A hydroxide exchange membrane fuel cell consisting of a nickel-based anode and a cobalt–manganese–oxide cathode is shown to achieve a power density of 488 mW cm–2 at 95 °C.
Nitrogen-doped carbon materials featuring atomically dispersed metal cations (M–N–C) are an emerging family of materials with potential applications for electrocatalysis. The electrocatalytic ...activity of M–N–C materials toward four-electron oxygen reduction reaction (ORR) to H2O is a mainstream line of research for replacing platinum-group-metal-based catalysts at the cathode of fuel cells. However, fundamental and practical aspects of their electrocatalytic activity toward two-electron ORR to H2O2, a future green “dream” process for chemical industry, remain poorly understood. Here we combined computational and experimental efforts to uncover the trends in electrochemical H2O2 production over a series of M–N–C materials (M = Mn, Fe, Co, Ni, and Cu) exclusively comprising atomically dispersed M–N x sites from molecular first-principles to bench-scale electrolyzers operating at industrial current density. We investigated the effect of the nature of a 3d metal within a series of M–N–C catalysts on the electrocatalytic activity/selectivity for ORR (H2O2 and H2O products) and H2O2 reduction reaction (H2O2RR). Co–N–C catalyst was uncovered with outstanding H2O2 productivity considering its high ORR activity, highest H2O2 selectivity, and lowest H2O2RR activity. The activity–selectivity trend over M–N–C materials was further analyzed by density functional theory, providing molecular-scale understandings of experimental volcano trends for four- and two-electron ORR. The predicted binding energy of HO* intermediate over Co–N–C catalyst is located near the top of the volcano accounting for favorable two-electron ORR. The industrial H2O2 productivity over Co–N–C catalyst was demonstrated in a microflow cell, exhibiting an unprecedented production rate of more than 4 mol peroxide gcatalyst –1 h–1 at a current density of 50 mA cm–2.
In the past 5 years, advances in anion-conductive membranes have opened the door for the development of advanced anion-exchange membrane fuel cells (AEMFCs) as the next generation of affordable fuel ...cells. Several recent works have shown that AEMFCs currently achieve nearly identical beginning-of-life performance as state-of-the-art proton exchange membrane fuel cells. However, until now, these high AEMFC performances have been reached with platinum-group metal (PGM)-based anode and cathode catalysts. In order to fulfill the potential of AEMFCs, such catalysts should in the near future be free of PGMs and, eventually, free of critical raw materials. Although great progress has been achieved in the development of PGM-free catalysts for the oxygen reduction reaction in basic media, significantly less attention has been paid to the catalysis of the hydrogen oxidation reaction (HOR). The much lower HOR activity of Pt in basic media compared with that in acid was itself revealed only relatively recently. While several PGM-based composite materials have shown improved HOR activity in basic media, the HOR kinetics remains slower than necessary for an ideal nonpolarizable electrode. In addition, attempts to move away from PGMs have hitherto resulted in high anode overpotentials, significantly reducing the performance of PGM-free AEMFCs. This would be a major barrier for the large-scale deployment of this technology once the other technological hurdles (e.g., membrane stability) have been overcome. A fundamental understanding of the HOR mechanism in basic media and of the main energy barriers needs to be firmly established to overcome this challenge. This review presents the current understanding of the HOR electrocatalysis in basic media and critically discusses the most recent material approaches. Promising future research directions in the development of the HOR electrocatalysts for alkaline electrolytes are also outlined.
One bottleneck hampering the widespread use of fuel cell vehicles, in particular of proton exchange membrane fuel cells (PEMFCs), is the high cost of the cathode where the oxygen reduction reaction ...(ORR) occurs, due to the current need of precious metals to catalyze this reaction. Electrochemists tackle this issue in the short/medium term by developing catalysts with improved utilization or efficiency of platinum, and in the longer term, by developing catalysts based on Earth-abundant elements. Considerable progress has been achieved in the initial performance of Metal-nitrogen-carbon (Metal-N-C) catalysts for the ORR, especially with Fe-N-C materials. However, until now, this high performance cannot be maintained for a sufficiently long time in an operating PEMFC. The identification and mitigation of the degradation mechanisms of Metal-N-C electrocatalysts in the acidic environment of PEMFCs has therefore become an important research topic. Here, we review recent advances in the understanding of the degradation mechanisms of Metal-N-C electrocatalysts, including the recently identified importance of combined oxygen and electrochemical potential. Results obtained in a liquid electrolyte and a PEMFC device are discussed, as well as insights gained from in situ and operando techniques. We also review the mitigation approaches that the scientific community has hitherto investigated to overcome the durability issues of Metal-N-C electrocatalysts.
Active and inexpensive catalysts for oxygen reduction are crucially needed for the widespread development of polymer electrolyte fuel cells and metal–air batteries. While iron–nitrogen–carbon ...materials pyrolytically prepared from ZIF-8, a specific zeolitic imidazolate framework (ZIF) with sodalite topology, have shown enhanced activities toward oxygen reduction in acidic electrolyte, the rational design of sacrificial metal–organic frameworks toward this application has hitherto remained elusive. Here, we report for the first time that the oxygen reduction activity of Fe–N–C catalysts positively correlates with the cavity size and mass-specific pore volume in pristine ZIFs. The high activity of Fe–N–C materials prepared from ZIF-8 could be rationalized, and another ZIF structure leading to even higher activity was identified. In contrast, the ORR activity is mostly unaffected by the ligand chemistry in pristine ZIFs. These structure–property relationships will help identifying novel sacrificial ZIF or porous metal–organic frameworks leading to even more active Fe–N–C catalysts. The findings are of great interest for a broader application of the class of inexpensive metal–nitrogen–carbon catalysts that have shown promising activity also for the hydrogen evolution (Co–N–C) and carbon dioxide reduction (Fe–N–C and Mn–N–C).
