Fe-based catalysts for oxygen reduction in proton exchange membrane fuel cells with cyanamide as nitrogen precursor and/or pore-filler

Fe/N/C catalysts for oxygen reduction reaction were synthesized via impregnation or ballmilling. The role of cyanamide (CM) as nitrogen precursor and/or pore-filler for a highly microporous carbon (Black Pearls 2000) was investigated. The use of CM in this work resulted in two main differences compared with phenanthroline from our previous work; (i) ballmilling the precursors did not result in improved activity of the resulting catalysts, and (ii) the activity after the first pyrolysis in argon was relatively high, but did not increase after a second pyrolysis in NH₃. These differences may be explained by TGA measurements of both pore-fillers, where complete gasification of CM is observed at temperatures above 750 °C in Ar, while pyrolysis of phenanthroline in Ar results in 20 wt% residual carbon-based material. Consequently, when using CM as pore-filler with a highly microporous carbon support, the maximum microporous surface area and nitrogen content is reached after only a single pyrolysis in Ar. The most active catalyst prepared with CM was obtained by pyrolysing in Ar at 950 °C a catalyst precursor containing 1 wt% Fe, 80 wt% CM and Black Pearls 2000. This catalyst possessed about 1/6th the catalytic activity of best reported using phenanthroline as a pore-filler. Changing the carbon support had effects on the activity and stability of the catalysts. The catalysts made with a non-porous furnace black (N330) or carbon nanotubes as a carbon support were more stable but less performing than those using carbon supports having high microporous surface area like Black Pearls 2000 or Ketjenblack. The desirable properties for a pore-filler molecule used in the synthesis of Fe/N/C-catalysts by the pore-filling method are discussed.     

Cobalt based non-precious electrocatalysts for oxygen reduction reaction in proton exchange membrane fuel cells

Cobalt based non-precious metal catalysts were synthesized using chelation of cobalt (II) by imidazole followed by heat-treatment process and investigated as a promising alternative of platinum (Pt)-based electrocatalysts in proton exchange membrane fuel cells (PEMFCs). Transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements were used to characterize the synthesized CoN_x/C catalysts. The activities of the catalysts towards oxygen reduction reaction (ORR) were investigated by electrochemical measurements and single cell tests, respectively. Optimization of the heat-treatment temperature was also explored. The results indicate that the as-prepared catalyst presents a promising electrochemical activity for the ORR with an approximate four-electron process. The maximum power density obtained in a H₂/O₂ PEMFC is as high as 200 mW cm⁻² with CoN_x/C loading of 2.0 mg cm⁻².     

A comparative study of PtCo, PtCr, and PtCoCr catalysts for oxygen electro-reduction reaction

A comparative study of carbon supported Pt₃₀Co₇₀, Pt₃₀Cr₇₀, and Pt₃₀Co₃₀Cr₄₀ catalysts for oxygen electro-reduction reaction (ORR) activity was performed. In alloy catalysts synthesized via NaBH₄ reduction, more than a 3-fold improvement was observed in ORR specific activity compared with that of Pt/C catalyst, while mass activities did not show significant improvement. After annealing at 900 °C under reducing conditions, the ORR specific activities of the alloy catalysts increased to give a relative ORR catalytic activity ordering of PtCo/C-900 (2800 μA ) > PtCoCr/C-900 (1770 μA ) > PtCr/C-900 (871 μA ) > Pt/C-900 (393 μA ) > Pt/C (334 μA ). On the other hand, the ORR mass activity followed an order of PtCr/C-900 (140 mA ) > PtCoCr/C-900 (111 mA ) > PtCo/C-900 (84.1 mA ). Cyclic voltammetry results suggest that incorporation of Cr resulted in a large electrochemically active surface area producing higher mass activity in the PtCr/C-900 catalyst although it showed the lowest specific activity among the alloy catalysts. The intermediate EAS and ORR activity values of the PtCoCr/C-900 catalyst suggest that the characteristics of the PtCo/C-900 (low mass and high specific activities) and PtCr/C-900 (high mass and low specific activities) are combined by alloying of Pt with both Co and Cr.     

