Controlling Electrons and Protons through Theory: Molecular Electrocatalysts to Nanoparticles

Recent development and applications of differential electrochemical mass spectrometry in emerging energy conversion and storage solutions

Electrochemical energy conversion and storage are playing an increasingly important role in shaping the sustainable future. Differential electrochemical mass spectrometry (DEMS) offers an operando and cost-effective tool to monitor the evolution of gaseous/volatile intermediates and products during these processes. It can deliver potential-, time-, mass- and space-resolved signals which facilitate the understanding of reaction kinetics. In this review, we show the latest developments and applications of DEMS in various energy-related electrochemical reactions from three distinct perspectives. (I) What is DEMS addresses the working principles and key components of DEMS, highlighting the new and distinct instrumental configurations for different applications. (II) How to use DEMS tackles practical matters including the electrochemical test protocols, quantification of both potential and mass signals, and error analysis. (III) Where to apply DEMS is the focus of this review, dealing with concrete examples and unique values of DEMS studies in both energy conversion applications (CO2 reduction, water electrolysis, carbon corrosion, N-related catalysis, electrosynthesis, fuel cells, photo-electrocatalysis and beyond) and energy storage applications (Li-ion batteries and beyond, metal-air batteries, supercapacitors and flow batteries). The recent development of DEMS-hyphenated techniques and the outlook of the DEMS technique are discussed at the end. As DEMS celebrates its 40th anniversary in 2024, we hope this review can offer electrochemistry researchers a comprehensive understanding of the latest developments of DEMS and will inspire them to tackle emerging scientific questions using DEMS.

Electrochemical CO2 Reduction by Urea Hangman Mn Terpyridine species

Based on our previous study in chemical subtleties of the proton tunneling distance for metal hydride formation (PTD-MH) to regulate the selectivity of CO2 reduction reaction (CO2RR), we have developed a family of Mn terpyridine derivatives, in which urea groups functions as multipoint hydrogen-bonding hangman to accelerate the reaction rate. We found that such changes to the second coordination sphere significantly increased the turnover frequency (TOF) for CO2 reduction to ca. 360s - 1 ${{s}^{-1}}$ with this family of molecular catalysts while maintaining high selectivity (ca. 100 %±3) for CO even in the presence of a large amount of phenol as proton source. Notably, the compounds studied in this manuscript all exhibit large value fori c a t / i p ${{{\bf i}}_{{\bf c a t}}/{{\bf i}}_{{\bf p}}}$ as that achieved by Fe porphyrins derivates, while saving up to 0.55 V in overpotential with respect to the latter.

Unified quantum theory of electrochemical kinetics by coupled ion-electron transfer

A general theory of coupled ion-electron transfer (CIET) is presented, which unifies Marcus kinetics of electron transfer (ET) with Butler-Volmer kinetics of ion transfer (IT). In the limit of large reorganization energy, the theory predicts normal Marcus kinetics of "electron-coupled ion transfer" (ECIT). In the limit of large ion transfer energies, the theory predicts Butler-Volmer kinetics of "ion-coupled electron transfer" (ICET), where the charge transfer coefficient and exchange current are connected to microscopic properties of the electrode/electrolyte interface. In the ICET regime, the reductive and oxidative branches of Tafel's law are predicted to hold over a wide range of overpotentials, bounded by the ion-transfer energies for oxidation and reduction, respectively. The probability distribution of transferring electron energies in CIET smoothly interpolates between a shifted Gaussian distribution for ECIT (as in the Gerischer-Marcus theory of ET) to an asymmetric, fat-tailed Meixner distribution centered at the Fermi level for ICET. The latter may help interpret asymmetric line shapes in x-ray photo-electron spectroscopy (XPS) and Auger electron spectroscopy (AES) for metal surfaces in terms of shake-up relaxation of the ionized atom and its image polaron by ICET. In the limit of large overpotentials, the theory predicts a transition to inverted Marcus ECIT, leading to a universal reaction-limited current for metal electrodes, dominated by barrierless quantum transitions. Uniformly valid, closed-form asymptotic approximations are derived that smoothly transition between the limiting rate expressions for ICET and ECIT for metal electrodes, using simple but accurate mathematical functions. The theory is applied to lithium intercalation in lithium iron phosphate (LFP) and found to provide a consistent description of the observed current dependence on overpotential, temperature and concentration. CIET theory thus provides a critical bridge between quantum electrochemistry and electrochemical engineering, which may find many other applications and extensions.

