Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies.

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Detection Technologies for Reactive Oxygen Species: Fluorescence and Electrochemical Methods and Their Applications.

Reactive oxygen species (ROS) have been found in plants, mammals, and natural environmental processes. The presence of ROS in mammals has been linked to the development of severe diseases, such as diabetes, cancer, tumors, and several neurodegenerative conditions. The most common ROS involved in human health are superoxide (O2•-), hydrogen peroxide (H2O2), and hydroxyl radicals (•OH). Organic and inorganic molecules have been integrated with various methods to detect and monitor ROS for understanding the effect of their presence and concentration on diseases caused by oxidative stress. Among several techniques, fluorescence and electrochemical methods have been recently developed and employed for the detection of ROS. This literature review intends to critically discuss the development of these techniques to date, as well as their application for in vitro and in vivo ROS detection regarding free-radical-related diseases. Moreover, important insights into and further steps for using fluorescence and electrochemical methods in the detection of ROS are presented.

Preparation of Nonprecious Metal Electrocatalysts for the Reduction of Oxygen Using a Low-Temperature Sacrificial Metal.

Non-platinum group metal (non-PGM) electrocatalysts for the oxygen reduction reaction (ORR) are generally composed of iron, nitrogen, and carbon synthesized through high-temperature pyrolysis. Among the various types of precursors, metal-organic frameworks (MOFs), zeolitic imidazolate framework (ZIF)-8 in particular, have often been used in the synthesis. The pyrolysis of ZIF-8 precursor relies on the use of Zn as a sacrificial metal (SM), and the optimal processing temperatures often exceed 1000 °C to generate active non-PGM catalysts. The high pyrolysis temperature tends to result in heterogeneous active moieties ranging from Fe single atoms to nanoparticles. In this study, we present the synthesis of non-PGM catalysts using Cd as the sacrificial metal instead of Zn. By using Cd, we were able to generate active non-PGM electrocatalysts from the MOF precursors at a low pyrolysis temperature of 750 °C, which helps preserve the single atomic iron active sites.

Understanding water management in platinum group metal-free electrodes using neutron imaging.

Platinum group metal-free (PGM-free) catalysts are a low-cost alternative to expensive PGM catalysts for polymer electrolyte fuel cells. However, due to the low volumetric activity of PGM-free catalysts, the catalyst layer thickness of the PGM-free catalyst electrode is an order of magnitude higher than PGM based electrodes. The thick PGM-free electrodes suffer from increased transport resistance and poor water management, which ultimately limits the fuel cell performance. This manuscript presents the study of water management in the PGM-free electrodes to understand the transport limitations and improve fuel cell performance. In-operando neutron imaging is performed to estimate the water content in different components across the fuel cell thickness. Water saturation in thick PGM electrodes, with similar catalyst layer thickness to PGM-free electrodes, is lower than in the PGM-free electrodes irrespective of the operating conditions, due to high water retention by PGM-free catalysts. Improvements in fuel cell performance are accomplished by enhancing water removal from the flooded PGM-free electrode in three ways: (i) enhanced water removal with a novel microporous layer with hydrophilic pathways incorporated through hydrophilic additives, (ii) water removal through anode via novel GDL in the anode, and (iii) lower water saturation in PGM-free electrode structures with increased catalyst porosity.

Heat-Treated Aerogel as a Catalyst for the Oxygen Reduction Reaction.

Aerogels are fascinating materials that can be used for a wide range of applications, one of which is electrocatalysis of the important oxygen reduction reaction. In their inorganic form, aerogels can have ultrahigh catalytic site density, high surface area, and tunable physical properties and chemical structures-important features in heterogeneous catalysis. Herein, we report on the synthesis and electrocatalytic properties of an iron-porphyrin aerogel. 5,10,15,20-(Tetra-4-aminophenyl)porphyrin (H2 TAPP) and FeII were used as building blocks of the aerogel, which was later heat-treated at 600 °C to enhance electronic conductivity and catalytic activity, while preserving its macrostructure. The resulting material has a very high concentration of atomically dispersed catalytic sites (9.7×1020  sites g-1 ) capable of catalyzing the oxygen reduction reaction in alkaline solution (Eonset =0.92 V vs. RHE, TOF=0.25 e- site-1 s-1 at 0.80 V vs. RHE).

PGM-Free Cathode Catalysts for PEM Fuel Cells: A Mini-Review on Stability Challenges.

