Dispersed Transition Metal Research Articles

P-Block Aluminum Single-Atom Catalyst for Electrocatalytic CO2 Reduction with High Intrinsic Activity.

Atomically dispersed transition metal sites on nitrogen-doped carbon catalysts hold great potential for the electrochemical CO2 reduction reaction (CO2RR) to CO due to their encouraging selectivity. However, their intrinsic activity is restricted by the hurdle of the high energy barrier of either *COOH formation or *CO desorption due to the scaling relationship. Herein, we discover a p-block aluminum single-atom catalyst (Al-NC) featuring an Al-N4 site that enables disentangling this hurdle, which endows a moderate reaction kinetic barrier for *COOH formation and *CO desorption, as validated by in situ attenuated total reflection infrared spectroscopy and theoretical simulations. As a result, the developed Al-NC shows a CO Faradaic efficiency (FECO) of up to 98.76% at -0.65 V vs RHE and an intrinsic catalytic turnover frequency of 3.60 s-1 at -0.99 V vs RHE, exceeding those of the state-of-the-art Ni-NC and Fe-NC counterparts. Moreover, it also delivers a partial CO current of 309 mA·cm-2 at 93.65% FECO and 605 mA at >85% FECO in a flow cell and membrane electrode assembly (MEA), respectively. Strikingly, when using low-concentration CO2 (30%) as the feedstock, this catalyst can still deliver a partial CO current of 240 mA at >80% FECO in the MEA. Considering the earth-abundant character of the Al element and the high intrinsic activity of the Al-NC catalyst, it is a promising alternative to today's transition metal-based single-atom catalysts.

Atomically dispersed asymmetric cobalt electrocatalyst for efficient hydrogen peroxide production in neutral media

Electrochemical hydrogen peroxide (H2O2) production (EHPP) via a two-electron oxygen reduction reaction (2e- ORR) provides a promising alternative to replace the energy-intensive anthraquinone process. M-N-C electrocatalysts, which consist of atomically dispersed transition metals and nitrogen-doped carbon, have demonstrated considerable EHPP efficiency. However, their full potential, particularly regarding the correlation between structural configurations and performances in neutral media, remains underexplored. Herein, a series of ultralow metal-loading M-N-C electrocatalysts are synthesized and investigated for the EHPP process in the neutral electrolyte. CoNCB material with the asymmetric Co-C/N/O configuration exhibits the highest EHPP activity and selectivity among various as-prepared M-N-C electrocatalyst, with an outstanding mass activity (6.1 × 105 A gCo−1 at 0.5 V vs. RHE), and a high practical H2O2 production rate (4.72 mol gcatalyst−1 h−1 cm−2). Compared with the popularly recognized square-planar symmetric Co-N4 configuration, the superiority of asymmetric Co-C/N/O configurations is elucidated by X-ray absorption fine structure spectroscopy analysis and computational studies.

Acetate Ions Facilitated Immobilization of Highly Dispersed Transition Metal Oxide Nanoclusters in Mesoporous Silica.

The immobilization of tiny active species within inert mesoporous silica imparts a range of functions, enhancing their applicability. A significant obstacle is the spontaneous migration and aggregation of these species within the mesopores, which threaten their uniform distribution. To address this, we propose a postmodification method that involves grafting transition metal oxide nanoclusters into silica mesopores via interfacial condensation, catalyzed by acetate ions. Specifically, CuO nanoclusters, in the form of oligomeric [O1-x-Cu2-(OH) 2x]n2+, have a strong interaction with the silica framework. This interaction inhibits their growth and prevents mesopore blockage. Theoretical calculation results reveal that the acetate ion promotes proton transfer among various hydroxy species, lowering the free energy and thereby facilitating the formation of Cu-O-Si bonds. This technique has also been successfully applied to the encapsulation of four other types of transition metal oxide nanoclusters. Our encapsulation strategy effectively addresses the challenge of dispersing transition metal oxides in mesoporous silica, offering a straightforward and widely applicable method for enhancing the functionality of mesoporous materials.

