Oxygen Reduction Reaction Overpotential Research Articles

First-principles calculation on effects of oxygen vacancy on α-MnO2 and β-MnO2 during oxygen reduction reaction for rechargeable metal-air batteries

MnO2 is widely applied as oxygen reduction reaction (ORR) electrocatalysts in different metal-air batteries (MABs). Enhancing the ORR activity of MnO2-based catalysts is necessary for improving the performance of MABs. Defect-engineering of catalyst materials is a key approach for enabling the high performance of ORR. Here, the defect-engineering of α-MnO2 (211) and β-MnO2 (110) by oxygen vacancy (OV) is investigated using the first-principles density functional theory calculation. The geometric structure, adsorption, electronic conductivity, and oxygen reduction reaction (ORR) activity are studied. As a result, the OV induces the geometric structure that the Mn–Mn and Mn–O distances are closer when the catalysts lose the oxygen atom(s) on the top-layer surfaces. The presence of OV not only enhances the adsorption energy of *OOH, *O, and *OH, but also increases the electronic conductivity analyzed via the electron transfer. The Bader charge analysis demonstrates that the Mn(IV) can be altered to Mn(III) by the electron accumulation from OV. The volcano plot of ORR overpotential indicates that having the excess OV concentration on MnO2 surfaces cannot enhance the ORR activity. The excellent activity is yielded by 12.50 % OV α-(211) and 66.66 % OV β-(110) with the ORR overpotential of 0.31 V and 0.60 V, respectively. The results demonstrate that Ov is an essential parameter defining the existence of Mn(III) and Mn(IV) on the surface of MnO2-based catalysts. The optimal ratio of Mn(IV):Mn(III) is in challenge developing the α-MnO2 and β-MnO2 electrocatalyst cathode for metal-air batteries. This study provides a guideline for developing the potential cathode catalyst for MABs, used for harvesting energy.

Wood-Derived Monolithic Catalysts with the Ability of Activating Water Molecules for Oxygen Electrocatalysis.

Oxygen reduction reaction (ORR) is the key reaction on cathode of rechargeable zinc-air batteries (ZABs). However, the lack of protons in alkaline conditions limits the rate of ORR. Herein, an activating water strategy is proposed to promote oxygen electrocatalytic activity by enhancing the proton production from water dissociation. FeP nanoparticles (NPs) are coupled on N-doped wood-derived catalytically active carbon (FeP-NWCC) to associate bifunctional active sites. In alkaline, FeP-NWCC possesses outstanding catalytic activities toward ORR (E1/2 = 0.86V) and Oxygen evolution reaction (OER) (overpotential is 310mV at 10mA cm-2 ). The liquid ZABs assembled by FeP-NWCC deliver superior peak power density (144mW cm-2 ) and cycle stability (over 450 h). The quasi-solid-state ZABs based on FeP-NWCC also display excellent performances. Theoretical calculation illustrates that the superb bifunctional performance of FeP-NWCC results from the elevated dissociation efficiency of water via FeP NPs to assist the oxygen catalytic process. The strategy of activating water provides a new perspective for the design of ORR/OER bifunctional catalysts. This work is a model for the application of forest biomass.

Low-carbon CeOx/Ru@RuO2 nanosheets as bifunctional catalysts for lithium-oxygen batteries

Lithium-oxygen (Li-O2) batteries are facing challenges in capacity, cycling stability, and kinetics for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Advanced catalysts should have a high and bifunctional catalytic activity for ORR and OER. Moreover, the trade-off carbon in catalysts should have good oxidation resistance and high electronic conductivity and be used in a tiny amount. Up to now, it is still a challenge. Therefore, this work introduces a low-carbon CeOx/Ru@RuO2 nanosheet as efficient cathode catalysts for Li-O2 batteries and studies the preparation and working mechanisms using a variety of characterization and electrochemical techniques. The air oxidation treatment oxidizes Ru into Ru@RuO2 and simultaneously removes unstable C into CO2, leaving a tiny amount of stable carbon. CeOx/Ru@RuO2 has enormous mesopores, well-distributed sub-5 nm CeOx and Ru@RuO2 nanocrystals, a tiny amount of anti-oxidation carbon (1.2 %), and a high specific surface area of 159.3 m² g−1. These aspects optimize the Li-O reaction and regulate the Li2O2 nucleation to form uniform and ultrathin Li2O2 nanoflakes. As a result, the ORR and OER overpotentials are only 0.17 V and 0.45 V, respectively. This work provides a novel material combination and structure design for developing bifunctional catalyst materials in Li-O2 batteries.

