Ligand-modified metal nanoclusters (NCs) have emerged as candidate materials for catalysis owing to their well-defined yet tunable structure and their metal centers' high nuclearity. We posited that NC-based catalytic behavior will depend on ligand properties, the accessibility of active sites, and their atomic configuration. We synthesized a series of Cu NC-based catalysts, tuned local hydrophobicity through ligand adjustment, balanced the ligand coverage and active site exposure, and found that we were, in this way, able to engender efficient electrosynthesis of acetate via CO electroreduction. Computation and operando spectroscopy show that asymmetric Cu-Cu sites, which determine the CO binding strength, impact the bifurcation step after C-C coupling. The best of these catalysts, Cu13Nap, achieved an acetate Faradaic efficiency (FE) of 86% and an energy efficiency of 29% in a 5 bar system, exceeding the single C2+ FE of <50% previously achieved by NC-based catalysts.

Water oxidation is the holy grail reaction of natural and artificial photosynthesis. How to design an efficient water-oxidation catalyst remains a long-term challenge for solar fuel production. The rate of water oxidation in photosystem II by the oxygen-evolving complex (OEC) Mn4CaO5 cluster is as high as 100-400 s−1. Mimicking the structures of the OEC is a straightforward strategy to design water-oxidation catalysts. However, the high efficiency of the OEC relies on not only its highly active site but also its holistic system for well-organized electron transfer and proton transport. Lacking such a holistic functional system makes δ-MnO2 a poor water-oxidation catalyst, although the local structure of δ-MnO2 is similar to that of the Mn4CaO5 cluster. Electrocatalysts simultaneously imitating the catalytically active sites, fast electron transfer, and promoted proton transport in a natural OEC have been rarely reported. The significance of the synergy of a holistic system is underrated in the design of water-oxidation catalysts. In this work, we fabricated holistic functional biomimetic composites of two-dimensional manganese oxide nanosheets and pyridyl-modified graphene (MnOx-NS/py-G) for electrocatalytic water oxidation. MnOx-NS/py-G simultaneously imitates the synergy of catalytically active sites, fast electron transfer, and promoted proton transport in a natural OEC, resulting in overall 600 times higher activity than that of typical δ-MnO2. This work demonstrates the significance of holistic functional biomimetic design and guides the development of highly active electrocatalysts for small molecule activation related to solar energy storage.

Water oxidation is an important reaction for multiple renewable energy conversion and storage-related devices and technologies. High-performance and stable electrocatalysts for the oxygen evolution reaction (OER) are urgently required. Bimetallic (oxy)hydroxides have been widely used in alkaline OER as electrocatalysts, but their activity is still not satisfactory due to insufficient active sites. In this research, A unique and efficient approach of sacrificial W to prepare CoFe (oxy)hydroxides with abundant active species for OER is presented. Multiple ex situ and operando/in situ characterizations have validated the self-reconstruction of the as-prepared CoFeW sulfides to CoFe (oxy) hydroxides in alkaline OER with synchronous W etching. Experiments and theoretical calculations show that the sacrificial W in this process induces metal cation vacancies, which facilitates the in situ transformation of the intermediate metal hydroxide to CoFe-OOH with more high-valence Co(III), thus creating abundant active species for OER. The Co(III)-rich environment endows the in situ formed CoFe oxyhydroxide with high catalytic activity for OER on a simple flat glassy carbon electrode, outperforming those not treated by the sacrificial W procedure. This research demonstrates the influence of etching W on the electrocatalytic performance, and provides a low-cost means to improve the active sites of the in situ self-reconstructed bimetallic oxyhydroxides for OER.

Bismuth vanadate (BiVO4) is one of the most fascinating building blocks for the design and assembly of highly efficient artificial photosynthesis devices for solar water splitting. Our recent report has shown that borate treated BiVO4 (B-BiVO4) results in an improved water oxidation performance. In this study, further improvement of both the photoelectrochemical (PEC) activity and stability of B-BiVO4 was successfully achieved by introducing NiFeV LDHs as an oxygen evolution catalyst and interfacial hole transporter. Benefiting from the synergistic effect of co-catalyst and borate pretreatment, the as-prepared NiFeV/B-BiVO4 exhibited a high photocurrent density of 4.6 mA cm−2 at 1.23 VRHE and an outstanding onset potential of ∼0.2 VRHE with good long-term stability. More importantly, NiFeV was found to play a pivotal role in the critically efficient suppression of charge combination on the BiVO4 surface and acceleration of charge transfer rather than a mere electrocatalyst for water oxidation.

