Oxygen Reduction Reaction Overpotential Research Articles

A High-Entropy Single-Atom Catalyst Toward Oxygen Reduction Reaction in Acidic and Alkaline Conditions.

The design of high-entropy single-atom catalysts (HESAC) with 5.2 times higher entropy compared to single-atom catalysts (SAC) is proposed, by using four different metals (FeCoNiRu-HESAC) for oxygen reduction reaction (ORR). Fe active sites with intermetallic distances of 6.1Å exhibit a low ORR overpotential of 0.44V, which originates from weakening the adsorption of OH intermediates. Based on density functional theory (DFT) findings, the FeCoNiRu-HESAC witha nitrogen-doped sample were synthesized. The atomic structures are confirmed with X-ray photoelectron spectroscopy (XPS), X-ray absorption (XAS), and scanning transmission electron microscopy (STEM). The predicted high catalytic activity is experimentally verified, finding that FeCoNiRu-HESAC has overpotentials of 0.41and 0.37V with Tafel slopes of 101 and 210mVdec-1 at the current density of 1mAcm-2 and the kinetic current densities of 8.2 and 5.3mAcm-2, respectively, in acidic and alkaline electrolytes. These results are comparable with Pt/C. The FeCoNiRu-HESAC is used for Zinc-air battery applications with an open circuit potential of 1.39V and power density of 0.16Wcm-2. Therefore, a strategy guided by DFT is provided for the rational design of HESAC which can be replaced with high-cost Pt catalysts toward ORR and beyond.

High-throughput screening of dual atom catalysts for oxygen reduction and evolution reactions and rechargeable zinc-air battery

We provide the rational design of dual atom catalysts (DACs) supported on nitrogen-doped graphene for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) through high-throughput computational screening of M1M2N6-DAC systems, where M1 and M2 represent Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, or Pt metals. We predict that FeRuN6-DAC at the summit of the volcano plot exhibits a low theoretical ORR overpotential (ηORR) of 0.24 V and a low theoretical OER overpotential (ηOER) of 0.19 V. The low ηORR and ηOER result from the catalytic performance of the Fe site being tuned to electronic properties that facilitate adsorption and desorption of the OH* intermediate. Inspired by these hybrid density functional theory (DFT) computational and machine learning (ML) results, we synthesized FeRuN6-DAC, FeN4-SAC, and RuN4-SAC and characterized them using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), and in-situ electron spin resonance (ESR). Our in-situ ESR spectroscopy signifies that the spin of the Fe active site increases with increasing applied potential due to the increase in the concentration of OH* intermediate on Fe. We verified experimentally the predicted catalytic performances, finding that FeRuN6-DAC leads to an experimental ORR overpotential of 0.29 V with a Tafel slope of 104 mVdec−1 and an OER overpotential of 0.27 V with a Tafel slope of 124 mVdec−1. The rechargeable Zinc-air battery setup was fabricated with FeRuN6-DAC in place of the cathode, showing a maximum power density of 0.45 W/cm2 at the current density of 0.44 A/cm2 and good stability after 120 cycles. According to our findings, we demonstrate that DFT-guided strategies are useful for designing advanced DACs applicable to ORR, OER, and Zinc-air battery applications.

Fe, N‐Inducing Interfacial Electron Redistribution in NiCo Spinel on Biomass‐Derived Carbon for Bi‐functional Oxygen Conversion

AbstractHerein, an interfacial electron redistribution is proposed to boost the activity of carbon‐supported spinel NiCo2O4 catalyst toward oxygen conversion via Fe, N‐doping strategy. Fe‐doping into octahedron induces a redistribution of electrons between Co and Ni atoms on NiCo1.8Fe0.2O4@N‐carbon. The increased electron density of Co promotes the coordination of water to Co sites and further dissociation. The generation of proton from water improves the overall activity for the oxygen reduction reaction (ORR). The increased electron density of Ni facilitates the generation of oxygen vacancies. The Ni−VO−Fe structure accelerates the deprotonation of *OOH to improve the activity toward oxygen evolution reaction (OER). N‐doping modulates the electron density of carbon to form active sites for the adsorption and protonation of oxygen species. Fir wood‐derived carbon endows catalyst with an integral structure to enable outstanding electrocatalytic performance. The NiCo1.8Fe0.2O4@N‐carbon express high half‐wave potential up to 0.86 V in ORR and low overpotential of 270 mV at 10 mA cm−2 in OER. The zinc‐air batteries (ZABs) assembled with the as‐prepared catalyst achieve long‐term cycle stability (over 2000 cycles) with peak power density (180 mWcm−2). Fe, N‐doping strategy drives the catalysis of biomass‐derived carbon‐based catalysts to the highest level for the oxygen conversion in ZABs.

