreatment for oxygen reduction in alkaline electrolyte

The effects of different media on carbon pretreatments for oxygen reduction in alkaline electrolyte without application of active electrocatalysts were examined. Low surface area Vulcan XC-72 and high surface area Ketjenblack EC-300 were subjected to aqueous acid (flouric or formic), gaseous (H 2, N 2 or CO 2) and thermal treatments at 600 or 900 °C. Though non-scrubbed air was used, as a result of which carbonate build-up was high and peroxide concentration increased due to the electrode reaction, some of the electrodes resulted in life-tests of more than 2000 h at 65 °C, 6 M KOH with a constant load of 50 mA cm −2 and intermittent polarisations at higher current densities. BET-surface areas and pH changes of more than 60% and weight losses of up to 15% of the carbon blacks were observed after the pretreatment steps. Electrochemical characterisation of the carbons showed that pretreatment steps of the carbon blacks have a significant effect on the long-term stability and activity of the gas diffusion electrodes in alkaline electrolyte.

MnO x/C composites as electrode materials II. Reduction of oxygen on bifunctional catalysts based on manganese oxides

MnO x /C composites prepared by the reduction of KMnO 4 by active carbon black and doped with Ca(II), Mg(II), Ni(II), Bi(III), and Cr(III) ions were tested as catalysts of oxygen reduction in alkaline electrolyte. The polarisation curves were analysed by logarithmic wave analysis. MnO x doped with the transition metal salts are slightly more active than those with Ca(II) and Mg(II).

Carbon-based air electrodes carrying MnO 2 in zinc–air batteries

Catalysts prepared from the carbon black impregnated with manganous nitrate solution and then heated at temperature from 270°C to 450°C were investigated. It was found that the impregnated catalysts heated at temperature of 340°C exhibited the best catalytic activity for oxygen reduction in alkaline electrolyte. It was also found that the XRD spectra of pyrolytic MnO 2 from manganous nitrate over 340°C were different from those below 340°C. The enhanced catalysis of air electrodes was ascribed to the formation of MnO 2 crystal with d-value of 2.72 Å as the impregnated-catalysts was heated at temperature of 340°C. The other factors in preparation of air electrodes were also discussed.