Manipulating metal atoms in a controllable way for the synthesis of materials with the desired structure and properties is the holy grail of chemical synthesis. The recent emergence of single atomic site catalysts (SASC) demonstrates that we are moving toward this goal. Owing to the maximum efficiency of atom-utilization and unique structures and properties, SASC have attracted extensive research attention and interest. The prerequisite for the scientific research and practical applications of SASC is to fabricate highly reactive and stable metal single atoms on appropriate supports. In this review, various synthetic strategies for the synthesis of SASC are summarized with concrete examples highlighting the key issues of the synthesis methods to stabilize single metal atoms on supports and to suppress their migration and agglomeration. Next, we discuss how synthesis conditions affect the structure and catalytic properties of SASC before ending this review by highlighting the prospects and challenges for the synthesis as well as further scientific researches and practical applications of SASC.

Chemical Synthesis of Single Atomic Site Catalysts.

Metal-based electrocatalysts with different sizes (single atoms, nanoclusters, and nanoparticles) show different catalytic behaviors for various electrocatalytic reactions. Regulating the coordination environment of active sites with precision to rationally design an efficient electrocatalyst is of great significance for boosting electrocatalytic reactions. This review summarizes the recent process of heterogeneous supported single atoms, nanoclusters, and nanoparticles catalysts in electrocatalytic reactions, respectively, and figures out the construct strategies and design concepts based on their strengths and weaknesses. Specifically, four key factors for enhancing electrocatalytic performance, including electronic structure, coordination environment, support property, and interfacial interactions are proposed to provide an overall comprehension to readers in this field. Finally, some insights into the current challenges and future opportunities of the heterogeneous supported electrocatalysts are provided.

Design concept for electrocatalysts

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–N4 sites into Fe–N4 sites. Characterization by multiple techniques shows that all Fe–N4 sites formed via this approach are gas-phase and electrochemically accessible. As a result, the Fe–N–C catalyst has an active site density of 1.92 × 1020 sites per gram with 100% site utilization. This catalyst delivers an unprecedented oxygen reduction reaction activity of 33 mA cm−2 at 0.90 V (iR-corrected; i, current; R, resistance) in a H2–O2 proton exchange membrane fuel cell at 1.0 bar and 80 °C. Replacing platinum with metal–nitrogen–carbon catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells has been impeded by low activity. These limitations have now been overcome by the trans-metalation of Zn–N4 sites into Fe–N4 sites.

https://hal.archives-ouvertes.fr/hal-03024544/document

Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites.

This contribution reports the discovery and analysis of a p-block Sn-based catalyst for the electroreduction of molecular oxygen in acidic conditions at fuel cell cathodes; the catalyst is free of platinum-group metals and contains single-metal-atom actives sites coordinated by nitrogen. The prepared SnNC catalysts meet and exceed state-of-the-art FeNC catalysts in terms of intrinsic catalytic turn-over frequency and hydrogen–air fuel cell power density. The SnNC-NH3 catalysts displayed a 40–50% higher current density than FeNC-NH3 at cell voltages below 0.7 V. Additional benefits include a highly favourable selectivity for the four-electron reduction pathway and a Fenton-inactive character of Sn. A range of analytical techniques combined with density functional theory calculations indicate that stannic Sn(iv)Nx single-metal sites with moderate oxygen chemisorption properties and low pyridinic N coordination numbers act as catalytically active moieties. The superior proton-exchange membrane fuel cell performance of SnNC cathode catalysts under realistic, hydrogen–air fuel cell conditions, particularly after NH3 activation treatment, makes them a promising alternative to today’s state-of-the-art Fe-based catalysts. For oxygen reduction and hydrogen oxidation reactions, proton-exchange membrane fuel cells typically rely on precious-metal-based catalysts. A p-block single-metal-site tin/nitrogen-doped carbon is shown to exhibit promising electrocatalytic and fuel cell performance.

