Platinum Group Metal Catalysts Research Articles

Insight into the hydrogen oxidation electrocatalytic performance enhancement on Ni via oxophilic regulation of MoO2

Abstract Exploring platinum-group-metal (PGM) free electrocatalysts for hydrogen oxidation reaction (HOR) in alkaline media is essential to the progress of anion exchange membrane fuel cells (AEMFCs). In this work, a Ni/MoO2 heterostructure catalyst with comparable HOR activity in alkaline electrolyte with PGM catalyst was prepared by a simple hydrothermal-reduction method. Remarkably, the Ni/MoO2 presents a mass kinetic current density of 38.5 mA mgNi−1 at the overpotential of 50 mV, which is higher than that of the best PGM free HOR catalyst reported by far. Moreover, the HOR performance of Ni/MoO2 under 100 ppm CO shows negligible fading, together with the superior durability, render it significant potential for application in AEMFCs. A particular mechanistic study indicates that the excellent HOR performance is ascribed to the accelerated Volmer step by the incorporation of MoO2. The function of MoO2 was further confirmed by CO striping experiment on Pt/C-MoO2 that MoO2 can facilitated OH adsorption thus accelerate the HOR process. On account of the high performance and low cost, the Ni/MoO2 electrocatalyst encourages the establishment of high performance PGM free catalyst and shows significant potential for application in AEMFCs.

Microstructure Engineering of Fe/Fe3C-Decorated Metal-Nitrogen-Carbon Mesoporous Nanospheres via a Self-Template Method for Enhancing Oxygen Reduction Activity.

We report a self-template and facile pyrolysis method to synthesize Fe/Fe3C-decorated metal-nitrogen-carbon mesoporous nanospheres, of which preserved plum-like and hollow structures can be simply engineered via controlling the thickness of the outermost polydopamine layer in the precursors. The preserved plum-like structure is demonstrated to show a large electrochemically active surface area and facilitate fast charge transfer, in comparison with the hollow one. The catalytic activities of metal-nitrogen-carbon and nitrogen-doped carbon active sites in the outer carbon layer toward oxygen reduction are improved under the activation of the encased Fe species. Hence, preserved plum-like structures exhibit excellent catalytic kinetics toward the oxygen reduction reaction in alkaline media. The mass activity of 21.0 mA mgcatalyst-1 at 0.9 V vs RHE is achieved and the half-wave potential is 50 mV more positive than that of the Pt/C catalyst with the same mass loading. Moreover, the outer carbon layer endows the tolerance of strong acidic and alkaline environments, resulting in good durability. Our study proposes a simple strategy for the rational design of novel transition metal carbide-based catalysts, making it a promising candidate for replacing platinum-group metal catalysts in low-temperature fuel cells.

(Invited) Non-PGM Electrocatalysts for PEM Fuel Cells: Origin of Their Instability

Proton exchange membrane fuel cells (PEMFCs) have earned substantial commercial and government interests worldwide as highly promising renewable energy power sources for various applications (e.g., electric vehicles). However, a key contributor to their high cost remains the use of Platinum Group Metals (PGM) catalysts, especially at the cathode where the oxygen reduction reaction (ORR) is much more sluggish than the hydrogen oxidation reaction (HOR) occurring at the anode. The replacement of PGM catalysts by PGM-free catalysts at the cathode has so far been mainly hampered by the poor stability of the most active and performing PGM-free catalysts [1,2] that would otherwise become serious contenders to PGM catalysts. It is therefore important to determine the origin of the instability plaguing the PGM-free catalysts [3-8]. Based on our highly performing catalyst (Nature Commun., 2011, 2, 416) [2], INRS team has devoted, during recent years, much attention to the instability of this PGM-free catalyst [3-8]. This communication will present our very recent work on the catalyst stability which is summarized in two publications in Energy and Environmental Science [6, 8]. Our catalyst was obtained by ball milling ZIF-8, FeAc and 1,10 Phenanthroline, followed by two heat-treatments in Ar and in NH3. Via a systematic study of the instability behavior of the catalyst at different fuel cell potentials from 0.8 to 0.2 V at 80 °C and 25 °C, we discovered that the decay of the current density always involves the superposition of a fast and a slow exponential decay. With the combination of various characterization (e.g., BET, NAA and Mössbauer spectroscopy), we concluded that the fast exponential decay of the current density was the result of the specific demetalation of FeNx sites located in the micropores of the catalyst. Following this work, we explored the behavior of this catalyst before and after fluorination by F2 at room temperature. We discovered that all Fe-based catalytic sites were poisoned by reaction with F2, but not the ORR active CNx sites (and edge carbon sites) located at the surface of the catalyst carbonaceous support. Consequently, the instability behavior of fluorinated catalysts was found to be different from that of pristine catalyst, but similar to that of MOF_CNx_Ar+NH3 (a catalyst devoid of Fe, with only CNx sites and edge carbon sites). Further, the F2-poisoned catalysts can be partially reactivated under different heat-treatments. With a systematic study by fuel cell, BET, IR, TGA, XPS, XAS, as well as DFT and thermodynamic calculations, all the results demonstrate that the root-causes of instability of the FeN4 and CNx catalytic sites do not have the same origin. The two models (from INRS and Los Alamos) proposed so far to describe the instability curves of Fe-based catalysts will also be discussed.

