Metalloporphyrin- and metallocorrole-based catalysts for the oxygen reduction reaction: from molecules to materials.

Defective or heteroatoms-doped metal-free carbon materials (MFCMs) are regarded as efficient oxygen reduction reaction (ORR) catalysts in the past decade. However, the active site structures for ORR in MFCMs are hard to be precisely confirmed and controllably synthesized through the common methods such as high temperature pyrolysis or heteroatom doping process. To verify the precise structure acted as active center for ORR, we firstly report two crystalline metal-free thiophene-sulfur covalent organic frame-works (MFTS-COFs) as ORR catalysts. The MFTS-COFs show more positive catalytic capability than that of the thiophene-free COF, indicating pentacyclic thiophene-sulfur building blocks acted as active centers induce ORR catalytic activity. MFTS-COFs with higher numbers of thiophene-sulfur exhibits better ORR performance. The experimental identification is supported by density functional theory calculations. These results thus demonstrate that rational design and precise synthesis of metal-free crystal-line organic materials can promote the development of new ORR catalysts.

A bifunctional oxygen reduction reaction (ORR)/ oxygen evolution reaction (OER) catalyst is essential for rechargeable metal-air batteries and regenerative fuel cells. Platinum (Pt) and iridium oxide (IrO2) are the state-of-the-art ORR and OER catalysts, respectively. However the high price and scarcity of these platinum group metals (PGMs) has been an obstacle for wide spread application of these catalysts. Recently, in alkaline media, carbon based ORR and perovskite OER catalysts have demonstrated similar or even better catalytic activities compared to the counterpart PGM catalysts [1, 2]. Therefore, if we combine these two non-PGM catalysts, a non-PGM bifunctional ORR/OER catalyst can be obtained. A hindrance in this approach is the vulnerability of carbon-based ORR catalysts to oxidation in the OER potential range, i.e., potentials > 1.5 V vs. RHE. Thus development of robust ORR catalysts under practical OER conditions is a key to realize this kind of bifunctional catalysts. The carbon support used in our ORR catalysts was black pearl (BP) 2000 [1]. In preliminary tests, however, we found that BP 2000 undergoes oxidization at potentials around ca. 1.2 V vs. RHE and above (data not shown). In this work, we used reduced graphene oxide (rGO) as an alternative support to synthesize oxidation resistant ORR catalysts. The OER catalyst we chose was a perovskite (La1-xSrx)CoO3-δ (LSC). Pre-synthesized LSC was added into the initial solution of the rGO based ORR catalyst synthesis process, and after drying and heat-treatment, bifunctional (LSC + rGO) catalysts were obtained. In measuring the OER activity of the LSC catalyst, acetylene black (AB) carbon was added to the LSC (LSC + AB) to increase the electrical conductivity. Fig. 1 shows the comparison of ORR/OER activities between (LSC + AB) and (LSC + rGO). As expected, the ORR activity of (LSC + rGO) is greatly improved by ca. 200 mV in terms of E½ , in comparison to that of (LSC + AB). Interestingly even the OER activity of (LSC + rGO) becomes higher than that of (LSC + AB). Thanks to the enhancement of both ORR and OER activities with (LSC + rGO), highly active bifunctional catalysts are obtained. In this talk, material analysis results and diverse electrochemical performances of the (LSC + rGO) catalysts will be presented. Acknowledgements Support from the Directed Research of the Los Alamos National Laboratory’s Laboratory Directed Research & Development (LDRD-DR) is greatly acknowledged. References Chung et al., Nat. Commun. 4, 1922 (2013).Suntivich et al., Science, 334, 1383 (2011). Figure 1

Metal-free catalyst for efficient pH-universal oxygen reduction electrocatalysis in microbial fuel cell

