Oxygen Reduction Reaction Kinetics Research Articles

Fe doped Co9S8 nanoparticles embedded in N, S co-doped porous carbon as an efficient bifunctional electrocatalyst for rechargeable Zn-air batteries

Transition metal sulfides embedded in co-doped porous carbon materials with nitrogen and sulfur are a promising catalyst that can effectively solve the problem of slow oxygen reduction reaction (ORR) kinetics. However, monometallic sulfides consisting of a single active component only have monofunctional catalytic activity and cannot effectively increase the rate of oxygen evolution reaction (OER), thus limiting the energy conversion efficiency of Zn-air batteries (ZAB). Herein, a unique silica template assisted in situ pyrolysis strategy is used to synthesize iron (Fe)-doped cobalt sulfide (Co9S8) nanoparticle embedded in a honeycomb nitrogen (N), sulphur (S) co-doped porous carbon (Fe/Co9S8@NSC) electrocatalyst. The introduction of Fe increases the active component of the catalyst and further enhances the catalytic ability of Co9S8 by optimizing the electronic structure through charge transfer effect. Therefore, the rich active sites of Fe/Co9S8 nanoparticles anchored in N, S co-doped carbon materials with rich pore structure enable the catalysts to have good bifunctional catalytic activities of ORR and OER. In particular, the Fe/Co9S8@NSC has a high half-wave potential of 0.82 V for catalytic ORR and only needs low overpotential of 332 mV at a current density of 10 mA cm−2 for catalytic OER. In addition, the Fe/Co9S8@NSC catalyst based rechargeable ZAB also exhibits a significant specific capacity of 734 mAh g−1 and prolonged stability of 270 h, which outperforms the bulk of recently reported catalysts based on transition metal sulfides-based catalysts. This research offers a workable method for producing metal sulfides catalysts with bifunctional catalytic activity.

Importance of the catalyst–water Coulomb interaction for oxygen reduction reaction kinetics

This work identifies an important new factor affecting oxygen reduction reaction kinetics (i.e., the catalyst–water Coulomb interaction).

Prussian blue-derived FeNx/Fe-based N-doped carbon nanotube catalysts with high ORR electrochemical performance

Prussian blue-derived FeNx/Fe-based N-doped carbon nanotube catalysts with high ORR electrochemical performance.

Sulfur-bridge ligands altering the microenvironment of single-atom CoN3S sites to boost the oxygen reduction reaction.

We report here an asymmetric N,S-coordinated cobalt-based single-atom catalyst with sulfur (S)-bridge ligands (Co-N/S-C) for the oxygen reduction reaction (ORR). The Co-N/S-C exhibits a half-wave potential (E1/2) of 0.908 V versus RHE, outperforming most state-of-the-art ORR catalysts. Theoretical calculations indicate that the CoN3SC10-S moiety facilitates the ORR kinetics by optimizing the adsorption of intermediates. This work provides new insights into the design of single-atom catalysts for electrocatalysis through heteroatom-bridge ligand engineering.

Nano-engineered catalysts for high-performance oxygen reduction reaction

The efficient energy conversion of fuel cells is greatly constrained by the slow oxygen reduction reaction (ORR) kinetics, which necessitates the use of highly active metal catalysts such as platinum (Pt). The critical challenge limiting large-scale usage of Pt is the capital cost that can be addressed through a prototypical approach by embedding metal nanoparticles (NPs), e.g., Pt NPs, in the conductive framework. However, previously reported embedding approaches are sophisticated and suffer from limited yields, leading to higher chemical process costs and remaining distant from commercial viability. Here, we report a facile, cost-effective and time-efficient structural tuning approach to synthesizing ultrafine Pt NPs impregnated within a conductive and highly porous carbon framework via a microwave-assisted polyol reduction method. Pt NPs with a uniform size of ∼2.27 nm can be successfully integrated within the pores of the carbon framework, enabling homogeneous dispersion. Benefiting from these highly dispersed and ultrafine Pt NPs, the electrochemical surface area (ECSA) is improved to 142.98 m²/gPt, 2.25 times higher than that of the commercial counterpart (63.52 m²/gPt). Furthermore, our structurally optimized catalyst composite features a remarkably catalytic activity with a high half-wave potential (E1/2) of 0.895 V and an improved mass activity (MA) of 0.2289 A/mgPt, 2.39-fold improvement compared to the commercial counterpart. In addition, orthogonal experiments were designed to identify the key process parameters for fabricating Pt/C catalysts, offering insights for scaled-up and industrial production.

Real-time diagnostic synthesis of nitrogen-doped carbon-coated VC@C(Nx) core-shell nanoparticles by DC arc plasma as efficient ORR electrocatalysts

The high complexity and cost of synthesis methods, along with the instability of catalytic activity, pose significant challenges in the search for viable non-precious metal alternatives to commercial Pt/C catalysts for oxygen reduction reaction (ORR). In this study, we employed a highly efficient and cost-effective method, namely, DC arc plasma, to synthesize carbon-coated vanadium carbide (VC@C) core-shell nanoparticles in a one-step process. The synthesized VC@C material was subsequently nitrogen-doped using urea as a nitrogen (N) source in a tube furnace, resulting in the formation of N-doped carbon-coated VC [VC@C(Nx)] catalysts for ORR electrocatalysis in alkaline media. Real-time optical emission spectroscopy (OES) analysis offers an in-depth comprehension of the energy states involved in the synthesis process of VC@C nanocrystalline powders. Highlighting the plasma states at various stages throughout the synthesis, provides a solid basis for dissecting the formation of VC@C nanoparticles and elucidating the intricate nature of their core-shell structures. Compared to the VC@C catalyst, the VC@C(N10) catalyst exhibited a significant improvement in both half-wave and onset potential, approaching the performance levels comparable to Pt/C catalysts. Moreover, VC@C(N10) demonstrated superior ORR kinetics, stability, and methanol tolerance. The exceptional performance of VC@C(N10) could be primarily attributed to the introduction of nitrogen doping, which generated more active sites and enhanced the synergistic cooperation between the VC core and the nitrogen-doped carbon shell. Additionally, the protective effect from the carbon shell prevented corrosion and oxidation of the VC core. This study represents a new exploration in the quest for the development of stable and efficient non-precious metal catalysts for ORR applications.