Fe, N-Inducing Interfacial Electron Redistribution in NiCo Spinel on Biomass-Derived Carbon for Bi-functional Oxygen Conversion.

Herein, an interfacial electron redistribution is proposed to boost the activity of carbon-supported spinel NiCo2O4 catalyst toward oxygen conversion via Fe, N-doping strategy. Fe-doping into octahedron induces a redistribution of electrons between Co and Ni atoms on NiCo1.8Fe0.2O4@N-carbon. The increased electron density of Co promotes the coordination of water to Co sites and further dissociation. The generation of proton from water improves the overall activity for the oxygen reduction reaction (ORR). The increased electron density of Ni facilitates the generation of oxygen vacancies. The Ni-VO-Fe structure accelerates the deprotonation of *OOH to improve the activity toward oxygen evolution reaction (OER). N-doping modulates the electron density of carbon to form active sites for the adsorption and protonation of oxygen species. Fir wood-derived carbon endows catalyst with an integral structure to enable outstanding electrocatalytic performance. The NiCo1.8Fe0.2O4@N-carbon express high half-wave potential up to 0.86 V in ORR and low overpotential of 270 mV at 10 mA cm-2 in OER. The zinc-air batteries (ZABs) assembled with the as-prepared catalyst achieve long-term cycle stability (over 2000 cycles) with peak power density (180 mWcm-2). Fe, N-doping strategy drives the catalysis of biomass-derived carbon-based catalysts to the highest level for the oxygen conversion in ZABs.

Heterojunction of MXenes and MN4–graphene: Machine learning to accelerate the design of bifunctional oxygen electrocatalysts

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential for the development of excellent bifunctional electrocatalysts, which are key functions in clean energy production. The emphasis of this study lies in the rapid design and investigation of 153 MN4–graphene (Gra)/ MXene (M2NO) electrocatalysts for ORR/OER catalytic activity using machine learning (ML) and density functional theory (DFT). The DFT results indicated that CoN4–Gra/Ti2NO had both good ORR (0.37 V) and OER (0.30 V) overpotentials, while TiN4–Gra/M2NO and MN4–Gra/Cr2NO had high overpotentials. Our research further indicated orbital spin polarization and d-band centers far from the Fermi energy level, affecting the adsorption energy of oxygen-containing intermediates and thus reducing the catalytic activity. The ML results showed that the gradient boosting regression (GBR) model successfully predicted the overpotentials of the monofunctional catalysts RhN4–Gra/Ti2NO (ORR, 0.39 V) and RuN4–Gra/W2NO (OER, 0.45 V) as well as the overpotentials of the bifunctional catalyst RuN4–Gra/W2NO (ORR, 0.39 V; OER, 0.45 V). The symbolic regression (SR) algorithm was used to construct the overpotential descriptors without environmental variable features to accelerate the catalyst screening and shorten the trial-and-error costs from the source, providing a reliable theoretical basis for the experimental synthesis of MXene heterostructures.

Single Transition-Metal Atom Anchored on a Rhenium Disulfide Monolayer: An Efficient Bifunctional Electrocatalyst for the Oxygen Evolution and Oxygen Reduction Reactions.

