Lior Elbaz

Metallocorroles as Nonprecious‐Metal Catalysts for Oxygen Reduction

The future of affordable fuel cells strongly relies on the design of earth-abundant (non-platinum) catalysts for the electrochemical oxygen reduction reaction (ORR). However, the bottleneck in the overall process occurs therein. We have examined herein trivalent Mn, Fe, Co, Ni, and Cu complexes of β-pyrrole-brominated corrole as ORR catalysts. The adsorption of these complexes on a high-surface-area carbon powder (BP2000) created a unique composite material, used for electrochemical measurements in acidic aqueous solutions. These experiments disclosed a clear dependence of the catalytic activity on the metal center of the complexes, in the order of Co>Fe>Ni>Mn>Cu. The best catalytic performance was obtained for the Co(III) corrole, whose onset potential was as positive as 0.81 V versus the reversible hydrogen electrode (RHE). Insight into the properties of these systems was gained by spectroscopic and computational characterization of the reduced and oxidized forms of the metallocorroles.

Heat-Treated Non-precious-Metal-Based Catalysts for Oxygen Reduction

Non-precious metal catalysts have shown good activity towards oxygen reduction reaction, both in basic and acidic media. The use of NPMCs in fuel cells and metal–air batteries has been hampered by two main issues: the synthesis complexity, translating into a high fabrication cost, and by relatively low stability when compared to platinum-based catalysts, especially in acidic media. In order to overcome these issues, a new class of non-precious metal oxygen reduction catalysts was developed that involves heat treatment as a key step in the NPMC synthesis. This chapter provides a review of the progress in research on heat-treated non-precious metal catalysts of oxygen reduction since the early 1970s until today. The focus of this chapter is on the activity and morphology of the state-of-the-art heat-treated ORR catalysts and trends in the development of more active and durable materials.

Metallocorroles as Non-Precious Metal Electrocatalysts for Highly Efficient Oxygen Reduction in Alkaline Media

A series of non‐precious metal complexes, composed of five first‐row transition‐metal complexes with β‐pyrrole‐brominated 5,10,15‐tris(pentafluorophenyl)corroles [M(tpfcBr8), M=Mn, Fe, Co, Ni, and Cu], was investigated as catalysts for oxygen reduction in an alkaline solution (0.1 m KOH). The corroles were adsorbed on a high surface area carbon powder (BP2000) prior to electrochemical measurements to create a unique composite material. The comparison between the different metal complexes revealed a high oxygen reduction reaction (ORR) catalytic performance in the case of the Fe‐ and Co‐corroles. These complexes reduce oxygen at very low overpotentials (with E1/2=0.79 V and 0.77 V vs. RHE, respectively), which is better than other well‐defined molecular catalysts and comparable to that of Pt on carbon (XC‐72). The mechanism by which the most active complexes catalyze the ORR in alkaline solutions was also studied, disclosing that the dominant reaction path is a four‐electron reduction of molecular oxygen to hydroxide.

High Surface Area Molybdenum Nitride Support for Fuel Cell Electrodes

Alternative supports for polymer electrolyte membrane fuel cells were synthesized and catalytic activity was explored using electrochemical analysis. High surface area, molybdenum nitride supports were synthesized by rapidly heating a gel of polyethyleneimine bound molybdenum in a tube furnace under a forming gas atmosphere. Subsequent disposition of platinum through an incipient wetness approach lead to dispersed crystallites of platinum on the conductive support. All the ceramic materials were characterized with XRD, SEM, TEM and electrochemical analysis. The supports without platinum are highly stable to acidic aqueous conditions and show no signs of oxygen reduction reactivity (ORR). However, once the 20 wt % platinum is added to the material, ORR activity comparable to XC72 based materials is observed.

Mediation at High Potentials for the Reduction of Oxygen to Water by Cobalt Porphyrin–Quinone Systems in Porous Aerogel Carbon Electrodes

Reduced p-, m-, and o-benzoquinones: hydroquinone (HQ), resorcinol (Res), and catechol (Cat), undergo irreversible monolayer adsorption in aerogel carbon (AEC) electrodes with rates of 1.7 × 10 ―4 , 7.1 × 10 ―5 , and 1.4 × 10 ―4 s ―1 for HQ, Res, and Cat, respectively. The adsorbed species showed electrochemical quasi-reversible behavior in 1 M H 2 SO 4 with half-wave potentials (E 1/2 ) of +0.45, +0.31, and +0.58 V vs Ag/AgCl/KCl (saturated) for AEC/HQ, AEC/Res, and AEC/Cat, respectively. Upon adsorption of Co(III) tetra(p-sulfonatophenyl)porphyrin in these electrodes, a single reduction wave was observed and its E 1/2 (∼+0.45 V) was independent of the nature of the adsorbed reduced quinone. This was attributed to a metalloporphyrin/ quinone complex, which formed and stabilized at the electrode surface. This species, after being reduced, reacted with oxygen with a rate of 1.8 × 10 5 M ―1 s ―1 . Mediation of oxygen reduction by these systems occurred at a relatively high potential (∼+0.5 V) almost completely via a four-electron-transfer process.