The hangman effect boosts hydrogen production by a manganese terpyridine complex

The manganese terpyridine complex 1 with a coordinated carboxylate in the axial position was obtained in situ. By virtue of a hangman effect, complex 1 catalyzes electrochemical hydrogen evolution from phenol in acetonitrile solution with a turnover frequency of 525 s^-1 at a low overpotential of ca. 230 mV.

Multiple-Site Concerted Proton-Electron Transfer in a Manganese-Based Complete Functional Model for [FeFe]-Hydrogenase

The active site of [FeFe]-hydrogenase (H₂ ase) is preorganized with an amine (azadithiolate) as a proton relay and a [4Fe4S] subunit as an electron reservoir, which together lower the overpotential for proton reduction and hydrogen oxidation by multiple-site concerted proton-electron transfer (MS-CPET). Herein, we report a mononuclear manganese complex, fac-[Mn(CO)₃ (6-(2-hydroxyphenol)-2-pyridine-2-quinoline) Br] (1), as a rare model to fully mimic the functions of the H₂ ase. In 1, a redox-active bidentate ligand with a pendent phenol replicates the roles of the electron reservoir and the proton relay in the enzyme. Experimental and theoretical studies revealed two consecutive MS-CPET processes in the catalytic cycle, in each of which an electron stored in the reductive ligand and a proton at the proximal phenol moiety are transferred to the Mn center in a concerted way. By virtue of this mechanism, complex 1 exhibited a low overpotential comparable to that of natural enzyme in electrochemical hydrogen production using phenol as a proton source.

Structural, Electronic and Thermochemical preference for multi-PCET reactivity of Ruthenium(II)-Amine and Ruthenium(IV)-Amido Complexes

The multiredox reactivity of bioinorganic cofactors is often coupled to proton transfers. Here we investigate the structural, thermochemical, and electronic structure of ruthenium-amino/amido complexes with multi- proton-coupled electron transfer reactivity. The bis(amino)ruthenium(II) and bis(amido)ruthenium(IV) complexes [Ru^II(bpy)(en*)₂]²⁺ (Ru^II-H₀ ) and [Ru^IV(bpy)(en*-H₂)₂]²⁺ (Ru^IV-H₂ ) interconvert reversibly with the transfer of 2e^-/2H⁺ (bpy = 2,2'-bipyridine, en* = 2,3-diamino-2,3-dimethylbutane). X-ray structures allow correlations between the structural and electronic parameters, and the thermochemical data of the 2e^-/2H⁺ multi-square grid scheme. Redox potentials, acidity constants and DFT calculations reveal potential intermediates implicated in 2e^-/2H⁺ reactivity with organic reagents in non-protic solvents, which shows a strong inverted redox potential favouring 2e^-/2H⁺ transfer. This is suggested to be an attractive system for potential one-step (concerted) transfer of 2e^-and 2H⁺ due to the small changes of the pseudo-octahedral geometries and the absence of charge change, indicating a relatively small overall reorganization energy.