Developing efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) bifunctional electrocatalysts is attractive for rechargeable metal-air batteries. Meanwhile, single metal atoms embedded in 2D layered transition metal chalcogenides (TMDs) have become a very promising catalyst. Recently, many attentions have been paid to the 2D ReS2 electrocatalyst due to its unique distorted octahedral 1T' crystal structure and thickness-independent electronic properties. Here, the catalytic activity of different transition metal (TM) atoms embedded in ReS2 using the density functional theory is investigated. The results indicate that TM@ReS2 exhibits outstanding thermal stability, good electrical conductivity, and electron transfer for electrochemical reactions. And the Ir@ReS2 and Pd@ReS2 can be used as OER/ORR bifunctional electrocatalysts with a lower overpotential for OER (ηOER ) of 0.44 V and overpotentials for ORR (ηORR ) of 0.26 V and 0.27 V, respectively. The excellent catalytic activity is attributed to the optimal adsorption strength for oxygen intermediates coming from the effective modulation of the electronic structure of ReS2 after Ir/Pd doping. The results can help to deeply understand the catalytic activity of TM@ReS2 and develop novel and highly efficient OER/ORR electrocatalysts.

Enhanced Oxygen Reduction Activity of Platinum Micropore

AbstractThe high overpotential of oxygen reduction reaction (ORR) prevents the wide commercialization of fuel cells and metal‐air batteries. To accelerate the reaction rate, porous electrocatalysts draw attention to obtain a higher specific surface area. However, the effect of micropores on ORR kinetics has not been understood because of the non‐uniform pore sizes, length, and tortuosity of practical electrocatalysts. In this study, we evaluate ORR activity in a micropore using platinum model electrodes with arrays of cylindrical pores with a uniform pore diameter of 1.8 nm. The model electrodes having pore lengths of 45, 100, and 380 nm are fabricated, and their ORR performances are examined by electrochemical measurements and numerical simulations in 0.1 mol dm−3 KOH aqueous solution. The intrinsic ORR activity of the micropore is successfully obtained with the electrode having a pore length of 45 nm, where oxygen transport resistance in the micropore has little effect on ORR. It is found that the intrinsic ORR activity of the micropore is higher than that of a planar Pt surface. X‐ray photoelectron spectroscopy and CO stripping voltammetry indicate a downshift of the d‐band center, which can be the origin of the high intrinsic ORR activity.

First-principle calculations study of the ORR/OER electrocatalytic activity of ruthenium polyphthalocyanine axially modified with aliphatic thiol groups.

In this study, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic activity of ruthenium polyphthalocyanine axially modified with different aliphatic thiol groups, RuPPc-SR (SR = -SCH3, -SC2H5, -SC3H7, -SC4H9, -SC5H11, and -SC6H13), in an acidic medium were simulated using DFT. All -SR groups can effectively enhance the ORR and OER catalytic activities of RuPPc. The ORR and OER overpotentials of RuPPc-SC4H9 are 0.237 V and 0.436 V, respectively, which are far lower than those of RuPPc (0.960 V and 0.903 V). For RuPPc-SC4H9, the four C and S atoms of the -SC4H9 chain and Ru atom are coplanar, and thus, conjugate effects and inductive effects exist between the -SC4H9 chain and Ru atom. This makes the Ru atom exhibit the least positive Bader charge and smallest spin density, and the anti-bonding orbitals of dxz, dyz, and dz2 of the Ru atom shift below the Fermi level (Ef). This makes the adsorption strength of RuPPc-SC4H9 toward ORR and OER intermediates the weakest, which accelerates the reaction process, thus resulting in better ORR and OER catalytic activity. Therefore, the introduction of the aliphatic thiol groups might effectively improve the OER/ORR catalytic activity of RuPPc.

Transition metals anchored on nitrogen-doped graphdiyne for an efficient oxygen reduction reaction: a DFT study.