In recent years, significant progress has been achieved in the development of platinum group metal-free (PGM-free) oxygen reduction reaction (ORR) catalysts for proton exchange membrane (PEM) fuel cells. At the same time the limited durability of these catalysts remains a great challenge that needs to be addressed. This mini-review summarizes the recent progress in understanding the main causes of instability of PGM-free ORR catalysts in acidic environments, focusing on transition metal/nitrogen codoped systems (M-N-C catalysts, M: Fe, Co, Mn), particularly MNx moiety active sites. Of several possible degradation mechanisms, demetalation and carbon oxidation are found to be the most likely reasons for M-N-C catalysts/cathodes degradation.

Progress in the Development of Fe-Based PGM-Free Electrocatalysts for the Oxygen Reduction Reaction.

Development of alternative energy sources is crucial to tackle challenges encountered by the growing global energy demand. Hydrogen fuel, a promising way to store energy produced from renewable power sources, can be converted into electrical energy at high efficiency via direct electrochemical conversion in fuel cells, releasing water as the sole byproduct. One important drawback to current fuel-cell technology is the high content of platinum-group-metal (PGM) electrocatalysts required to perform the sluggish oxygen reduction reaction (ORR). Addressing this challenge, remarkable progress has been made in the development of low-cost PGM-free electrocatalysts synthesized from inexpensive, earth-abundant, and easily sourced materials such as iron, nitrogen, and carbon (Fe-N-C). PGM-free Fe-N-C electrocatalysts now exhibit ORR activities approaching that of PGM electrocatalysts but at a fraction of the cost, promising to significantly reduce overall fuel-cell technology costs. Herein, recent developments in PGM-free electrocatalysis, demonstrating increased fuel-cell performance, as well as efforts aimed at understanding the key limiting factor, i.e., the nature of the PGM-free active site, are summarized. Further improvements will be accomplished through the controlled and/or rationally designed synthesis of materials with higher active-site densities, while at the same time establishing methods to mitigate catalyst degradation.

Highly Graphitic Mesoporous Fe,N-Doped Carbon Materials for Oxygen Reduction Electrochemical Catalysts.

The synthesis, characterization, and electrocatalytic properties of mesoporous carbon materials doped with nitrogen atoms and iron are reported and compared for the catalyzed reduction of oxygen gas at fuel cell cathodes. Mixtures of common and inexpensive organic precursors, melamine, and formaldehyde were pyrolyzed in the presence of transition-metal salts (e.g., nitrates) within a mesoporous silica template to yield mesoporous carbon materials with greater extents of graphitization than those of others prepared from small-molecule precursors. In particular, Fe,N-doped carbon materials possessed high surface areas (∼800 m2/g) and high electrical conductivities (∼19 S/cm), which make them attractive for electrocatalyst applications. The surface compositions of the mesoporous Fe,N-doped carbon materials were postsynthetically modified by acid washing and followed by high-temperature thermal treatments, which were shown by X-ray photoelectron spectroscopy to favor the formation of graphitic and pyridinic nitrogen moieties. Such surface-modified materials exhibited high electrocatalytic oxygen reduction activities under alkaline conditions, as established by their high onset and half-wave potentials (1.04 and 0.87 V, respectively vs reversible hydrogen electrode) and low Tafel slope (53 mV/decade). These values are superior to many similar transition-metal- and N-doped carbon materials and compare favorably with commercially available precious-metal catalysts, e.g., 20 wt % Pt supported on activated carbon. The analyses indicate that inexpensive mesoporous Fe,N-doped carbon materials are promising alternatives to precious metal-containing catalysts for electrochemical reduction of oxygen in polymer electrolyte fuel cells.

Direct atomic-level insight into the active sites of a high-performance PGM-free ORR catalyst.

Platinum group metal-free (PGM-free) metal-nitrogen-carbon catalysts have emerged as a promising alternative to their costly platinum (Pt)-based counterparts in polymer electrolyte fuel cells (PEFCs) but still face some major challenges, including (i) the identification of the most relevant catalytic site for the oxygen reduction reaction (ORR) and (ii) demonstration of competitive PEFC performance under automotive-application conditions in the hydrogen (H2)-air fuel cell. Herein, we demonstrate H2-air performance gains achieved with an iron-nitrogen-carbon catalyst synthesized with two nitrogen precursors that developed hierarchical porosity. Current densities recorded in the kinetic region of cathode operation, at fuel cell voltages greater than ~0.75 V, were the same as those obtained with a Pt cathode at a loading of 0.1 milligram of Pt per centimeter squared. The proposed catalytic active site, carbon-embedded nitrogen-coordinated iron (FeN4), was directly visualized with aberration-corrected scanning transmission electron microscopy, and the contributions of these active sites associated with specific lattice-level carbon structures were explored computationally.