Prediction of the standard potentials for one-electron oxidation of N,N,N',N' tetrasubstituted p-phenylenediamines by calculation

The formal potentials for the reversible one-electron oxidation of N,N,N',N' tetrasubstituted p-phenylenediamines in acetonitrile have been applied as a test set for benchmarking computational methods for a series of compounds with only small structural differences. The aim of the study is to propose a simple method for calculating the standard oxidation potentials, and therefore, the protocol is progressively developed by adding more terms in the energy expression. In addition, the effect of including implicit solvation models (IEFPCM, CPCM, and SMD), larger basis sets, and correlation methods are investigated. The oxidation potentials calculated using the G3MP2B3 approach with IEFPCM resulted in the best fit (R² = 0.9624), but the slope of the correlation line, 0.74, is far from the optimal value, 1.00. B3LYP/6-311++G(d,p) and TPSSh/6-311++G(2d,p) yielded only slightly less consistent data (R² = 0.9388 and R² = 0.9425), but with much better slopes, 1.00 and 0.94, respectively. We conclude that it is important to investigate the basis set size and treatment of electron correlation when calculating oxidation potentials for N,N,N',N' tetrasubstituted p-phenylenediamines.

Photochemistry of carbon nitrides and heptazine derivatives

We explore the photochemistry of polymeric carbon nitride (C₃N₄), an archetypal organic photocatalyst, and derivatives of its structural monomer unit, heptazine (Hz). Through spectroscopic studies and computational analysis, we have observed that Hz derivatives can engage in non-innocent hydrogen bonding interactions with hydroxylic species. The photochemistry of these complexes is influenced by intermolecular nπ*/ππ* mixing of non-bonding orbitals of each component and the relative energy of intermolecular charge-transfer (CT) states. Coupling of the former to the latter appears to facilitate proton-coupled electron transfer (PCET), resulting in biradical products. We have also observed that Hz derivatives exhibit an extremely rare inverted singlet/triplet energy splitting (ΔE_ST). In violation of Hund's multiplicity rules, the lowest energy singlet (S₁) is stabilized relative to the lowest triplet (T₁) electronic excited state. Exploiting this unique inverted ΔE_ST character has obvious implications for transformational discoveries in solid-state OLED lighting and photovoltaics. Harnessing this inverted ΔE_ST, paired with light-driven intermolecular PCET reactions, may enable molecular transformations relevant for applications ranging from solar energy storage to new classes of non-triplet photoredox catalysts for pharmaceutical development. To this end, we have explored the possibility of optically controlling the photochemistry of Hz derivatives using ultrafast pump-push-probe spectroscopy. In this case, the excited state branching ratios among locally excited states of the chromophore and the reactive intermolecular CT state can be manipulated with an appropriate secondary "push" excitation pulse. These results indicate that we can predictively redirect chemical reactivity with light in this system, which is an avidly sought achievement in the field of photochemistry. Looking forward, we anticipate future opportunities for controlling heptazine photochemistry, including manipulating PCET reactivity with a diverse array of substrates and optically delivering reducing equivalents with, for example, water as a partial source of electrons and protons. Furthermore, we wholly expect that, over the next decade, materials such as Hz derivatives, that exhibit inverted ΔE_ST character, will spawn a significant new research effort in the field of thin-film optoelectronics, where controlling recombination via triplet excitonic states can play a critical role in determining device performance.

Trends in computational molecular catalyst design

Computational methods have emerged as a powerful tool to augment traditional experimental molecular catalyst design by providing useful predictions of catalyst performance and decreasing the time needed for catalyst screening. In this perspective, we discuss three approaches for computational molecular catalyst design: (i) the reaction mechanism-based approach that calculates all relevant elementary steps, finds the rate and selectivity determining steps, and ultimately makes predictions on catalyst performance based on kinetic analysis, (ii) the descriptor-based approach where physical/chemical considerations are used to find molecular properties as predictors of catalyst performance, and (iii) the data-driven approach where statistical analysis as well as machine learning (ML) methods are used to obtain relationships between available data/features and catalyst performance. Following an introduction to these approaches, we cover their strengths and weaknesses and highlight some recent key applications. Furthermore, we present an outlook on how the currently applied approaches may evolve in the near future by addressing how recent developments in building automated computational workflows and implementing advanced ML models hold promise for reducing human workload, eliminating human bias, and speeding up computational catalyst design at the same time. Finally, we provide our viewpoint on how some of the challenges associated with the up-and-coming approaches driven by automation and ML may be resolved.