The search for highly active and low-cost single-atom catalysts for the oxygen reduction reaction (ORR) is essential for the widespread application of proton exchange membrane fuel cells. Transition metals anchored on nitrogen-doped graphdiyne (GDY) have attracted considerable interest as potentially excellent catalysts for the ORR. However, the relationship between the active site and nitrogen-doped GDY remains unclear. In this work, we conducted a systematic investigation of sp-hybridized N atoms anchoring single transition metal atoms of 3d and 4d on GDY (TMC2N2) as electrocatalysts for the ORR. Firstly, 18 kinds of TMC2N2 were determined to have good thermodynamic stability. Due to the extremely strong adsorption of *OH, TMC2N2 exhibits inferior ORR performance compared to traditional Pt(111). Considering that *OH adsorption hinders the catalytic activity of TMC2N2, we modified the OH ligand of TMC2N2 to develop the high-valent metal complex (TMC2N2-OH) aiming to enhance the electrocatalytic activity. The adsorption of intermediates on most TMC2N2-OH is weakened after the modification of the OH ligand, especially for the adsorption of *OH. Thus, by comparing the ORR overpotential of catalysts before and after ligand modification, we find that the catalytic activity of different TMC2N2-OHs improves to various degrees. MnC2N2-OH, TMC2N2-OH, and TcC2N2-OH exhibit relatively high ORR catalytic activity, with overpotentials of 0.93 V, 1.19 V, and 0.92 V, respectively. Furthermore, we investigated the cause of improved catalytic activity of TMC2N2-OH and found that the modified coordination environment of the catalyst led to adjusted adsorption of ORR intermediates. In summary, our work sheds light on the relationship between nitrogen-doped GDY and transition metal sites, thus contributing to the development of more efficient catalysts.

Elucidation of the Interplay between the Features of the Precursor and the Physicochemical Properties of “Core-Shell” Hierarchical Carbon Nitride Electrocatalysts for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is a major bottleneck in the operation of proton-exchange membrane fuel cells (PEMFCs) since it introduces large overpotentials (no less than 250-300 mV), which degrade significantly the energy conversion efficiency of the device. In addition, the electrocatalysts (ECs) yielding the lowest ORR overpotentials in the acidic environment found at the PEMFC cathode are based on Pt, a very scarce critical raw material (CRM). Pt raises concerns as it might trigger serious supply bottlenecks in the event of a practical widespread implementation of PEMFCs in the framework of today’s energy transition. Hence, the development of high-performing and durable ECs for the ORR is a major stepping stone towards the large-scale rollout of PEMFCs.In our research group a new breakthrough approach has been devised for the synthesis of ORR ECs, that is completely different form the preparation routes that are commonly adopted in the state of the art [1, 2]. Briefly, the approach consists in the pyrolysis and subsequent treatment of a precursor obtained by coupling suitable carbon species/sacrificial supports with a zeolitic inorganic-organic polymer electrolyte (Z-IOPE). The latter comprises anionic complexes including the Pt “catalyst” and other “co-catalysts”, which are bridged by organic binders [1,2].On one hand, the proposed approach has already shown its potential to yield ECs exhibiting a performance and durability improved with respect to Pt/C benchmark ECs. On the other hand, the proposed approach is extremely flexible, especially with respect to the synthesis of the initial EC precursor. In principle, it is possible to modulate a number of crucial features of such EC precursor, with a particular reference to: (i) the chemical composition (e.g., in terms of Pt, other “co-catalysts” and N; the latter plays a crucial role to stabilize the active sites into “coordination nests” on the EC surface, raising durability); and (ii) the morphology, that can be “templated” on suitable species (e.g., carbon species, sacrificial supports). The very flexibility of the proposed synthetic process needs to be leveraged appropriately to maximize the performance of the final ORR ECs, at the same time minimizing the efforts wasted on approaches with a low development potential.Herein it is elucidated how the modulation in the parameters involved in the synthesis of the EC precursor (e.g., the binder and the approaches to couple the Z-IOPE with the support) impact the physicochemical and electrochemical properties of the final ECs, with a particular reference to the chemical composition, morphology and distribution of the ORR active sites. An extensive characterization of the final ECs is carried out. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and microanalysis are adopted to determine the bulk chemical composition. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is used to elucidate the surface chemical composition and the chemical states of the surface elements. Ultra-high resolution scanning electron microscopy (UHR-SEM) and transmission electron microscopy (TEM) allow for the elucidation of EC morphology. The porosimetric features are probed by automatic gas adsorption instrumentations. Wide-angle X-ray diffraction (WAXD) is crucial to resolve the crystal structure. Cyclic voltammetry with the rotating ring-disk electrode method (CV-TF-RRDE) is used to clarify the “ex-situ” electrochemical performance and ORR reaction mechanism. Finally, the most promising ECs are used in the fabrication of single PEMFCs, that are tested as a function of the operating conditions.It is revealed that, in comparison with the Pt/C benchamark, some ECs are characterized by a much higher intrinsic kinetic performance due to the electronic and bifunctional effects bestowed by the carbon nitride support and the «co-catalysts». Finally, the binder has a marked effect on the stoichiometry, size, and distribution of the PtMx aggregates bearing the active sites.

