Operando Raman Spectroscopy Reveals Degradation Byproducts from Ionomer Oxidation in Anion Exchange Membrane Water Electrolyzers.

This work showcases the discovery of degradation mechanisms for nonplatinum group metal catalyst (PGM free) based anion exchange membrane water electrolyzers (AEMWE) that utilize hydroxide ion conductive polymer ionomers and membranes in a zero gap configuration. An entirely unique and customized test cell was designed from the ground up for the purposes of obtaining Raman spectra during potentiostatic operation. These results represent some of the first operando Raman spectroscopy explorations into the breakdown products that are generated from high oxidative potential conditions with carbonate electrolytes.

Exploring the Role of an Electrolyte Additive in Suppressing Surface Reconstruction of a Ni-Rich NMC Cathode at Ultrahigh Voltage via Enhanced In Situ and Operando Characterization Methods.

Vinylene carbonate (VC) is a widely used electrolyte additive in lithium-ion batteries for enhanced solid electrolyte interphase formation on the anode side. However, the cathode electrolyte interphase (CEI) formation with VC has received a lot less attention. This study presents a comprehensive investigation employing advanced in situ/operando-based Raman and X-ray absorption spectroscopy (XAS) to explore the effect of electrolyte composition on the CEI formation and suppression of surface reconstruction of LiNiMnCoO (NMC) cathodes.

Bypassing Formation of Oxide Intermediate via Chemical Vapor Deposition for the Synthesis of an Mn-N-C Catalyst with Improved ORR Activity.

A significant barrier to the commercialization of proton exchange membrane fuel cells (PEMFCs) is the high cost of the platinum-based oxygen reduction reaction (ORR) cathode electrocatalysts. One viable solution is to replace platinum with a platinum-group metal (PGM) free catalyst with comparable activity and durability. However, PGM-free catalyst development is burdened by a lack of understanding of the active site formation mechanism during the requisite high-temperature synthesis step, thus making rational catalyst design challenging.

Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N sites.

Replacing scarce and expensive platinum (Pt) with metal-nitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells has largely been impeded by the low oxygen reduction reaction activity of M-N-C due to low active site density and site utilization. Herein, we overcome these limits by implementing chemical vapour deposition to synthesize Fe-N-C by flowing iron chloride vapour over a Zn-N-C substrate at 750 °C, leading to high-temperature trans-metalation of Zn-N sites into Fe-N sites. Characterization by multiple techniques shows that all Fe-N sites formed via this approach are gas-phase and electrochemically accessible.

Evolution Pathway from Iron Compounds to Fe(II)-N Sites through Gas-Phase Iron during Pyrolysis.

Pyrolysis is indispensable for synthesizing highly active Fe-N-C catalysts for the oxygen reduction reaction (ORR) in acid, but how Fe, N, and C precursors transform to ORR-active sites during pyrolysis remains unclear. This knowledge gap obscures the connections between the input precursors and the output products, clouding the pathway toward Fe-N-C catalyst improvement. Herein, we unravel the evolution pathway of precursors to ORR-active catalyst comprised exclusively of single-atom Fe(II)-N sites via in-temperature X-ray absorption spectroscopy.

Alkaline Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer.

Alkaline anion-exchange membrane (AAEM) fuel cells have attracted significant interest in the past decade, thanks to the recent developments in hydroxide-anion conductive membranes. In this article, we compare the performance of current state of the art AAEM fuel cells to proton-exchange membrane (PEM) fuel cells and elucidate the sources of various overpotentials. While the continued development of highly conductive and thermally stable anion-exchange membranes is unambiguously a principal requirement, we attempt to put the focus on the challenges in electrocatalysis and interfacial charge transfer at an alkaline electrode/electrolyte interface.

Unifying the Hydrogen Evolution and Oxidation Reactions Kinetics in Base by Identifying the Catalytic Roles of Hydroxyl-Water-Cation Adducts.

Despite the fundamental and practical significance of the hydrogen evolution and oxidation reactions (HER/HOR), their kinetics in base remain unclear. Herein, we show that the alkaline HER/HOR kinetics can be unified by the catalytic roles of the adsorbed hydroxyl (OH)-water-alkali metal cation (AM) adducts, on the basis of the observations that enriching the OH abundance via surface Ni benefits the HER/HOR; increasing the AM concentration only promotes the HER, while varying the identity of AM affects both HER/HOR. The presence of OH-(HO) -AM in the double-layer region facilitates the OH removal into the bulk, forming OH-(HO) -AM as per the hard-soft acid-base theory, thereby selectively promoting the HER.

X-Ray Absorption Spectroscopy Characterizations on PGM-Free Electrocatalysts: Justification, Advantages, and Limitations.

Transition metals embedded in nitrogen-doped carbon matrices (denoted as M-N-C) are the leading platinum group metal (PGM)-free electrocatalysts for the oxygen reduction reaction (ORR) in acid, and are the most promising candidates for replacing platinum in practical devices such as fuel cells. Two of the long-standing puzzles in the field are the nature of active sites for the ORR and the reaction mechanism. Poor understanding of the structural and mechanistic basis for the exceptional ORR activity of M-N-C electrocatalysts impedes rational design for further improvements.