Tautomerism in N-confused porphyrins as the basis of a novel fiber-optic humidity sensor

It is shown that N-confused porphyrins exhibit tautomerism not only in organic solvents, as already reported, but also after incorporation in dry/humid Nafion films. This allows the development of a new fiber-optic humidity sensor which exhibits long-term stability and a linear response over the humidity range 0 to at least 4000 ppm.

First-Principles Investigation of the Formation of Pt Nanorafts on a Mo2C Support and Their Catalytic Activity for Oxygen Reduction Reaction

We use first-principles calculations to study the formation of Pt nanorafts and their oxygen reduction reaction (ORR) catalytic activity on Mo2C. Due to the high Pt binding energy on C atoms, Pt forms sheet-like structures on the Mo2C surface instead of agglomerating into particles. We find that the disordered Mo2C surface carbon arrangement limits the Pt sheet growth, leading to the formation of 4-6 atom Pt nanorafts. The O-O repulsion between the O atoms on the Mo2C and O adsorbate enhances the ORR activity by weakening the O adsorption energy. We find a significant change from the usual scaling of the energies of the intermediates in the ORR pathway and a strong interaction between the nanoraft and water that lead to a high activity of the Pt nanorafts. Fundamentally, our work demonstrates that the activity of metal catalysts can be strongly affected by manipulation of the atomic arrangement of the supporting carbide surface.

Comparison of new metal organic framework-based catalysts for oxygen reduction reaction

In this article, we collected the most significant and recent data in brief in the field of metal organic frameworks oxygen reduction reaction catalysts, obtained from some of the most recent research papers in the field. We present lists of materials and their key parameters that are relevant to the cathode catalysts in polymer electrolyte membrane fuel cells. All the materials listed in this paper are composed of metal organic frameworks, zeolitic imidazolate frameworks, or their derivatives. These are divided into two main groups: pristine MOFs and MOF-derived materials. The data in this article is a summary of more extensive review (Gonen and Elbaz, 2018) [1].

Bioinspired Electrocatalysis of Oxygen Reduction Reaction in Fuel Cells Using Molecular Catalysts

One of the most important chemical reactions for renewable energy technologies such as fuel cells and metal-air batteries today is oxygen reduction. Due to the relatively sluggish reaction kinetics, catalysts are necessary to generate high power output. The most common catalyst for this reaction is platinum, but its scarcity and derived high price have raised the search for abundant nonprecious metal catalysts. Inspired from enzymatic processes which are known to catalyze oxygen reduction reaction efficiently, employing transition metal complexes as their catalytic centers, many are working on the development of bioinspired and biomimetic catalysts of this class. This research news article gives a glimpse of the recent progress on the development of bioinspired molecular catalyst for oxygen reduction, highlighting the importance of the molecular structure of the catalysts, from advancements in porphyrins and phthalocyanines to the most recent work on corroles, and 3D networks such as metal-organic frameworks and polymeric networks, all with nonpyrolyzed, well-defined molecular catalysts for oxygen reduction reaction.

Direct Electro-oxidation of Dimethyl Ether on Pt-Cu NanoChains

A new platinum-copper alloy electrocatalyst for the direct electro-oxidation of dimethyl ether (DME) has been synthesized in an easy and low-cost approach and studied by using an array of techniques, including X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and elemental analysis. Structural characterization revealed that the synthesized PtCu nanoparticles (3 nm on average) formed homogeneous nanochains without aggregation of metallic platinum or copper. The catalysts activity towards electro-oxidation of DME was tested using cyclic voltammetry (CV) and in membrane-electrode assembly (MEA) in a full cell and was found to be promising. The direct DME fuel cell (DDMEFC) studied in this work has relatively high energy density, of 13.5 mW cm-1 and thus shows great potential as fuel for low power fuel cells. The newly synthesized PtCu catalyst exhibited almost double the performance of commercial PtRu in electrocatalytic DME oxidation.