Extensive research has been undertaken to make intermittent clean and renewable energy sources more reliable. Bifunctional oxygen cathodes which can catalyze both ORR (oxygen reduction reaction) and OER (oxygen evolution reaction) are the backbone of rechargeable metal-air batteries as well as regenerative fuel cells [1-3]. Although noble metal family elements and alloys such as Pt , Pt-Au and Pt-Co are known as the best catalysts for ORR in alkaline media, their poor electrocatalytic activity toward OER as well as high price, comparing to non-PGM (non-precious group metal) compounds, limit their usage as a cost effective and active catalyst in bifunctional oxygen cathodes [2, 4]. Manganese oxides have been vastly employed as a robust cost-effective multifunctional and environmental friendly electrode material in battery industry, from primary to rechargeable metal-air batteries, as well as alkaline fuel cells and capacitors [3]. The electrolytic manganese dioxide (γ-MnO2) is known as the most electrochemically active crystallographic form of MnO2 for ORR in alkaline media with tafel slope of 40 mV dec-1 and a low overpotential of -375 mV [1, 5]. However, poor OER electrocatalytic activity of MnOx-based oxides in alkaline media diminish the hope of finding an exclusive bi-functional catalyst for both ORR and OER [1].Our research is aimed at the development of highly active MnO2-based catalysts for both ORR and OER with long cycle life by adding non-precious metal compounds as co-catalysts and intercalating potassium ions into the catalyst layer. The mechanisms for OER and ORR of the mixed catalysts and the role of K+are investigated by a combination of surface characterization methods and electrochemical techniques.In the present work, LaCoO3 were synthesized via co-precipitation methods. MnO2-based Gas diffusion electrodes (GDE) were then prepared using the method described elsewhere [1]. In order to intercalate the potassium ions into the catalyst layer, a simple yet effective method has been used. The GDEs were kept in in 6M KOH solution at open cicuit potential while rotating the sample at 400 RPM for 6 days at 313 K.Cyclic voltammetry tests were performed in O2 saturated 6 M KOH at 293 K to investigate the electrocatalytic activity of the MnO2-LaCoO3 catalyst for both OER and ORR. The longer-term durability of the electrodes was also investigated by performing 100 repeated OER-ORR voltammetric cycles. Electron energy loss spectroscopy (EELS) has been also used to further analyze the catalyst layer and study the Mn valance changes during both OER and ORR. X-ray photoelectron spectroscopy (XPS) was also used to confirm the existence of intercalated K+ions in the catalyst layer.Interestingly, huge enhancement is observed in the OER/ORR performance of the activated catalyst comparing to the fresh electrodes without any K+ activation. Fig 1-a and 1-b show the OER and ORR voltammograms, respectively, of the fresh and activated MnO2-LaCoO3 GDEs as well as fresh MnO2 electrodes. The OER overpotential for MnO2-LaCoO3 decreases to 193 from 367 mV after intercalating potassium ions in the catalyst layer (Fig. 1-a). In the ORR region, activated MnO2-LaCoO3 electrodes still provides the lowest ORR overpotential of -321 mV (Fig. 1-b), which is far better than the ORR behavior reported in the literature for Core-Corena Bifunctional Catalyst (CCBC), needle-like MnOx thin film, CoMn2O4 and even nanostructured Mn oxide thin film [1].Moreover, the activated catalyst performs almost without any degradation in OER region while suffering up to 61% decrease in the ORR current density at -250 mV (vs MOE) after 100 cycles of severe durability tests. Our results indicate that even 12 hrs of activation in 6 M KOH could enhance the electrocatalytic activity of the degraded catalyst after severe cycling to some point, mentioned before as “healing effect” [1]. The EELS analysis reveals that the Mn valance decreases during severe cycling from about 4 to 2.7 for the cycle No. 1 and 100, respectively. This confirms that the loss in the ORR electrocatalytic activity of the activated sample after 100 cycles is associated with the transformation of MnO2 to Mn3O4during severe cycling.

Electrochemical methods are promising technical routes for future clean energy storage and conversion. Most of the electrochemical methods involve oxygen reactions. Unfavorable kinetics and sluggish reactions are the main challenges for these processes. We report here a facile synthesis of highly efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts. The catalysts are synthesized through the fine-tuning of metal ions (M, specifically Co, Ni, Zn, and Cu) in Prussian blue analogues (PBAs) and thus termed as M-PBAs. The CoNi-PBA-2 catalyst shows the highest activity toward OER with an onset potential at 280 mV and a Tafel slope of 63 mV dec-1. Zn-PBA catalysts demonstrate high selectivity in two-electron-transfer ORR. The H2O2 yield is as high as 88% at 0 V vs RHE. Density functional theory (DFT) calculations also confirm the high selectivity of Zn-PBA toward H2O2 in ORR.

In recent years, the development of sustainable and environmentally friendly catalysts for various electrochemical processes has become a major focus in the fields of energy storage and fine chemicals. Efficient and cost-effective oxygen reduction reaction (ORR) catalysts are crucial for the advancement of fuel cells and metal-air batteries. This study explores the use of rice husk-based porous carbon (RHPC) with a hierarchically porous structure as a support material for sustainable ORR catalysts. The performance of RHPC was compared with other commercial carbon materials, such as acetylene black (AB) and coconut shell carbon (YP-50), evaluating key properties including particle size, specific surface area, oxygen-containing functional groups, degree of graphitization, and hydrophilicity/hydrophobicity. Compared to AB, which has higher conductivity, and YP-50, which has a greater abundance of oxygen functional groups, RHPC demonstrated significant advantages as a catalyst support. The resulting Fe-NS/RHPC catalyst exhibited high activity (E1/2 = 0.858 V vs RHE, J = 4.83 mA cm−2), outperforming the standard Pt/C (E1/2 = 0.844 V vs RHE, J = 4.99 mA cm−2). When tested in a liquid Zn-air battery, the Fe-NS/RHPC catalyst achieved a peak power density of 116.2 mW cm−2 and a capacity of up to 792.5 mAh g−1.