Influence of Ionomer Composition and Distribution on PEM Fuel Cell Performance of PGM-Free Catalyst

One of the effective ways to reduce the cost of polymer electrolyte membrane fuel cells (PEMFCs) is to reduce the amount of platinum-group metal (PGM) catalysts used for the oxygen reduction reaction (ORR). Recently, highly active platinum group metal-free (PGM-free) catalysts were developed and applied for the ORR reaction [1-3]. Significant developments have been made on the chemistry of PGM-free catalysts and their kinetic performance [4-6]. However, despite all the progress, some challenges remain before they are commercially viable. Mass-transport within the very thick catalyst layer (CL) displays a key challenge for further improvement of the fuel cell performance of PGM-free catalysts. Understanding of the structure of CL including the distribution of active sites and the ionomer inside the CL play an important role in the development of stable PGM-free cathodes capable of high power density. In the present work, our objective has been to demonstrate the effects of the ionomer content, solvents, and equivalent weight (EW) on electrode performance. The structure of the ionomer (i.e., short-side-chain vs long-side chain ionomer) as well as the effect of the relative humidity (RH) have been considered. To increase the proton conductivity of the cathode and improve the catalysts’ effectiveness across its thickness, it is important to optimize the ionomer type and its EW. In this work, we used the nanoscale X-ray computed tomography (nano-CT) method to characterize the morphology and transport properties of PGM-free cathodes. Our results show the sensitivity of the cathode’s performance to the hydrophilicity of the ionomer films. We found that electrodes with a lower ionomer EW have a tendency to retain more water than electrodes with a higher EW. Thus, at high current density conditions where significant water is being generated, the lower EW ionomers showed reduced performances due to electrode flooding when using the same processing and ionomer loading. In general, higher MEA performance at 100% RH was achieved using more hydrophobic ionomers (higher EW) with a greater proportion of hydrophobic fluoropolymer backbone or by using ink formulations that enhanced the hydrophobicity of a particular ionomer.AcknowledgmentThis material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office (FCTO) under Award Number DE-EE0008076.

Improving Stability and Kinetics of Alkaline HOR Catalysts – Towards Reduced System Cost

Anion exchange membrane fuel cells (AEMFCs) show great promise in overcoming high cost of currently state of the art proton exchange membrane fuel cells, that rely on expensive Pt catalysts for both the hydrogen oxidation reaction (HOR) as well as the oxygen reduction reaction (ORR). With recent advancements in demonstrating high ORR activity and stability of Fe-N-C catalysts in alkaline environments,1 the HOR suddenly became the bottleneck reaction towards high power density platinum group metal (PGM) -low or -free AEMFCs. While application of PGM-free ORR catalysts is an eventual goal, current state of the art NiM nanoparticles show relatively poor performance in terms of both activity and irreversible oxidation and dissolution.2 Hence, at least for some time, one will need to rely on PGM catalysts for alkaline HOR, where the biggest challenge will be lowering the loading and meanwhile increasing the stability in an alkaline environment. To this end, fundamental understanding of surface processes is required.In this work we investigate model CeOx covered Pt and Pd electrocatalysts as benchmark materials for the HOR reaction3 , 4. To prepare such model electrodes, we use physical vapor deposition for the formation of PGM and atomic layer deposition subsequently, to yield CeOx surface layers of variable thickness. Activity and stability of the CeOx@Pt and CeOx@Pd electrodes are studied in common rotating disk electrode and on-line inductively coupled plasma mass spectrometry setups, respectively. External analytics, e.g. X-ray photoelectron spectroscopy, are used to further characterize the electrodes before and after degradation studies. We show that stability of the electrodes at the higher anodic potentials can be greatly improved by tuning the thickness of the CeOx layer. Interestingly, this can be achieved without sacrificing and even with a significant enhancement in the HOR activity. This unique performance is explained by assuming that the deposited CeOx films form an ion selective semipermeable passivation layer on the surface of Pd and Pt. We anticipate that, as a next logical step, such layers can be used to stabilize PGM-free Ni alloys and reach the goal of having completely PGM-free AEMFCs.