Resolving Electrode Morphology's Impact on Platinum Group Metal-Free Cathode Performance Using Nano-CT of 3D Hierarchical Pore and Ionomer Distribution.

This article reports on the characterization of polymer electrolyte fuel cell (PEFC) cathodes featuring a platinum group metal-free (PGM-free) catalyst using nanoscale resolution X-ray computed tomography (nano-CT) and morphological analysis. PGM-free PEFC cathodes have gained significant interest in the past decade since they have the potential to dramatically reduce PEFC costs by eliminating the large platinum (Pt) raw material cost. However, several challenges remain before they are commercially viable. Since these catalysts have lower volumetric activity, the PGM-free cathodes are thicker and subject to increased gas and proton transport resistances that reduce the performance. To better understand the efficacy of the catalyst and improve electrode performance, a detailed understanding the correlation between electrode fabrication, morphology, and performance is crucial. In this work, the pore/solid structure and the ionomer distribution was resolved in three dimensions (3D) using nano-CT for three PGM-free electrodes of varying Nafion loading. The associated transport properties were evaluated from pore/particle-scale simulations within the nano-CT-imaged structure. These characterizations are then used to elucidate the microstructural origins of the dramatic changes in fuel cell performance with varying Nafion ionomer loading. We show that this is primarily a result of distinct changes in ionomer's spatial distribution. The significant impact of electrode morphology on performance highlights the importance of PGM-free electrode development in concert with efforts to improve catalyst activity and durability.

Critical role of intercalated water for electrocatalytically active nitrogen-doped graphitic systems.

Graphitic materials are essential in energy conversion and storage because of their excellent chemical and electrical properties. The strategy for obtaining functional graphitic materials involves graphite oxidation and subsequent dissolution in aqueous media, forming graphene-oxide nanosheets (GNs). Restacked GNs contain substantial intercalated water that can react with heteroatom dopants or the graphene lattice during reduction. We demonstrate that removal of intercalated water using simple solvent treatments causes significant structural reorganization, substantially affecting the oxygen reduction reaction (ORR) activity and stability of nitrogen-doped graphitic systems. Amid contrasting reports describing the ORR activity of GN-based catalysts in alkaline electrolytes, we demonstrate superior activity in an acidic electrolyte with an onset potential of ~0.9 V, a half-wave potential (E ½) of 0.71 V, and a selectivity for four-electron reduction of >95%. Further, durability testing showed E ½ retention >95% in N2- and O2-saturated solutions after 2000 cycles, demonstrating the highest ORR activity and stability reported to date for GN-based electrocatalysts in acidic media.

Experimental Observation of Redox-Induced Fe-N Switching Behavior as a Determinant Role for Oxygen Reduction Activity.

The commercialization of electrochemical energy conversion and storage devices relies largely upon the development of highly active catalysts based on abundant and inexpensive materials. Despite recent achievements in this respect, further progress is hindered by the poor understanding of the nature of active sites and reaction mechanisms. Herein, by characterizing representative iron-based catalysts under reactive conditions, we identify three Fe-N4-like catalytic centers with distinctly different Fe-N switching behaviors (Fe moving toward or away from the N4-plane) during the oxygen reduction reaction (ORR), and show that their ORR activities are essentially governed by the dynamic structure associated with the Fe(2+/3+) redox transition, rather than the static structure of the bare sites. Our findings reveal the structural origin of the enhanced catalytic activity of pyrolyzed Fe-based catalysts compared to nonpyrolyzed Fe-macrocycle compounds. More generally, the fundamental insights into the dynamic nature of transition-metal compounds during electron-transfer reactions will potentially guide rational design of these materials for broad applications.

A simple synthesis of nitrogen-doped carbon micro- and nanotubes.