Asymmetric Atomic Tin Catalysts with Tailored p‐Orbital Electron Structure for Ultra‐Efficient Oxygen Reduction

AbstractAtomically dispersed transition metal–nitrogen–carbon (M–N–C) catalysts guide by the d‐band center theory have been extensively studied for oxygen reduction reaction (ORR) in various energy conversion and storage processes. However, asymmetric p‐block metal single‐atom catalysts (SACs) toward ORR have rarely been reported, and the origin of their catalytic activity is still unclear. Here, an asymmetric N, O coordinated Sn SAC is developed as an efficient ORR electrocatalyst. Remarkably, the optimized Sn SAC (e.g., Sn–N/O–C) exhibit outstanding ORR performance with a half‐wave potential of 0.910 V in alkaline media, outperforming most state‐of‐the‐art ORR catalysts. More importantly, the Sn–N/O–C possesses a long‐term durability in both alkaline and acidic electrolytes. Besides, Zn–air batteries based on the Sn–N/O–C cathode also show a higher energy density (254 mW cm‐2) than that of their reported M–N–C counterparts. Theoretical calculations suggest that the asymmetric N, O coordinated atomic Sn sites have a stronger binding interaction with O2 and better charge transfer ability compared with the symmetric SnN4 sites, thereby facilitating the ORR process. This work provides a nitrogen‐, oxygen‐coordinated engineering strategy for the rational design of highly active and durable carbon‐based catalysts with atomic p‐block metal sites for ORR and beyond.

Exploring the synergistic effect of palladium nanoparticles and highly dispersed transition metals on carbon nitride/super-activated carbon composites for boosting electrocatalytic activity

In the present work, multifunctional electrocatalysts formed by palladium nanoparticles (Pd NPs) loaded on Fe or Cu-containing composite supports, based on carbon nitride (C3N4) and super-activated carbon with a high porosity development (SBET 3180 m2/g, VDR 1.57 cm3/g, and VT 1.65 cm3/g), were synthesised. The presence of Fe or Cu sites favoured the formation of Pd NPs with small average particle size and a very narrow size distribution, which agreed with Density Functional Theory (DFT) calculations showing that the interaction of Pd clusters with C3N4 flakes is weaker than with Cu- or Fe-C3N4 sites. The electroactivity was also dependent on the composition and, as suggested by preliminary DFT calculations, the Pd-Cu catalyst showed lower overpotential for hydrogen evolution reaction (HER) while bifunctional oxygen reduction reaction/ oxygen evolution reaction (ORR/OER) behaviour was superior in Pd-Fe sample. The Pd-Fe electrocatalyst was studied in a zinc-air battery (ZAB) for 10 h, showing a performance similar to a commercial Pt/C + RuO2 catalyst with a high content of precious metal. This study demonstrates the synergistic effect between Pd species and transition metals and shows that transition metals anchored on C3N4-based composite materials promote the electroactivity of Pd NPs in HER, ORR and OER due to the interaction between both species.

A p-d block synergistic effect enables robust electrocatalytic oxygen evolution

Oxygen evolution reaction (OER), occurring at the anode of electrochemical water splitting requires a comprehensive understanding of oxygen electrocatalysis mechanism to optimize its efficiency. Atomically dispersed transition metal supported by nitrogen-doped carbon is featured with excellent catalytic performance. Herein, we report a Mg/Co bimetal site which utilizes Mg 3p electrons with strong binding of *OH (the first key reaction intermediates in the free energy diagram) to trigger the OER reaction and Co 3d itinerant character to regulate the binding strength of *O. Benefiting from the fine-tuned adsorption/desorption possesses, the optimized catalyst delivers superior OER activity with low overpotential, i.e., 310 mV at a current density of 10 mA/cm2 and 455 mV at 100 mA/cm2. Moreover, the current density is able to be maintained at 10 mA/cm2 for 10 h, consistent with the theoretical simulations for oxidization process, which demonstrates stable configurations after multiple *OH modification, revealing robust applicability in alkaline medium.