Molecular catalysts possess numerous advantages over conventional heterogeneous catalysts in precise structure regulation, in-depth mechanism understanding, and efficient metal utilization. Various molecular catalysts have been reported that efficiently catalyze reactions involved in artificial photosynthesis, however, these catalysts have been rarely considered in view of practical applications. With this review, firstly we demonstrate in the introduction that molecular catalysts can bring new opportunities to proton exchange membrane (PEM) electrolyzers. In the following parts, we provide an overview of molecular catalyst modified carbon materials developed for electrochemical water oxidation, proton reduction, and CO2 reduction reactions. These materials and the involved immobilization strategies as well as characterization techniques may be directly employed in the investigations of application of molecular catalysts in PEM electrolyzers. The future scientific perspectives and challenges to advance this promising, yet underdeveloped technology for solar fuel production, integrating PEM electrolyzer with molecular-level catalysis, are discussed in the conclusions.

The development of highly active and durable catalysts for water oxidation under acidic conditions is necessary but challenging for renewable energy conversion. Ir-based catalysts are highly efficient for water oxidation in acid, but their large scale application is hindered by the high cost and scarcity of iridium. Herein, we use an amorphous WO3 induced lattice distortion (AWILD) strategy to reduce the Ir content to only 2 wt% in the final material. The optimized hybrid nitrogen-doped carbon (NC)/WO3/IrO2 can efficiently catalyze water oxidation with a low overpotential of 270 mV at 10 mA cm(-2) current density (eta (10)) and a high turnover frequency of over 2 s(-1) at 300 mV overpotential in 0.5 M H2SO4, a performance that surpasses that of commercial IrO2 significantly. Introducing the layer of amorphous WO3 between IrO2 nanoparticles and NC can distort the lattice of IrO2, exposing more highly active sites for water oxidation. The AWILD effect compensates for the lower Ir content and dramatically reduces the cost of the catalyst without sacrificing the catalytic activity. Additionally, this catalyst also exhibits high activity in acid for hydrogen evolution with only 65 mV of eta (10) attributed to the AWILD effect, exhibiting efficient bifunctionality as a Janus catalyst for overall water splitting. The AWILD approach provides a novel and efficient strategy for low-cost and highly efficient electrocatalysts for acidic overall water splitting with an extremely low content of noble metals.

Materials design of efficient electrochemical micro-reactors is challenging, although hierarchically structured, self-standing electrodes with catalyst arrays offer promise. Herein, catalyst function in compact micro-reactor electrodes is designed by nanostructural tailoring of carbonized wood for efficient water splitting. Specifically, NiFe rod tipped, N-doped graphitic carbon nanocapsule arrays are self-assembled in hierarchical wood, and the benefit of this unique presentation and its promotive effect on accessibility of the catalyst surfaces is apparent. This report also comprises the first wood based micro-reactor electrodes for electrocatalytic water oxidation demonstrating excellent performance. The overpotential for oxygen evolution reaction was as low as 180 mV for 10 mA cm−2 current density and TOFredox was high at a level of 5.8 s−1 (at 370 mV overpotential). This hierarchical electrode can also work as bifunctional catalyst (both as anodic and as cathodic electrode) for total water splitting with a cell potential of 1.49 V for 10 mA cm−2 in alkaline solution, suggestive of their potential also in other electrochemical applications.

Ammonia production consumes ~2% of the annual worldwide energy supply, therefore strategic alternatives for the energy-intensive ammonia synthesis through the Haber-Bosch process are of great importance to reduce our carbon footprint. Inspired by MoFe-nitrogenase and the energy-efficient and industrially feasible electrocatalytic synthesis of ammonia, we herein establish a catalytic electrode for artificial nitrogen fixation, featuring a carbon fiber cloth fully grafted by boron-doped molybdenum disulfide (B-MoS2/CFC) nanosheets. An excellent ammonia production rate of 44.09 μg h–1 cm–2 is obtained at −0.2 V versus the reversible hydrogen electrode (RHE), whilst maintaining one of the best reported Faradaic efficiency (FE) of 21.72% in acidic aqueous electrolyte (0.1 M HCl). Further applying a more negative potential of −0.25 V renders the best ammonia production rate of 50.51 μg h–1 cm–2. A strong-weak electron polarization (SWEP) pair from the different electron accepting and back-donating capacities of boron and molybdenum (2p shell for boron and 5d shell for molybdenum) is proposed to facilitate greatly the adsorption of non-polar dinitrogen gas via N≡N bond polarization and the first protonation with large driving force. In addition, for the first time a visible light driven photo-electrochemical (PEC) cell for overall production of ammonia, hydrogen and oxygen from water + nitrogen, is demonstrated by coupling a bismuth vanadate BiVO4 photo-anode with the B-MoS2/CFC catalytic cathode.