Developing efficient and anti-corrosive oxygen reduction reaction (ORR) catalysts is of great importance for the applications of proton exchange membrane fuel cells (PEMFCs). Herein, we report a novel approach to prepare metal oxides supported intermetallic Pt alloy nanoparticles (NPs) via the reactive metal-support interaction (RMSI) as ORR catalysts, using Ni-doped cubic ZrO2 (Ni/ZrO2) supported L10-PtNi NPs as a proof of concept. Benefiting from the Ni migration during RMSI, the oxygen vacancy concentrations in the support are increased, leading to an electron enrichment of Pt. The optimal L10-PtNi-Ni/ZrO2-RMSI catalyst achieves remarkably low mass activity (MA) loss (17.8 %) after 400,000 accelerated durability test cycles in a half-cell and exceptional PEMFC performance (MA=0.76 A mgPt -1 at 0.9 V, peak power density=1.52/0.92 W cm-2 in H2-O2/-air, and 18.4 % MA decay after 30,000 cycles), representing the best reported Pt-based ORR catalysts without carbon supports. Density functional theory (DFT) calculations reveal that L10-PtNi-Ni/ZrO2-RMSI requires a lower energetic barrier for ORR than L10-PtNi-Ni/ZrO2 (direct loading), which is ascribed to a decreased Bader charge transfer between Pt and *OH, and the improved stability of L10-PtNi-Ni/ZrO2-RMSI compared to L10-PtNi-C can be contributed to the increased adhesion energy and Ni vacancy formation energy within the PtNi alloy.

Developing high performance electrocatalysts to improve the sluggish kinetics of oxygen reduction reaction (ORR) at the cathode is essential in expanding fuel cell applications. Metal organic frameworks (MOFs), given their high surface area, tunable pore size, and diverse metal choices, are promising candidates to catalyze such a reaction. Herein, we report a family of nine 2D-phthalocyanine MOF-based catalysts for alkaline ORR, where M = Co, Ni, Cu. Structurally, these bimetallic materials have two distinct metal sites, where M1 is a phthalocyanine moiety and M2 is a node, existing as M-N4 moiety. X-ray Diffraction (XRD) shows that all as-prepared MOFs have similar structures and inter-layer spacing. X-ray Photoelectron Spectroscopy (XPS) shows slight shifts in binding energy when the same element is placed in a M1 or M2 site, which indicates that the M1 and M2 sites are chemically distinct.Through cyclic voltammetry we evaluate the role of the metal sites for ORR activity and selectivity in 0.1 M KOH. All materials are active for alkaline ORR, while Ni (M1) – Co (M2) has the best overall kinetic activity performance of - 5 mA cm-2 at 0.7 V vs. RHE. Cu – Cu, on the other hand, is the least active combination under the same testing conditions. Within composition series where M2 = Co, the activity trend for M1 is Ni > Cu ~ Co. From a 2-electron selectivity perspective, M1 = Ni > Cu > Co. From these general trends we hypothesize that Co M2 and Ni M1 sites are essential in promoting 4- and 2-electron selectively, respectively. On average we find a nominal ORR performance trend where M2 = Co > Cu > Ni. Interestingly, we note that the M1 identity also plays a non-linear role in activity and selectivity modulation. Though we identify one of the metal sites (M1 or M2) as the dominating site for activity and/or selectivity, the other site also contributes to the total performance. The density functional theory (DFT) calculation supports our experimental results and finds out that Ni as M1 is highly selective for H2O2 formation whereas Co as M2 shows excellent activity for ORR. Our predicted volcano plot shows that MOF with Ni – Co and Cu – Co combinations at Co active site offering lowest ORR overpotential among all other combinations, which is also in good agreement with experimental findings.By evaluating different metal sites performance through electrochemical testing, we can better understand the active species in the MOF catalytic system. By pairing theory with experiment, we gain active site insight to design new specific enhanced active motifs. We believe that these performance trends can assist in better designing efficient MOF-based ORR catalysts that can lead to advances in clean energy technology for sustainable development.