CO‐Assisted Direct Methane Conversion into C1 and C2 Oxygenates over ZSM‐5 Supported Transition and Platinum Group Metal Catalysts Using Oxygen as an Oxidant

AbstractConversion of methane towards chemical feedstocks such as oxygenates and hydrocarbons has been studied along with the development of natural gas technology. In this study, CO‐assisted direct conversion of methane into C1 and C2 oxygenates was demonstrated over ZSM‐5 supported transition and platinum group metal catalysts. Besides previously investigated Rh, other platinum group metals (e. g., Ru and Ir) were found to be a potential element as the catalyst. Presence of CO was critical for the reaction, which would work as a ligand, a reductant, and a reactant. On the other hand, the competing undesired CO oxidation toward CO2 was found to be the major issue. Partial oxidation toward methanol or formic acid (C1 oxygenates formation) was parallel to oxidative carbonylation to acetic acid (C2 oxygenate formation).

Low-PGM and PGM-Free Catalysts for Proton Exchange Membrane Fuel Cells: Stability Challenges and Material Solutions.

Fuel cells as an attractive clean energy technology have recently regained popularity in academia, government, and industry. In a mainstream proton exchange membrane (PEM) fuel cell, platinum-group-metal (PGM)-based catalysts account for ≈50% of the projected total cost for large-scale production. To lower the cost, two materials-based strategies have been pursued: 1) to decrease PGM catalyst usage (so-called low-PGM catalysts), and 2) to develop alternative PGM-free catalysts. Grand stability challenges exist when PGM catalyst loading is decreased in a membrane electrode assembly (MEA)-the power generation unit of a PEM fuel cell-or when PGM-free catalysts are integrated into an MEA. More importantly, there is a significant knowledge gap between materials innovation and device integration. For example, high-performance electrocatalysts usually demonstrate undesired quick degradation in MEAs. This issue significantly limits the development of PEM fuel cells. Herein, recent progress in understanding the degradation of low-PGM and PGM-free catalysts in fuel cell MEAs and materials-based solutions to address these issues are reviewed. The key factors that degrade the MEA performance are highlighted. Innovative, emerging material concepts and development of low-PGM and PGM-free catalysts are discussed.

Temperature Impacts on Oxygen Reduction Reaction Measured by the Rotating Disk Electrode Technique

The response of the catalyst oxygen reduction reaction (ORR) activity to temperature is an important basis for identifying and selecting ORR catalysts which include platinum group metal (PGM) catalysts and PGM-free catalysts (including non-precious metal catalysts and metal-free catalysts). Rotating Disk Electrode (RDE) technology is the most commonly used method for assessing ORR activity. Herein, the ORR activity parameters of three representative catalysts Pt/C, Fe-N-C and N/P-doped carbon nanomaterials were measured by the RDE technique over a wide temperature range of 0−80 °C for the first time. Experimental results indicate that due to the significant decrease in oxygen concentration with increasing temperature, Pt/C exhibits the highest activity at ~4 °C, while Fe-N-C and N/P-C achieve optimum performances at ~40 and 60 °C respectively, at which the ORR kinetics and oxygen concentration in electrolyte are balanced.