Electrocatalytic Activities of Sr2BO3Cl (B = Mn, Fe, Co, Ni) for Oxygen Reactions in Alkaline Solutions

The practical application of zinc-air rechargeable batteries, which are expected to be the next-generation rechargeable batteries, is challenged by the kinetically very slow oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Therefore, an air electrode catalyst with high activity for OER and ORR is required, and perovskite oxides have attracted attention as a candidate. To improve the catalytic activity of perovskite oxides, optimization of metal cations has been studied. On the other hand, anion substitution has also been reported to improve activity, and the oxychloride Sr2CoO3Cl with the Ruddlesden-Popper (RP) structure (composition: An+1BnO3n+1), which is a type of layered perovskite structure showed superior catalytic activities. However, there are still few studies on oxychlorides and the effect of anion substitution on the catalytic activity is still unknown. In this study, we compared the air electrode catalytic activity of RP-type acid chlorides Sr2 BO3Cl (B = Mn, Fe, Co, Ni) and LaSrBO4, an oxide with the same structure and valence, and investigated the B-site element dependence of the chloride ion substitution effect.The ORR activity measurement results show that the oxychlorides with Co and Mn at the B site exhibited lower ORR overpotentials than the oxides, and that the ORR reaction electron number calculated from the Koutecký-Levich plots increases. Comparing the results for the same B-site elements, the oxychlorides had higher OER activity for all B-site elements. This suggests that chloride ion substitution may be effective in improving OER activity for all B-site elements, not only Co.

4-6 carbophene: A two-dimensional carbon material with Dirac cone and superior activity for electrochemical oxygen reduction reactions

The positive exploration of efficient and cost-effective electrocatalysts for the oxygen reduction reaction (ORR) is crucial for the sustainable development of energy conversion and storage technologies. Recently, a new type of two-dimensional (2D) carbon crystals, called 4-6 carbophene, has been successfully synthesized. It consists of a four-carbon ring and a six-carbon ring. In this work, we have discovered that 4-6 carbophene is a two-dimensional Dirac cone material through density-functional theory (DFT) calculations. Therefore, we further investigated the oxygen reduction reaction on 4-6 carbophene to understand its catalytic properties. The results demonstrate that on 4-6 carbophene, the ORR prefers a direct four-electron pathway. Under acidic conditions, the ORR overpotential of 4-6 carbophene is as low as 0.59 V, which is comparable to some metal benchmark catalysts, demonstrating that 4-6 carbophene is a metal-free ORR candidate catalyst. In addition, the richly charged carbon atoms on 4-6 carbophene exhibit remarkable charge transfer when binding to the adsorbed intermediates. This results in a strong binding strength. Thus, 4-6 carbophene has favorable ORR catalytic activity. This work not only provides promising low-cost catalysts in pure carbon materials for ORR, promoting the development of metal-free ORR catalysts, but also enhances the understanding of Dirac cone two-dimensional materials for applications in catalysis.

C48, B24N24, CNT(8, 0) and BNNT(8, 0) as catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)

The capacities and performances of Mn doped nanostructures as nano-catalysts for ORR and OER are investigated. The mechanism of ORR by four-electron reaction is opposite of OER and in fuel cell the change in Gibbs free energy of OER is calculated on various surfaces of catalysts. Results shown that the over-potential of ORR on C48, B24N24 and C nanostructures and BN nanostructures are 1.14, 1.10, 1.05 and 0.99 V. Results indicated that the over-potential of OER on C48, B24N24 and C nanostructures and BN nanostructures are 1.10, 1.05, 0.98 and 0.92 V. Results shown that the ΔGreaction of ORR on C48, B24N24 and C nanostructures and BN nanostructures are −0.68, −0.65, −0.57 and −0.52 eV. Results indicated that the ΔGreaction of OER on C48, B24N24 and C nanostructures and BN nanostructures are −0.38, −0.35, −0.29 and −0.27 eV. The Mn-BNNT(8, 0) with high performances is suggested as new catalysts of ORR and OER reactions by theoretical models.