Effect of defects and framework Sn on the stability and activity of Pt clusters for ethane dehydrogenation in chabazite zeolite

With increasing interest in new catalytic materials based on atomically dispersed transition metals on various supports (e.g., zeolites or oxides), it is necessary to have an atomic level understanding of...

Non-PGM Cathode Electrocatalysts for PEM Fuel Cells

Polymer electrolyte membrane fuel cells, PEMFCs, are considered the most ideal next generation energy conversion device, able to provide power to a broad range of applications; automotive, combined heat and power, portable electronic devices. Pt-based electrocatalysts are regarded as ideal for promoting the desired redox reactions of a fuel cell, especially the sluggish oxygen reduction at the cathode, ORR. High expense and scarcity of Pt are the greatest impediments to successful commercialization of this technology. The replacement of unsustainable noble-metal catalysts with platinum group metal (PGM)-free catalysts for oxygen reduction reaction (ORR) is essential for material wise and market value viable PEMFCs.Among the different attempts towards non-PGM catalysts, incorporating atomically dispersed transition metal atoms (Fe, Mn, etc.) and heteroatoms (N, S, etc.) into carbon frameworks is the most promising [1,2]. Large specific surface area, high intrinsic ORR electrocatalytic activity, efficient porosity and sufficient stability in acid media and/or in the actual working environment are the least requirements to consider when designing the catalysts. So far, despite the tremendous efforts, many drawbacks still persist that include poor understanding of the nature of active centres and their control over the synthesis, as well as fast decay of the structure under operation.In this work, we will present a rationally designed strategy to prepare hybrid composite catalysts by developing Fe–N–C materials onto different 2D and 3D templates [3,4] with differentiated surface chemistry (silicon and heteroatom-doped carbon structures). Despite structural and physicochemical characterization, the study includes extensive electrochemical characterization in terms of activity and durability in relative to the application conditions. An example is depicted in Figure 1 for a single atom Fe-N-C catalyst synthesized using porous silica nanotubes as the sacrificial template resulting in a hierarchically porous structure with a surface area of 370 m2/g. Electrochemical study revealed a half-wave potential of 0.72 V and an onset potential of 0.88 V with high stability in acidic environment.Very importantly, the different findings are correlated to one another, thereby establishing a composition to function relationship to provide a pathway for the integrated engineering of novel structures with improved performance. Acknowledgments Funding from the Operational Program “Competitiveness, Entrepreneurship & Innovation” (EPAnEK), NSRF 2014-2020 through the project Solar2HyP Τ2ΕΔΚ-01877 References [1] L. Osmieri, et al., Current Opinion in Electrochemistry 25 (2020) 100627.[2] T. Asset, et al., Joule 4 (2019) 33-44.[3] Jin-Cheng Li et al., Asia Materials 10 (2018) e461[4] Nan Zhang et al., Energy Environ. Sci. 13 (2020) 111-118 Figure 1

Synergistic effect of nickel and calcium dual-atomic sites to facilitate electrochemical CO2 reduction to methanol: Combining high activity and selectivity

Atomically dispersed transition metal sites embedded into nitrogen-doped graphene provide promising potential for the electrochemical CO2 reduction to CO, but the catalytic efficiency for further hydrogenation to methanol is seriously restricted. Herein, combining the advantage of single transition metal, alkaline-earth metal, and multiple active sites, we designed nickel and calcium dual-atomic site catalysts (Ni-Ca DACs) supported on N-doped graphene to enable CO2 hydrogenation. First-principles calculations reveal that the electron-rich Ni atom serves as the carbon adsorption site, and the adjacent Ca atom with electron-deficient properties acts as the oxygen adsorption site, which largely stabilizes many CHxOy species through the O-Ca bond, especially the *OCHO, thus facilitating CO2 reduction. More importantly, a synergistic adsorption process on Ni-Ca sites promote the formation of *CHO species that is considered as the key intermediate for generation of multi-electron products. Different from the single Ni or Ca catalysts that produce mainly CO or HCOOH, the Ni-Ca DACs can selectively catalyze CO2 reduction to CH3OH, with a low limiting potential of −0.52 V, while suppressing the hydrogen evolution reaction. The outstanding catalytic activity originates from the tuned polarized charge and d-band center of active sites by synergistic interaction of adjacent Ni and Ca atoms.