Cu Electrodeposition on Nanostructured MoS2 and WS2 and Implications for HER Active Site Determination

Cu electrodeposition in both underpotential and overpotential regimes on nanostructured MoS2 and WS2 prepared by plasma-enhanced atomic layer deposition has been studied in detail. A combination of electrochemical methods, advanced characterization by X-ray absorption spectroscopy (XAS) as well as theoretical modelling were employed to reveal Cu adsorption modes on transition metal dichalcogenides (TMDs) from initial stages until bulk deposition. Since Cu UPD on TMDs has been used recently to evaluate the number of electrochemically active sites (NAS) for H2 evolution reaction, we evaluate and discuss here the implications of the Cu electrodeposition phenomena on nanostructured MoS2 and WS2 gauging the general applicability of the Cu UPD method for number of HER active sites determination in TMDs. Although an apparently better correlation of HER current density with Cu UPD charge than with double layer capacitance is found, the Cu UPD method cannot be used quantitatively because of the absence of a clear H UPD phenomenon on the studied nanostructured TMDs. This is in contrast to platinum group metal catalysts where H UPD and Cu UPD sites are strongly correlated.

Establishing Performance Baselines for the Oxygen Evolution Reaction in Alkaline Electrolytes

This paper establishes baseline performance of several platinum group metal (PGM) and non-PGM catalysts in alkaline oxygen evolution (OER). As OER catalyst development efforts increase, there is a need to standardize testing and baseline performance to compare catalysts between different studies, better direct materials development, and understand how performance improvements translate to the device. Of the catalysts tested, metals tend to have higher half-cell activity than their fully oxidized counterparts. In single-cells, however, metal activities approach oxides, likely due to the elevated temperature, higher potential, and longer time needed to condition membrane electrode assemblies (MEAs) relative to rotating disk electrodes (RDEs). In RDEs, cobalt (Co) and ruthenium nanoparticles are the most OER active. Due to high ruthenium dissolution rates, however, iridium (Ir) is used as a PGM baseline. Activity differences between materials in RDE (Ir 320 A g‒1, Co 12 A g‒1 at 1.55 V) further appear to translate to MEAs (Ir 1370 A g‒1, Co 101 A g‒1 at 1.5 V), indicating that half-cell testing can be useful in the early stages of catalyst development to predict kinetics at the device-level.

Recent Advances in Single-Atom Electrocatalysts for Oxygen Reduction Reaction.

Oxygen reduction reaction (ORR) plays significant roles in electrochemical energy storage and conversion systems as well as clean synthesis of fine chemicals. However, the ORR process shows sluggish kinetics and requires platinum-group noble metal catalysts to accelerate the reaction. The high cost, rare reservation, and unsatisfied durability significantly impede large-scale commercialization of platinum-based catalysts. Single-atom electrocatalysts (SAECs) featuring with well-defined structure, high intrinsic activity, and maximum atom efficiency have emerged as a novel field in electrocatalytic science since it is promising to substitute expensive platinum-group noble metal catalysts. However, finely fabricating SAECs with uniform and highly dense active sites, fully maximizing the utilization efficiency of active sites, and maintaining the atomically isolated sites as single-atom centers under harsh electrocatalytic conditions remain urgent challenges. In this review, we summarized recent advances of SAECs in synthesis, characterization, oxygen reduction reaction (ORR) performance, and applications in ORR-related H2O2 production, metal-air batteries, and low-temperature fuel cells. Relevant progress on tailoring the coordination structure of isolated metal centers by doping other metals or ligands, enriching the concentration of single-atom sites by increasing metal loadings, and engineering the porosity and electronic structure of the support by optimizing the mass and electron transport are also reviewed. Moreover, general strategies to synthesize SAECs with high metal loadings on practical scale are highlighted, the deep learning algorithm for rational design of SAECs is introduced, and theoretical understanding of active-site structures of SAECs is discussed as well. Perspectives on future directions and remaining challenges of SAECs are presented.

Linear Activation Energy-Reaction Energy Relations for LaBO3 (B = Mn, Fe, Co, Ni) Supported Single-Atom Platinum Group Metal Catalysts for CO Oxidation.

Single-atom catalysts are at the center of attention of the heterogeneous catalysis community because they exhibit unique electronic structures distinct from nanoparticulate forms, resulting in very different catalytic performance combined with increased usage of often costly transition metals. Proper selection of a support that can stably keep the metal in a high dispersion is crucial. Here, we employ spin-polarized density functional theory and microkinetics simulations to identify optimum LaBO3 (B = Mn, Fe, Co, Ni) supported catalysts dispersing platinum group metals as atoms on their surface. We identify a strong correlation between the CO adsorption energy and the d-band center of the doped metal atom. These CO adsorption strength differences are explained in terms of the electronic structure. In general, Pd-doped surfaces exhibit substantially lower activation barriers for CO2 formation than the Rh- and Pt-doped surfaces. Strong Brønsted–Evans–Polanyi correlations are found for CO oxidation on these single-atom catalysts, providing a tool to predict promising compositions. Microkinetics simulations show that Pd-doped LaCoO3 is the most active catalyst for low-temperature CO oxidation. Moderate CO adsorption strength and low reaction barriers explain the high activity of this composition. Our approach provides guidelines for the design of highly active and cost-effective perovskite supported single-atom catalysts.

