Revealing Significant Electronic Effects on the Oxygen Reduction Reaction with Iron Porphyrins

Understanding electronic effects on catalysis from a mechanism point of view is of fundamental significance but is also challenging. We herein report on electronic effects on the oxygen reduction reaction (ORR) with Fe porphyrins. By using FeIII tetraphenylporphyrin (TPP-Fe) and FeIII tetra(pentafluorophenyl)porphyrin (TPFP-Fe), we showed their different electrochemical and chemical behaviors for ORR. Mechanism studies revealed that the FeIII-superoxo species of TPP-Fe can undergo smooth protonation with trifluoroacetic acid (TFA) but the electron-deficient FeIII-superoxo species of TPFP-Fe cannot be protonated with TFA. The FeIII-superoxo reactivity difference between TPP-Fe and TPFP-Fe is the origin of their different catalytic ORR behaviors.

Investigation of Iron(III) Tetraphenylporphyrin as a Redox Flow Battery Anolyte: Unexpected Side Reactivity with the Electrolyte

Redox flow batteries (RFBs) present an opportunity to bridge the gap between the intermittent availability of green energy sources and the need for on-demand grid level energy storage. While aqueous vanadium-based redox flow batteries have been commercialized, they are limited by the constraints of using water as an electrochemical solvent. Nonaqueous redox flow battery systems can be used to produce high voltage batteries due to the larger electrochemical window in nonaqueous solvents and the ability to tune the redox properties of active materials through functionalization. Iron porphyrins, a class of organometallic macrocycles, have been the subject of many studies for their photocatalytic and electrocatalytic properties in nonaqueous solvents. Often, iron porphyrins can undergo multiple redox events making them interesting candidates for use as anolytes in asymmetrical redox flow batteries or as both catholyte and anolyte in symmetrical redox flow battery systems. Here the electrochemical properties of Fe(III)TPP species relevant to redox flow battery electrolytes are investigated including solubility, electrochemical properties, and charge/discharge cycling. Commonly used support electrolyte salts can have reactivities that are often overlooked beyond their conductivity properties in nonaqueous solvents. Parasitic reactions with the cations of common support electrolytes are highlighted herein, which underscore the careful balance required to fully assess the potential of novel RFB electrolytes.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Effects of functional groups in iron porphyrin on the mechanism and activity of oxygen reduction reaction

The activity of the oxygen reduction reaction (ORR) on the cathode is one of the dominant factors in the performance of proton exchange membrane fuel cells. Iron porphyrin has low cost, environmental benignity, and maximum efficiency of metal usage. Therefore, this material can be a promising single-atomic metal dispersion catalyst for fuel cell cathodes. The variation of functional groups was proven to effectively modify the activity of the ORR on the iron porphyrin. However, the influences of functional groups on the mechanisms of the ORR remain ambiguous. This work paid attention to the substitution of carboxyl (-COOH), methyl (-CH₃), and amino (-NH₂) functional groups at the meso positions of the porphyrin ring. By using van der Waals density functional theory (vdW-DF) calculations, we found that the ORR mechanisms can follow the associative and dissociative pathways, respectively. The Gibbs free energy diagrams revealed that the rate-limiting step occurs at the second hydrogenation step for the first pathway and the O₂ dissociation step for the second pathway for all considered functional groups. The thermodynamic energy barrier at the rate-limiting step was found to be in the following order: porphyrin-(CH₃)₄ < porphyrin-(NH₂)₄ < original porphyrin < porphyrin-(COOH)₄ for the associative mechanism and porphyrin-(NH₂)₄ < porphyrin-(CH₃)₄ < porphyrin-(COOH)₄ < original porphyrin for the dissociative pathway. The findings suggested that porphyrin-(CH₃)₄ and porphyrin-(NH₂)₄ should be the best choices among the considered substrates for the oxygen reduction reaction. Furthermore, the interaction between the ORR intermediates and the substrates was attributed to the resonance of the d _{z ²} , d _xz , and d _yz components of the Fe d orbital and the C and N p orbitals of the substrates with the p orbitals of the oxygen atoms in the intermediates. Finally, the nature of the interaction between the adsorbent and adsorbate was charge transfer.

Recent developments in the synthesis of bio-inspired iron porphyrins for small molecule activation

Nature utilizes a diverse set of tetrapyrrole-based macrocycles (referred to as porphyrinoids) for catalyzing various biological processes. Investigation of the differences in electronic structure and reactivity in these reactions have revealed striking differences that lead to diverse reactivity from, apparently, similar looking active sites. Therefore, the role of the different heme cofactors as well as the distal superstructure in the proteins is important to understand. This article summarizes the role of a few synthetic metallo-porphyrinoids towards catalyzing several small molecule activation reactions, such as the ORR, NiRR, CO₂RR, etc. The major focus of the article is to enlighten the synthetic routes to the well-decorated active-site mimic in a tailor-made fashion pursuing a retrosynthetic approach, learning from the biosynthesis of the cofactors. Techniques and the role of the second-sphere residues on the reaction rate, selectivity, etc. are incorporated emulating the basic amino acid residues fencing the active sites. These bioinspired mimics play an important role towards understanding the role of the prosthetic groups as well as the basic residues towards any reaction occurring in Nature.