Transition Metal (Fe, Zn, Co, and Ni) Single-Atom Catalysts Anchored on N,S-Codoped Hybrid Nanocarbons for Oxygen Reduction Reaction

Atomically dispersed transition metal catalysts have served as promising candidates to substitute noble metals in energy conversion. A majority of carbon-supported transition metal single-atom catalysts (SACs) have been synthesized via the high-temperature sintering method, in which the complex heating and cooling processes are time- and energy-consuming. Herein, we have developed a simple and highly effective microwave-assisted heating method to prepare a series of isolated transition metal SACs anchored on a N,S-codoped nanocarbon matrix, which are denoted as M-NSC-MW, M = Fe, Zn, Co, and Ni. Among the various M-NSC-MW, Fe-NSC-MW possesses the highest ORR electrochemical activity in alkaline electrolytes. Its onset potential (Eonset) is 0.955 V, and its half-wave potential (E1/2) can reach 0.852 V, both of which are the same as those of commercial Pt/C. Moreover, Fe-NSC-MW also exhibits much superior ORR catalytic efficiency than conventional pyrolysis samples. It is worth noting that Zn-NSC–MW is prepared for the first time, and it also shows a high ORR performance with an Eonset of 0.945 V and an E1/2 of 0.832 V. This work has revealed the general application of the microwave-assisted heating method to prepare well-dispersed transition metal SACs anchored on the N,S-codoped graphene/carbon hybrid matrix.

Impact of catalyst loading of atomically dispersed transition metal catalysts on H2O2 electrosynthesis selectivity

Electrosynthesis of hydrogen peroxide (H2O2) via a two-electron oxygen reduction reaction (2e− ORR) has emerged as a promising alternative to the current anthraquinone process. Atomically dispersed transition metal catalysts (M–N/C catalysts; M=transition metal) have received particular attention as H2O2 electrosynthesis catalysts. Among the factors affecting catalytic properties, the catalyst loading on the electrode has significant impacts on the ORR activity and selectivity of M–N/C catalysts in particular and electrocatalysts in general. However, the loading effect has been largely neglected and underexplored in literature. In this study, we investigated the impacts of the catalyst loading and the metal center in M–N/C catalysts on the 2e− ORR activity and selectivity. We prepared a series of mesoporous carbons comprising M (Fe, Co, Ni)-based atomically dispersed species, meso‑M–N/C catalysts. At a fixed low catalyst loading, the meso‑Co–N/C and meso‑Ni–N/C catalysts exhibited the best 2e−ORR performance under acidic and alkaline electrolytes, respectively. At high catalyst loadings, the H2O2 production activity of the meso‑Co–N/C catalyst dramatically declined to ∼20% compared to that at the low loading. In the thick catalyst layer, the generated H2O2 is accumulated due to poor mass transport, leading to reduction and/or decomposition. In contrast, the H2O2 synthesis activity meso‑Ni–N/C catalyst was nearly insensitive to the catalyst loading, which can be ascribed to the high porosity and well-developed mesopores of the meso‑Ni–N/C catalyst, promoting mass transport.

