The Fenton-like process presents one of the most promising strategies to generate reactive oxygen-containing radicals to deal with the ever-growing environmental pollution. However, developing improved catalysts with adequate activity and stability is still a long-term goal for practical application. Herein, we demonstrate single cobalt atoms anchored on porous N-doped graphene with dual reaction sites as highly reactive and stable Fenton-like catalysts for efficient catalytic oxidation of recalcitrant organics via activation of peroxymonosulfate (PMS). Our experiments and density functional theory (DFT) calculations show that the CoN4 site with a single Co atom serves as the active site with optimal binding energy for PMS activation, while the adjacent pyrrolic N site adsorbs organic molecules. The dual reaction sites greatly reduce the migration distance of the active singlet oxygen produced from PMS activation and thus improve the Fenton-like catalytic performance.

Single Cobalt Atoms Anchored on Porous N-Doped Graphene with Dual Reaction Sites for Efficient Fenton-like Catalysis

In recent years, nanocrystals of metal sulfide materials have attracted scientific research interest for renewable energy applications due to the abundant choice of materials with easily tunable electronic, optical, physical and chemical properties. Metal sulfides are semiconducting compounds where sulfur is an anion associated with a metal cation; and the metal ions may be in mono-, bi- or multi-form. The diverse range of available metal sulfide materials offers a unique platform to construct a large number of potential materials that demonstrate exotic chemical, physical and electronic phenomena and novel functional properties and applications. To fully exploit the potential of these fascinating materials, scalable methods for the preparation of low-cost metal sulfides, heterostructures, and hybrids of high quality must be developed. This comprehensive review indicates approaches for the controlled fabrication of metal sulfides and subsequently delivers an overview of recent progress in tuning the chemical, physical, optical and nano- and micro-structural properties of metal sulfide nanocrystals using a range of material fabrication methods. For hydrogen energy production, three major approaches are discussed in detail: electrocatalytic hydrogen generation, powder photocatalytic hydrogen generation and photoelectrochemical water splitting. A variety of strategies such as structural tuning, composition control, doping, hybrid structures, heterostructures, defect control, temperature effects and porosity effects on metal sulfide nanocrystals are discussed and how they are exploited to enhance performance and develop future energy materials. From this literature survey, energy conversion currently relies on a limited range of metal sulfides and their composites, and several metal sulfides are immature in terms of their dissolution, photocorrosion and long-term durability in electrolytes during water splitting. Future research directions for innovative metal sulfides should be closely allied to energy and environmental issues, along with their advanced characterization, and developing new classes of metal sulfide materials with well-defined fabrication methods.

Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond

Replacing precious platinum with earth-abundant materials for the oxygen reduction reaction (ORR) in fuel cells has been the objective worldwide for several decades. In the last 10 years, the fastest-growing branch in this area has been carbon-based metal-free ORR electrocatalysts. Great progress has been made in promoting the performance and understanding the underlying fundamentals. Here, a comprehensive review of this field is presented by emphasizing the emerging issues including the predictive design and controllable construction of porous structures and doping configurations, mechanistic understanding from the model catalysts, integrated experimental and theoretical studies, and performance evaluation in full cells. Centering on these topics, the most up-to-date results are presented, along with remarks and perspectives for the future development of carbon-based metal-free ORR electrocatalysts.

Carbon-Based Metal-Free ORR Electrocatalysts for Fuel Cells: Past, Present, and Future.

A comprehensive review of recent advances in the field of oxygen reduction electrocatalysis utilizing nonprecious metal (NPM) catalysts is presented Progress in the synthesis and characterization of pyrolyzed catalysts, based primarily on the transition metals Fe and Co with sources of N and C, is summarized Several synthetic strategies to improve the catalytic activity for the oxygen reduction reaction (ORR) are highlighted Recent work to explain the active-site structures and the ORR mechanism on pyrolyzed NPM catalysts is discussed Additionally, the recent application of Cu-based catalysts for the ORR is reviewed Suggestions and direction for future research to develop and understand NPM catalysts with enhanced ORR activity are provided

Nonprecious Metal Catalysts for Oxygen Reduction in Heterogeneous Aqueous Systems

A Review of Precious-Metal-Free Bifunctional Oxygen Electrocatalysts: Rational Design and Applications in Zn−Air Batteries

This study demonstrates promising bifunctionality of nitrogen-doped carbon nanostructures (CNx) for ORR and OER in acidic medium. Although CNx catalysts are not as active as Pt/C in ORR and Ir/C in OER, they exhibit significantly lower combined overpotential for ORR and OER relative to those shown by the two commercial catalysts, which are highly active only for one of the two reactions, but not both. The effect of various nitrogen functionalities on the ORR and OER activity of CNx catalysts was also studied. CNx samples with higher pyridinic-N site density exhibited higher ORR and OER activity.