Nitrogen-doped carbon nanotubes (N-CNTs) have shown promising performance in a variety of applications: catalysts, catalyst supports, lithium-ion batteries and supercapacitors. While chemical vapor deposition continues to be the most common method for N-CNT fabrication, its complexity and high cost have prevented the practical implementation of N-CNTs on a larger scale. Here, we describe a new and simple method for N-CNT synthesis. This method is not only potentially scalable to any required level but also allows for the control of the tube size (20-1000 nm in diameter) and nitrogen doping levels (4-9 at%) through the selection of most suitable transition metal and heat-treatment conditions.

High-activity PtRuPd/C catalyst for direct dimethyl ether fuel cells.

Dimethyl ether (DME) has been considered as a promising alternative fuel for direct-feed fuel cells but lack of an efficient DME oxidation electrocatalyst has remained the challenge for the commercialization of the direct DME fuel cell. The commonly studied binary PtRu catalyst shows much lower activity in DME than methanol oxidation. In this work, guided by density functional theory (DFT) calculation, a ternary carbon-supported PtRuPd catalyst was designed and synthesized for DME electrooxidation. DFT calculations indicated that Pd in the ternary PtRuPd catalyst is capable of significantly decreasing the activation energy of the CO and CH bond scission during the oxidation process. As evidenced by both electrochemical measurements in an aqueous electrolyte and polymer-electrolyte fuel cell testing, the ternary catalyst shows much higher activity (two-fold enhancement at 0.5 V in fuel cells) than the state-of-the-art binary Pt50 Ru50 /C catalyst (HiSPEC 12100).

Ozonated graphene oxide film as a proton-exchange membrane.

Graphene oxide (GO) contains several chemical functional groups that are attached to the graphite basal plane and can be manipulated to tailor GO for specific applications. It is now revealed that the reaction of GO with ozone results in a high level of oxidation, which leads to significantly improved ionic (protonic) conductivity of the GO. Freestanding ozonated GO films were synthesized and used as efficient polymer electrolyte fuel cell membranes. The increase in protonic conductivity of the ozonated GO originates from enhanced proton hopping, which is due to the higher content of oxygenated functional groups in the basal planes and edges of ozonated GO as well as the morphology changes in GO that are caused by ozonation. The results of this study demonstrate that the modification of dispersed GO presents a powerful opportunity for optimizing a nanoscale material for proton-exchange membranes.

Nanostructured nonprecious metal catalysts for oxygen reduction reaction.

Platinum-based catalysts represent a state of the art in the electrocatalysis of oxygen reduction reaction (ORR) from the point of view of their activity and durability in harnessing the chemical energy via direct electrochemical conversion. However, because platinum is both expensive and scarce, its widespread implementation in such clean energy applications is limited. Recent breakthroughs in the synthesis of high-performance nonprecious metal catalysts (NPMCs) make replacement of Pt in ORR electrocatalysts with earth-abundant elements, such as Fe, Co, N, and C, a realistic possibility. In this Account, we discuss how we can obtain highly promising M-N-C (M: Fe and/or Co) catalysts by simultaneously heat-treating precursors of nitrogen, carbon, and transition metals at 800-1000 °C. The activity and durability of resulting catalysts depend greatly on the selection of precursors and synthesis chemistry. In addition, they correlate quite well with the catalyst nanostructure. While chemists have presented no conclusive description of the active catalytic site for this class of NPMCs, they have developed a designed approach to making active and durable materials, focusing on the catalyst nanostructure. The approach consists of nitrogen doping, in situ carbon graphitization, and the usage of graphitic structures (possibly graphene and graphene oxides) as carbon precursors. Various forms of nitrogen, particularly pyridinic and quaternary, can act as n-type carbon dopants in the M-N-C catalysts, assisting in the formation of disordered carbon nanostructures and donating electrons to the carbon. The CNx structures are likely a crucial part of the ORR active site(s). Noteworthy, the ORR activity is not necessarily governed by the amount of nitrogen, but by how the nitrogen is incorporated into the nanostructures. Apart from the possibility of a direct participation in the active site, the transition metal often plays an important role in the in situ formation of various carbon nanostructures by catalyzing the decomposition of the nitrogen/carbon precursor. We can control the formation of different nanostructures during the synthesis of M-N-C catalysts. For example, in situ formed nitrogen-doped graphene-sheets can only be derived from polyaniline (PANI), probably due to structural similarities between the aromatic structures of PANI and graphene. Highly-graphitized carbon nanostructures may serve as a matrix for the formation of ORR-active groups with improved catalytic activity and durability, containing nitrogen and most probably also metal atoms. In the future, we will likely focus NPMC synthesis approaches on precise control of interactions between precursors of the metal and carbon/nitrogen during the heat treatment. The main purposes will be to maximize the number of active sites, optimize nitrogen doping levels, and generate morphologies capable of hosting active and stable ORR sites.