Illuminating Oxygen Reduction Reaction Kinetics in High Temperature Polymer Electrolyte Membrane Fuel Cells Using EIS

Owing to their notable power efficiency and zero greenhouse emissions, High Temperature Polymer Electrolyte Membrane Fuel Cells (HTPEMFCs) hold immense promise as a prospective candidate for generating clean energy. However, their commercialization is still impeded by the cathodic Oxygen Reduction Reaction (ORR) kinetics, which after almost half a century not only remain elusive, but constitute the most energy-intensive process, requiring large amounts of the expensive Pt catalyst to mitigate.In this work we provide solid experimental evidence, that the simple three elementary step Dissociative Adsorption Pathway (DAP) with two intermediate species (Oad and OHad), can accurately capture both the steady state (IV) and the Electrochemical Impedance Spectra (EIS) as well.Detailed EIS and IV experimental data were recorded at 180 °C under differential conditions, using a single serpentine HTPEMFC with a 4 cm2 active area. The fuel cell operated at the activation regime, where ORR potential losses dominate but mass transport limitations are negligible. The experimental data were successfully fitted with the derived analytic IV and EIS microkinetic model.The use of the DAP in accordance with the Transitions State Theory (TST), allowed the extraction and analysis of ORR kinetics and energetics. The deconvoluted EIS outlined the important role of the intrinsic kinetic inertia , i.e. the outcome of the competitive nature of the elementary reaction steps on the adsorbed intermediate species. Kinetic inertia not only dominated EIS and the polarization resistance, but demonstrated that the commonly used Butler Volmer and apparent Tafel slope kinetic approach is incapable of describing the multistep ORR kinetics. With the help of the Degree of Rate Control (DRC) analysis, we successfully identified the O2(g) dissociative adsorption as the rate limiting step.Finally, from an energetics perspective, by employing the TST kinetics formulation, we were able to calculate the free Gibbs’ transition states activation (kinetics) and reaction steps’ energies (thermodynamics). These results clarified that, the high ORR overpotential losses stem from the combined strength of both kinetically and thermodynamically imposed barriers, which originate from the high bonding strength of Oad on the Pt surface and the rather high activation energy of the O2(g) adsorption step.

Enhanced Kinetics of Oxygen Reduction Reactions in Alkaline Solutions inside Mesoporous Electrodes

Oxygen reduction reactions (ORR) in alkaline solutions attract increasing attention for their use as the cathodic reaction in metal-air batteries and alkaline fuel cells. However, the commercialization of these systems is impeded by high activation and concentration overpotentials of ORR, which originate from the sluggish kinetics on the catalyst surface and oxygen transport in the electrolyte, respectively. Although the practical electrode enables a decrease of the overpotentials with the porous structure, the effect of inner pores has not been understood. Among various pore sizes, it is crucial to investigate an electrochemical behavior in mesopores with a size of a few nm since such confined structures possibly change the intrinsic catalytic kinetics.1 In this study, we evaluated the ORR activity in the mesopore using Pt model electrodes with arrays of cylindrical pores with uniform pore sizes (Fig. a). We investigated the activity dependence on the electrode thickness to separate activation and concentration overpotentials, and found that the activation overpotential inside the mesopore with a size of 1.8 nm is smaller than that measured with the planer electrode. Pt model electrodes were electrodeposited from platinum salt in a liquid crystalline phase of a surfactant.2 The liquid crystalline phase was obtained by mixing 29 wt% of H2O, 29 wt% of hexachloroplatinic acid, and 42 wt% of octaethylene glycol monododecyl ether. The three-electrode cell consisted of an Au disk (φ 4 mm) working electrode, a Pt wire counter electrode, and an Ag|AgCl|sat’d KClaq reference electrode. The deposition was conducted at –0.055 V and 25 °C, and the charge densities were adjusted to obtain the electrode thickness of ca. 45, 100, 380, and 900 nm. Transmission electron microscopy (TEM) was performed to observe the pore structure. Electrochemical surface area (ECSA) and ORR activities were evaluated by cyclic voltammetry in N2/O2-saturated 0.1 mol dm–3 KOH solutions with a rotating disk electrode system, consisting of the porous model or a polished planar Pt working electrode, a Pt wire counter electrode, and a Hg|HgO reference electrode. The scan rate and rotation speed were 20 mV s–1 and 2000 rpm, respectively. TEM image exhibits the honeycomb arrays of mesopores with a pore size of 1.8 nm and a pore distance of 4.9 nm (Fig. a). The model electrodes possessed polycrystalline facets, and ECSA was calculated from the hydrogen adsorption peak area in blank voltammetry (Fig. b). The ECSA matched closely with the geometrically calculated surface area, suggesting that the cylindrical pores are uniformly distributed on the electrodes with defined pore lengths (Fig. c). ORR activities were evaluated with the Tafel plots (Fig. d), with current densities corrected for ECSA and concentration overpotential outside the mesopores. Since the oxygen transport resistance or the concentration overpotential is higher in longer pores, the electrodes with longer pore lengths caused smaller ORR current densities. In contrast, the concentration overpotential was suppressed enough to extract the activation overpotential when the pore length was 45 nm and 100 nm, and these electrodes led to higher ORR current density than the planar electrode. The result indicates that the activation overpotential inside the mesopore with a size of 1.8 nm is smaller than that measured with the planer electrode. We will report the origin of the enhanced kinetics inside the mesopore from the viewpoint of the electronic structure change.