Computational Design of Ni6@Pt1M31 Clusters for Multifunctional Electrocatalysts.

High-efficiency and low-cost multifunctional electrocatalysts for hydrogen evolution reaction (HERs), oxygen evolution reaction (OERs) and oxygen reduction reaction (ORRs) are important for the practical applications of regenerative fuel cells. The activity trends of core-shell Ni6@M32 and Ni6@Pt1M31 (M = Pt, Pd, Cu, Ag, Au) were investigated using the density functional theory (DFT). Rate constant calculations indicated that Ni6@Pt1Ag31 was an efficient HER catalyst. The Volmer-Tafel process was the kinetically favorable reaction pathway for Ni6@Pt1M31. The Volmer-Heyrovsky reaction mechanism was preferred for Ni6@M32. The Pt active site reduced the energy barrier and changed the reaction mechanism. The ORR and OER overpotentials of Ni6@Pt1Ag31 were calculated to be 0.12 and 0.33 V, indicating that Ni6@Pt1Ag31 could be a promising multifunctional electrocatalyst. Ni6@Pt1M31 core-shell clusters present abundant active sites with a moderate adsorption strength for *H, *O, *OH and *OOH. The present study shows that embedding a single Pt atom onto a Ni@M core-shell cluster is a rational strategy for designing an effective multifunctional electrocatalyst.

Electronic structure modification of FeWO4 through F doping for enhanced oxygen reduction performance in zinc–air batteries

Exploring efficient and stable oxygen reduction reaction (ORR) catalysts for zinc-air batteries (ZABs) holds tremendously significant. Tungsten-based materials have attracted the attention of researchers thanks to abundant resources and excellent stability. Nevertheless, due to the strong adsorption energy for O-containing intermediates, *O and *OH are difficult to desorb from tungsten-based materials during ORR. Here, an F-doping strategy was employed to construct F-doped FeWO4 species composited on a multi-dimensional N-doped carbon (F–FeWO4/NC) as the ORR catalyst for ZABs. The incorporation of F atoms into the main lattice of FeWO4 enhanced the polarity of chemical bonds in FeWO4 through hydrogen bond interactions between H2O molecules and F atoms, further enhancing the polarity and hydrophilicity of F–FeWO4/NC. These changes favored the adsorption and desorption of intermediates as well as the diffusion of electrolyte, and ultimately reduced the ORR overpotential. Therefore, the F–FeWO4/NC catalyst demonstrated excellent ORR activity (E1/2 = 0.85 V vs. RHE). Furthermore, the ZAB with F–FeWO4/NC yielded satisfactory a specific capacity of 800.2 mAh g−1, a peak power density of 173.5 mW cm−2, and charge-discharge cycling stability (over 170 h at 5 mA cm−2). This study is beneficial to improve the ORR intrinsic activity of cheap tungsten-based materials and provides theoretical guidance for their practical application in ZABs.

Introducing highly polarizable cation in M-N-C type catalysts to boost their oxygen reduction reaction performance

The metal-nitrogen-carbon (M-N-C) type oxygen reduction reaction (ORR) catalysts are promising for hydroxide exchange membrane fuel cells, but catalysts with further improved performance are challenging. Here, we report the efficient improvement of the ORR activity of M-N-C catalysts by employing highly polarizable metal cation Ag+. Ag, Fe single atomic sites embedded in concave nitrogen doped carbon (Ag1Fe1/CNC) is successfully synthesized and shows high performance towards ORR, indicating by both the ultra-high half-wave potential of 0.917 V and membrane electrolyte assembly performance of peak power density up to 1.26 W cm−2. The binding energy of the ORR intermediates on the highly polarizable Ag site in Ag1Fe1/CNC was significantly tuned by the adjacent Fe site by more than 0.2 eV, and thus leading to a low theoretical ORR overpotential of only 0.398 V. The employment of the highly polarizable metal cation brings a novel and efficient approach to construct highly active catalysts.