Study on oxygen reduction mechanism of S-doped Fe-NC monatomic catalyst

Heteroatom doped Fe-NC catalyst shows excellent performance in oxygen reduction reaction. In this work, density functional theory was used to study the mechanism of S atom doping on the regulation of electronic structure of Fe-NC monatomic catalyst and the promotion of oxygen reduction reaction. The stable configuration of Fe-NC catalyst after sulfur atom doping, the regulation of electronic structure of fen4 active site by S atom, and the mechanism of oxygen adsorption and oxygen reduction reaction were analyzed. The results show that doping a small amount of S atoms around the fen4 active site can improve the stability of the catalyst. The mechanism of S atom doping to improve oxygen reduction performance is as follows: (1) the doping of S atom reduces the band gap of the catalyst, improves the conductivity of the catalyst, and is conducive to the electrocatalytic oxygen reduction reaction; (2) the doping of S atom can improve the ability of the catalyst to adsorb oxygen, which is conducive to oxygen reduction reaction; (3) the introduction of four S atoms into the system can reduce the overpotential of oxygen reduction reaction and improve the catalytic activity of fen4 site for oxygen reduction reaction. This work may provide a new idea for the regulation of heteroatom doping on carbon-based monatomic catalysts.

Nanostructured Mn-doped Zn N C @reduced graphene oxide as high performing electrocatalyst for oxygen reduction reaction

Future fuel cells and metal-air batteries will benefit greatly from revealing self-supported electrodes with efficient oxygen reduction reaction (ORR) activity and durability. However, noble metal catalysts are expensive and unstable, necessitating research into novel metal-organic framework (MOF)-based topologies that can catalyse oxygen reduction reactions (ORR). In this work, the synthesis of nitrogen-doped carbon (NC) is derived from zeolitic imidazolate frameworks (ZIFs) and a novel Mn-doped ZnN[email protected] is synthesized using a self-templated solvothermal method and their performance in an alkaline medium for oxygen reduction reaction (ORR) is studied. The nanostructured composite outperformed the commercial Pt/C catalyst in terms of both material resources and application efficacy. The Mn/ZnNC @30% rGO exhibit outstanding performance for ORR in KOH, with a more positive cathodic peak of 0.78 V vs RHE and an onset potential of 0.97 V vs RHE, which are characteristics that suggest the possibility of reducing ORR overpotentials. The improved electrochemical performance, small Tafel slopes and methanol tolerance are ascribed to the interdependent effect of the N-doped carbon (NC) and the Mn/Zn active sites. Novel architecture, tunable porosities, template directed growth and remarkable electrocatalytic performance of Mn-doped ZnN[email protected] make it a good aspirant for energy applications.

Bifunctional catalytic activity of 2D boron monochalcogenides BX (X = S, Se, Te)

Photocatalysis and electrocatalysis are two sustainable and renewable technologies that can meet global energy demands in environmentally friendly ways. According to recent research, 2D boron monochalcogenides in the 1 T and 2 H phases are stable, strong, and broad bandgap semiconductors. Our calculations show a strong UV absorption ability and suitable band edge positions for water splitting oxidation and reduction, making it a good choice for an efficient photocatalyst. The development of bifunctional electrocatalysts has piqued the interest of researchers working in the field of electrocatalysts for fuel cells. The electrocatalytic properties of 2D boron monochalcogenides are also investigated for catalyzing both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). The calculated overpotentials for OER/ORR mechanism are found to be 0.92/1.09 for BS (1 T), 1.00/0.59 for BS (2 H), 0.96/1.05 for BSe (1 T), 0.92/0.85 for BSe (2 H), and 1.10/0.92 for BTe (1 T), which are close to benchmark catalysts. The ORR overpotential of BS (2H) is 0.59 V, near well-known catalyst Pt (0.45 V). Therefore, our investigations indicate that the family of 2D materials, boron monochalcogenides, are promising photocatalyst and electrocatalyst candidates for OER and ORR.