Modeling the Environment-Dependent Kinetics of Oxygen Reduction Reaction – a Continuum Model for Electric Double Layer

For proton-exchange-membrane fuel cells (PEMFCs) to achieve broad commercialization, improved energy-conversion efficiency with minimal Pt-based electrocatalyst is required [1]. Because the sluggish rate of oxygen reduction reaction (ORR) limits the efficiency of PEMFCs [2], the efficiency improvement requires a better understanding of ORR kinetics and mechanism to design better catalyst. To understand the ORR mechanism, theoretical and experimental analyses have been conducted. While previous studies reasonably explained the catalyst-dependent activity on single crystal catalysts in 0.1 M perchloric acid solution [3, 4], the explicit effect of electrolyte and related microenvironments [5, 6] is not thoroughly understood. The change in the electrolyte alters the electric-double-layer (EDL) structure and thus the local microenvironment at the electrode/electrolyte interface. Thus, the structure of the EDL should be carefully analyzed to uncover the electrolyte-dependent reaction kinetics. In this talk, we propose a multiscale continuum model to predict the EDL structure and examine the effect of perchloric acid concentration on ORR activity on Pt (111). The model includes Density Potential Functional Theory (DPFT) for electron density and Modified Poisson Boltzmann equation for species’ density and electric potential [7]. Also, the interaction between adsorbents and electric field is taken into account by minimizing the grand potential. After model validation with experimentally measured double-layer capacity data as a function of applied potential and concentration, the effect of the perchloric acid concentration (0.02 M – 0.2 M) on ORR activity is analyzed and discussed. It is shown that the model reproduces the specific activity obtained in the experiments when assuming the oxygen adsorption is limiting the rate, which can be attributed to the large energetic barrier for solvent reorganization [8]. Then, extension of the model to PEMFC ionomer electrolytes will be introduced. Overall, the model framework and findings provide insights into the ORR mechanism and guidance on how to tailor catalyst materials for increased PEMFC performance. Acknowledgements: This work was partially supported by the Hydrogen and Fuel Cell Technologies Office (HFTO), Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE) through the Million Mile Fuel Cell Truck (M2FCT) consortia, technology managers G. Kleen and D. Papageorgopoulos, under contract number DE-AC02- 05CH11231, as well as by a CRADA with Toyota Central R&D Labs., Inc.. Reference [1] D. A. Cullen, K. C. Neyerlin, R. K. Ahluwalia, R. Mukundan, K. L. More, R. L. Borup, A. Z. Weber, D. J. Myers and A. Kusoglu, Nature Energy, 6, 462 (2021).[2] I. E. L. Stephens, A. S. Bondarenko, U. Grønbjerg, J. Rossmeisl and I. Chorkendorff, Energy & Environmental Science, 5 (2012).[3] M. Shibata, M. Inaba, K. Shinozaki, K. Kodama and R. Jinnouchi, Journal of The Electrochemical Society, 169 (2022).[4] H. A. Hansen, V. Viswanathan and J. K. Nørskov, The Journal of Physical Chemistry C, 118, 6706 (2014).[5] K. Kodama, K. Motobayashi, A. Shinohara, N. Hasegawa, K. Kudo, R. Jinnouchi, M. Osawa and Y. Morimoto, ACS Catalysis, 8, 694 (2017).[6] M. Luo and M. T. M. Koper, Nature Catalysis, 5, 615 (2022).[7] J. Huang, J Chem Theory Comput, 19, 1003 (2023).[8] D. T. Limmer, A. P. Willard, P. Madden and D. Chandler, Proceedings of the National Academy of Sciences, 110, 4200 (2013).