H2 Evolution from Electrocatalysts with Redox-Active Ligands: Mechanistic Insights from Theory and Experiment vis-à-vis Co-Mabiq

Electrocatalytic hydrogen production via transition metal complexes offers a promising approach for chemical energy storage. Optimal platforms to effectively control the proton and electron transfer steps en route to H₂ evolution still need to be established, and redox-active ligands could play an important role in this context. In this study, we explore the role of the redox-active Mabiq (Mabiq = 2-4:6-8-bis(3,3,4,4-tetramethlyldihydropyrrolo)-10-15-(2,2-biquinazolino)-[15]-1,3,5,8,10,14-hexaene1,3,7,9,11,14-N₆) ligand in the hydrogen evolution reaction (HER). Using spectro-electrochemical studies in conjunction with quantum chemical calculations, we identified two precatalytic intermediates formed upon the addition of two electrons and one proton to [Co^II(Mabiq)(THF)](PF₆) (Co_Mbq). We further examined the acid strength effect on the generation of the intermediates. The generation of the first intermediate, Co_Mbq-H¹, involves proton addition to the bridging imine-nitrogen atom of the ligand and requires strong proton activity. The second intermediate, Co_Mbq-H², acquires a proton at the diketiminate carbon for which a weaker proton activity is sufficient. We propose two decoupled H₂ evolution pathways based on these two intermediates, which operate at different overpotentials. Our results show how the various protonation sites of the redox-active Mabiq ligand affect the energies and activities of HER intermediates.

A Bioinspired Disulfide/Dithiol Redox Switch in a Rhenium Complex as Proton, H Atom, and Hydride Transfer Reagent

The transfer of multiple electrons and protons is of crucial importance in many reactions relevant in biology and chemistry. Natural redox-active cofactors are capable of storing and releasing electrons and protons under relatively mild conditions and thus serve as blueprints for synthetic proton-coupled electron transfer (PCET) reagents. Inspired by the prominence of the 2e^-/2H⁺ disulfide/dithiol couple in biology, we investigate herein the diverse PCET reactivity of a Re complex equipped with a bipyridine ligand featuring a unique SH···^-S moiety in the backbone. The disulfide bond in fac-[Re(^S-Sbpy)(CO)₃Cl] (1, ^S-Sbpy = [1,2]dithiino[4,3-b:5,6-b']dipyridine) undergoes two successive reductions at equal potentials of -1.16 V vs Fc^+|0 at room temperature forming [Re(^S2bpy)(CO)₃Cl]^2- (1^2-, ^S2bpy = [2,2'-bipyridine]-3,3'-bis(thiolate)). 1^2- has two adjacent thiolate functions at the bpy periphery, which can be protonated forming the S-H···^-S unit, 1H^-. The disulfide/dithiol switch exhibits a rich PCET reactivity and can release a proton (ΔG°_H⁺ = 34 kcal mol^-1, pK_a = 24.7), an H atom (ΔG°_H^• = 59 kcal mol^-1), or a hydride ion (ΔG°_H^- = 60 kcal mol^-1) as demonstrated in the reactivity with various organic test substrates.

Successes, challenges, and opportunities for quantum chemistry in understanding metalloenzymes for solar fuels research

Overview of the rich and diverse contributions of quantum chemistry to understanding the structure and function of the biological archetypes for solar fuel research, photosystem II and hydrogenases.

Acid-triggering of light-induced charge-separation in hybrid organic/inorganic molecular photoactive dyads for harnessing solar energy

H⁺ modulated charge-transfer in photoexcited covalently linked W and Mo Keggin-bodipy conjugates is demonstrated using transient absorption spectroscopy and photoluminescence. Adding acid switches on (W) or accelerates (Mo) charge separation.