Computational Prediction of Graphdiyne-Supported Three-Atom Single-Cluster Catalysts

Computational Prediction of Graphdiyne-Supported Three-Atom Single-Cluster Catalysts

Shifting Target Reaction from Oxygen Reduction to Superoxide Disproportionation by Tuning Isomeric Configuration of Quinone Derivative as Redox Mediator for Lithium-Oxygen Batteries.

Quinones having a fully conjugated cyclic dione structure have been used as redox mediators in electrochemistry. 2,5-Ditert-butyl-1,4-benzoquinone (DBBQ or DB-p-BQ) as a para-quinone derivative is one of the representative discharge redox mediators for facilitating the oxygen reduction reaction (ORR) kinetics in lithium-oxygen batteries (LOBs). Herein, we presented that the redox activity of DB-p-BQ for electron mediation was possibly used for facilitating superoxide disproportionation reaction (SODR) by tuning the isomeric configuration of the carbonyl groups of the substituted quinone to change its reduction potentials. First, we expected a molecule having its reduction potential between oxygen/superoxide at 2.75 V versus Li/Li+ and superoxide/peroxide at 3.17 V to play a role of the SODR catalyst by transferring an electron from one superoxide (O2-) to another superoxide to generate dioxygen (O2) and peroxide (O22-). By changing the isomeric configuration from para (DB-p-BQ) to ortho (DB-o-BQ), the reduction potential of the first electron transfer (Q/Q-) of the ditert-butyl benzoquinone shifted positively to the potential range of the SODR catalyst. The electrocatalytic SODR-promoting functionality of DB-o-BQ kept the reactive superoxide concentration below a harmful level to suppress superoxide-triggered side reaction, improving the cycling durability of LOBs, which was not achieved by the para form. The second electron transfer process (Q-/ Q2-) of the DB-o-BQ, even if the same process of the para form was not used for facilitating ORR, played a role of mediating electrons between electrode and oxygen like the Q/Q- process of the para form. The ORR-promoting functionality of the ortho form increased the LOB discharge capacity and reduced the ORR overpotential.

Electrochemical Acceleration of Redox Reaction Cycles on the Surface of Fe2O3-MnO2 Cathode to Activate the Peroxymonosulfate for the Efficient Removal of Levofloxacin

Here, we establish an electrochemically promoted peroxymonosulfate (PMS) activation system based on Fe2O3-MnO2 cathode for the degradation of levofloxacin (LEV). Compared with the single materials Fe2O3 and MnO2, Fe2O3-MnO2 exhibits more active sites and excellent electrochemical characteristics, including faster electron transfer and lower overpotential of oxygen reduction reaction (ORR). The degradation efficiency of the system can reach 92.1% within 60 min under the optimal conditions of 30 mA cm−2, natural pH, 10 mM PMS, and 25 °C. The efficient degradation performance is due to the fact that electrochemistry can accelerate the electron transfer and further improve the cycle of the redox reactions of FeⅡ/FeⅢ and MnⅡ/MnⅢ/MnⅣ, thereby, activating PMS to generate more active species (e.g., ·OH, SO4 −· and 1O2 etc). And the 1O2 is found to be the main reactive substance. Besides, the degradation pathway of LEV is inferred based on the identification of reaction intermediates, including defluorination, decarboxylation, destruction of piperazinyl groups, and oxidation of quinolone rings. This research provides a reliable method for the effective removal of refractory organic pollutants.

Valence Engineering of Polyvalent Cobalt Encapsulated in a Carbon Nanofiber as an Efficient Trifunctional Electrocatalyst for the Zn-Air Battery and Overall Water Splitting.

The rapid development of electrochemical power systems has prompted high demand for nonprecious trifunctional electrocatalysts with superior performance, prolonged stability, and low cost for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). Here, a valence engineering strategy is devised to construct a morphology with polyvalent cobalt encapsulated in nitrogen-doped carbon nanofibers (Co/N-CNFs). The diverse cobalt valence states of the Co/N-CNF catalysts contribute to their excellent catalytic effect and high durability in multiple electrochemical processes. The optimal Co/N-CNF catalyst fabricated exhibits a high half-wave potential of ORR (0.86 V) and low overpotentials of OER (380 mV) and HER (241 mV) at 10 mA cm-2. The Co/N-CNF-based Zn-air battery possesses a high charge-discharge efficiency as well as a good cycle stability (50 h at 10 mA cm-2 and 120 h at 20 mA cm-2), much superior to the Pt/C-based batteries. Furthermore, the Co/N-CNF catalyst could perform efficient overall water splitting.