Nanoscale Investigations of the Electric Double Layer in Protic Ionic Liquids

A hydrogen-based energy storage system will be the backbone of a future energy grid using renewable energies. Polymer electrolyte membrane fuel cells (PEMFCs) are a key element in this energy system as they convert chemical energy stored as hydrogen into electrical energy on demand. PEMFC systems, especially for automotive application, could be significantly improved by increasing the operation temperature above 100 °C. Protic ionic liquids are promising candidates as non-aqueous protic electrolytes for next-generation high-temperature polymer electrolyte membrane fuel cells. These fuel cells have a target operation temperature of 160 °C and allowing for a more efficient water and heat management compared to conventional Nafion®-based PEMFCs, which operate at temperatures below 80 °C [1].In order to ensure a reliable and efficient operation an electrolyte with a high electrochemical performance and stability has to be selected. For this purpose, protic ionic liquids have been proposed and first fuel cell tests have shown promising results [2]. Hence, we aim on understanding the properties of this class of novel electrolytes on an atomistic level, which would allow designing suitable material combinations and predicting their properties for an efficient fuel cell operation. As ionic liquids are molten salts, which are liquid below 100 °C, their electrochemical properties differ significantly from those of aqueous solutions. Instead of a classical electric double layer, which can be described by the models provided by Helmholtz, Gouy-Chapman and Stern, the interface structure formed between the electrolyte and a charged electrode is governed by the interplay between coulomb interaction and steric effects between the (large) molecular ions [3]. In order to understand the formation of this double layer on a metallic electrode, we employ atomic force microscopy and infrared spectroscopy in combination with molecular dynamics simulations. Our results show that in the interface region between the prototype protic ionic liquid diethylmethylammonium triflate ([Dema][TfO]) and a Pt electrode, a dense layered structure consisting of alternating anion and cation layers is present, that extends several nanometres into the bulk of the electrolyte [4]. The composition and structure changes with applied potential due to a preferential attraction of anions or cations depending on the electrode charge. When water is added to the ionic liquid, the layered structure becomes distorted and water molecules appear near the electrode. Since the presence of water will also influence the relevant electrochemical processes such as the oxygen reduction reaction (ORR), the analysis of the double layer structure on an atomistic scale is necessary in order to understand the subtle interactions between the molecules in the electrolyte and to propose design routes for novel more efficient ionic liquid-based electrolytes. Wippermann, K.; Suo, Y.; Korte, C. Oxygen Reduction Reaction Kinetics on Pt in Mixtures of Proton-Conducting Ionic Liquids and Water: The Influence of Cation Acidity. J. Phys. Chem. C 2021, 125, 4423–4435, doi:10.1021/acs.jpcc.0c11374.Skorikova, G.; Rauber, D.; Aili, D.; Martin, S.; Li, Q.; Henkensmeier, D.; Hempelmann, R. Protic Ionic Liquids Immobilized in Phosphoric Acid-Doped Polybenzimidazole Matrix Enable Polymer Electrolyte Fuel Cell Operation at 200 °C. Journal of Membrane Science 2020, 608, 118188, doi:10.1016/j.memsci.2020.118188.Rodenbücher, C.; Wippermann, K.; Korte, C. Atomic Force Spectroscopy on Ionic Liquids. Applied Sciences 2019, 9, 2207, doi:10.3390/app9112207.Rodenbücher, C.; Chen, Y.; Wippermann, K.; Kowalski, P.M.; Giesen, M.; Mayer, D.; Hausen, F.; Korte, C. The Structure of the Electric Double Layer of the Protic Ionic Liquid [Dema][TfO] Analyzed by Atomic Force Spectroscopy. International Journal of Molecular Sciences 2021, 22, 12653, doi:10.3390/ijms222312653. Figure 1

Metal-Oxygen Covalency Competition of Unsymmetrical Metal Pairs for Enhanced Bifunctional Oxygen Electrocatalysis

Rechargeable zinc-air batteries are considered promising energy conversion devices due to their inherent safety and high energy-to-cost ratio. However, the sluggish kinetics of oxygen evolution reaction and oxygen reduction reaction during the charging-discharging process lead to low round trip efficiency and power density. The synthesis of efficient and stable bifunctional oxygen electrocatalysts constitutes the bottleneck to construct applicable ZABs.Here, we report an Mn-doped ruthenium oxide electrocatalyst that exhibits remarkable ORR/OER activity, with a low potential gap of 0.62V between the half-wave potential of ORR and the OER potential at 10mA cm-2. The Mn-RuO2 assembled ZABs present a high power density of 179 mW cm-2 and long lifespans of 800 h. Metal covalency competition induces surface Mn atoms shows more favorable adsorption behavior for ORR and Ru atoms shows more favorable adsorption behavior for OER. The charge redistribution between asymmetrical Ru-O-Mn pairs results in dual actives sites towards OER/ORR. The surface ruthenium stabilizes the O* intermediate due to the weaker hybridization of the lattice Ru-O bond. The upshift of the dz2 orbital relieves the over-binding of the critical intermediates OH* on the Mn atom. Such dual Ru-Mn sites minimized the OER/ORR activity gap restricted by scaling relationships. Our findings rationalize the double active sites mechanism in Mn-RuO2 electrocatalyst and provide a strategy to enhance bifunctional catalytic performance. Figure 1

Boosting Oxygen Reduction Reaction with Fe-Based Single-Atom Catalysts: The Role of Cluster Interactions