Active and stable carbon nanotube/nanoparticle composite electrocatalyst for oxygen reduction.

Nanostructured carbon-based materials, such as nitrogen-doped carbon nanotube arrays, Co3O4/nitrogen-doped graphene hybrids and carbon nanotube-graphene complexes have shown respectable oxygen reduction reaction activity in alkaline media. Although certainly promising, the performance of these materials does not yet warrant implementation in the energy conversion/storage devices utilizing basic electrolytes, for example, alkaline fuel cells, metal-air batteries and certain electrolysers. Here we demonstrate a new type of nitrogen-doped carbon nanotube/nanoparticle composite oxygen reduction reaction electrocatalyst obtained from iron acetate as an iron precursor and from cyanamide as a nitrogen and carbon nanotube precursor in a simple, scalable and single-step method. The composite has the highest oxygen reduction reaction activity in alkaline media of any non-precious metal catalysts. When used at a sufficiently high loading, this catalyst also outperforms the most active platinum-based catalysts.

A carbon-nanotube-supported graphene-rich non-precious metal oxygen reduction catalyst with enhanced performance durability.

A non-precious metal catalyst for oxygen reduction in acid media, enriched in graphene sheets/bubbles during a high-temperature synthesis step, has been developed from an Fe precursor and in situ polymerized polyaniline, supported on multi-walled carbon nanotubes. The catalyst showed no performance loss for 500 hours in a hydrogen/air fuel cell. The improved durability is correlated with the graphene formation, apparently enhanced in the presence of carbon nanotubes.

Nitrogen-doped graphene-rich catalysts derived from heteroatom polymers for oxygen reduction in nonaqueous lithium-O2 battery cathodes.

In this work, we present a synthesis approach for nitrogen-doped graphene-sheet-like nanostructures via the graphitization of a heteroatom polymer, in particular, polyaniline, under the catalysis of a cobalt species using multiwalled carbon nanotubes (MWNTs) as a supporting template. The graphene-rich composite catalysts (Co-N-MWNTs) exhibit substantially improved activity for oxygen reduction in nonaqueous lithium-ion electrolyte as compared to those of currently used carbon blacks and Pt/carbon catalysts, evidenced by both rotating disk electrode and Li-O(2) battery experiments. The synthesis-structure-activity correlations for the graphene nanostructures were explored by tuning their synthetic chemistry (support, nitrogen precursor, heating temperature, and transition metal type and content) to investigate how the resulting morphology and nitrogen-doping functionalities (e.g., pyridinic, pyrrolic, and quaternary) influence the catalyst activity. In particular, an optimal temperature for heat treatment during synthesis is critical to creating a high-surface-area catalyst with favorable nitrogen doping. The sole Co phase, Co(9)S(8), was present in the catalyst but plays a negligible role in ORR. Nevertheless, the addition of Co species in the synthesis is indispensable for achieving high activity, due to its effects on the final catalyst morphology and structure, including surface area, nitrogen doping, and graphene formation. This new route for the preparation of a nitrogen-doped graphene nanocomposite with carbon nanotube offers synthetic control of morphology and nitrogen functionality and shows promise for applications in nonaqueous oxygen reduction electrocatalysis for Li-O(2) battery cathodes.

High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt.

The prohibitive cost of platinum for catalyzing the cathodic oxygen reduction reaction (ORR) has hampered the widespread use of polymer electrolyte fuel cells. We describe a family of non-precious metal catalysts that approach the performance of platinum-based systems at a cost sustainable for high-power fuel cell applications, possibly including automotive power. The approach uses polyaniline as a precursor to a carbon-nitrogen template for high-temperature synthesis of catalysts incorporating iron and cobalt. The most active materials in the group catalyze the ORR at potentials within ~60 millivolts of that delivered by state-of-the-art carbon-supported platinum, combining their high activity with remarkable performance stability for non-precious metal catalysts (700 hours at a fuel cell voltage of 0.4 volts) as well as excellent four-electron selectivity (hydrogen peroxide yield <1.0%).