Catalytic activity of titanium oxide for oxygen reduction reaction as a non-platinum catalyst for PEFC

A non-platinum cathode electrocatalyst must have the stability and catalytic activity for the oxygen reduction reaction (ORR) in order to be used in polymer electrolyte fuel cells (PEFCs). Titanium oxide catalysts as the non-platinum catalyst were prepared by the heat treatment of titanium sheets in the temperature range from 600 to 1000 °C. The prepared catalysts were chemically and electrochemically stable in 0.1 mol dm⁻³ H₂SO₄. The titanium oxide catalysts showed different catalytic activities for the ORR. The ORR of the catalysts heat-treated at around 900 °C occurred at the potential of about 0.65 V versus RHE. It is considered that the deference in the catalytic activity for the ORR of the heat-treated titanium oxide catalysts was due to the fact that the heat-treatment condition changed the material property of the catalyst surface. In particular, it was found that the catalytic activity for the ORR of the Ti oxide catalysts increased with the increase in the specific crystalline structure, such as the TiO₂ (rutile) (1 1 0) plane and the work function. It is considered that a surface state change, such as the crystalline structure and work function, might affect the catalytic activity for the ORR.     

Average turn-over frequency of O2 electro-reduction for Fe/N/C and Co/N/C catalysts in PEFCs

This work aims to evaluate the average turn-over frequency of O₂ electro-reduction for the catalytic sites resulting from the heat-treatment of iron- or cobalt-acetate and carbon black in ammonia at high temperature. This task is complex because at least three factors may control the activity of such catalysts: their metal content, nitrogen content and micropore specific area. In this work, the activity was measured for metal contents from 0.005 to 5 wt.%. The time of heat-treatment was tuned to keep the micropore specific area constant. At Fe content ≤0.2 wt.% the activity increases linearly with Fe content, thus enabling the average turn-over frequency of the Fe/N/C site to be determined. At Co content ≤1.0 wt.% the activity increases approximately as the square root of the Co content. Because no linear relation was found, the turn-over frequency of the Co/N/C site could not be determined. For both Fe and Co catalysts, the activity drops dramatically at contents >1 wt.%. This is concurrent with a drop in the micropore specific area of the catalysts.     

Influence of sputtering power on oxygen reduction reaction activity of zirconium oxides prepared by radio frequency reactive sputtering

Zirconium oxides (ZrO_2−x) have been investigated as new cathodes for direct methanol fuel cells without platinum. ZrO_2−x films were prepared using a radio frequency (RF) magnetron sputtering at RF powers from 75 to 175 W. The influence of the RF power on the catalytic activity for the oxygen reduction reaction (ORR) and properties of the ZrO_2−x films were examined. The ORR activity of the ZrO_2−x catalyst increased with the RF power in the range we studied. The onset potential for ORR over ZrO_2−x deposited at 175 W was 0.88 V vs RHE. In addition, the relationship between the ORR activity and the composition, crystallinity, electric conductivity, as well as the ionization potential has been investigated. The zirconium oxide with an oxygen defected state and the higher electric conductivity showed the higher ORR activity, and the electrocatalytic activity for ORR increased with the decreasing in the ionization potential of the ZrO_2−x catalyst.     

The effect of magnetic field on the oxygen reduction reaction and its application in polymer electrolyte fuel cells

The effect of magnetic field gradients on the electrochemical oxygen reduction was studied with relevance to the cathode gas reactions in polymer electrolyte fuel cells. When a permanent magnet was set behind a cathode, i.e. platinum foil or Pt-dispersed carbon paper for both electrochemical and rotating electrode experiments and oxygen was supplied to the uphill direction of the magnetic field, electrochemical flux was enhanced and the current increased with increasing the absolute value of magnetic field. This magnetic effect can be explained by the magnetic attractive force toward O₂ gas. When magnet particles were included in the catalyst layer of the cathode and the cathode was magnetized, the current of oxygen reduction was higher than that of nonmagnetized cathode. A new design of the cathode catalyst layer incorporating the magnet particles was tested, demonstrating a new method to improve the fuel cell performance.     