Fuel cells (FCs) have emerged as a promising alternative for efficient electricity production in the future [1]. However, their widespread commercialization is hindered by their prohibitive cost due to the large amount of platinum used in these electrochemical systems [2]. Platinum is used to accelerate the unfavorable kinetics of the cathodic oxygen reduction reaction (ORR) [3]. To achieve large-scale and cost-effective development of FCs, it is essential to find high-performance platinum-free catalysts. Two solutions are mainly reported in the literature: the development of non-precious metal-based carbon catalysts and metal-free carbon catalysts. However, the work reported on these two types of carbon catalysts is sometimes contradictory regarding the actual nature of the active sites and the mechanisms of reduction of oxygen molecules on the catalyst [4].Pyrolyzed metal-nitrogen-carbon (M-N-C) catalysts for the ORR, especially single-atom catalysts (SACs), have been widely studied because they exhibit excellent electrocatalytic performance, thanks to the high content of metal atoms anchored on a carbon support [5]. Recent research has indicated that the combination of single-atom active sites with metal nanoclusters can further improve the catalytic activity, but the underlying mechanism remains unclear [6]. Therefore, it is necessary to develop model catalysts based on both single-atoms and nanoclusters to better understand their catalytic activity.In this work, we prepared nitrogen-doped and iron-doped carbon catalysts (called Fe-NC below) with precise control of the type of iron in the material. First, the functionalization of an activated carbon was performed by carbonizing it under air in the presence of urea. Then, the sample was placed in an autoclave for hydrothermal treatment in the presence of FeCl3.6H2O to allow the incorporation of iron, and thermal post-treatment was applied to synthesize model catalysts (Fig. 1a). The precursor ratio and final pyrolysis temperature were adjusted and optimized to produce either iron-based single atoms (SA) (here referred to as FeSA-NC) or atomic clusters (AC) associated with single atoms (here referred to as FeSA+AC-NC). The results show that the iron-based catalysts exhibit outstanding catalytic activity (Fig. 1b) and durability (Fig. 1c) in alkaline medium for the ORR, even better than commercial Pt (20 wt.%)/C in the case of FeSA+AC-NC. The analysis provided insight into the development of highly efficient multifunctional electrocatalysts, where the presence of iron nanoclusters was found to be a crucial determinant of the catalytic activity for the ORR.[1] M.K. Debe, Electrocatalyst approaches and challenges for automotive fuel cells, Nature. 486 (2012) 43–51. https://doi.org/10.1038/nature11115.[2] C. Sealy, The problem with platinum, Mater. Today. 11 (2008) 65–68. https://doi.org/10.1016/S1369-7021(08)70254-2.[3] M. Borghei, J. Lehtonen, L. Liu, O.J. Rojas, Advanced Biomass-Derived Electrocatalysts for the Oxygen Reduction Reaction, Adv. Mater. 30 (2018) 1703691. https://doi.org/10.1002/adma.201703691.[4] A. Dessalle, J. Quílez-Bermejo, V. Fierro, F. Xu, A. Celzard, Recent progress in the development of efficient biomass-based ORR electrocatalysts, Carbon. 203 (2023) 237–260. https://doi.org/10.1016/j.carbon.2022.11.073.[5] K. Liu, J. Fu, Y. Lin, T. Luo, G. Ni, H. Li, et al., Insights into the activity of single-atom Fe-N-C catalysts for oxygen reduction reaction, Nat. Commun. 13 (2022) 2075. https://doi.org/10.1038/s41467-022-29797-1.[6] X. Wan, Q. Liu, J. Liu, S. Liu, X. Liu, L. Zheng, et al., Iron atom–cluster interactions increase activity and improve durability in Fe–N–C fuel cells, Nat. Commun. 13 (2022) 2963. https://doi.org/10.1038/s41467-022-30702-z. Figure 1

New Materials for Alkaline Fuel Cells from Electroplating Industries Waste Solution Oxygen Reduction Reaction Catalyzed by Pd on Carbon Nanotubes, Graphene and Carbon Black

Issues concerning the energy sector are the focus of international discussions; the progressive depletion of fossil reserves and the increase in energy demand have led to the rise in fuel and electricity prices. In this context, fuel cells are promising candidates for a sustainable future. Oxygen Reduction Reaction (ORR) is the bottle-neck strategic reaction ruling the fuel cell efficiency process. This reaction allows the release of chemical energy in the form of electricity without producing any waste. However, the slow kinetics of ORR require highly effective electrocatalysts for proper boosting. It is well known that best available catalysts, in terms of raw bulk elements, belong to the so-called platinum group metals (PGMs). PGMs are very expensive, hence lowering the PGM content of ORR catalysts is among the most prevalent research focuses [1,2].We propose two different ways to obtain carbon-based ORR catalyst with a low continent of PMGs. The first approach involves the use of Single-Ion catalysts designed by carbon nanotubes functionalized with palladium(II) complexes show [3]. The second approach is totally innovative and aims at recovering precious metals to reduce industrial waste and promote the development of a circular economy. Large quantities of wastewater are produced in electroplating industries. This wastewater not only has to be properly processed before it is released into the environment, but also contains metals like palladium [4]. Using carbon materials and an electroless deposition, palladium can be recovered from the wastewater of the electroplating industry. We focus on carbon black (CB) and two different types of graphene-type materials named as (G4) and (G7). It was found that CB and G4 can recover up to 97% palladium. In this way we obtained hybrid materials containing palladium that could be promising candidates for use as catalysts for the ORR in alkaline fuel cells.New catalysts were treated by preparing a nafion polymeric dispersion and tested with electroanalytical techniques. Rotating ring-disk electrode (RRDE) experiments were performed to investigate the catalytic pathway of ORR and asses the number of exchanged electrons per oxygen molecule. Results confirm that tested materials promote the four-electron reaction pathway and reduce the formation of hydrogen peroxide. These approaches provide the basis for further developments to improve the efficiency of future low-cost catalysts for alkaline fuel cells.[1] Passaponti, M.; Lari, L.; Bonechi, M.; Bruni, F.; Giurlani, W.; Sciortino, G.; Rosi, L.; Fabbri, L.; Vizza, M.; Lazarov, V.K.; et al. Optimisation Study of Co Deposition on Chars from MAP of Waste Tyres as Green Electrodes in ORR for Alkaline Fuel Cells. Energies 2020, 13, 5646, doi:10.3390/en13215646.[2] Bonechi, M.; Giurlani, W.; Vizza, M.; Savastano, M.; Stefani, A.; Bianchi, A.; Fontanesi, C.; Innocenti, M. On the Oxygen Reduction Reaction Mechanism Catalyzed by Pd Complexes on 2D Carbon. A Theoretical Study. Catalysts 2021, 11, 764, doi:10.3390/catal11070764.[3] Savastano, M.; Passaponti, M.; Giurlani, W.; Lari, L.; Calisi, N.; Delgado-Pinar, E.; Serrano, E.S.; Garcia-España, E.; Innocenti, M.; Lazarov, V.K.; et al. Linear, Tripodal, Macrocyclic: Ligand Geometry and ORR Activity of Supported Pd(II) Complexes. Inorganica Chim. Acta 2021, 518, 120250, doi:10.1016/j.ica.2021.120250.[4] De Beni, E.; Giurlani, W.; Fabbri, L.; Emanuele, R.; Santini, S.; Sarti, C.; Martellini, T.; Piciollo, E.; Cincinelli, A.; Innocenti, M. Graphene-Based Nanomaterials in the Electroplating Industry: A Suitable Choice for Heavy Metal Removal from Wastewater. Chemosphere 2022, 292, 133448, doi:10.1016/j.chemosphere.2021.133448.