Co Nanoparticles Encapsulated in N‐Doped Carbon Nanotubes Grafted CNTs as Electrocatalysts for Enhanced Oxygen Reduction Reaction

AbstractThe exploration of high‐efficiency and inexpensive electrocatalysts for oxygen reduction reaction (ORR) is of critical significance for renewable energy conversion. Herein, an in situ catalytic transformation strategy toward a unique hierarchical nanostructure is reported. In the architecture, Co nanoparticles encapsulated at the tip of bamboo‐like N‐doped carbon nanotubes (NCNTs) are grafted on N‐doped polypyrrole‐derived CNTs. Thanks to the smart design of unique 3D architecture, the NPCN@Co‐NCNTs catalyst displays an extraordinary ORR activity in 0.1 m KOH solution (the onset and half‐wave potentials are 0.96 and 0.90 V vs RHE, respectively), which is similar to commercial Pt/C catalyst (0.99 and 0.88 V vs RHE, respectively). Meanwhile, the catalyst shows the low Tafel slope of 78 mV dec−1 and long‐time stability. Experimental and theoretical results verify that the improved ORR performance is mainly related to the existence of Co nanoparticles protected by pyridinic‐N‐doped carbon, which lowers the theoretical overpotential of ORR. Density functional theory calculations reveal that Pyridinic‐NCCo site is the reactive site with the lowest overpotential for ORR (0.57 V). These results unambiguously indicate that the NPCN@Co‐NCNTs represent a low‐cost yet high‐efficiency electrocatalyst for the electrocatalytic ORR.

First principles study on the oxygen reduction reaction of Ir@Pt core-shell structure

The core-shell nanostructure can combine the properties of the core and the shell, thereby improving the performance of the oxygen reduction reaction (ORR) catalyst. The [email protected] core–shell nanostructure (CSNS) is considered to be a potential ORR catalyst due to its properties both of Ir and Pt. In this paper, a complete series of [email protected] core-shell nanostructure with different exposure surfaces and monolayers (ML) of Pt shell are designed and scrutinized for ORR activity. The results show that the [email protected] CSNSs can effectively reduce the binding energy of all intermediates, and most CSNSs can reduce the overpotential of ORR. At the same time, the formation of CSNS will strengthen the association mechanism but weaken the dissociation mechanism. It is worth noting that the reduction in binding energy can not simply be the same as the reduction in overpotential. The strain effect and the electronic effect act on the change of the overpotential at the same time. However, the electronic effect plays the main role in the overpotential change caused by the change of the number of layers. The analysis of the deformed charge density and charge transfer shows that the electronic effect is only effective within 2ML, the reason for the decrease in binding energy is that a part of electrons in the Ir core transfer to the surface and combine with the intermediates. Among all CSNSs, [email protected] 1ML and 2ML have significantly reduced the overpotential of ORR, which indicates that [email protected] is a potential ORR catalyst.

Unveiling the HER and ORR activity origin of isolated Co sites supported on N-doped carbon

Electrocatalysis plays very important role in clean energy conversion. In which, developing high active and robust electrocatalysts for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) are highly crucial and challenging. Using density functional theory (DFT) calculations, the Gibbs free energy diagrams and electronic structure of N-graphene, Ir-N4 and Co-N4 are investigated in this work. The results show that Co-N4 have the appropriate H adsorption and lower ORR overpotentials, which gives expectation of high multifunctional electrocatalytic performance. This theoretical study provides vital insights into the enhanced electrocatalytic mechanism of Co sites supported on N-doped carbon.