Surface-oxidized Fe–Co–Ni alloys anchored to N-doped carbon nanotubes as efficient catalysts for oxygen reduction reaction

Electronic structure regulation of noble metal-free materials toward alkaline oxygen electrocatalysis

Etching engineering on controllable synthesis of etched N-doped hierarchical porous carbon toward efficient oxygen reduction reaction in zinc–air batteries

Achieving the optimal balance between cost-efficiency and stability of oxygen reduction reaction (ORR) catalysts is currently among the key research focuses aiming at reaching the broader implementation of proton-exchange membrane fuel cells (PEMFCs). To address this challenge, we combine two well-established strategies. Specifically, we prepare ternary PtNi–Au alloys, where each alloying element plays a distinct role: Ni reduces costs and boosts ORR activity [1], while Au enhances stability [2,3].A systematic comparative analysis of the activity-stability relationship for compositionally tuned PtNi–Au model layers, prepared by magnetron co-sputtering, was conducted using a diverse range of complementary characterization techniques and electrochemistry [4]. Our study reveals that a progressive increase of the Au concentration in the Pt50Ni50 alloy catalyst from 3 to 15 at.% led to opposing activity and stability trends. Specifically, we observed a decrease in the ORR activity accompanied by an increase in catalyst stability, manifested in the suppression of both Pt and Ni dissolution. Despite the reduced activity compared to PtNi, the PtNi–Au alloy with 15 at.% Au still exhibited nearly three times the activity of monometallic Pt. It also demonstrated significantly improved stability against Pt and Ni dissolution compared to the PtNi alloy and even monometallic Pt.Our findings provide valuable insights into the intricate balance between activity and stability in multimetallic ORR catalysts, guiding the design of cost-effective and durable catalysts for PEMFCs cathode.[1] Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K. Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts. Nat. Chem. 2009, 1 (7), 552–556.[2] Lopes, P. P.; Li, D.; Lv, H.; Wang, C.; Tripkovic, D.; Zhu, Y.; Schimmenti, R.; Daimon, H.; Kang, Y.; Snyder, J.; et al. Eliminating Dissolution of Platinum-Based Electrocatalysts at the Atomic Scale. Nat. Mater. 2020, 19 (11), 1207–1214.[3] Xie, X.; Briega-Martos, V.; Farris, R.; Dopita, M.; Vorokhta, M.; Skála, T.; Matolínová, I.; Neyman, K. M.; Cherevko, S.; Khalakhan, I. Optimal Pt–Au Alloying for Efficient and Stable Oxygen Reduction Reaction Catalysts. ACS Appl. Mater. Interfaces 2023, 15, 1192–1200.[4] Xie, X.; Briega-Martos, V.; Alemany, P.; Mohandas Sandhya, A.L.; Skála, T.; Gamón Rodríguez, M.; Nováková, J.; Dopita, M.; Vorochta, M.; Bruix, A.; Cherevko, S.; Neyman, K.M.; Matolínová, I.; Khalakhan, I. Balancing activity and stability through compositional engineering of ternary PtNi–Au alloy ORR catalysts.ACS Catal. 2025, 15, 234–245

Bimetallic niobium-iron oxynitride as a highly active catalyst towards the oxygen reduction reaction in acidic media

Considerable efforts have been exerted in the development of non‐noble metal catalysts (NNMCs) for oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs). The effects of the preparation strategy, including carbon support, metal and nitrogen precursors, as well as heat‐treatment conditions, on the ORR activity for such NNMCs have also been extensively explored. In this review, we mainly focused on the recent advances in carbon‐incorporated NNMCs, specifically carbon‐incorporated iron nitride (FeCN)‐based catalysts. Influences of pyrolysis temperature on the crystalline and local structures, chemical environment, morphology, and ORR activity of FeCN‐based catalysts were discussed, and the ORR mechanism was also proposed.

Efficient oxygen reduction reaction (ORR) catalysts play a key role in the development of electrochemical energy devices, such as fuel cells and metal-air batteries. In order to replace or reduce the usage of the precious Pt-based ORR catalysts, carbon nanotubes (CNTs) have recently been explored as efficient ORR electrocatalysts due to their unique electrical and thermal properties, wide availability, corrosion resistance, and large surface area. In particular, the electrocatalytic activities of CNTs can be significantly enhanced by heteroatom doping, especially nitrogen doping. This chapter focuses on recent progresses in the development of doped CNTs as advanced ORR electrocatalysts. An emphasis is placed on recent breakthroughs in the design and development of various heteroatom (e.g., N, B, P, and S)-doped CNTs with high electrocatalytic activities, along with the experimental observation and fundamental understanding of the active sites and catalytic mechanism for ORR.