Fe is considered as a promising alternative for OER catalysts owing to its high natural abundance and low cost. Due to the low conductivity and sluggish catalytic kinetics, the catalytic efficiency of Fe-rich catalysts is far from less abundant Ni, Co-rich alternatives and has been hardly improved without the involvement of Ni or Co. The lower activity of Fe-rich catalysts renders the real active center of state-of-the-art NiFe, CoFe catalyst in long-term scientific debate, despite of detection of Fe-based active intermediates in these catalysts during catalytic process. In the present work, we fabricated a series of sub-5 nm Fe1-yCryOx nanocatalysts via a simple solvothermal method, achieving systematically promoted high-valent Fe(VI) species generation by structural and electronic modulation, displaying highly active OER performance without involvement of Ni or Co. Detailed investigation revealed that the high OER activity is related to the ultrasmall nanoparticle size that promotes abundant edge- and corner-site exposure at catalyst surface, which involves in OER as highly reactive site; and the incorporated Cr ions that remarkably accelerate the charge transfer kinetics, providing an effective conduit as well as suitable host for high-valent active intermediate. This work reveals the structural prerequisites for efficient Fe-rich OER catalyst fabrication, inspiring deeper understanding of the structure-activity relationship as well as OER mechanism of Fe-based catalysts.

Artificial photosynthesis provides a promising strategy for sustainable energy harvesting, yet its overall efficiency is limited by the water oxidation reaction. The subject of this thesis focuses on the exploration of highly efficient cost-effective heterogeneous catalysts for water oxidation, and the investigation of essential catalyst structure-activity relationships.

Chapters 1 and 2 present a brief introduction on heterogeneous catalysts for water oxidation, including selected state-of-the-art catalysts, methodologies for activity improvement, and mechanistic investigations. The characterization methods used in this thesis are also demonstrated.

In chapter 3, a molecular functionalization approach is developed to rationally modify the electronic structure of NiO catalyst, by which the water oxidation activity is systematically tailored. These studies correspond to the question: “How to rationally adjust the catalytic performance of heterogeneous catalysts?”

In chapter 4, to lower the catalyst cost, a Fe-based Fe_0.65Cr_0.35O_x nanocatalyst is fabricated by structural and electronic modulation, which shows considerable water oxidation activity. These studies target the question: “How to fabricate an efficient Fe-based water oxidation catalyst?”

In chapter 5, a bio-inspired Mn-based catalyst is presented. The catalyst successfully imitates the key features of the natural oxygen evolving complex, achieving dramatically improved water oxidation activity. These studies correspond to the question: “How to improve the catalytic activity of Mn-based water oxidation catalysts?”

Finally, in chapter 6, a 3D NiFeCr/Cu nanoarray electrode is constructed by structural engineering, which exhibits extremely high water oxidation activity. These studies correspond to the question: “How to fabricate an efficient catalytic electrode for water oxidation?”

Artificiell fotosyntes erbjuder en lovande strategi för den hållbara energiomvandlingen, men dess totala effektivitet begränsas av vattenoxidationsreaktionen. Den här avhandlingen behandlar utvecklingen av aktiva och kostnadseffektiva heterogena katalysatorer för vattenoxidation och utforskning av de väsentliga struktur-aktivitetsförhållandena.

Kapitlen 1 och 2 omfattar en kort introduktion till de heterogena katalysatorerna för vattenoxidation, inklusive de utvalda toppmoderna katalysatorerna, metoder för aktivitetsförbättring och mekanistiska undersökningar. Karakteriseringsmetoderna som används i denna avhandling demonstreras också.

I kapitel 3, en molekylär funktionaliseringsmetod utvecklas för att rationellt modulera den elektroniska strukturen av NiO-katalysator, genom vilken vattenoxidationsaktiviteten systematiskt anpassas. Dessa studier motsvarar frågan: “Hur kan man rationellt anpassa den katalytiska prestanda hos heterogena katalysatorer?”

I kapitel 4, för att sänka katalysatorkostnaden, en Fe-baserad Fe_0.65Cr_0.35O_x nanokatalysator tillverkas genom strukturell och elektronisk modulering och utför betydande vattenoxidationsaktivitet. Dessa studier riktar sig till frågan: “Hur man tillverkar en effektiv Fe-rik vattenoxidationskatalysator?”

I kapitel 5, en bioinspirerad Mn-baserad katalysator presenteras. Den utvecklade katalysatorn imiterar framgångsrikt nyckelfunktionerna av det naturliga syreutvecklande komplexet och uppnår dramatiskt förbättrad vattenoxidationsaktivitet. Dessa studier motsvarar frågan: “Hur kan man förbättra den katalytiska aktiviteten hos Mn-baserad vattenoxidationskatalysator?”

Till sist, i kapitel 6 beskrivs konstruktionen utav en 3D NiFeCr/Cu NA (nanoarray) elektrod som uppvisar extremt hög vattenoxidation aktivitet. Dessa studier motsvarar frågan: “Hur kan man tillverka en effektiv katalytisk elektrod för vattenoxidation?”