Bipyridine-based polybenzimidazole as a nitrogen-rich ionomer and a platinum nanoparticle support for enhanced fuel cell performance

High density nitrogen supporters, in particular nitrogen-based polymeric supporters of metal catalysts is a topic of high interest to boost the catalytic activity of oxygen reduction reaction (ORR), and accordingly improve fuel cell (FC) performance. Here, we offer a promising membrane electrode assembly (MEA) of a nitrogen-rich and highly conductive polymer; namely, poly(bipyridine-5,5′-bibenzimidazole) (BipyPBI). The MEA comprised BipyPBI as the membrane and platinum(Pt)/BipyPBI-functionalized multiwalled carbon nanotubes as the electrodes. The fabricated materials characterized by NMR, XPS, TEM, XRD, and TGA, and catalytically evaluated by CV, LSV, current-potential, and power density measurements. The fuel cell performance diagnosed by electrochemical impedance spectroscopy (EIS). The use of BipyPBI provided highly active fuel cell electrodes with a homogenous distribution of the Pt catalyst on the surface of carbon nanotubes. Notably, the onset overpotential of ORR lowered by ∼10 mV, the half-wave potential positively shifted by a 41 mV, and the diffusion-limiting current increased by 36 mA/mgPt, when the BipyPBI-based electrode compared to the conventional PBI-based electrode. Importantly, the power density of BipyPBI-based MEA reached 0.893 W/cm2 (1.48 W/mgPt) compared to 0.549 W/cm2 (0.94 W/mgPt) for the PBI-MEA. The EIS results indicated an improvement in the ohmic and charge transfer resistances of BipyPBI-based MEA, thanks to the high density nitrogen structure of BipyPBI which enhanced the proton conduction and the reaction kinetics. These results enrich the fuel cell research, and stimulate researchers engaged in related fields.

Tunable properties of ZnSe/graphene heterostructure as a promising candidate for photo/electro-catalyst applications

The graphene-based Van der Waals (vdW) heterostructures show great potential for the next generation of photoelectronic nanodevices. Here, we constructed the ZnSe/graphene vdW heterojunction and investigated its structural, electronic, optical, transportable, and catalytic properties by using density functional theory (DFT). The results show that ZnSe is a direct semiconductor with a bandgap of 3.24 eV. The electron mobility of ZnSe is seven times higher than that of the hole, which is beneficial for the photogenerated hole-electron separation. Besides, the n-type Schottky to Ohmic contact transformation can be realized by applying an external electric field of 0.2 V/Å, and the underlying mechanism is discussed. Finally, the potential ability of photo/electrocatalysis for graphene/ZnSe heterostructure and electrocatalysis for ZnSe monolayer is also explored. The hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) can spontaneously activate in acid (PH = 1,2) and alkaline (PH > 9) media under light irradiation. The overpotential of oxygen reduction reaction reaches 0.42 eV, which indicates graphene/ZnSe heterostructure would be a potential electrocatalyst. For the HER process of ZnSe monolayer, the overpotential is significantly decreased to 0.23 eV by doping 3d transition metal atom. Consequently, we hope our work can provide useful guidance for designing practical ZnSe-based applications.

Mesoporous N-P Codoped Carbon Nanosheets as Superior Cathodic Catalysts of Neutral Metal-Air Batteries.

Development of high-efficiency oxygen reduction reaction (ORR) catalysts under neutral conditions has made little research progress. In this work, we synthesized a three-dimensional porous N/P codoped carbon nanosheet composites (CNP@PNS) by high-temperature thermal treatment of dicyandiamide, starch, and triphenylphosphine and subsequent porous structure-making treatment using the NaCl molten salt template. In the neutral solution, the electrocatalytic performance of the CNP@PNS-4 catalyst exhibits an onset potential of 0.98 V (vs reversible hydrogen electrode) and a half-wave potential of 0.91 V for ORR, which greatly surpasses commercial Pt/C (40%). Three kinds of neutral metal-air batteries (Zn-air, Al-air, and Fe-air) using the prepared samples as cathodic catalysts were constructed, corresponding to the maximum power density of 120.2, 78.3, and 18.9 mW·cm-2, respectively. Also, they reveal outstanding discharge stability under different current densities. The density functional theory calculation depicts the reduction of the free energy of the determining step and subsequent decline of the overpotential for ORR.