(Keynote) Interplay between Physicochemical Properties of Precursors and “Core-Shell” Hierarchical Carbon Nitride Electrocatalysts for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is a major bottleneck in the operation of proton-exchange membrane fuel cells (PEMFCs) since it introduces large overpotentials (no less than 250-300 mV), which degrade significantly the energy conversion efficiency of the device. In addition, the electrocatalysts (ECs) yielding the lowest ORR overpotentials in the acidic environment found at the PEMFC cathode are based on Pt, a very scarce critical raw material (CRM). Pt raises concerns as it might trigger serious supply bottlenecks in the event of a practical widespread implementation of PEMFCs in the framework of today’s energy transition. Hence, the development of high-performing and durable ECs for the ORR is a major stepping stone towards the large-scale rollout of PEMFCs.In our research group a new breakthrough approach has been devised for the synthesis of ORR ECs, that is completely different form the preparation routes that are commonly adopted in the state of the art [1, 2]. Briefly, the approach consists in the pyrolysis and subsequent treatment of a precursor obtained by coupling suitable carbon species/sacrificial supports with a zeolitic inorganic-organic polymer electrolyte (Z-IOPE). The latter comprises anionic complexes including the Pt “catalyst” and other “co-catalysts”, which are bridged by organic binders [1,2].On one hand, the proposed approach has already shown its potential to yield ECs exhibiting a performance and durability improved with respect to Pt/C benchmark ECs. On the other hand, the proposed approach is extremely flexible, especially with respect to the synthesis of the initial EC precursor. In principle, it is possible to modulate a number of crucial features of such EC precursor, with a particular reference to: (i) the chemical composition (e.g., in terms of Pt, other “co-catalysts” and N; the latter plays a crucial role to stabilize the active sites into “coordination nests” on the EC surface, raising durability); and (ii) the morphology, that can be “templated” on suitable species (e.g., carbon species, sacrificial supports). The very flexibility of the proposed synthetic process needs to be leveraged appropriately to maximize the performance of the final ORR ECs, at the same time minimizing the efforts wasted on approaches with a low development potential.Herein it is elucidated how the modulation in the parameters involved in the synthesis of the EC precursor (e.g., the binder and the approaches to couple the Z-IOPE with the support) impact the physicochemical and electrochemical properties of the final ECs, with a particular reference to the chemical composition, morphology and distribution of the ORR active sites. An extensive characterization of the final ECs is carried out. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and microanalysis are adopted to determine the bulk chemical composition. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is used to elucidate the surface chemical composition and the chemical states of the surface elements. Ultra-high resolution scanning electron microscopy (UHR-SEM) and transmission electron microscopy (TEM) allow for the elucidation of EC morphology. The porosimetric features are probed by automatic gas adsorption instrumentations. Wide-angle X-ray diffraction (WAXD) is crucial to resolve the crystal structure. Cyclic voltammetry with the rotating ring-disk electrode method (CV-TF-RRDE) is used to clarify the “ex-situ” electrochemical performance and ORR reaction mechanism. Finally, the most promising ECs are used in the fabrication of single PEMFCs, that are tested as a function of the operating conditions.It is revealed that, in comparison with the Pt/C benchamark, some ECs are characterized by a much higher intrinsic kinetic performance due to the electronic and bifunctional effects bestowed by the carbon nitride support and the «co-catalysts». Finally, the binder has a marked effect on the stoichiometry, size, and distribution of the PtMx aggregates bearing the active sites.