Roles of Mo Surface Dopants in Enhancing the ORR Performance of Octahedral PtNi Nanoparticles.

Doping with a transition metal was recently shown to greatly boost the activity and durability of PtNi/C octahedral nanoparticles (NPs) for the oxygen reduction reaction (ORR), but its specific roles remain unclear. By combining electrochemistry, ex situ and in situ spectroscopic techniques, density functional theory calculations, and a newly developed kinetic Monte Carlo model, we showed that Mo atoms are preferentially located on the vertex and edge sites of Mo-PtNi/C in the form of oxides, which are stable within the wide potential window of the electrochemical cycle. These surface Mo oxides stabilize adjacent Pt sites, hereby stabilizing the octahedral shape enriched with (111) facets, and lead to increased concentration of Ni in subsurface layers where they are protected against acid dissolution.

Experimental Proof of the Bifunctional Mechanism for the Hydrogen Oxidation in Alkaline Media.

Realization of the hydrogen economy relies on effective hydrogen production, storage, and utilization. The slow kinetics of hydrogen evolution and oxidation reaction (HER/HOR) in alkaline media limits many practical applications involving hydrogen generation and utilization, and how to overcome this fundamental limitation remains debatable. Here we present a kinetic study of the HOR on representative catalytic systems in alkaline media.

Identification of catalytic sites in cobalt-nitrogen-carbon materials for the oxygen reduction reaction.

Single-atom catalysts with full utilization of metal centers can bridge the gap between molecular and solid-state catalysis. Metal-nitrogen-carbon materials prepared via pyrolysis are promising single-atom catalysts but often also comprise metallic particles. Here, we pyrolytically synthesize a Co-N-C material only comprising atomically dispersed cobalt ions and identify with X-ray absorption spectroscopy, magnetic susceptibility measurements and density functional theory the structure and electronic state of three porphyrinic moieties, CoNC, CoNC and CoNC.

Resolving the Iron Phthalocyanine Redox Transitions for ORR Catalysis in Aqueous Media.

Metal macrocycles are among the most important catalytic systems in electrocatalysis and biocatalysis owing to their rich redox chemistry. Precise understanding of the redox behavior of metal macrocycles in operando is essential for fundamental studies and practical applications of this catalytic system. Here we present electrochemical data for the representative iron phthalocyanine (FePc) in both aqueous and nonaqueous media coupled with in situ Raman and X-ray absorption analyses to challenge the traditional notion of the redox transition of FePc at the low potential end in aqueous media by showing that it arises from the redox transition of the ring.

Unraveling the Nature of Sites Active toward Hydrogen Peroxide Reduction in Fe-N-C Catalysts.

Fe-N-C catalysts with high O reduction performance are crucial for displacing Pt in low-temperature fuel cells. However, insufficient understanding of which reaction steps are catalyzed by what sites limits their progress. The nature of sites were investigated that are active toward H O reduction, a key intermediate during indirect O reduction and a source of deactivation in fuel cells.

Metal and Metal Oxide Interactions and Their Catalytic Consequences for Oxygen Reduction Reaction.

Many industrial catalysts are composed of metal particles supported on metal oxides (MMO). It is known that the catalytic activity of MMO materials is governed by metal and metal oxide interactions (MMOI), but how to optimize MMO systems via manipulation of MMOI remains unclear, due primarily to the ambiguous nature of MMOI. Herein, we develop a Pt/NbO/C system with tunable structural and electronic properties via a modified arc plasma deposition method.

Highly Active and Stable Fe-N-C Catalyst for Oxygen Depolarized Cathode Applications.

Anion immunity toward the oxygen reduction reaction (ORR) has tremendous implications in electrocatalysis with applications for fuel cells, metal-air batteries, and oxygen depolarized cathodes (ODCs) in the anodic evolution of chlorine. The necessity of exploring ORR catalysts with immunity to anion adsorption is particularly significant considering that platinum group metal (PGM) catalysts are costly and highly vulnerable to impurities such as halides. Herein, we report a metal organic framework (MOF)-derived Fe-N-C catalyst that exhibits a dramatically improved half-wave potential of 240 mV compared to the state-of-the-art RhS/C catalyst in a rotating disk electrode in the presence of Cl.

Asymmetric Volcano Trend in Oxygen Reduction Activity of Pt and Non-Pt Catalysts: In Situ Identification of the Site-Blocking Effect.

Proper understanding of the major limitations of current catalysts for oxygen reduction reaction (ORR) is essential for further advancement. Herein by studying representative Pt and non-Pt ORR catalysts with a wide range of redox potential (E) via combined electrochemical, theoretical, and in situ spectroscopic methods, we demonstrate that the role of the site-blocking effect in limiting the ORR varies drastically depending on the E of active sites; and the intrinsic activity of active sites with low E have been markedly underestimated owing to the overlook of this effect. Accordingly, we establish a general asymmetric volcano trend in the ORR activity: the ORR of the catalysts on the overly high E side of the volcano is limited by the intrinsic activity; whereas the ORR of the catalysts on the low E side is limited by either the site-blocking effect and/or intrinsic activity depending on the E.

