Enhancing the sluggish kinetics of electrochemical hydrogen-oxidation reaction in high pH environments is of crucial importance considering its applications in alkaline-membrane fuel cells (AMFC) and ...regenerative hydrogen electrodes for energy storage. Alkaline H2-oxidation to form water involves reaction between H-adsorbed intermediates and hydroxide anions wherein the nature/source of the latter plays a crucial role. Here, we take a systematic approach to understand why H2-oxidation kinetics is slower in alkaline media compared to acid. While recently reported models focus on surface-adsorbate bond strength optimization, we herein show that the alkaline H2-oxidation mechanism is fundamentally different due to a complex interplay between electrocatalysis and electrochemical double-layer structure. A heretofore unknown modern rendition of the double-layer structure is proposed wherein specifically adsorbed (M-OHad) and quasi-specifically adsorbed (M-Had/upd…OHq-ad) reactive hydroxide-species localized in the compact part of the electrochemical double-layer is shown to define H2-oxidation kinetics on monometallic and bimetallic catalyst surfaces at high pH.
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•Fundamental mechanistic causes of sluggishness of HOR in alkaline electrolyte.•Catalysis mechanisms and parameters to design non-precious electrocatalysts for HOR.•Interplay between adsorbed intermediates in electrochemical double-layer structure.
Two Fe-N-C catalysts comprising only atomically-dispersed FeNx moieties were prepared, differing only in the fact that the second catalyst (Fe0.5-NH3) was obtained by subjecting the first one ...(Fe0.5-Ar) to a short pyrolysis in ammonia. While the initial ORR activity in acid medium in rotating disk electrode is similar for both catalysts, the activity in alkaline medium is significantly higher for Fe0.5-NH3. Time-resolved Fe dissolution reveals a circa 10 times enhanced Fe leaching rate in acidic electrolyte for Fe0.5-NH3 relative to Fe0.5-Ar. Furthermore, for the former, the leaching rate is strongly enhanced when the electrochemical potential is in the range 0.75-0.3 V vs. RHE. This may explain the reduced stability of ammonia-pyrolyzed Fe-N-C catalysts in operating PEMFCs. In alkaline medium in contrast, Fe0.5-NH3 is more active and more stable, with minimized Fe leaching during electrochemical operation in load-cycling mode. Operando X-ray absorption spectroscopy measurements in alkaline electrolyte reveals similar trends of the XANES and EXAFS spectra as a function of the electrochemical potential for both catalysts, but the magnitude of change is much less for Fe0.5-NH3, as evidenced by a Δμ analysis. This is interpreted as a lower average oxidation state of FeNx moieties in Fe0.5-NH3 at open circuit potential.
It is unclear why the hydrogen evolution and oxidation reactions (HER/HOR) of platinum (Pt) are much slower in base than in acid. Neither is it clear why the sluggish HER/HOR of Pt in a base can be ...improved by mixing Pt with some transition metals. Herein, we constructed dealloyed carbon-supported Pt–Cu nanoclusters (D-PtCu3/C) with a core–shell Cu@Pt structure wherein the Pt–Cu alloying core is surrounded by Pt shells as a model HER/HOR catalyst to interpret these puzzles. Combined microscopy and in situ X-ray absorption spectroscopy verified the Cu@Pt structure in association with the compressive strain in D-PtCu3/C during the HER/HOR. The superior oxygen reduction reaction activity of the D-PtCu3/C to that of Pt/C in both acid and alkaline solution confirmed that the compressive strain weakens the Pt–O binding energy (E Pt–O) of the D-PtCu3/C. The D-PtCu3/C with compressed Pt shells exhibited inferior HER/HOR activity and a positive shift of the sharp hydrogen adsorption/desorption peaks toward higher potential in comparison with Pt/C in alkaline solution. These results verified that the compressive strain reduces the HER/HOR activity of Pt by weakening E Pt–O. This conclusion indicates that the HER/HOR kinetics of Pt in a base is mainly limited by the overly weak E Pt–O rather than overly strong Pt–H binding energy.