Noble Metal-Added Titanate Nanosheets for PEFC Cathode

A large overpotential of oxygen reduction reaction (ORR) at a cathode in polymer electrolyte fuel cells (PEFCs) is one of the serious problems to decrease the energy conversion efficiency. Nørskov’s group reported that even Pt which is currently used as an ORR catalyst cannot reach the theoretical oxygen electrode potential of 1.23 V at 298 K [1]. On the other hand, Yamamoto et al. recently suggested that noble metal doped titanium dioxides (TiO2) can reach the theoretical potential by changing the absorption energies of the ORR intermediates according to the first-principle calculations [2]. However, the first-principle calculations did not consider the continuous electron supply to proceed the ORR. It is difficult to form the sufficient electron conduction path from supports to active sites on the TiO2 because TiO2 has low conductivity due to a large band gap. Therefore, we focused on the formation of the active sites on the TiO2 surface by doping noble metal such as Rh and Pd and the electron conduction path from carbon supports to the active sites by preparing the titanate nanosheets.Na2Ti3O7 was synthesized as precursor of anionic titanate nanosheets([Ti3O7]2- ;TiNS) according to the previous paper [3]. Na2Ti3O7 was treated through ion-exchange reaction with hydrochloric acid and tetramethylammonium aqueous solution, finally TiNS were obtained. A TEM observation revealed that the formation of very large sheet-like materials with a size of approximately upwards of ten micrometers, resulting that TiNS were completely synthesized.Next, we focused on the formation of the electron conduction path through nanosheets from supports using the electron tunneling effect. When the nanosheets were directly supported on the glassy carbon electrode, the ORR activity was the same as that of GC, and almost no ORR current flowed even low potential region. Then, vapor grown carbon fibers (VGCFs) as purchased, with immersion of mixed acid (MA; H2SO4 and HNO3) at 170 oC for 24 h and PDDA (Poly(diallyl dimethylammonium chloride)) as polycation were attempted to use as supports. The TiNS were immersed in the distilled water dispersed with the treated VGCFs to be supported by them. These catalysts were designated as TiNS/VGCF, TiNS/VGCF(MA), and TiNS/VGCF(MA-PDDA).The ORR current of TiNS/VGCF(MA-PDDA) was larger than that of TiNS/VGCF, TiNS/VGCF(MA). In addition, the ORR onset potential of the TiNS/VGCF(MA-PDDA) increased by 0.3 V compared to that of TiNS/VGCF. These results suggest that surface treatment of VGCFs was effective to form the electron conduction path from VGCF to active sites of nanosheet surface. The electrostatic interaction between VGCF surface positively charged by PDDA and anionic TiNS contributed the formation of the electron conduction path. However, according to the TEM observation, TiNS were partially contacted with VGCF and some TiNS were agglomerated. It is necessary to obtain higher ORR activity that a control of size of TiNS and contact with supports such as VGCF.On the other hand, we attempted to dope Rh and Pd to TiNS through solid-state reaction. Rh-,Pd-doped TiNS (designated as Rh-TiNS and Pd-TiNS) were synthesized the same way as previous paper [3]. XRD patterns of the precursors revealed that Rh2O3 or PdO were existed with Na2Ti3O7 above the addition of 5 atomic %. Rh-TiNS and Pd-TiNS were supported by VGCF(MA-PDDA) after Rh-TiNS and Pd-TiNS were treated TMAOH aqueous solution. The ORR activities of the samples with the addition of 5at% Rh or 10at% Pd were higher than those of the samples which Rh2O3 or PdO were not observed in XRD patterns. The ORR active sites were estimated by Rh2O3 or PdO deposited on the surface of the TiNS. This result showed that an amount of doped Rh or Pd was limited to Na2Ti3O7. The limitation of doped Rh or Pd was 5at% or 10at%, respectively. Therefore, the doped amounts of noble metals such as Rh or Pd to TiNS should be appropriately controlled.AcknowledgementThe authors thank the New Energy and Industrial Technology Development Organization (NEDO) for financial support. This work was partly supported by TOKYO CITY UNIVERSITY Interdisciplinary Research Center for Nano Science and Technology for instrumental analysis.Reference(1) A. Kulkarni et al., Chem. Rev., 118, 2302 (2018).(2) Y. Yamamoto et al., J. Phys. Chem. C., 123, 19486 (2019).(3) W. Soontornchaiyakul et al., RSC Adv., 7, 21790 (2017). Figure 1

Solving the Trifunctional Activity Challenge of Catalysts in Unitized Regenerative Fuel Cells via 1T-MoS2-Coordinated Single Pd Atoms.