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.

A Search for the Optimum Lithium Rich Layered Metal Oxide Cathode Material for Li-Ion Batteries.

We report the results of a comprehensive study of the relationship between electrochemical performance in Li cells and chemical composition of a series of Li rich layered metal oxides of the general formula xLiMnO · (1-x)LiMnNiCoO in which x = 0,1, 0.2, 0,3, 0.5 or 0.

Improved Oxygen Reduction Activity and Durability of Dealloyed PtCo Catalysts for Proton Exchange Membrane Fuel Cells: Strain, Ligand, and Particle Size Effects.

The development of active and durable catalysts with reduced platinum content is essential for fuel cell commercialization. Herein we report that the dealloyed PtCo/HSC and PtCo/HSC nanoparticle (NP) catalysts exhibit the same levels of enhancement in oxygen reduction activity (~4-fold) and durability over pure Pt/C NPs. Surprisingly, ex situ high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) shows that the bulk morphologies of the two catalysts are distinctly different: D-PtCo/HSC catalyst is dominated by NPs with solid Pt shells surrounding a single ordered PtCo core; however, the D-PtCo/HSC catalyst is dominated by NPs with porous Pt shells surrounding multiple disordered PtCo cores with local concentration of Co.

Composite Ni/NiO-CrO Catalyst for Alkaline Hydrogen Evolution Reaction.

We report a Ni-Cr/C electrocatalyst with unprecedented mass-activity for the hydrogen evolution reaction (HER) in alkaline electrolyte. The HER kinetics of numerous binary and ternary Ni-alloys and composite Ni/metal-oxide/C samples were evaluated in aqueous 0.1 M KOH electrolyte.

Spectroscopic Measurements of the Relative Pt Skin Thicknesses and Porosities of Dealloyed PtM (Ni, Co) Electrocatalysts.

X-ray adsorption near edge structure (XANES) data at the Co or Ni K-edge, analyzed using the Δ difference procedure, are reported for dealloyed PtCo and PtNi catalysts (six different catalysts at different stages of life). All catalysts meet the 2017 DOE beginning of life target Pt mass activity target (>0.44 A mg), but exhibit varying activities and durabilities.

Highly active oxygen reduction non-platinum group metal electrocatalyst without direct metal-nitrogen coordination.

Replacement of noble metals in catalysts for cathodic oxygen reduction reaction with transition metals mostly create active sites based on a composite of nitrogen-coordinated transition metal in close concert with non-nitrogen-coordinated carbon-embedded metal atom clusters. Here we report a non-platinum group metal electrocatalyst with an active site devoid of any direct nitrogen coordination to iron that outperforms the benchmark platinum-based catalyst in alkaline media and is comparable to its best contemporaries in acidic media. In situ X-ray absorption spectroscopy in conjunction with ex situ microscopy clearly shows nitrided carbon fibres with embedded iron particles that are not directly involved in the oxygen reduction pathway.

Activity descriptor identification for oxygen reduction on platinum-based bimetallic nanoparticles: in situ observation of the linear composition-strain-activity relationship.

Despite recent progress in developing active and durable oxygen reduction catalysts with reduced Pt content, lack of elegant bottom-up synthesis procedures with knowledge over the control of atomic arrangement and morphology of the Pt-alloy catalysts still hinders fuel cell commercialization. To follow a less empirical synthesis path for improved Pt-based catalysts, it is essential to correlate catalytic performance to properties that can be easily controlled and measured experimentally. Herein, using Pt-Co alloy nanoparticles (NPs) with varying atomic composition as an example, we show that the atomic distribution of Pt-based bimetallic NPs under operating conditions is strongly dependent on the initial atomic ratio by employing microscopic and in situ spectroscopic techniques.

Elucidating Oxygen Reduction Active Sites in Pyrolyzed Metal-Nitrogen Coordinated Non-Precious-Metal Electrocatalyst Systems.

Detailed understanding of the nature of the active centers in non-precious-metal-based electrocatalyst, and their role in oxygen reduction reaction (ORR) mechanistic pathways will have a profound effect on successful commercialization of emission-free energy devices such as fuel cells. Recently, using pyrolyzed model structures of iron porphyrins, we have demonstrated that a covalent integration of the Fe-N sites into π-conjugated carbon basal plane modifies electron donating/withdrawing capability of the carbonaceous ligand, consequently improving ORR activity. Here, we employ a combination of X-ray spectroscopy and electrochemical methods to identify the various structural and functional forms of the active centers in non-heme Fe/N/C catalysts.

The Role of OOH Binding Site and Pt Surface Structure on ORR Activities.

We present experimentally observed molecular adsorbate coverages (e.g., O(H), OOH and HOOH) on real operating dealloyed bimetallic PtM (M = Ni or Co) catalysts under oxygen reduction reaction (ORR) conditions obtained using X-ray absorption near edge spectroscopy (XANES).