Stable Fe2P2S6 Nanocrystal Catalyst for High‐Efficiency Water Electrolysis

AbstractA crucial step toward clean hydrogen (H2) energy production through water electrolysis is to develop high‐stability catalysts, which can be reliably used at high current densities for a long time. So far, platinum group metals (PGM) and their oxides, for example, Pt and iridium oxide (IrO2) have been well‐regarded as the criterion for hydrogen and oxygen evolution reactions (HER and OER) electrocatalysts. However, the PGM catalysts usually undergo severe performance decay during the long‐term operation. Herein, the in situ growth of iron phosphosulfate (Fe2P2S6) nanocrystals (NCs) catalysts on carbon paper synthesized by combing chemical vapor deposition with solvent‐thermal treatment is reported to show competitive performance and stability as compared to the state‐of‐the‐art PGM catalysts in a real water electrolyzer. A current density of 370 mA cm−2 is achieved at 1.8 V when using Fe2P2S6 NCs as bifunctional catalysts in an anion exchange membrane water electrolyzer. The Fe2P2S6 NCs also show much better stability than the Pt‐IrO2 catalysts at 300 mA cm−2 for a continuous 24 h test. The surface generated FeOOH on Fe2P2S6 is the real active site for OER. These results indicate that the Fe2P2S6 NCs potentially can be used to replace PGM catalysts for practical water electrolyzers.

A metal-organic framework-derived Fe–N–C electrocatalyst with highly dispersed Fe–Nx towards oxygen reduction reaction

Metal and nitrogen co-doped catalysts have been promising alternatives to platinum group metal (PGM) catalysts for oxygen reduction reaction (ORR) over the past few decades. Herein, we have synthesized an efficient Fe–N–C catalyst by the co-calcination of NH2-MIL-101@PDA and melamine. The best Fe–N–C shows a positive half-wave potential of 0.844 V, which is 14 mV higher than that of Pt/C catalyst, as well as superior methanol resistance and long-term durability in alkaline electrolyte. In addition, Fe–N–C also exhibits pronounced catalytic activity and a direct 4e− reaction pathway in acid electrolyte. We ascribed the excellent ORR performance of Fe–N–C to its crumpled structure, large specific surface area (584.6 m2 g−1) and high content of Fe-Nx sites (1.22 at. %). This study provides a simple way for the fabrication of excellent PGM-free ORR catalysts.

AEM Electrolysis Testing Update and Industrial Device Requirements

Membrane (PEM)-based electrolysis provides an ideal near-term technology to address the growing energy storage demands as intermittent renewable energy becomes more prevalent. While proton exchange membrane (PEM) systems are often viewed as preferred solutions to legacy KOH systems due to the lack of corrosive electrolyte, small footprint, dynamic operating range and ability to generate hydrogen at differential pressure, they still present capital cost challenges. In particular, the platinum group metal catalysts and valve metals for separator and flow field construction required for stability in the acidic environment become a significant cost driver at scale. Alkaline exchange membrane (AEM)-based electrolysis is therefore viewed as a potential replacement to the current PEM systems as a means of driving down the high capital cost. This cost reduction is accomplished by replacing the typical high cost catalyst and materials of cell construction with lower cost metals that would corrode in the acidic PEM environments, but are stable in the alkaline chemistries. In theory, this approach is highly promising, but testing of the current state-of the art has shown the technology gaps between the two platforms in terms of stability, durability, and operating electrical efficiency. Modeling has shown that savings realized in capital cost are typically off-set by an increase in operating expense resulting from the lower activity of the non-PGM catalysts and poor conductivity of the AEM. Additionally, thermal instability of the AEMs require stacks to be operated at lower temperatures than a typical PEM system, impacting conductivity further and slowing kinetics. Recent activities have looked at improving the mechanical properties, through the addition of mechanical reinforcements, to support pressurized operation, while operating temperature and varied concentrations of liquid electrolyte to the water feed have been explored to operationally evaluate the impact on performance and durability. The latter of these operating parameters have been shown by some groups to be effective in reducing cell operating potentials and providing overall voltage stability. However, this solution adds management and additional maintenance of handling the caustics typically used. Efforts at Proton continue to show improvements in short duration operational tests under milder conditions and potential pathways to achieve targets required to compete with incumbent technology. In this talk, some of the advancements achieved and how these have impacted the cost of the hydrogen will be discussed, as well as the longer-term targets that need to be reached in order to be a viable commercial application. Additionally, cost models will be compared along with sensitivity analysis to show how the impact of operating parameters, with regard to the capital and operational expense for the AEM electrolysis technology cost, show the critical areas to focus on to establish equivalence with current PEM systems and what is required to go beyond. Figure 1

