A Methodology for the Analysis of Water Oxidation Electrocatalysts in the Absence of Limiting Current that Avoids the Pitfalls of Existing Methods.

An all-inorganic, oxidatively and thermally stable, homogeneous water oxidation catalyst based on redox-active (vanadate(V)-centered) polyoxometalate ligands, Na10[Co4(H2O)2(VW9O34)2]·35H2O (Na101-V2, sodium salt of the polyanion 1-V2), was synthesized, thoroughly characterized and shown to catalyze water oxidation in dark and visible-light-driven conditions. This synthetic catalyst is exceptionally fast under mild conditions (TOF > 1 × 10(3) s(-1)). Under light-driven conditions using [Ru(bpy)3](2+) as a photosensitizer and persulfate as a sacrificial electron acceptor, 1-V2 exhibits higher selectivity for water oxidation versus bpy ligand oxidation, the final O2 yield by 1-V2 is twice as high as that of using [Co4(H2O)2(PW9O34)2](10-) (1-P2), and the quantum efficiency of O2 formation at 6.0 μM 1-V2 reaches ∼68%. Multiple experimental results (e.g., UV-vis absorption, FT-IR, (51)V NMR, dynamic light scattering, tetra-n-heptylammonium nitrate-toluene extraction, effect of pH, buffer, and buffer concentration, etc.) confirm that the polyanion unit (1-V2) itself is the dominant active catalyst and not Co(2+)(aq) or cobalt oxide.

Trinuclear Nickel Catalyst for Water Oxidation: Intramolecular Proton-Coupled Electron Transfer Triggered Trimetallic Cooperative O–O Bond Formation

A virtually identical cubical or cubane core structure appears to be a key commonality in several active homogeneous, heterogeneous and enzymatic water oxidation catalysts. We show that X-ray crystal structures of the CaMn3O4 core of the water oxidizing complex of Photosystem II coincide remarkably closely with comparable structures in a range of non-biological Co or Mn-based homogeneous and heterogeneous water oxidation catalysts. Included amongst these are molecular Co4O4 and Mn4O4 cubanes, as well as Co and Mn spinels containing cubical arrangements. This observation supports the existence of an optimum structural element for water oxidation catalysis. It also offers a potential avenue for comparing and relating homogeneous, heterogeneous and enzymatic catalysis to each other.

Review: 46 refs.

Linear free energy scaling relationships (LFESRs) and regression analysis may predict the catalytic performance of heterogeneous and recently, homogenous water oxidation catalysts (WOCs). This study analyses twelve homogeneous Ru-based catalysts – some, the most active catalysts studied: the Ru(tpy-R)(QC) and Ru(tpy-R)(4-pic)2 catalysts, where tpy is 2,2:6,2-terpyridine, QC is 8-quinolinecarboxylate and 4-pic is 4-picoline. Typical relationships studied among heterogenous and solid-state catalysts cannot be broadly applied to homogeneous catalysts. This subset of structurally similar catalysts with impressive catalytic activity deserves closer computational and statistical analysis of energetics correlating with measured catalytic activity. We report general methods of LFESR analysis yield insufficiently robust relationships between descriptor variables. However, volcano plot-based analysis grounded in Sabatier’s principle reveals ranges of ideal relative energies of the RuIV=O and RuIV-OH intermediates and optimal changes in free energies of water nucleophilic attack on RuV=O. A narrow range of RuIV-OH to RuV=O redox potentials corresponding with the highest catalytic activities suggests facile access to the catalytically competent high-valent RuV=O state, often inaccessible from RuIV=O. Our work introduces experimental oxygen evolution rates into approaches of LFESR and Sabatier principle-based analysis, identifying a narrow yet fertile energetic landscape to bountiful oxygen-evolution activity, leading future rational design.

While computational screening with first-principles density functional theory (DFT) is essential for evaluating candidate catalysts, limitations in accuracy typically prevent the prediction of experimentally relevant activities. Exemplary of these challenges are homogeneous water oxidation catalysts (WOCs) where differences in experimental conditions or small changes in ligand structure can alter rate constants by over an order of magnitude. Here, we compute mechanistically relevant electronic and energetic properties for 19 mononuclear Ru transition-metal complexes (TMCs) from three experimental water oxidation catalysis studies. We discover that 15 of these TMCs have experimental activities that correlate with a single property, the ionization potential of the Ru(II)-O2 catalytic intermediate. This scaling parameter allows the quantitative understanding of activity trends and provides insight into the rate-limiting behavior. We use this approach to rationalize differences in activity with different experimental conditions, and we qualitatively analyze the source of distinct behavior for different electronic states in the other four catalysts. Comparison to closely related single-atom catalysts and modified WOCs enables rationalization of the source of rate enhancement in these WOCs.

While computational screening with first-principles density functional theory (DFT) is essential for evaluating mechanisms of candidate catalysts, limitations in accuracy typically prevent prediction of experimentally relevant activities. Exemplary of these challenges are homogeneous water oxidation catalysts (WOCs) where differences in experimental conditions along with small changes in ligand structure can alter rate constants by over an order of magnitude. To leverage computational screening for homogeneous WOC design, a distinct approach is needed. Here, we compute mechanistically-relevant electronic and energetic properties for 19 mononuclear Ru transition metal complexes (TMCs) from three experimental water oxidation catalysis studies. We discover that 15 of these TMCs have experimental activities that can be correlated to a single property, the ionization potential of the Ru(II)-O2 catalytic intermediate. This scaling parameter is well correlated with experimentally-reported rate constants, allowing quantitative understanding activity trends and insight into rate-limiting behavior. We use this approach to rationalize differences in activity with differing experimental conditions, and we qualitatively analyze the source of distinct behavior for differing electronic states in the other four catalysts. Comparison to closely related single-atom catalysts and modified WOCs enables rationalization of the source of rate enhancement in these experimental WOC catalysts.