Evaluation of the Synthetic Scope and the Reaction Pathways of Proton-Coupled Electron Transfer with Redox-Active Guanidines in C-H Activation Processes

Proton-coupled electron transfer (PCET) is currently intensively studied because of its importance in synthetic chemistry and biology. In recent years it was shown that redox-active guanidines are capable PCET reagents for the selective oxidation of organic molecules. In this work, the scope of their PCET reactivity regarding reactions that involve C-H activation is explored and kinetic studies carried out to disclose the reaction mechanisms. Organic molecules with potential up to 1.2 V vs. ferrocenium/ferrocene are efficiently oxidized. Reactions are initiated by electron transfer, followed by slow proton transfer from an electron-transfer equilibrium.

Metal nanoparticle alters adenine induced charge transfer kinetics of vitamin K3 in magnetic field

In this article, we highlight the alterations in the photoinduced electron transfer (ET) and hydrogen atom transfer (HAT) pathways between an anti-tumor drug vitamin-K3 (MQ) and a nucleobase adenine (ADN) in the presence of gold (Au) and iron (Fe) nanoparticles (NPs). Inside the confined micellar media, with laser flash photolysis corroborated with an external magnetic field (MF), we have detected the transient geminate radicals of MQ and ADN, photo-generated through ET and HAT. We observe that the presence of AuNP on the MQ-ADN complex (^AuMQ-ADN) assists HAT by limiting the ET channel, on the other hand, FeNP on the MQ-ADN complex (^FeMQ-ADN) mostly favors a facile PET. We hypothesize that through selective interactions of the ADN molecules with AuNP and MQ molecules with FeNP, a preferential HAT and PET process is eased. The enhanced HAT and PET have been confirmed by the escape yields of radical intermediates by time-resolved transient absorption spectroscopy in the presence of MF.

Computational Insights on the Electrocatalytic Behavior of [Cp*Rh] Molecular Catalysts Immobilized on Graphene for Heterogeneous Hydrogen Evolution Reaction

The heterogeneous metal-based molecular electrocatalyst can typically exhibit attractive features compared to its homogeneous analogue including recoverability and durability. As such, it is necessary to evaluate the electrocatalytic behavior of heterogenized molecular catalysts of interest toward gaining insights concerning the retainability of such behaviors while benefiting from heterogenization. In this work, we examined computationally the electrochemical properties of nanographene-based heterogenized molecular complexes of Rhodium. We assessed, as well, the electrocatalytic behavior of the heterogenized molecular catalyst for hydrogen evolution reaction (HER). Two electrochemical pathways were examined, namely one- and two-electron electrochemical reduction pathways. Interestingly, it is computationally demonstrated that [Rh^III(Cp*)(phen)Cl]⁺-Gr can exhibit redox and electrocatalytic properties for HER that are comparable to its homogeneous analogue via a two-electron reduction pathway. On the other hand, the one-electron reduction pathway is notably found to be less favorable kinetically and thermodynamically. Furthermore, molecular insights are provided with respect to the HER employing molecular orbitals analyses and mechanistic aspects. Importantly, our findings may provide insights toward designing more efficient graphene-based molecular heterogeneous electrocatalysts for more efficient energy production.

Kinetic Isotope Effects and Hydrogen Tunnelling in PCET Oxidations of Ascorbate: New Insights into Aqueous Chemistry?

Recent experimental studies of kinetic isotope effects (KIE-s) and hydrogen tunnelling comprising three proton-coupled electron transfer (PCET) oxidations of ascorbate monoanion, (a) in aqueous reaction solutions, (b) in the mixed water-organic cosolvent systems, (c) in aqueous solutions of various salts and (d) in fairly diluted aqueous solutions of the various partial hydrophobes are reviewed. A number of new insights into the wealth of the kinetic isotope phenomena in the PCET reactions have been obtained. The modulation of KIE-s and hydrogen tunnelling observed when partially hydrophobic solutes are added into water reaction solution, in the case of fairly diluted solutions is revealed as the strong linear correlation of the isotopic ratios of the Arrhenius prefactors Ah/Ad and the isotopic differences in activation energies ΔE_a (D,H). The observation has been proposed to be a signature of the involvement of the collective intermolecular excitonic vibrational dynamics of water in activation processes and aqueous chemistry.