Engineering Pt Coordination Environment with Atomically Dispersed Transition Metal Sites Toward Superior Hydrogen Evolution

AbstractMetal single‐atom (SA) catalysts have attracted immense attention due to the high catalytic efficiency given by the desired coordination environment of each metal atom. Yet, engineering the local electronic structure of SAs and multi‐atoms (MAs) still remains a challenge. Herein, an atomically dispersed catalyst comprised of Pt SAs, Pt‐Pt/V dual‐atoms, and small clusters supported on a vanadium and nitrogen co‐doped carbon (VNC) (denoted as Pt@VNC) surface is synthesized. In the Pt@VNC, both V and Pt atoms are evenly distributed on the surface of N‐doped carbon, while a few Pt atoms are linked to other Pt atoms via V, forming Pt clusters. The coordination structures of Pt atoms are modulated upon introducing atomically dispersed V sites (which generate small‐sized Pt clusters) and V2O5 clusters, showing extraordinary activity for the hydrogen evolution reaction (HER). Benefiting from the low charge transfer resistance, i.e., fast reaction kinetics, due to the synergistic effect of SAs and clusters, the Pt@VNC demonstrates superior catalytic efficiency and robust durability for the HER. It requires an overpotential of only 5 mV at a current density of 10 mA cm−2 and shows 15 times larger mass activity than the commercial 20 wt.% Pt/C catalyst. This novel catalyst‐design strategy paves a new way for maximizing catalytic efficiency by optimizing the coordination structure of metal atoms.

Attenuating metal-substrate conjugation in atomically dispersed nickel catalysts for electroreduction of CO2 to CO

Atomically dispersed transition metals on carbon-based aromatic substrates are an emerging class of electrocatalysts for the electroreduction of CO2. However, electron delocalization of the metal site with the carbon support via d-π conjugation strongly hinders CO2 activation at the active metal centers. Herein, we introduce a strategy to attenuate the d-π conjugation at single Ni atomic sites by functionalizing the support with cyano moieties. In situ attenuated total reflection infrared spectroscopy and theoretical calculations demonstrate that this strategy increases the electron density around the metal centers and facilitates CO2 activation. As a result, for the electroreduction of CO2 to CO in aqueous KHCO3 electrolyte, the cyano-modified catalyst exhibits a turnover frequency of ~22,000 per hour at −1.178 V versus the reversible hydrogen electrode (RHE) and maintains a Faradaic efficiency (FE) above 90% even with a CO2 concentration of only 30% in an H-type cell. In a flow cell under pure CO2 at −0.93 V versus RHE the cyano-modified catalyst enables a current density of −300 mA/cm2 with a FE above 90%.

Local Hydrophobicity of Atomically Dispersed Fe-N-C Catalysts and Its Impact on the Oxygen Reduction Reaction in Membrane Electrode Assembly

Non-precious metal electrocatalysts with atomically dispersed transition metal sites have shown promising performance for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. The Fe-N-C catalysts, with single iron atom coordinated by nitrogen atoms (FeNx) on carbonaceous supports, exhibits outstanding ORR activity as compared to its other metal counterparts (M-N-C). [1] The Fe-N-C catalysts synthesized via sacrificial support method (SSM) showed superior ORR performance and this family of Fe-N-C electrocatalysts is so far the only commercially available PGM-free catalysts for fuel cell application. [2] For membrane electrode assembly (MEA) performance, the pore structure [3] and surface hydrophobicity [4] of the carbon matrix are of particular significance, because the high current throughput requires good oxygen transport and well-controlled water management on cathodes. In this talk, we present the local hydrophobicity of SSM-based Fe-N-C electrocatalysts [1] and its impact on the ORR macrokinetics in MEA. Specifically, nitrogen physisorption and water vapor physisorption were used to study the pore morphology and the local surface hydrophobicity, respectively. As shown in Figure 1, the sacrificial templated Fe-N-C catalyst showed a large amount of mesoporosity while its surface was very hydrophobic, as indicated by the little water uptake through whole pressure range. Such porosity and local hydrophobicity features were further correlated to its macrokinetic performance in a study of water partial pressure dependence under MEA configuration. Reference [1] T. Asset and P. Atanassov, Joule, 2020, 4, 33.[2] https://pajaritopowder.com [3] Y. Huang, Y. Chen, M. Xu, T. Asset, P. Tieu, A. Gili, D. Kulkarni, V. de Andrade, F. de Carlo, H. S. Barnard, A. Doran, D. Y. Parkinson, X. Pan, P. Atanassov, and I. Zenyuk, Materials Today, 2021, 47, 53.[4] L. Liu, S. J. Tan, T. Horikawa, D. D. Do, D. Nicholson and J. Liu, Adv Colloid Interface Sci, 2017, 250, 64. Figure 1