Homogeneous electrocatalytic water oxidation catalyzed by a mononuclear nickel complex

The precise mechanisms of four-electron-transfer water oxidation processes remain to be further understood. Oxide-based precipitation from molecular catalysts as a frequent observation during water oxidation has raised extensive debates on the differentiation between homogeneous and heterogeneous catalysis. Although soluble cobalt salts are known to be active in water oxidation, CoOx species formed in situ were generally considered to be the true catalyst. Here we report on the possibility homogeneous water oxidation with cobalt chloride in acidic conditions, which prevent CoOx precipitation. Interestingly, both the buffer media and counteranions were found to significantly influence the oxygen evolution activity, and their roles in the water oxidation process were analyzed with various techniques. This study sheds new light on Co2+ ions in key transformation processes of homogeneous water oxidation catalysts.

In neutral medium (pH 7.0) [RuIIIRuII(µ-CO3)4(OH)]4− undergoes one electron oxidation to form [RuIIIRuIII(µ-CO3)4(OH)2]4− at an E1/2 of 0.85 V vs. NHE followed by electro-catalytic water oxidation at a potential ≥1.5 V. When the same electrochemical measurements are performed in bicarbonate medium (pH 8.3), the complex first undergoes one electron oxidation at an Epa of 0.86 V to form [RuIIIRuIII(µ-CO3)4(OH)2]4−. This complex further undergoes two step one electron oxidations to form RuIVRuIII and RuIVRuIV species at potentials (Epa) 1.18 and 1.35 V, respectively. The RuIVRuIII and RuIVRuIV species in bicarbonate solutions are [RuIVRuIII(µ-CO3)4(OH)(CO3)]4− and [RuIVRuIV(µ-CO3)4(O)(CO3)]4− based on density functional theory (DFT) calculations. The formation of HCO4− in the course of the oxidation has been demonstrated by DFT. The catalyst acts as homogeneous water oxidation catalyst, and after long term chronoamperometry, the absorption spectra does not change significantly. Each step has been found to follow a proton coupled electron transfer process (PCET) as obtained from the pH dependent studies. The catalytic current is found to follow linear relation with the concentration of the catalyst and bicarbonate. Thus, bicarbonate is involved in the catalytic process that is also evident from the generation of higher oxidation peaks in cyclic voltammetry. The detailed mechanism has been derived by DFT. A catalyst with no organic ligands has the advantage of long-time stability.

The development of homogeneous first–row transition metal (FRTM) catalysts for the water oxidation reaction is considerably more challenging than for second and third-row catalysts. Given that FRTM catalysts are, in general, more labile, additional design principles must be considered to develop robust and stable catalysts for the water oxidation reaction. In this review, we highlight important design criteria and summarize important lessons learned for FRTM water oxidation catalysts. • The design principles of homogeneous water oxidation catalysts are reviewed. • Additional design criteria that are required for first–row transition metal water oxidation catalysts are considered. • A link is made between operating conditions and catalyst homogeneity. • The utilization of redox-active ligands is discussed.

Inspired by the process of water oxidation during photosynthesis, researchers have developed mononuclear and polynuclear transition metal complexes as homogeneous water oxidation catalysts. While water oxidation catalysis by mononuclear copper-peptide complexes with N4 motif has been briefly explored, there is limited research on how systematically changing individual components of the ligand framework can enhance water oxidation catalysis. We report the synthesis and characterization of a library of twenty-eight copper (II) peptide/peptidomimetic complexes, each incorporating a systematic ligand framework change. Their electrocatalytic water oxidation properties were investigated through cyclic voltammetry studies in a pH 11 buffer medium. These studies offer novel insight into how to enhance the water oxidation catalysis abilities of mononuclear and polynuclear copper-peptide/peptidomimetic complexes by modifying the ligand framework.

We study the water oxidation mechanism of the cobalt aqua complex [Co(H2O)6]2+ in a photocatalytic setup by means of density functional theory. Assuming a water-nucleophilic-attack or radical coupling mechanism, we investigate how the oxidation state and spin configuration change during the catalytic cycle. In addition, different ligand environments are employed by substituting a water ligand with a halide, pyridine, or derivative thereof. This allows exploration of the effect of such ligands on the frontier orbitals and the thermodynamics of the water oxidation process. Moreover, the thermodynamically most promising water oxidation catalyst can be identified by comparing the computed free energy profiles to the one of an “ideal catalyst”. Examination of such simple (hypothetical) water oxidation catalysts provides a basis for the derivation of design guidelines, which are highly sought for the development of efficient homogeneous water oxidation catalysts.

This article reviews some of the recent work by fellows and associates of the Australian Research Council Centre of Excellence for Electromaterials Science (ACES) at Monash University and the University of Wollongong, as well as their collaborators, in the field of water oxidation and reduction catalysts. This work is focussed on the production of hydrogen for a hydrogen-based energy technology. Topics include: (1) the role and apparent relevance of the cubane-like structure of the Photosystem II Water Oxidation Complex (PSII-WOC) in non-biological homogeneous and heterogeneous water oxidation catalysts, (2) light-activated conducting polymer catalysts for both water oxidation and reduction, and (3) porphyrin-based light harvesters and catalysts.

Binuclear polyoxometalates based on abundant metals as efficient homogeneous photocatalytic water oxidation catalysts