Platinum-alloy nanostructured thin film catalysts for the oxygen reduction reaction

In an effort to study advanced catalytic materials for the oxygen reduction reaction (ORR), a number of metallic alloy nanostructured thin film (NSTF) catalysts have been characterized by rotating disk electrode (RDE). Optimal loadings for the ORR and activity enhancement compared to conventional carbon supported nanoparticles (Pt/C) were established. The most efficient catalyst was found to be PtNi alloy with 55 wt% of Pt. The enhancement in specific activity is more than one order of magnitude, while the improvement factor in mass activity is 2.5 compared to Pt/C. Further lowering of the platinum to nickel ratio in NSTF catalysts did not lead to increased mass activity values.     

Non-platinum oxygen reduction electrocatalysts based on pyrolyzed transition metal macrocycles

In this work pyrolyzed porphyrins were investigated for oxygen reduction electrocatalysis. Pyrolysis of non-supported cobalt and iron tetraphenylporphyrins in the temperature range of 500-800 °C generates high surface area catalysts with high degree of exposure of active sites to the reacting species. This is achieved through templating porphyrins on fumed amorphous silica that is removed after pyrolysis by etching with concentrated KOH.Detailed material characterization of the pyrolyzed materials is presented here. X-ray photoelectron spectroscopy (XPS) analysis of cobalt and iron porphyrins was used to elucidate the transformations of nitrogen, carbon, cobalt and iron species resulting from the heat treatment. Partial decomposition of the precursor material and formation of polymer-like network decorated by metal oxide particles are identified. Differences in the chemical composition of products of pyrolysis of FeTPP, CoTPP and Co/FeTPP are discussed. Transmission electron microscopy (TEM) imaging revealed the structure of the pyrolyzed porphyrins and was used to gain insight into the size of the metal crystals formed in the bulk. X-ray diffraction spectra (XRD) provided information about the type of crystals formed in the different formulations of the precursor porphyrins. Further, steady state polarization curves were obtained utilizing gas diffusion type electrodes in 0.5 M sulfuric acid and membrane electrode assembly (MEA) configurations under working PEM fuel cell conditions. This work revealed the necessity of the metal phases for the oxygen reduction process.     

Investigation into the activity of heteropolyacids towards the oxygen reduction reaction on PEMFC cathodes

A total of 18 heteropolyacids (HPAs) were investigated to determine their activity as non-Pt oxygen reduction reaction (ORR) catalysts in polymer electrolyte membrane fuel cell cathodes (PEMFCs). Polarization curves, cyclic voltammetry and impedance spectroscopy determined that, of the HPAs tested, only molybdenum based HPAs are active for the ORR and that vanadium substitutions improved the activity. The reduction potentials of the HPAs in the fuel cell environment were determined by cyclic voltammetry. This showed that no activity is seen above 0.55 V, as the catalysts must first be reduced in situ by 4e⁻ before the HPA can reduce oxygen. The potential at which the HPA can be reduced has been determined to be the limiting factor when using these catalysts for ORR in PEMFCs. Power densities of 67 mW/cm² at 0.2 V were obtained using H₅PMo₁₀V₂O₄₀. Molybdenum based HPAs were covalently bonded to the carbon achieving mass loadings 3× that obtained through adsorption. Using this approach catalyst, performance was improved to 86 mW/cm² at 0.2 V. The increased loadings did not significantly increase the potentials at which the HPA becomes active for the ORR. We were able to show that MEA degradation, as measured by F⁻ emission rates, using these catalysts are reduced during accelerated testing protocols.     

Studies of oxygen reduction reaction active sites and stability of nitrogen-modified carbon composite catalysts for PEM fuel cells

A non-precious nitrogen-modified carbon composite (NMCC) catalyst is synthesized by the pyrolysis of cobalt, iron-ethylenediamine-chelate complexes on silica followed by chemical and pyrolysis treatments. Pyrolysis temperature and time have a remarkable impact on the content and the type of the nitrogen-containing functional groups in the NMCC catalysts, which affect their catalytic activity and stability. Based on the analysis of the nitrogen functional groups before and after the stability tests, the ORR active sites of the NMCC catalysts are proposed to be pyridinic-N and quaternary-N functional groups. However the pyridinic-N group is not stable in the acidic environment due to the protonation reaction.     