Insights into oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) active sites for nitrogen-doped carbon nanostructures (CNx) in acidic media

To probe the active sites of nitrogen-doped carbon nanostructures (CNx), the effect of dihydrogen phosphate (H2PO4–) anion on their oxygen reduction reaction (ORR) performance was investigated by adding increasing concentrations of phosphoric acid in half-cell measurements. A linear decrease in specific kinetic current at 0.7 V was noted with increasing phosphate anion concentration. It was also found that the adsorption of phosphate species on CNx was strong and the corresponding ORR activity was not recovered when the catalyst was reintroduced to a fresh HClO4 solution. Trends similar to those noted upon addition of H3PO4 to the half-cell were observed when CNx catalysts were soaked in phosphoric acid. Adsorption of dihydrogen phosphate ions on the surface of CNx exposed to phosphoric acid was verified by transmission infrared (IR) and Raman spectroscopy as well as X-ray photoelectron spectroscopy (XPS). XPS results also showed a decrease in the surface concentration of pyridinic-N species accompanied b...

Probing the Oxygen Reduction Reaction Active Sites over Nitrogen-Doped Carbon Nanostructures (CNx) in Acidic Media Using Phosphate Anion

CO poisoning as a probe for oxygen reduction reaction (ORR) active sites was examined on both iron–nitrogen coordinated catalysts supported on carbon (FeNC) and nitrogen-doped carbon nanostructures (CNx). Rotating disk electrode (RDE) experiments show a partial decrease in the ORR activity of FeNC catalysts after CO exposure and no decrease for CNx catalysts. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy experiments after exposure to CO show a peak at 2033 cm–1 (associated with linearly bound CO) on the FeNC catalysts and no peaks associated with adsorbed CO on the CNx catalyst surface. Density functional theory (DFT) calculations incorporating dispersion interactions of the adsorption energy of O2 and CO were performed on a total of 16 proposed active sites for the FeNC and CNx catalysts. On CNx, all the sites show weak CO adsorption and only O2 molecules are expected to adsorb, which matches the experimental observation of no poisoning. Several FeNC sites show CO binding energies s...

CO Poisoning Effects on FeNC and CNx ORR Catalysts: A Combined Experimental–Computational Study

In this study, strontium-doped lanthanum ferrite perovskite oxides (LSF) with different A-site occupancies were used as cathode catalysts for CO2 electrolysis at 800 °C. XRD, EXAFS, XPS, FTIR, and temperature-programmed desorption were used to characterize the properties of the material. Fe K-edge EXAFS indicated that oxygen vacancy concentration and oxidation states of Fe atoms increased with concomitant improvement of electronic and ionic conductivities with a decrease in the A-site occupancy. XPS spectra revealed that Sr atoms segregated on the surface of A-site excess LSF92 to a greater degree than that on the A-site deficient LSF72. These segregated A-site metal oxides, especially SrO, are alkaline in nature and hence strongly bind to acidic CO2 forming stable SrCO3, which consequently facilitates carbon formation. This study highlights the effect of A-site stoichiometric modulation on carbon formation during electrolysis of CO2, which is crucial for improving the long- term performance of an SOEC.

Coke formation during high-temperature CO2 electrolysis over AFeO3 (A = La/Sr) cathode: Effect of A-site metal segregation

The ORR and OER performances of N-coordinated iron–carbon (FeNC), Ir/C, and Pt/C catalysts were compared using half-cell measurements. The results suggest significant ORR and OER activity for FeNC materials unlike state-of-the-art Ir/C or Pt/C which exhibited high catalytic activity only for one of the two reactions. Bifunctionality analysis on these three catalyst materials reveals significantly lower combined overpotential for FeNC catalysts compared to that for commercial Ir/C and Pt/C samples.

Deeksha Jain

Papers

Insights into oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) active sites for nitrogen-doped carbon nanostructures (CNx) in acidic media

Probing the Oxygen Reduction Reaction Active Sites over Nitrogen-Doped Carbon Nanostructures (CNx) in Acidic Media Using Phosphate Anion

CO Poisoning Effects on FeNC and CNx ORR Catalysts: A Combined Experimental–Computational Study

Coke formation during high-temperature CO2 electrolysis over AFeO3 (A = La/Sr) cathode: Effect of A-site metal segregation

Nitrogen-Coordinated Iron−Carbon as Efficient Bifunctional Electrocatalysts for the Oxygen Reduction and Oxygen Evolution Reactions in Acidic Media