Cathode Loading Impact on PEM Fuel Cell Performance Losses in Voltage Cycling Based Accelerated Stress Tests

In order to reach the target of carbon neutrality by the end of 2050 set by the European Union, hydrogen technologies will play an important role in supporting the transition to a renewable energy economy, especially for heavy duty applications, such as trucks and airplanes.1 For the latter, hydrogen is a viable alternative fuel compared to kerosene, offering a higher specific energy density as well as zero in-flight CO2 emissions.2 However during operation, proton exchange membrane fuel cells (PEMFCs) suffer from significant performance losses due to the instability of Pt-based catalysts for the oxygen reduction reaction (ORR) via the loss of electrochemically active surface area (ECSA).3 This induces substantial decay in the H2/air performance mainly due to the increasing impact of the local O2 transport resistance (RO2,local) with decreasing cathode roughness factor (rf, in units of cm2 Pt cm-2 MEA), which is especially pronounced for low Pt-loadings.4 Thus, for heavy duty applications requiring long-term durability, higher Pt loadings are required to reduce the impact of RO2,local.In this study, we analyse the H2/air performance loss contributions during voltage cycling based accelerated stress tests (ASTs) using cathodes with different Pt-loadings. ASTs were performed using 5 cm2 MEAs with cathode Pt loadings of 0.2, 0.4, and 0.8 mgPt cmMEA -2 (48.6 wt%, TEC10V50E, Tanaka Kikinzoku K.K. (TKK)). Full characterization of the MEA performance characteristics after different numbers of voltage cycles, was performed by measuring: i) H2/O2 and H2/air polarization curves; ii) cathode rf, determined by cyclic voltammetry and CO stripping; iii) O2 transport resistances (RO2,total) via limiting current measurements; and, iv) proton conduction resistances in the cathode catalyst layer (RH+,cath) via electrochemical impedance spectroscopy (EIS). Voltage cycling was performed under H2/N2 (200/75 nccm) at 80 °C, 95% RH, and ambient pressure using square wave profiles with lower and upper potential limits of 0.60 and 0.95 V, respectively, with hold times of 1 s at each potential.A strong H2/air performance dependency on the cathode rf is visible for the same Pt/C cathode catalyst with varying Pt loading. As shown in Figure 1, the same H2/air performance is reached at similar cathode rf values, independent of number of voltage cycles. This is due to the observation that the RO2,local values only depend on the cathode rf and not on the electrode’s history.Finally, when choosing an end-of-life (EoL) criterion of a more than 20 mV deviation of the measured cell voltages at a given current density from the theoretical ORR kinetics losses (c.f. red line in Figure 1 and gray areas for accepted cell voltage range), this EoL criterion is reached for the differently loaded MEAs at the same cathode rf but after different amounts of aging cycles. For example, at 1.5 A cm2 MEA this 20 mV deviation is reached at a cathode rf of 50-70 mgPt 2 cmMEA -2. However, for the 0.8 mgPt cmMEA -2 Pt/C cathode more than 200.000 cycles are needed to reach this value, compared to only 5.000 cycles for the 0.2 mgPt cmMEA -2 loaded cathode. Consequently, a 4-fold increase of the the cathode Pt loading increases its voltage cycling stability by »40-fold.

Mechanisms of Stabilization and Degradation of Transition Metal Oxygen Electroreduction Catalysts with in-Situ Electrochemical Flow Cell ICP-MS