Additive treatment effect of TiO2 as supports for Pt-based electrocatalysts on oxygen reduction reaction activity

In this study, we investigated the additive treatment effect of TiO₂ as alternative support materials to common carbon black for Pt-based electrocatalysts on electrocatalytic activity for oxygen reduction reaction (ORR). The shape of TiO₂ was varied by hydrothermal treatment with various additives, such as urea, thiourea, and hydrofluoric acid. From the results of transmission electron microscopy (TEM) images and ultraviolet-visible spectroscopy (UV-vis) spectra, it was identified that the morphology of hydrofluoric acid (HF)-treated TiO₂ was changed into a round shape having lower aspect ratio than other samples, and its band gap was decreased. Notably, the electronic state of HF-treated TiO₂ support was changed into highly reduced (electron rich) state which led to the increase of ORR activity, compared to other samples treated with different additives or before treatment. The electrocatalytic characteristics changes after treatment with various additives were investigated by using X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), cyclic voltammograms (CV), and rotating disk electrode (RDE) techniques.     

Partially oxidized niobium carbonitride as a non-platinum catalyst for the reduction of oxygen in acidic medium

Partially oxidized NbC_0.5N_0.5 has been evaluated as a non-platinum catalyst for the reduction of oxygen in acidic medium. NbC_0.5N_0.5 powder was partially oxidized in N₂ gas containing O₂ of 10⁻⁴ atm at the temperature range of 700-1000 °C. Partially oxidized NbC_0.5N_0.5 had a definite oxygen reduction reaction (ORR) activity, while as-prepared NbC_0.5N_0.5 and completely oxidized Nb₂O₅ had a poor catalytic activity for ORR. The onset potential of the partially oxidized NbC_0.5N_0.5 for the ORR achieved 0.92 V vs. RHE in 0.1 M H₂SO₄ at 30 °C. The results of X-ray absorption spectroscopy and ionization potential measurements suggested that oxygen-vacancy defects might be responsible for the oxygen reduction capability by creating electronically favorable oxygen adsorption sites.     

Nanostructured Pt-M/C (M = Fe and Co) catalysts prepared by a microemulsion method for oxygen reduction in proton exchange membrane fuel cells

Nanostructured Pt-M/C (M = Fe and Co) catalysts have been synthesized by a microemulsion method and a high-temperature route. They have been characterized by cyclic voltammetry in 1 M H₂SO₄ and for oxygen reduction in proton exchange membrane fuel cells (PEMFC). The Pt-M alloy catalysts synthesized by the microemulsion method show higher electrochemical active surface area than those prepared by the high-temperature route, and some of them exhibit improved catalytic activity towards oxygen reduction compared to pure Pt. Among the various alloy catalysts investigated, the Pt-Co/C catalyst prepared by the microemulsion method shows the best performance with the maximum catalytic activity and minimum polarization loss. Mild heat treatment of the catalysts prepared by the microemulsion method at moderate temperatures (200 °C) in reducing atmosphere is found to improve the catalytic activity due to a cleaning of the surface and an increase in the electrochemical surface area.     

Electrocatalysis of oxygen reduction on a carbon supported platinum-vanadium alloy in polymer electrolyte fuel cells

The electrocatalysis of the oxygen reduction reaction (ORR) on carbon supported Pt:V 1:1 catalyst in polymer electrolyte fuel cells (PEFC) was investigated. At an oxygen pressure of 1 atm results indicate a lower electrocatalytic activity for the ORR in the presence of vanadium. However, at an O₂ pressure ≥2 atm an enhanced electrocatalytic property of PtV/C compared with Pt/C is revealed. This result indicates the occurrence of a different electrocatalytic mechanism for the ORR on Pt/C and PtV/C. An increase of mass transport overpotentials is observed for the PtV/C catalyst, and this was related to the presence of vanadium oxide. Indeed, XRD analysis revealed that only about 30% of V present in the catalyst is alloyed with Pt, forming a face centred cubic (fcc) Pt₃V solid solution. A thermal treatment at 850 °C under reducing atmosphere leads to the formation of an ordered fcc Pt₂V phase. After this, the ORR activity of PtV/C at O₂ pressure 1 atm is higher than that of Pt/C.     