Mechanistic dichotomies in redox reactions of mononuclear metal–oxygen intermediates

This review article focuses on various mechanistic dichotomies in redox reactions of metal–oxygen intermediates with the emphasis on understanding and controlling their redox reactivity from experimental and theoretical points of view.

Redox-Active Guanidines in Proton-Coupled Electron-Transfer Reactions: Real Alternatives to Benzoquinones?

Guanidino-functionalized aromatics (GFAs) are readily available, stable organic redox-active compounds. In this work we apply one particular GFA compound, 1,2,4,5-tetrakis(tetramethylguanidino)benzene, in its oxidized form in a variety of oxidation/oxidative coupling reactions to demonstrate the scope of its proton-coupled electron transfer (PCET) reactivity. Addition of an excess of acid boosts its oxidation power, enabling the oxidative coupling of substrates with redox potentials of at least +0.77 V vs. Fc⁺ /Fc. The green recyclability by catalytic re-oxidation with dioxygen is also shown. Finally, a direct comparison indicates that GFAs are real alternatives to toxic halo- or cyano-substituted benzoquinones.

Molecular Electrocatalysts for the Hydrogen Evolution Reaction: Input from Quantum Chemistry

In the pursuit of carbon-free fuels, hydrogen can be considered as an apt energy carrier. The design of molecular electrocatalysts for hydrogen production is important for the development of renewable energy sources that are abundant, inexpensive, and environmentally benign. Over the last 20 years, a large number of electrocatalysts have been developed, and considerable efforts have been directed toward the design of earth-abundant, first-row transition-metal complexes capable of promoting electrocatalytic hydrogen evolution reaction (HER). In this context, numerical approaches have emerged as powerful tools to study the catalytic performances of these complexes. This review covers some of the most significant theoretical mechanistic studies of biomimetic and bioinspired homogeneous HER catalysts. The approaches employed to study the free energy landscapes are discussed and methods used to obtain accurate estimates of relevant observables required to study the HER are presented. Furthermore, the structural and electronic parameters that govern the reactivity, and are necessary to achieve efficient hydrogen production, are discussed in view of future research directions.

A Reaction-Induced Localization of Spin Density Enables Thermal C-H Bond Activation of Methane by Pristine FeC4. Chemistry 2019;25:12940-12945. [PMID: 31268193 PMCID: PMC6852486 DOI: 10.1002/chem.201902572]

The reactivity of the cationic metal-carbon cluster FeC₄ ⁺ towards methane has been studied experimentally using Fourier-transform ion cyclotron resonance mass spectrometry and computationally by high-level quantum chemical calculations. At room temperature, FeC₄ H⁺ is formed as the main ionic product, and the experimental findings are substantiated by labeling experiments. According to extensive quantum chemical calculations, the C-H bond activation step proceeds through a radical-based hydrogen-atom transfer (HAT) mechanism. This finding is quite unexpected because the initial spin density at the terminal carbon atom of FeC₄ ⁺ , which serves as the hydrogen acceptor site, is low. However, in the course of forming an encounter complex, an electron from the doubly occupied sp-orbital of the terminal carbon atom of FeC₄ ⁺ migrates to the singly occupied π*-orbital; the latter is delocalized over the entire carbon chain. Thus, a highly localized spin density is generated in situ at the terminal carbon atom. Consequently, homolytic C-H bond activation occurs without the obligation to pay a considerable energy penalty that is usually required for HAT involving closed-shell acceptor sites. The mechanistic insights provided by this combined experimental/computational study extend the understanding of methane activation by transition-metal carbides and add a new facet to the dizzying mechanistic landscape of hydrogen-atom transfer.

Promoting proton coupled electron transfer in redox catalysts through molecular design

Mini-review on using the secondary coordination sphere to facilitate multi-electron, multi-proton catalysis.