Cooperative non-covalent interactions-directed the supramolecular self-assembly and its derivative for bifunctional water electrocatalysis

The highly dispersed transition metal phosphides have gradually become viable electrocatalysts candidates for water splitting due to their high efficiency, ease of availability, and good stability. Herein, a novel electrocatalyst composed of Co2P embedded in N and P co-doped carbon (Co2P @ NPC) was designed using a tuned self-assembly strategy and high-temperature calcination. The former strategy was employed to prepare a supramolecular precursor self-assembled via cooperative non-covalent interactions. Specifically, Co2+ ions were introduced into the preorganized phytic acid (PA)-melamine (MA) hydrogen-bonded system, and competitively substituted hydrogen atoms to form coordinate bonds with PA. The Co2P @ NPC catalyst synthesized from the 800 ℃-calcinated precursor exhibited a well-modulated structure of carbon-entrapping Co2P nanoparticles, which promoted its efficient bifunctional electrocatalysis for hydrogen and oxygen evolution reactions (HER and OER). To achieve a current density of 10 mA cm−2, the electrocatalyst only required overpotentials of 288 mV for HER and 476 mV for OER.

Nitrogen Electroreduction on Borophene-Supported Atomic and Diatomic Transition Metals: Stability, Activity and Selectivity Improvements via Defect-Engineering.

The present work investigated the binding of atomically dispersed transition metals to the perfect and single/double vacancy (SV/DV)-containing defective β12 -borophenes and the catalytic performance of those corresponding single-atom catalysts (SACs) and diatomic catalysts (DACs) for nitrogen reduction reaction (NRR) by means of density functional theory calculations. Although previous theoretical studies proposed that the inherent hexagon hole of the defect-free β12 -borophene is capable of anchoring single metal atom for NRR, calculations suggested that the interaction between borophene and doped metal is not strong enough to avoid metal aggregation. For the defective β12 -borophene with SV, even though the single metal could be stabilized in an 8-membered ring, it was found that the SAC was still ineffective for NRR because of the competitive hydrogen evolution process. Regarding the DV-containing β12 -borophene, a defective configuration with an unexpected 11-membered hole was proved as the most stable structure, which possessed a very similar average atomic energy (6.25 eV atom-1 ) compared to that of the pristine β12 sheet (6.26 eV atom-1 ). Two metal atoms could be encapsulated into the confined space of the B11 ring. Compared to SACs, those corresponding DACs were more active for N2 fixation and hydrogenation, and the hydrogen evolution reaction could be passivated, attributing to the synergistic effect of dual metal centres. Among all candidates, the V2 /β12 -DV was predicted as the most promising catalyst for NRR, with the limiting potential of as low as -0.15 V.

Atomically Isolated Nickel–Nitrogen–Carbon Electrocatalysts Derived by the Utilization of Mg2+ ions as Spacers in Bimetallic Ni/Mg–Metal–Organic Framework Precursors for Boosting the Electroreduction of CO2