Proton exchange membrane hydrogen fuel cell (PEMFC) deployment is presently limited by the high material cost of oxygen reduction reaction (ORR) electrocatalysts at the cathode. Developing low-cost, earth-abundant alternatives for use in acidic environments presents a material stability challenge, especially for long-term device operation. Platinum (Pt)-based materials are used in typical commercial PEMFC cathode formulations due to their simultaneous high performance and long-term stability despite the prohibitively high material cost. Developing lower-cost, longer-life catalyst materials based on earth abundant transition metals can accelerate the adoption of PEMFC technology. The impact of potential, current density, and local microenvironment has been shown to cause degradation of non-Pt materials but standard ex situ measurements can only probe surface restructuring and cumulative dissolution after operation. Material-specific degradation mechanisms that are strong functions of the operating conditions cannot be ascertained without in situ quantification of material degradation under PEMFC-relevant conditions. Information about the degradation pathways of these non-precious catalysts can assist with material design for devices; however, most efforts towards developing an in-situ understanding of PEMFC cathode catalyst degradation under operating conditions are generally limited to Pt-based materials. Extensive in situ characterization has revealed Pt-specific mechanistic information about corrosion pathways as well as conditions that exacerbate Pt alloy leaching. It has been hypothesized that the Pt degradation processes are related to oxide formation under potential cycling. With this critical in-situ information, Pt conditioning protocols have been developed to enhance material lifetime; however, investigations on non-precious materials are less common. Insights into non-precious catalyst degradation in PEMFC operating conditions are not as well-established but could be essential in enabling low-cost materials for commercial use. There is a need for more insights into a vast array of materials which necessitates the development of faster, preferably benchtop techniques for degradation analysis.In this work, we use an in situ electrochemical flow cell assembly and coupled it with an inductively coupled plasma-mass spectrometer (ICP-MS) to quantify the loss of material in real time for several non-Pt materials under reaction conditions. The flow cell is fed by a peristaltic pump at 2.5 mL/min of gas-saturated acidic electrolyte for optimal mass transport. The cell has a three-electrode configuration with a compression-mounted metal foil working electrode, silver/silver chloride reference electrode, and Pt counter electrode downstream of other components. We evaluate a variety of transition metals across the spectrum of ORR activity: palladium, silver, nickel, copper, manganese, and cobalt. Each metal is tested under oxygen and nitrogen saturation in five electrolytes of identical pH: perchloric acid, sulfuric acid, nitric acid, hydrochloric acid, and hydrobromic acid. This setup allows for nearly real time measurement of ORR kinetics and in situ element-specific dissolution rates as a function of time. With careful design of experimental conditions and controlled variables, we are able to suggest mechanisms of degradation with specific reference to conditions under which material loss occurred such as high/low potential, oxygen-saturated/nitrogen-saturated electrolyte, or faradaic/non-faradaic current. We find some metals are stable under nitrogen-saturated electrolyte but corrode more under oxygen-saturated electrolyte only while ORR active. Furthermore, other metals are stable in nitrogen saturated electrolyte at all tested electrochemical conditions but unstable under oxygen saturated electrolyte but only outside the ORR range, implying that cathodic faradaic processes might be stabilizing the surface at low potentials. This stabilization where a thermodynamically unstable material (based on its Pourbaix diagram) exhibits greater immunity while performing ORR in certain electrolytes could inform the development stability mechanisms that direct the design of next generation, non-precious PEMFC cathode catalyst materials. With combined in situ catalyst stability analysis in five pH 1 electrolytes, we can recommend specific conditions for optimizing activity and stability based on the potential range of operation and the electrolyte composition/saturation. We can also distill mechanistic insights from these experiments by creating models that validate causes of catalyst degradation. These models are built using “graphical causal modeling”, which is a mathematical field that goes beyond correlative predictions by venturing into causal inference. We hope to expand this framework into other electrocatalytic reactions of interest. The in-situ ICP-MS electrochemical flow cell setup exhibits promise for accelerating materials stability analysis for enabling next generation Pt-free ORR electrocatalysts through practical experiments and direct comparability to established electrochemical techniques.

Temperature-Dependent Investigation of ORR Kinetics on Non-Precious Metal-Free Catalysts in Alkaline Media - Doping Effect on Electrocatalysis

The kinetic analysis of oxygen reduction reaction (ORR) on non-precious catalysts is cumbersome due to the formation of more than one major products ( and ). In the present study, ORR is investigated on Vulcan XC-72 (C) and a metal-free nitrogen-doped (~≤ 1%) carbon (N/C-900) catalysts in 0.1 M KOH [1]. The experiments are conducted in the temperature range of 293–323 K using a rotating ring disk electrode (RRDE) assembly to quantify the product distribution, viz., and . The measured kinetic current () is partitioned into and on the basis of the potential-dependent product distribution ( and ) .and is used in the estimation of kinetic parameters [2]. The α values are derived from Butler-Volmer (B-V) theory, Tafel equation at high overpotentials (η), and from the derivative of the change in Gibbs free energy of activation () with respect to η. The α values obtained from all the three methods are different. Therefore, the origin of the difference in the α values is the subject of the present investigation. The seemingly unreasonable values of (> 1.0)on N/C-900 may be due to the alternative lower energy path for the formation of activated complex along with the change in the free energy of the reactant [3,4]. On C, it does not provide any alternative pathway for the formation of activated complex and perhaps η influences only the energy state of the reactant. Therefore, the free energy of activation for the formation of activated complex is a stronger function of η on N/C-900 and an indication of strong electrocatalysis. References B. Bera, T. Kar, A. Chakraborty, M. Neergat, J. Electroanal. Chem., 805, 184–192 (2017).R. Das, D. Choudhury, R. Maurya, S. Sharma, M. Neergat, Langmuir,39 (12), 4351–4361 (2023).U. Stimming, J. Wang, A. Bund, ChemPhysChem, 20 (22), 3004–3009 (2019).R. Maurya, R. Das, A. K. Tripathi, M. Neergat, Phys. Chem. Chem. Phys., 25, 700–707 (2023). *Corresponding author: Tel.: +91 22 2576 7893; Fax: +91 22 2576 4890E-mail: nmanoj@iitb.ac.in (M. Neergat)

Metal–organic framework derived phosphide-hydroxide heterostructure as bifunctional oxygen electrocatalyst for zinc–air batteries