Reductive annealing for generating Se doped 20 wt% Ru/C cathode catalysts for the oxygen reduction reaction

Reductive annealing was chosen as a method for the syntheses of Se modified Ru/C catalysts. Initial preparation of a 20 wt% Ru/C was performed by impregnating RuCl₃·2H₂O on Vulcan XC72 with subsequent conditioning using H₂ at 250 °C for 4 h. Surface treatment of Ru/C by SeO₂ followed by reductive annealing produced Se modified Ru/C catalysts with a pre-determined Ru:Se = 1:0.15 and 1:1 a/o. Structural characterization was carried out using HRTEM while electrochemical characterization was performed using RDE measurements. It is concluded that the presence of Se on Ru has a positive effect on the oxygen reduction reaction of RuSe/C catalyst systems with an optimal loading of Se close to a Ru:Se ratio of 1:0.15 a/o. Overloading of selenium led to neutralization of its promoting effect.     

Advances in single-atom catalysts for oxygen electrodes

As the most promising energy conversion and storage devices, fuel cells and metal-air batteries are of great benefit in alleviating the energy and environmental problems. However, the sluggish oxygen electrode reactions, including oxygen reduction reaction (ORR) for fuel cell and ORR couple with oxygen evolution reaction (OER) for zinc-air batteries, seriously limit the efficient of both types of devices. In recent years, single-atoms catalysts (SACs) have been proposed to improve the kinetics of oxygen electrode reaction. Therefore, for these two types of oxygen electrode reactions, this review firstly summarized their possible mechanism. Then, the SACs were classified by the different metal elements for both ORR and OER. Thus, noble-metal-based and non-noble-metal-based catalysts have been summarized in these two reactions. At the same time, a summary of the dual-function catalyst and its application in zinc air batteries is also given. Finally, in view of the current problems and future development directions of SACs, suggestions are put forward, aiming to pave the way for the design and development of monoatomic oxygen electrode catalysts.     

超薄钯铜纳米片组装纳米花的构建及其氧还原性能

在原子水平上精确调控电催化剂的组成是增强其氧还原（ORR）性能的有效途径。本文采用简单的一步溶剂热还原法制备了由不同PdCu组成的超薄双金属纳米片自组装而成的三维（3D）合金纳米花（Pd₁Cu_x NCFs）。利用透射电子显微镜（TEM）、扫描透射电子显微镜-能谱仪（STEM-EDS）、X射线粉末衍射（XRD）和X射线光电子能谱（XPS）等手段系统表征了Pd₁Cu_x NCFs的形貌、晶体结构及成分。相比于传统二维（2D）纳米材料,Pd₁Cu_x NCFs丰富的快速传质路径、较高的Pd原子利用效率和更强的PdCu双金属间协同作用使得其在碱性介质中表现出增强的ORR性能。另外,本文还研究了Pd、Cu前体用量对ORR性能的影响。结果显示,Pd、Cu前体摩尔比为1∶0.5时,催化剂（Pd₁Cu_0.5NCFs）在碱性介质中ORR活性和电化学稳定性最佳。其半波电位E_1/2（0.937V）远高于商业Pt/C（0.851V）;在加速循环1000次扫描后,E_1/2几乎没有变化,说明其稳定性优异;0.90V电势下,Pd₁Cu_0.5 NCFs的质量比活性为1.09A/mg,是商业Pt/C的14.5倍。     

氧电极金属单原子催化剂的研究进展

燃料电池和金属-空气电池作为目前最具发展前景的能量转换和储存设备,对于缓解人类发展所面临的能源与环境问题大有裨益。然而,较差的氧电极反应,如燃料电池中的氧还原反应以及锌空电池中的氧还原及析氧反应,却限制着这两类装置的高效运行。近年来,人们提出了利用单原子催化剂(SACs)来提高氧电极反应的反应动力学。因此,针对两类氧电极反应,本综述根据构成活性位点的不同金属元素进行了分类总结,重点关注了各类催化剂的共性及进展。同时,还对具有双功能的催化剂及其在锌空电池的应用进行了总结。最后,针对SACs目前存在的问题和未来的发展方向提出了建议,旨在为单原子氧电极催化剂的设计及发展指明道路。