Electrochemical CO2 reduction (CO2R) is a promising avenue for the conversion of CO2 into fuels and beneficial chemicals. Significant efforts have been devoted to the development of active and selective electrocatalysts for CO2R. Atomically dispersed transition metal electrocatalysts have recently attracted consideration for CO2R due to their unique electronic and structural properties that can impart good catalytic performance and high active metal utilization. Among different precursors used for preparing these catalysts, metal–organic frameworks (MOFs) have been utilized as templates for generating atomically dispersed transition metal active sites as they provide well-defined structures that contain transition metals isolated from each other by organic ligands, along with high surface areas. However, maintaining the isolation of the transition metal species, achieving high surface concentrations of active sites, and imparting porosity during the high-temperature heat treatment of MOFs used during synthesis remain a challenge. In this report, Mg2+ ions have been employed as spacers in a bimetallic metal–organic framework (NiMg-MOF-74) to assist in preventing the coalescence of the Ni atoms into Ni-based particles during heat treatment, which combined with the use of urea as a nitrogen source resulted in the formation of isolated Ni–Nx/C active sites. Our findings demonstrated that Mg2+ ions play a crucial role in extending the distance between the adjacent Ni sites in the precursor structure and generating atomically isolated Ni sites. On the contrary, the utilization of Mg-free Ni-MOF-74 led to the formation of metallic Ni particles embedded in a carbon nanotube-based structure. Furthermore, we investigated the impact of pyrolysis temperature on the produced catalyst morphology. The generation of isolated Ni–Nx/C sites was promoted at higher pyrolysis temperatures (900 °C), while Ni-based particles were predominantly formed at a lower temperature (700 °C). The optimized atomically dispersed Ni–N–C catalyst exhibited excellent selectivity toward CO with a Faradaic efficiency of ∼90% and a current density of −4.2 mA/cm2 at −0.76 V vs a reversible hydrogen electrode.

A comparison study on single metal atoms (Fe, Co, Ni) within nitrogen-doped graphene for oxygen electrocatalysis and rechargeable Zn-air batteries

Single atom catalysts (SACs) with atomically dispersed transition metals on nitrogen-doped carbon supports have recently emerged as highly active non-noble metal electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), showing great application potential in Zn-air batteries. However, because of the complex structure-performance relationships of carbon-based SACs in the oxygen electrocatalytic reactions, the contribution of different metal atoms to the catalytic activity of SACs in Zn-air batteries still remains ambiguous. In this study, SACs with atomically dispersed transition metals on nitrogen-doped graphene sheets (M-N@Gs, M = Co, Fe and Ni), featured with similar physicochemical properties and M-N@C configurations, are obtained. By comparing the on-set potentials and the maximum current, we observed that the ORR activity is in the order of Co-N@G > Fe-N@G > Ni-N@G, while the OER activity is in the order of Co-N@G > Ni-N@G > Fe-N@G. The Zn-air batteries with Co-N@G as the air cathode catalysts outperform those with the Fe-N@G and Ni-N@G. This is due to the accelerated charge transfer between Co-N@C active sites and the oxygen-containing reactants. This study could improve our understanding of the design of more efficient bifunctional electrocatalysts for Zn-air batteries at the atomic level.

(Invited) Impact of Pore Morphology and Surface Hydrophobicity of the Carbon Matrix on the Macrokinetics of the Oxygen Reduction Reaction Performance for Atomically Dispersed Fe-N-C Catalysts