/pdf/p-block-single-metal-site-tin-nitrogen-doped-carbon-fuel-1n8dmgioul.pdf

P-block single-metal-site tin/nitrogen-doped carbon fuel cell cathode catalyst for oxygen reduction reaction

The modulation effect has been widely investigated to tune the electronic state of single-atomic M-N-C catalysts to enhance the activity of oxygen reduction reaction (ORR). However, the in-depth study of modulation effect is rarely reported for the isolated dual-atomic metal sites. Now, the catalytic activities of Fe-N4 moiety can be enhanced by the adjacent Pt-N4 moiety through the modulation effect, in which the Pt-N4 acts as the modulator to tune the 3d electronic orbitals of Fe-N4 active site and optimize ORR activity. Inspired by this principle, we design and synthesize the electrocatalyst that comprises isolated Fe-N4 /Pt-N4 moieties dispersed in the nitrogen-doped carbon matrix (Fe-N4 /Pt-N4 @NC) and exhibits a half-wave potential of 0.93 V vs. RHE and negligible activity degradation (ΔE1/2 =8 mV) after 10000 cycles in 0.1 M KOH. We also demonstrate that the modulation effect is not effective for optimizing the ORR performances of Co-N4 /Pt-N4 and Mn-N4 /Pt-N4 systems.

An Adjacent Atomic Platinum Site Enables Single-Atom Iron with High Oxygen Reduction Reaction Performance.

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 Fe1(II)-N4 sites via in-temperature X-ray absorption spectroscopy. The Fe precursor transforms to Fe oxides below 300 °C and then to tetrahedral Fe1(II)-O4 via a crystal-to-melt-like transformation below 600 °C. The Fe1(II)-O4 releases a single Fe atom that diffuses into the N-doped carbon defect forming Fe1(II)-N4 above 600 °C. This vapor-phase single Fe atom transport mechanism is verified by synthesizing Fe1(II)-N4 sites via "noncontact pyrolysis" wherein the Fe precursor is not in physical contact with the N and C precursors during pyrolysis.

/pdf/evolution-pathway-from-iron-compounds-to-fe1-ii-n4-sites-4n5by9lqgg.pdf

Evolution Pathway from Iron Compounds to Fe1(II)-N4 Sites through Gas-Phase Iron during Pyrolysis.

A Fe-N-C catalyst was synthesized via chemical vapor deposition (CVD) of gas phase FeCl3 onto a metal organic framework (MOF)-derived N-doped carbon (N-C) substrate at 750 ℃. This catalyst exhibits an unprecedented current density of 0.033 mA·cm-2 at 0.90 ViR-free (IR-corrected) and 0.044 mA·cm-2 at 0.89 ViR-free in a H2-O2 proton exchange membrane fuel cell under 1.0 bar and 80 ℃ conditions. The exceptional ORR activity of this catalyst is attributed to the ultra-high density of the Fe(II)-N4 sites. The high density of Fe(II)-N4 sites is realized by CVD that allows for the ready formation of Fe(II)-N4 sites via direct incorporation of gas phase FeCl3 into microporous N-C defects at relatively low temperatures. At these low temperatures, the doped N and Fe(II)-N4 are better preserved as compared to those in previous Fe-N-C catalysts synthesized via pyrolysis of the mixture of Fe, N, and C precursors at 1000 ± 100 ℃.

/pdf/high-performance-iron-based-orr-catalysts-synthesized-via-a1q06cp3ng.pdf

High-Performance Iron-Based ORR Catalysts Synthesized via Chemical Vapor Deposition

Replacing scarce and expensive platinum (Pt)
with metal-nitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction
(ORR) in proton
exchange membrane fuel cells (PEMFCs) has largely been
impeded by the low activity of M-N-C, in turn limited by low site density and
low site utilization. Herein, we overcome these limits by implementing chemical
vapor deposition (CVD) to synthesize Fe-N-C, an approach fundamentally different
from previous routes. The Fe-N-C catalyst, prepared by flowing iron chloride
vapor above a N-C substrate at 750 ℃, has a record Fe-N4 site
density of 2×1020 sites·gram-1 with 100% site utilization. A combination of
characterizations shows that the Fe-N4 sites formed via CVD are
located exclusively on the outer-surface, accessible by air, and
electrochemically active. This catalyst delivers an unprecedented current
density of 33
mA·cm-2 at 0.90 ViR-free (iR-corrected)
in an H2-O2 PEMFC at 1.0 bar and 80 ℃.

/pdf/chemical-vapor-deposition-of-fe-n-c-oxygen-reduction-3oeuoetthx.pdf

Lynne Larochelle Richard

Papers

Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites.

Evolution Pathway from Iron Compounds to Fe1(II)-N4 Sites through Gas-Phase Iron during Pyrolysis.

High-Performance Iron-Based ORR Catalysts Synthesized via Chemical Vapor Deposition

Chemical Vapor Deposition of Fe-N-C Oxygen Reduction Catalysts with Full Utilization of Dense Fe-N4 Sites