Durable Mn-Based PGM-Free Electrodes for Polymer Electrolyte Membrane Fuel Cells

The development of low cost and high performance cathode for the oxygen reduction reaction (ORR) remains a grand challenge for transportation applications of polymer electrolyte membrane fuel cells (PEMFCs). Fe and Co-based platinum group metal (PGM)-free catalysts have been previously studied, however facing poor long-term durability and may potentially accelerate membrane degradation caused by oxygen radicals1. To overcome the above weakness, Li et al.2 has developed Mn based PGM-free catalyst which can mitigate membrane degradation, meanwhile showing a competitive catalyst activity compared to Fe based catalyst. The activity of the Mn-N-C catalysts measured using rotating disk electrodes (RDEs) in acidic electrolytes approaches state-of-the-art Fe-N-C catalysts3. More importantly, the Mn-N-C catalysts have demonstrated enhanced stability using potential cycling (0.6-1.0 V) in O2-saturated acidic electrolytes. In addition to the Mn-based PGM-free catalyst development, the electrode design is also critical for the MEA performance. The oxygen reduction reaction (ORR) may occur at different interface in the catalyst layers for PGM and PGM-free catalysts. For PGM catalysts, the ORR takes place on the Pt/ionomer interface, while Pt is on the surface of the catalyst support. However, since the Mn active sites are embedded inside the metal organic framework (MOF), it is more difficult to establish the catalyst/ionomer interface PGM-free catalyst. Hereby, the ionomer with lower equivalent weight (EW=830) has been proposed to replace the conventional Nafion ionomer (EW=1100). The shorter chain of the low EW ionomer can penetrate into the micro-pores in the catalyst layer easier, which may promote their interaction with the catalyst active sites. In addition to the low EW ionomer, the ionomer to carbon (I/C) ratio also has a great impact on the proton, gas and water transports that can further affect the MEA performance. The particle size of the catalyst also affects the MEA performance. In the RDE studies, the Mn catalyst with a smaller average particle size (50 nm) shows a better ORR activity than the one with a larger average particle size (80nm). However, in the MEA studies, catalysts with larger average particle size seems to demonstrate better performance, likely due to optimal pore structures of electrodes. This work will provide a systematical understanding and guidance of the MEA design not only for Mn-based catalyst but also for other PGM-free catalysts. Acknowledgement: The project is financially supported by the Department of Energy’s Fuel Cell Technology Office under the Grant DE-EE0008075. Reference: Wang, X. X., Prabhakaran, V., He, Y., Shao, Y., Wu, G., Adv. Mater. 2019, 1805126.Li, J.; Chen, M.; Cullen, D. A.; Hwang, S.; Wang, M.; Li, B.; Liu, K.; Karakalos, S.; Lucero, M.; Zhang, H.; Lei, C.; Xu, H.; Sterbinsky, G. E.; Feng, Z.; Su, D.; More, K. L.; Wang, G.; Wang, Z.; Wu, G., Nature Catalysis 2018, 1 (12), 935-945.Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P., Science 2011, 332 (6028), 443-447.