Rational design and fabrication of high-efficiency electrocatalysts is essential for improving the kinetics of both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), so as to constructing high-performance rechargeable zinc-air batteries (ZABs). Among the promising candidates, transition metal-based (oxy)hydroxides have attracted extensive attention for their considerable catalytic potential on oxygen-involved reaction, while unsatisfactory intermediate adsorption/desorption strength and low conductivity limit their practical feasibility. In this work, interface engineering modification strategy is proposed, and a metal–organic framework derived heterostructure catalyst of CoFeOxHy-CoP@CC is fabricated. Comprehensively analyses demonstrate that the interaction between phosphide-(oxy)hydroxide induces the interfacial electronic redistribution, while the formed Schottky heterostructure facilitates rapid electron transfer. All these kinetically favorable for both OER and ORR, and leads to remarkable bifunctional performance on CoFeOxHy-CoP@CC, with the OER overpotential of 240 mV to deliver the current density of 10 mA·cm−2, which is better than CoP@CC, CC and RuO2. In practical applications, liquid zinc-air cells assembled by CoFeOxHy-CoP@CC deliver the peak power density up to 117.55 mW cm−2, which is close to that of 20% Pt/C catalyst (108.48 mW cm−2), and considerable operational performance has also been achieved in all solid-state flexible ZABs.

Accelerated Confined Mass Transfer of MoS2 1D Nanotube in Photo-Assisted Metal-Air Batteries.

Applying solar energy into energy storage battery systems is challenging in achieving green and sustainable development, however, the efficient progress of photo-assisted metal-air batteries is restricted by the rapid recombination of photogenerated electrons and holes upon the photocathode. Herein, a 1D-ordered MoS2 nanotube (MoS2-ONT) with confined mass transfer can be used to extend the lifetime of photogenerated carriers, which is capable of overcoming the challenge of rapid recombination of electron and holes. The tubular confined space cannot only promote the orderly separation and migration of charge carriers but also realize the accumulation of charge and the rapid activation of oxygen molecules. The concave surface of MoS2-ONT can improve the carrier separation ability and prolong the carrier lifetime. Meanwhile, the ordered tubular confined space can effectively realize the rapid transfer of charge, ion, and oxygen. Under light irradiation, a fast oxygen reduction reaction kinetics of 70mWcm-2 for photo-assisted Zn-air battery is achieved, which is the highest value reported for photo-assisted Zn-air batteries. Significantly, the photo-assisted Li-O2 battery based on MoS2-ONT also shows superior rate capability and other exciting battery performance. This work shows the universality of the confined carrier separation strategy in photo-assisted metal-air batteries.

Enhancing ORR activity and CO2 tolerance of Pr0.4Sr0.6Co0.2Fe0.8O3-δ-based SOFC cathode through synergistic doping and surface modification

The development of cathodes with high catalytic activity and stability remains a critical challenge for the commercialization of solid oxide fuel cells (SOFCs). In this study, a novel approach combining doping and surface modification was employed to enhance the catalytic activity and CO2 resistance of Pr0.4Sr0.6Co0.2Fe0.8O3-δ (PSCF) cathode. Initially, Ni was partially substituted for Fe in PSCF, resulting in the reduction of the average bond energy of metal–oxygen, thereby increasing oxygen vacancy concentration and electrochemical activity. The polarization resistance of PSCFN0.1 at 800 °C was found to be 0.025 Ω·cm2, representing a 32.4 % reduction compared to PSCF. Subsequently, Pr(NO3)3 solution was impregnated on the surface of the PSCFN0.1 using a spin-coating technique, resulting in the formation of a PrO2/PSCFN0.1 heterostructure that significantly improved the oxygen reduction reaction (ORR) kinetics. At 800 °C, the polarization resistance of PrO2-PSCFN0.1 was only 0.016 Ω·cm2, a 56.8 % reduction compared to PSCF. Moreover, the peak power density of PrO2-PSCFN0.1 reached 1190 mW·cm−2 at 800 °C, representing a substantial increase of 207.4 % compared to PSCF. Furthermore, PrO2-PSCFN0.1-based full cell exhibited excellent performance stability and CO2 resistance. This work provides an effective strategy to explore novel cathodes for SOFCs.

Mechanistic analysis of the electro-reduction of oxygen to water on electrode surfaces partially inhibited by electro-oxidation

A mechanistic model for the oxygen reduction reaction (ORR) on noble-metal electrodes was proposed, which considers the possible inhibition of active sites by electro-oxidation and the electro-reduction of dissolved oxygen to water on non-inhibited sites through the simultaneous occurrence of two parallel routes. Equations for describing steady state ORR polarization curves were derived by solving this mechanism without aprioristic restrictions on rate-determining steps. In this model the electro-oxidation of the metallic surface sites occurs in parallel with the ORR, causing the formation of subsurface oxides that block the active sites capable to adsorb ORR intermediates. For accounting the potential-dependent inhibition, metal electro-oxidation was described through a single-step reaction, assuming Frumkin-type interactions of the oxidized metal sites. On the other hand, reaction-rate equations were derived for the ORR operating through the simultaneous occurrence of two routes, so-called the dissociative (DP) and the associative (AP) pathways, which involve the adsorption of oxygen on the non-inhibited active sites as Oad and OOHad, respectively. An analysis of the effects of elementary kinetic and adsorption parameters allowed to visualize the effects that the electro-oxidation reaction causes, together with the ORR kinetics, on the polarization curves, particularly at low overpotentials. The obtained expressions were used to correlate reported experimental polarization curves measured on polycrystalline Pt microelectrodes in acid and alkaline media. It was verified that the DP is operative at low overpotentials and the current tends to reach a kinetically controlled limiting current just before the AP becomes effective. Additionally, it was concluded that while the ORR performance of Pt could be improved by shifting the electro-oxidation reaction toward more anodic values, it would be more efficient to increase its kinetic capacity for cleaving oxygen as Oad and for reducing its protonated form (OHad) to water.