Atomically dispersed transition metal catalysts are an important family of non-precious metal electrocatalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. The iron-nitrogen-carbon (Fe-N-C) catalyst, wherein the iron metal centers are coordinated by nitrogen atoms (FeNx) on carbonaceous supports, exhibits outstanding ORR activity in both rotating disk electrode (RDE) and membrane electrode assembly (MEA) configurations.[1] The pore structure[2] and surface hydrophobicity[3] of the carbon matrix are of particular significance to the MEA performance, where the high current throughput requires good mass transport capability and controlled water flooding on the electrodes. In this work, we employed two widely used Fe-N-C catalysts – one pyrolyzed from metal organic framework (MOF-derived)[4] and the other produced by using the sacrificial support method (sacrificial templated)[1]. Specifically, nitrogen physisorption and water vapor physisorption were used to study the pore morphology and the specific surface hydrophobicity, respectively. As shown in Figure 1, the sacrificial templated Fe-N-C catalyst showed a significant mesoporosity as compared to the MOF-derived counterpart, while its surface was much more hydrophobic, as indicated by the less water uptake through whole pressure range (Figure 1f). Such difference of carbon morphology was further correlated to their macrokinetic performance in a MEA configuration, to reveal the impact of the morphological and hydrophilic/hydrophobic balance on these two types of Fe-N-C catalysts. Reference [1] T. Asset and P. Atanassov, Joule, 2020, 4, 33.[2] Y. Huang, Y. Chen, M. Xu, T. Asset, P. Tieu, A. Gili, D. Kulkarni, V. de Andrade, F. de Carlo, H. S. Barnard, A. Doran, D. Y. Parkinson, X. Pan, P. Atanassov, and I. Zenyuk, Materials Today, 2021, 47, 53.[3] L. Liu, S. J. Tan, T. Horikawa, D. D. Do, D. Nicholson and J. Liu, Adv Colloid Interface Sci, 2017, 250, 64.[4] K. Kumar, T. Asset, X. Li, Y. Liu, X. Yan, Y. Chen, M. Mermoux, X. Pan, P. Atanassov, F. Maillard and L. Dubau, ACS Catalysis, 11, 484. Figure 1. XRD (a) and AC-STEM (b-d) images of the Fe-N-C catalyst produced by the sacrificial support method. Nitrogen (e) and water vapor (f) physisorption isotherms of MOF-derived and sacrificial templated Fe-N-C catalysts. (g) Pore size distribution of the two catalysts by the BJH method based on the N2 adsorption branch. Figure 1

The Role of Atomically Dispersed Transition Metal Centers for the Electrochemical Nitrate Reduction Reaction Towards Ammonia Synthesis

The electrochemical reduction of oxidized nitrogen species, nitrate (NO3 -) and nitrite (NO2 -), both of which are environmental pollutants, to benign nitrogen gas (N2) has been extensively researched for water remediation applications. Recently, a large research focus has shifted to reducing these oxidized nitrogen species to ammonia (NH3).1 In this way, a dual benefit is achieved by eliminating the environmentally hazardous nitrates, while also creating a value-added product, ammonia. Moreover, the nitrate reduction reaction (NO3RR) can aid in closing the nitrogen cycle, by reforming nitrates introduced into the environment through nitrate-based fertilizers originating from the Haber-Bosch process, back into NH3, in a carbon neutral fashion.The NO3RR is a complex, 8e-, 9H+ transfer process with multiple possible intermediate species (NO2 -, NO, N2O, N2H4, NH2OH, NH3) and rate limiting steps. The NO3RR mechanism and its activity descriptors over various metals is not fully understood, and this is especially true over atomically dispersed sites, where less than a handful of studies exist.2,3 Our previous work found that atomically dispersed nitrogen coordinated Fe-N4 and Mo-N4 sites displayed distinct associative adsorption and dissociative adsorption of the nitrate molecule in the NO3RR pathways, respectively. Specifically, Fe-N4 sites adsorb and reduce NO3 - into NH3through an 8e- pathway on the surface, while Mo-N4 sites dissociate NO3 - and release NO2 - into the bulk electrolyte. These active sites were then integrated into a single bi-metallic catalyst (FeMo-N-C), creating a catalytic cascade yielding a significantly improved yield rate and Faradaic efficiency for NH3.4 In this talk, building on our previous work, we will present a series of transition metal atomically dispersed nitrogen doped carbon (M-N-C) electrocatalysts (M = Mn, Fe, Co, Ni, Cu...) for the systematic investigation of NO3RR activities over the different metal centers. In this work we leverage physical (ICP-MS, gas physisorption), electrochemical (e.g., NO3RR) and computational (DFT) characterization to create a set of NO3RR activity descriptors. A detailed set of activity descriptors can aid in the selection of atomically dispersed metals for creating efficient reaction cascades to synergize favorable reaction pathways. Figure 1