Selective electrochemical reduction of CO2 into energy-dense organic compounds is a promising strategy for using CO2 as a carbon source. Herein, we investigate a series of iron-based catalysts ...synthesized by pyrolysis of Fe-, N-, and C-containing precursors for the electroreduction of CO2 to CO under aqueous conditions and demonstrate that the selectivity of these materials for CO2 reduction over proton reduction is governed by the ratio of isolated FeN4 sites vs Fe-based nanoparticles. This ratio can be synthetically tuned to generate electrocatalysts producing controlled CO/H2 ratios. It notably allows preparing materials containing only FeN4 sites, which are able to selectively reduce CO2 to CO in aqueous solution with Faradaic yields of over 90% and at low overpotential.
While platinum has hitherto been the element of choice for catalysing oxygen electroreduction in acidic polymer fuel cells, tremendous progress has been reported for pyrolysed Fe-N-C materials. ...However, the structure of their active sites has remained elusive, delaying further advance. Here, we synthesized Fe-N-C materials quasi-free of crystallographic iron structures after argon or ammonia pyrolysis. These materials exhibit nearly identical Mössbauer spectra and identical X-ray absorption near-edge spectroscopy (XANES) spectra, revealing the same Fe-centred moieties. However, the much higher activity and basicity of NH3-pyrolysed Fe-N-C materials demonstrates that the turnover frequency of Fe-centred moieties depends on the physico-chemical properties of the support. Following a thorough XANES analysis, the detailed structures of two FeN4 porphyrinic architectures with different O2 adsorption modes were then identified. These porphyrinic moieties are not easily integrated in graphene sheets, in contrast with Fe-centred moieties assumed hitherto for pyrolysed Fe-N-C materials. These new insights open the path to bottom-up synthesis approaches and studies on site-support interactions.
Electrocatalytic conversion of nitrogen oxides to value-added chemicals is a promising strategy for mitigating the human-caused unbalance of the global nitrogen-cycle, but controlling product ...selectivity remains a great challenge. Here we show iron-nitrogen-doped carbon as an efficient and durable electrocatalyst for selective nitric oxide reduction into hydroxylamine. Using in operando spectroscopic techniques, the catalytic site is identified as isolated ferrous moieties, at which the rate for hydroxylamine production increases in a super-Nernstian way upon pH decrease. Computational multiscale modelling attributes the origin of unconventional pH dependence to the redox active (non-innocent) property of NO. This makes the rate-limiting NO adsorbate state more sensitive to surface charge which varies with the pH-dependent overpotential. Guided by these fundamental insights, we achieve a Faradaic efficiency of 71% and an unprecedented production rate of 215 μmol cm
h
at a short-circuit mode in a flow-type fuel cell without significant catalytic deactivation over 50 h operation.
Anion-exchange membrane fuel cells and electrolyzers offer a unique opportunity of using non-noble metal electrocatalysts for catalyzing the sluggish oxygen reduction and oxygen evolution reactions ...(ORR and OER). In recent years, various Mn-based oxides were identified as promising catalysts for both reactions. While electrocatalytic activity of such oxides is well addressed, their stability is still to be proven. Herein, we investigate the stability of four main manganese oxide allotropes by following their Mn dissolution rate in operando ORR and OER conditions. Using an electrochemical on-line inductively coupled plasma mass spectrometer, we uncover unexpected instability of this class of catalysts, with different degradation mechanisms identified under OER and ORR conditions. The reason for their degradation is shown to be related to the production of hydrogen peroxide species on manganese oxides during ORR. Furthermore, we discuss how limits in thermodynamically stable windows of each Mn oxidation state lead to increased dissolution during applications with high potential perturbations, that is, change in load, start/stop conditions, and especially bifunctional application. Therefore, we recommend clear guidelines for future development of platinum group metal-free electrocatalysts for affordable alkaline energy conversion technologies.
Despite the promising activity of Fe-N-C catalysts at the beginning of life in proton-exchange membrane fuel cells (PEMFCs), their poor durability in operating PEMFCs remains a great challenge for ...the successful replacement of commercial Pt-based catalysts. One of the key reasons for this poor operando durability is the surface oxidation of carbonaceous supports via Fenton(-like) reactions between the Fe centers and the intermediate product of the oxygen reduction reaction (ORR) in an acidic medium, H2O2. In the present study, we have investigated the pH effect on the chemical deactivation of Fe-N-C catalysts by contacting them with a controlled amount of H2O2. Covering the entire pH range 0–14, we reveal a strong pH dependence of the H2O2-induced deactivation. Especially, acidic H2O2 treatment leads to a severe decrease in ORR activity while almost negligible deactivation is found after a treatment in a sufficiently strong alkaline electrolyte. An electron paramagnetic resonance (EPR) study reveals a positive correlation between the magnitude of Fe-N-C activity decrease and the signal intensity of the hydroxyl radical spin adduct after H2O2 treatment at a given pH. A reactive oxygen species (ROS) such as the hydroxyl radical is identified as a key deactivating agent of Fe-N-C catalysts operating from acidic to neutral pH environments. This result suggests that controlling the formation and lifetime of ROS at such pH is crucial to secure durable fuel cell operation with Fe-N-C cathodes. Alternatively, fuel cell operation under highly alkaline environment could also be considered to improve the catalytic durability, by virtue of a different Fenton(-like) reaction pathway at such pH.
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