Chemical Structure Control of Metal-Reduced Organic Framework-Supported Nitrogen-Doped Graphene Catalyst for Oxygen Reduction Reaction

Nitrogen-doped graphene (N-G) has a huge potential in many electrochemical systems such as fuel cells due to the high raw material cost of platinum group metal (PGM) catalysts. However, there is a significant gap between the current level of electrochemical performance of N-G catalysts and that of PGM catalysts. Here we report an advanced metal-reduced organic framework-supported N-G catalyst (N-G/MOF-x) prepared by functionalizing ZIF-8 and N-G using nanoscale high energy wet ball milling method. The chemical structure control of the N-G/MOF was studied by characterizing the variation of elemental composition and chemical bonding composition of synthesized samples throughout targeted grinding speeds. The results proved that the chemical interaction between ZIF-8 and N-G caused the reduction of nitrogen, oxygen and zinc atoms, and also the variation of chemical bonding composition in N-G/MOF-x. The reduction rate of zinc was gradually increased with the increasing grinding speed, and reached to 82% of zinc loss at 650 RPM. The characterization of carbon and nitrogen bonding composition confirmed that the loss of nitrogen, oxygen and zinc atoms was caused by the decomposition of the C-N-Zn heteroatom contents in ZIF-8 and the O-containing functional groups in N-G, and influenced by the grinding speed. The decomposition ZIF-8 not only influenced the pore structure, but also modified the chemical structure and the surface distribution of N functional groups-constituted active sites in N-G/MOF-x. The N-G/MOF catalyst has comparable electrochemical performance to 10 wt% Pt/C catalyst. The successful accomplishment of the N-G/MOF catalysts will provide the substantial way to the cost-effective and fuel-efficient energy conversion system.

Towards a High-Performance DMFC Membrane Electrode Assembly Using Iron-Doped Metal Organic Framework Catalysts on the Cathode

Direct methanol fuel cells (DMFC) have several advantages for a broad spectrum of energy density demanding applications, ranging from portable devices to stationary backup power. DMFCs deliver power at high efficiencies compared to internal combustion technologies and use high energy density methanol as the fuel. However, DMFCs face several challenges, including methanol crossover at the cathode and slow methanol oxidation reaction (MOR) kinetics at anode that impede its performance. Hence, high loadings of a platinum group metal (PGM) catalyst are normally needed on both the anode and cathode, leading to high raw material catalyst costs. In this work, a PGM-free iron catalyst derived from metal organic framework precursors (Fe-MOF) is used on the cathode side to catalyze the oxygen reduction reaction (ORR). The PGM-free cathode has shown a high tolerance to methanol crossover relative to standard PGM catalysts and high performance relative to previously reported DMFC PGM-free cathode performances in the field. A wide range of operating conditions were tested, and we evaluated the maximum power density using pure oxygen and air with varying methanol concentration. We conducted our tests with a custom, integrated H2 reference electrode that separately provided the anode and cathode overpotentials. This is necessary for cell and electrode optimization since DMFC anode overpotentials are large relative to that of H2 PEMFCs and cannot be used as a quasi-reference electrode. Furthermore, we performed long term durability tests to evaluate PGM-free catalyst stability in the DMFC environment. This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office (FCTO) under Award Number DE-EE0008440.

Thin Film Electrocatalytic Coat of Polythiophene Derivatives for Oxygen Reactions

