Electrocatalysis with Single‐Metal Atom Sites in Doped Carbon Matrices

Abstract

While supported metal nanoparticles cannot achieve full electrochemical utilisation of metal atoms, catalysts featuring single metal atom sites may offer this possibility, along with advantages in selectivity. However, the passage from nanometric to atomic dimension is not without consequences. It first raises the question of efficient and robust synthesis methods, and underlines the need of cutting-edge characterization techniques that can target single metal atoms. These analytical tools are also pivotal to gain insights into the structure of the active sites, and establish atomic structure-catalytic activity-selectivity-stability relationships. Herein, we illustrate these topics for electrocatalysis, with a particular focus on metal-nitrogen-carbon single metal atom catalysts, for which a fantastic leap forward has been achieved in the last 15 years, triggered by the growing interest in sustainable energy storage and conversion systems.C SMAC prepared via pyrolysis in the following decades. Hence, we focus mainly on the synthesis methods of Fe-and Co-N-C SMAC in this section. While in those early days, the use of Metal-N4 macrocycles without pyrolysis secured the well-defined Metal-N4 active centres, the different approaches used to interface them with a conductive support impacted the local environment of metal cations and their accessibility. Phthalocyanines and porphyrins are poorly soluble in water, and different solvents or suspensions in concentrated sulphuric acid were used, removing the solvent by evaporation or precipitating the macrocycle on the support. The dispersion quality of metal macrocycles on supports were shown to depend strongly on the morphology, specific surface area (SSA) but also on acido-basic properties of carbon supports [17,18]. For example, the characterization by 57 Fe Mössbauer spectroscopy, a technique which allows identifying the different coordinations and spin states of Fe (discussed in more detail in 13.3), of iron phthalocyanine (Fe-Pc) precipitated on a carbon powder revealed multiple coordinations and spin states for Fe, whereas a single coordination and electronic state was seen for the Fe-Pc monomer [17]. Controlled deposition or solvent removal however resulted in Fe-Pc/C composites with a single (or vast majority of one) 57 Fe Mössbauer spectroscopic signature [19]. For example, removal of the pyridine solvent for Fe-Pc by boiling it off at 420 °C resulted in a Fe-Pc/C composite with 95 % of the spectral signal assigned to one specific doublet [19]. The latter can nowadays be assigned to an O2-Fe(III)-N4 coordination [20], implying high dispersion and accessibility to O2 of the Fe-N4 sites.While some heterogeneities of metal coordination and site accessibility were exemplified above for SMAC prepared via interfacing Metal-N4 macrocycles without high-temperature pyrolysis, the extent of heterogeneities was progressively increased, and the true nature of the active site blurred for several decades, with the introduction of high-temperature treatments of macrocycles in a...

Spontaneous aerobic ageing of Fe–N–C materials and consequences on oxygen reduction reaction kinetics

“…To replace PGMs at the PEMFC cathode, research focuses on transition metal atoms embedded into nitrogen-doped carbon matrix (Metal-N-C) catalysts, and Fe-N-C has been identified as the most active Metal-N-C catalyst for the ORR [1][2][3]. Fe-N-C catalysts can be prepared by: i) pyrolysis of Fe macrocycles (phthalocyanines, porphyrins, tetraazaannulene, etc), carbon (C) and nitrogen (N) precursors [4,5] ii) pyrolysis of Fe-N-C precursors prepared from Fe salts, C and N organic precursors and securing mesoporosity via a hard templating with silica, [6] iii) pyrolysis of Fe-N-C precursors prepared from iron salt and metal-organic frameworks (MOF) (soft templating), [7][8][9] iv) pyrolysis of Fe-N-C precursors prepared from Fe salts and polymers [10] or v) vapor deposition of Fe onto a preexisting N-C support [11][12][13].…”

Spontaneous Demetallation of Mn(II)Nx Catalytic Sites in Proton Exchange Membrane Fuel Cell Conditions

“…The difference is also evident in terms of maximum power density, although in this case, ionic resistance and transport phenomena are limiting. In H 2 /air cells, the maximum power density reported for a Mn/N/C catalyst, ranges from 0.15 to 0.50 W cm –2 , ,,− whereas one of the best maximum power densities for the same type of cell, but with Fe/N/C, is 0.6 W cm –2 . It is, of course, more difficult to compare activity and performance in a fuel cell, where many variables, in addition to catalyst type, may also come into play.…”

“…However, the scarcity and cost of Pt, and the almost exclusive location of abundant Pt-containing mineral deposits in a small number of countries, mean that alternatives for producing Pt-free catalysts are being pursued relentlessly around the world. Molecular catalysts (Me/N/C) based on transition metals have been the subject of research for over 50 years. − They are obtained by high-temperature pyrolysis in an inert atmosphere of a metal precursor (Fe, Co, Mn), a nitrogen precursor, and carbon (or a carbon precursor). The catalytic sites thus obtained are of the MeN x type, in which the metal is in ionic form.…”

“…Among the metals used to produce these catalysts, Fe has attracted the most attention. As early as 2009, 5 Fe/N/C catalysts were shown to possess high catalytic activity, enabling such a catalyst, with a loading of 4 mg cm 2 at the cathode to achieve, in H 2 /O 2 fuel cells, currents equivalent to those obtained from a commercial Pt electrode with a Pt loading of 0.4 mg cm −2 . The comparison stopped, however, at a current density of ∼0.1 A cm −2 obtained at about 0.85 V. Beyond this limit, the Fe/N/C polarization curve no longer coincided with that of the Pt electrode.…”

Benefits of Using Rapid Microwave Heating in the Synthesis of Metal-Free Carbon Electrocatalysts

“…Nitrogen-doped carbons have emerged as promising metal-free electrocatalysts for applications such as oxygen reduction reaction (ORR) in hydrogen fuel cells. − They can also be used as catalyst supports for atomically dispersed metals for various electro- and thermocatalytic processes, as specific doped nitrogen sites act as anchoring sites for metal atoms − and metal nanoparticles. − Nitrogen-doped carbons are commonly synthesized by pyrolyzing a mixture of carbon and nitrogen precursors under an inert atmosphere, typically at temperatures ranging from 600 to 1100 °C. A second method involves introducing oxygen functional groups onto a carbon material using strong acids, followed by heating in NH 3 to incorporate nitrogen species through a reaction with oxygen functional groups. − The heat treatment is crucial in the synthesis of these materials as it impacts the type and concentration of heteroatom dopants, ,, structural defects, , π-electron delocalization and associated Lewis basicity, acid–base properties, , electrical properties, , surface polarity, and porosity of the materials.…”