Coinage metals doped 1T′ WS2 as efficient bifunctional electrocatalyst towards ORR and HER: A first principles study

The electrocatalytic activities of Cu, Ag and Au doped 1T′ WS2 as electrocatalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) in acidic medium are explored by means of first principles calculations. The stability of these doped systems are inspected in terms of formation energy and charge transfer analysis upon doping. Highest amount of charge transfer towards the dopant is observed for systems with lowest formation energy. For all the doped systems, charge is transferred to the dopant that substitutes the anionic S atom validating the doping scheme and stability of the doped systems. ORR proceeds only via the energetically efficient four electron pathway on Cu doped 1T′ WS2 with a monotonically exothermic free energy profile. In the case of Ag and Au doped structures, ORR occurs competitively via both two and four electron pathways due to the marginal free energy difference in the 2nd protonation step between these two pathways and produces both H2O or H2O2 competitively as the end product. Study of the effect of electrode potential variation on the free energy profiles reveals the overpotential for ORR on Cu, Ag and Au doped 1T′ WS2 to be 0.85 V, 0.80 V and 0.66 V respectively via the four electron pathway. Proneness of Cu doped 1T′ WS2 towards only the four electron pathway and low overpotential for ORR makes it the promising cathode electrocatalyst for ORR among the considered systems. Further study of the HER activity of Cu, Ag and Au doped 1T′ WS2 reveals the Gibbs free energy of H+ ion adsorption on Cu doped 1T′ WS2, Ag doped 1T′ WS2 and Au doped 1T′ WS2 to be −0.14 eV, −0.17 eV and 0.18 eV respectively indicating high efficiency of those system towards HER in acidic medium.

Insights into local coordination environment of main group metal-nitrogen-carbon catalysts for enhanced oxygen reduction reaction

For the 4e– oxygen reduction reaction (ORR) on main group metal-nitrogen-carbon catalysts, the effect of the local coordination environment of main group metals on ORR has rarely been reported. By performing density functional theory (DFT) calculations, we screen 18 MN4–xOx (x = 0–2, M = Mg, Ca, Sr, Al, Ga, In) candidates with different metal coordination environments and construct volcano curve between the adsorption free energy of OH* (ΔGOH*) and ORR overpotential, yielding MgN2O2 at the peak of the volcano with the overpotential as low as 0.55 V. The lower ORR overpotential on Mg coordinated with 2 N and 2 O atoms than other coordination environments is well explained by the decreased ΔGOH* because of more shifts of p-band position of Mg away from the Fermi energy level as well as the lower O highest occupied energy level when the number of the coordinated O atoms increases. Our DFT work provides theoretical guidance for engineering ORR performance of main group metal-nitrogen-carbon catalysts by modulating coordination environments.

A Surrogate Machine Learning Model for the Design of Single-Atom Catalyst on Carbon and Porphyrin Supports towards Electrochemistry

We apply the machine learning (ML) tool to calculate the Gibbs free energy (ΔG) of reaction intermediates rapidly and accurately as a guide for designing porphyrin- and graphene-supported single-atom catalysts (SACs) toward electrochemical reactions. Based on the 2105 DFT calculation data from the literature, we trained a support vector machine (SVR) algorithm. The hyperparameters were optimized using Bayesian optimization along with 10-fold cross-validation to avoid overfitting. Based on the Shapley Additive exPlanation (SHAP) and permutation methods, the feature importance analysis suggests that the most important parameters are the number of pyridinic nitrogen (Npy), the number of d electrons (θd), and the number of valence electrons of reaction intermediates. Inspired by this feature importance analysis and the Pearson correlation coefficient, we found a linear dependent, simple, and general descriptor (φ) to describe ΔG of reaction intermediates (e.g., ΔGOH* = 0.020φ – 2.190). Using the trained SVR algorithm, ΔGOH*, ΔGO*, ΔGOOH*, ΔGOO*, ΔGH*, ΔGCOOH*, ΔGCO*, and ΔGN2* intermediates are predicted for the oxygen reduction reaction (ORR), the oxygen evolution reaction (OER), the hydrogen evolution reaction (HER), and the CO2 reduction reaction (CO2RR). The SVR model predicts an ORR overpotential of 0.51 V and an HER overpotential of 0.22 V for FeN4-SAC. Moreover, we used the SVR algorithm for high-throughput screening of SACs, suggesting new SACs with low ORR overpotentials. This strategy provides a data-driven catalyst design method that significantly reduces the costs of DFT calculations while providing the means for designing SACs for electrocatalysis and beyond.