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 x /C system with tunable structural and electronic properties via a modified arc plasma deposition method. We unravel the nature of MMOI by characterizing this system under reactive conditions utilizing combined electrochemical, microscopy, and in situ spectroscopy. We show that Pt interacts with the Nb in unsaturated NbO x owing to the oxygen deficiency in the MMO interface, whereas Pt interacts with the O in nearly saturated NbO x , and further interacts with Nb when the oxygen atoms penetrate into the Pt cluster at elevated potentials. While the Pt–Nb interactions do not benefit the inherent activity of Pt toward oxygen reduction reaction (ORR), the Pt–O interactions improve the ORR activity by shortening the Pt–Pt bond distance. Pt donates electrons to NbO x in both Pt–Nb and Pt–O cases. The resultant electron eficiency stabilizes low-coordinated Pt sites, hereby stabilizing small Pt particles. This determines the two characteristic features of MMO systems: dispersion of small metal particles and high catalytic durability. These findings contribute to our understandings of MMO catalytic systems.
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. The Fe precursor transforms to Fe oxides below 300 °C and then to tetrahedral Fe
(II)-O
via a crystal-to-melt-like transformation below 600 °C. The Fe
(II)-O
releases a single Fe atom that diffuses into the N-doped carbon defect forming Fe
(II)-N
above 600 °C. This vapor-phase single Fe atom transport mechanism is verified by synthesizing Fe
(II)-N
sites via "noncontact pyrolysis" wherein the Fe precursor is not in physical contact with the N and C precursors during pyrolysis.
•The hydrogen binding energy theory centers on the inherent properties of the surface.•The bifunctional mechanism further considers the interactions between surface intermediate adsorbates (Had and ...OHad).•The pzfc theory considers the interfacial double layer environment and the transportation of reaction intermediates (OHad).•The 2B theory further incorporates the cation effects into the picture of the interfacial double layer region in alkaline electrochemistry.
The hydrogen evolution and oxidation reactions (HER/HOR) in alkaline media are the most fundamental electrochemical reactions in alkaline electrochemistry. Surprisingly, their kinetics is still elusive and under extensive debates. Herein, a critical review of the four schools of thoughts of the alkaline HER/HOR kinetics is given, focusing on the key discrepancies in the underlying mechanisms and experimental supports within these thoughts.
Developing efficient and inexpensive catalysts for the sluggish oxygen reduction reaction (ORR) constitutes one of the grand challenges in the fabrication of commercially viable fuel cell devices and ...metal–air batteries for future energy applications. Despite recent achievements in designing advanced Pt-based and Pt-free catalysts, current progress primarily involves an empirical approach of trial-and-error combination of precursors and synthesis conditions, which limits further progress. Rational design of catalyst materials requires proper understanding of the mechanistic origin of the ORR and the underlying surface properties under operating conditions that govern catalytic activity. Herein, several different groups of iron-based catalysts synthesized via different methods and/or precursors were systematically studied by combining multiple spectroscopic techniques under ex situ and in situ conditions in an effort to obtain a comprehensive understanding of the synthesis-products correlations, nature of active sites, and the reaction mechanisms. These catalysts include original macrocycles, macrocycle-pyrolyzed catalysts, and Fe−N–C catalysts synthesized from individual Fe, N, and C precursors including polymer-based catalysts, metal organic framework (MOF)-based catalysts, and sacrificial support method (SSM)-based catalysts. The latter group of catalysts is most promising as not only they exhibit exceptional ORR activity and/or durability, but also the final products are controllable. We show that the high activity observed for most pyrolyzed Fe-based catalysts can mainly be attributed to a single active site: non-planar Fe–N4 moiety embedded in distorted carbon matrix characterized by a high potential for the Fe2+/3+ redox transition in acidic electrolyte/environment. The high intrinsic ORR activity, or turnover frequency (TOF), of this site is shown to be accounted for by redox catalysis mechanism that highlights the dominant role of the site-blocking effect. Moreover, a highly active MOF-based catalyst without Fe–N moieties was developed, and the active sites were identified as nitrogen-doped carbon fibers with embedded iron particles that are not directly involved in the oxygen reduction pathway. The high ORR activity and durability of catalysts involving this second site, as demonstrated in fuel cell, are attributed to the high density of active sites and the elimination or reduction of Fenton-type processes. The latter are initiated by hydrogen peroxide but are known to be accelerated by iron ions exposed to the surface, resulting in the formation of damaging free-radicals.
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•The activity for various Fe-based catalysts is attributed to a single Fe–N4 active site.•The active site is non-planar Fe–N4 in distorted carbon with high Fe2+/3+ redox potentials.•The high intrinsic ORR activity of this site is accounted for by redox catalysis mechanism.•N-doped carbon with embedded Fe not directly involved in the ORR is identified as an active site.
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