Energy conversion technologies relying on platinum group metal (PGM) catalysts suffer from the disadvantage of high cost. Additionally, they are susceptible to be poisoned by the reaction byproducts. Thus, to realize cost-effective, clean and renewable electrochemical devices, such as metal-air batteries, there is an urgent need to develop non-PGM catalysts. In the past ten years, a wide range of materials–as alternatives to PGM-based catalysts–have been developed. Examples of these materials are transition metal oxides, covalent- and metal-organic frameworks, and metal-free carbon-based electrocatalysts [1]. Among these materials, metal-free carbon-based catalysts–such as graphene and carbon nanotubes (CNTs) doped with nitrogen or sulfur–are promising candidates due to their high activity, durability, and low cost. Nonetheless, the synthetic approaches utilized to prepare and process these catalysts are multi-steps. These complex schemes lead to a lack of control over materials structure and chemical functionality. As a consequence, the electroactivity of the synthesized materials is being compromised [2]. Here, we investigate the properties of conjugated polymers thin films as electrocatalysts. Although there are a few reports on the electroactivity of conjugated polymers toward oxygen reduction reaction (ORR), the reported values are not sufficient for the device fabrication. Additionally, the origin of the electrocatalytic activities of these materials are not well understood and often attributed to the residual metal atoms remained in the polymeric domains. Here, we show that by tailoring the chemistry of monomers, we can tune the electrochemical properties of the polymers. For instance, in the case of thiophene-based conjugated polymers, i.e. poly(thiophene) and poly(3,4-ethylenedioxythiophen) (PEDOT), the onset of the ORR overpotential are measured to be 0.61 V and 0.76 V vs reversible hydrogen electrode (RHE) [3]. Populating the monomer structure with heteroatoms, we recorded an enhanced electroactivity–comparable to Pt/C–for polymeric domains of poly(3,4-ethylenedithiathiophene) (PEDTT). More interestingly, PEDTT shows high electroactivity toward oxygen evolution reaction (OER) with overpotential onset of 1.72 V vs RHE. This observation makes PEDTT, the first conjugated polymer with bifunctional electroactivity for oxygen reactions. To demonstrate the bifunctionality of PEDTT, we assembled a rechargeable zinc-air battery using polymer in the cathode. We evaluated the performance of the assembled battery and measured an open circuit voltage of 1.4 V and charge and discharge voltages of 2.1 V and 1.1 V, respectively, when the cell was operated at 2 mA/cm2. The device demonstrated a stable cycling performance for over 100 cycles of charge-discharge. Fu, G., Y. Tang, and J.M. Lee, Recent Advances in Carbon ‐Based Bifunctional Oxygen Electrocatalysts for Zn− Air Batteries. ChemElectroChem, 2018. 5(11): p. 1424-1434.Guo, D., et al., Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science, 2016. 351(6271): p. 361-365.Kaviani, S., et al., Electroactive and Conformal Coatings of oCVD Polymers for Oxygen Electroreduction. ACS Applied Polymer Materials, 2019. Figure 1

One-Step Synthesis of Manganese Doped Single Atom Catalysts for Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells

Current commercial catalysts for polymer electrolyte membrane fuel cells (PEMFCs) require platinum group metal (PGM) catalysts that lead to high costs, thus preventing viable commercialization of PEM fuel cells. Many investigations into PGM-free catalysts have led to the identification of single metal atom sites coordinated with 4 nitrogens as being highly active. One of the best performing metal centers in the acidic environment of the PEMFC is Iron (Fe). Highly active iron-based catalysts have been synthesized easily via a one-step method of doping Iron into a ZIF-8 framework; however, there have been membrane degradation concerns due to the Fenton reaction products that are generated by Fe. Manganese (Mn) has recently been proposed as a potential metal center for the highly active metal-N4 catalyst site as theory predicts that it will have good activity and increased stability1 . The best performing Mn-based catalysts have been synthesized from a Mn doped ZIF-8 structure, but due to the high oxophilicity of Mn and difficulty in incorporating Mn into the ZIF structure, a laborious Mn catalyst synthesis route has been adopted, which requires acid leaching, secondary doping, and multiple pyrolysis steps2. This causes problems in scaling up the synthesis and limits the active site density when compared to the Fe catalyst. We aim to use highly reactive Mn metal precursors in an air-free environment to incorporate Mn into the metal sites of ZIF. High temperature pyrolysis (1100°C) leads to manganese catalyst synthesis in one step. We show that using a highly active manganese borohydride precursor allows for the formation of a metal organic framework (MOF) when combined with 2-methylimidazole, which can subsequently be pyrolyzed to form the target catalyst. Using zinc borohydride as a secondary metal source we can also form bimetallic MOF structures with increased thermal stability and varied Mn loading. With this strategy Mn can be incorporated directly into the metal MOF sites with a wide range of Mn loading. The utilization of an air-free environment prevents metal oxide formation thus giving a facile one step synthesis of a manganese-based catalyst. (1) Liu, K.; Qiao, Z.; Hwang, S.; Liu, Z.; Zhang, H.; Su, D.; Xu, H.; Wu, G.; Wang, G. Mn- and N- Doped Carbon as Promising Catalysts for Oxygen Reduction Reaction_ Theoretical Prediction and Experimental Validation. 2018. (2) Li, J.; Chen, M.; Cullen, D. A.; Hwang, S.; Wang, M.; Li, B.; Liu, K.; Karakalos, S.; Lucero, M.; Zhang, H.; et al. Atomically Dispersed Manganese Catalysts for Oxygen Reduction in Proton-Exchange Membrane Fuel Cells. Nat. Catal. 2018, 1 (12), 935–945.