Developing high-efficiency and low-cost multifunctional electrocatalysts is the core of unitized regenerative fuel cells (URFC), yet it remains a great challenge. Here, by performing first-principles calculations, we report the atomic-level electrocatalytic activity mechanism of 3d, 4d, and 5d monoatomic transition metals (TM) bound to the 1T-MoS2 monolayer for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Their structural stabilities are evaluated via the formation energy, elastic constant, and molecular dynamics simulations. Compared with the Co–N4–C single atom catalyst (SAC), the resulting Pd@1T-MoS2 SAC exhibits better bifunctional catalytic activity, with OER overpotential as low as 0.43 V and an ORR overpotential of 0.40 V. The dual volcano plot demonstrates that the bifunctional OER and ORR activities of Pd@1T-MoS2 originate from the neither strong nor weak OH* adsorption and the suitable d-band center (−1.83 eV) of the Pd active center. In conjunction with the intrinsic activity of the 1T-MoS2 monolayer for hydrogen evolution reaction, the Pd@1T-MoS2 SAC is a competitive and promising trifunctional electrocatalyst for sustainable energy conversion and storage systems.

Boosting the catalytic efficiency of platinum nanoparticles supported on pristine carbon nanotubes: Synergistic effects of conducting polymers

An advanced energy conversion catalyst comprising pristine carbon-nanotubes and conductive polyaniline polymer successfully synthesized and characterized for high temperature and non-humidified polymer electrolyte membrane fuel cells. The fabricated polyaniline-based catalyst shown a good distribution of platinum nanoparticles (Pt-NPs), and displayed an electrochemical surface area of 645 cm2/mgPt. The catalyst indicated remarkable catalytic activity towards the oxygen reduction reaction (ORR). The overpotential of ORR, the open-circuit voltage, and the power density of the polyaniline-based catalyst declined by a 30 mV, increased by a 50 mV, and enhanced by 26.5%, respectively, when compared to the polybenzimidazole-based catalyst. The impedance measurements have emphasized the improved catalytic activity and the enhanced fuel cell performance of the polyaniline-based catalyst, showing a remarkable reduction of 45% in the charge transfer resistance. This study revealed the advantages of using polyaniline as an anchor to improve the Pt-catalytic activity, opening the door for the wide applications of hydrogen energy.

Insight into photoelectrocatalytic mechanisms of bifunctional cobaltite hollow-nanofibers towards oxygen evolution and oxygen reduction reactions for high-energy zinc-air batteries

• Synthesis of high-efficient hollow nanofibers of bifunctional photoelectrocatalyst. • Photoelectrocatalytic activity depends on band structures of catalysts. • Photogenerated electron and hole (NiCo 2 O 4 *) in the conduction band and valence band play very important role. • NiCo 2 O 4 exhibit the highest photoelectrocatalytic activity towards OER and ORR for high-performance Zinc-air batteries. • In situ XAS reveals that light can enhance both Ni and Co catalytic activities but Ni is found to be the main active species for OER. Using the photoelectrocatalyst to facilitate the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) for the charge storage enhancement of Zn-air batteries (ZABs) has not yet been widely investigated. This work presents bimetal cobaltite hollow nanofiber electrocatalysts as the photocathodes in the ZABs. Among all catalysts, the NiCo 2 O 4 hollow nanofiber exhibits superior performances towards OER and ORR. Under light irradiation (AM1.5), the NiCo 2 O 4 can absorb photon energy and generate free electrons and photogenerated carriers or holes, reducing the energy barrier enhancing the kinetic activity of OER/ORR. The NiCo 2 O 4 hollow nanofiber decreases the OER and ORR overpotentials for 14.3% and 8.1%, respectively and improves the OER and ORR kinetics for 19.3% and 20.8%, respectively compared with those under dark condition. The in situ electrochemical X-ray absorption spectroscopy results reveal that light can enhance both Ni and Co catalytic activities but Ni is found to be the main active species for the OER. The as-fabricated ZAB cell with the NiCo 2 O 4 photocathode exhibits the decreasing voltage gap from 0.85 V (Dark) to 0.64 V (Light) due to the highly active hole and the excited electron increasing the specific capacity up to 11.4% as compared to that under dark condition.