Exploration of efficient water oxidation catalysts (WOCs) is the primary challenge in conversion of renewable energy into fuels. Here we report a molecularly well-defined heterogeneous WOC with Aza-fused, pi-conjugated, microporous polymer (Aza-CMP) coordinated single cobalt sites (Aza-CMP-Co). The single cobalt sites in Aza-CMP-Co exhibited superior activity under alkaline and near-neutral conditions. Moreover, the molecular nature of the isolated catalytic sites makes Aza-CMP-Co a reliable model for studying the heterogeneous water oxidation mechanism. By a combination of experimental and theoretical results, a pH-dependent nucleophilic attack pathway for O-O bond formation was proposed. Under alkaline conditions, the intramolecular hydroxyl nucleophilic attack (IHNA) process with which the adjacent -OH group nucleophilically attacks Co4+=O was identified as the rate-determining step. This process leads to lower activation energy and accelerated kinetics than those of the intermolecular water nucleophilic attack (WNA) pathway. This study provides significant insights into the crucial function of electrolyte pH in water oxidation catalysis and enhancement of water oxidation activity by regulation of the IHNA pathway.

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.

Photoelectrochemical (PEC) water splitting is a promising technology for converting solar energy into green hydrogen. The development of highly efficient, robust and cost-effective photoanodes has been established to be of essential importance for PEC water oxidation. BiVO4 has been deemed as one of the most up-and-coming metal oxide-based photoanode materials for PEC water splitting. Development of new surface engineering techniques for BiVO4 is therefore the subject of this thesis. 

In Chapter 1, a general introduction that centers on the solar fuel production by BiVO4-based PEC cells is presented. It concerns the working principles of PEC systems, current status of BiVO4-based photoanodes, and modification strategies for enhancement of the PEC activity. 

In Chapter 2, the characterization methods used in this thesis and the preparation of BiVO4 photoelectrode are introduced. 

In Chapter 3, a postsynthetic borate treatment is developed to decorate the BiVO4 surface. The PEC performance of as-prepared B-BiVO4 photoanode is evaluated and the mechanism of the PEC enhancement is subsequently investigated. Moreover, the layered double hydroxide-based cocatalyst is integrated with the B-BiVO4 substrate. The synergistic effects of borate treatment and cocatalyst on improvement of the PEC activity are discussed. 

In Chapter 4, a conjugated microporous polymer-based heterogeneous catalyst is applied to the surface modification of BiVO4. The PEC performance of the BiVO4/eCMP-Co hybrid photoanode is discussed. Furthermore, the origin of the PEC enhancement is investigated by charge kinetics studies.

In Chapter 5, a metal-organic complex, cobalt phytate, is introduced on BiVO4 by photo-assisted electrodeposition in the form of an ultra-thin nanolayer. The PEC performance of the BiVO4/CoPhy integrated photoanode and the role of CoPhy in interfacial charge transfer is investigated.

Fotoelektrokemisk (PEC) vattensplittring är en lovande teknik för att omvandla solenergi till grön vätgas. Utvecklingen av effektiva, robusta och kostnadseffektiva fotoanoder har fastställts vara av avgörande betydelse för PEC-baserad vattenoxidation. BiVO4 har ansetts vara ett av de mest framväxande metalloxidbaserade fotoanodmaterialen för PEC-baserad vattenoxidation. Att utveckla nya tekniker för ytmodifiering på BiVO4 är därför ämnet för denna avhandling.

I kapitel 1 presenteras en allmän introduktion som fokuserar på produktion av solbränsle med BiVO4-baserade PEC-celler. Kapitlet beskriver arbetsprinciperna för PEC-system, nuvarande status för BiVO4-baserade fotoanoder och modifieringsstrategier för att förbättra PEC-aktiviteten.

I kapitel 2 introduceras de karakteriseringsmetoder som används i denna avhandling och tillverkningen av BiVO4-baserade fotoelektroder.

I kapitel 3 utvecklas en postsyntetisk boratbehandling för att dekorera BiVO4-ytan på molekylär nivå. PEC-prestandan för den beredda B-BiVO4-fotoanoden utvärderas och mekanismen för PEC-förbättring undersöks därefter. Dessutom är en skiktad dubbelhydroxidbaserad co-katalysator integrerad med B-BiVO4-substratet. Dessutom diskuteras de synergistiska effekterna av boratbehandlingen och co-katalysatorn på förbättring av PEC-aktiviteten.

I kapitel 4 appliceras en konjugerad mikroporös polymerbaserad heterogen katalysator för ytmodifiering av BiVO4. PEC-prestandan för BiVO4/eCMP-Co hybrid fotoanoden diskuteras. Sedan undersöks ursprunget till PEC-förbättringen med hjälp av laddningskinetikstudier.

I kapitel 5 introduceras ett organisk-metall komplex, koboltfytat, på BiVO4 genom fotoassisterad elektrodeponering i form av ett ultratunt nanolager. PEC-prestandan för den BiVO4/CoPhy integrerade fotoanoden och rollen för CoPhy vid gränssnitt laddningsöverföring undersöks också.

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.

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.

Water-splitting photoanodes based on semiconductor materials typically require a dopant in the structure and co-catalysts on the surface to overcome the problems of charge recombination and high catalytic barrier. Unlike these conventional strategies, a simple treatment is reported that involves soaking a sample of pristine BiVO4 in a borate buffer solution. This modifies the catalytic local environment of BiVO4 by the introduction of a borate moiety at the molecular level. The self-anchored borate plays the role of a passivator in reducing the surface charge recombination as well as that of a ligand in modifying the catalytic site to facilitate faster water oxidation. The modified BiVO4 photoanode, without typical doping or catalyst modification, achieved a photocurrent density of 3.5 mA cm−2 at 1.23 V and a cathodically shifted onset potential of 250 mV. This work provides an extremely simple method to improve the intrinsic photoelectrochemical performance of BiVO4 photoanodes.

The mechanical properties of cellulose nanofibrils network structure are essential for their applications in functional materials. In this work, an adhesive peptide consisting of just 11 amino acid residues with a hydrophobic core sequence of FLIVI (F – phenylalanine, L – leucine, I – isoleucine, V – valine) flanked by three lysine (K) residues was adsorbed to 2,2,6,6-Tetramethyl-1-piperidinyloxy radical (TEMPO) oxidized cellulose nanofibrils (TO-CNF). Composite films were prepared by solution casting from water suspensions of TO-CNF adsorbed with the adhesive peptide. The nanofibrils network structure of the composite was characterized by atomic force microscopy (AFM). The structure of the peptide in the composites and the interactions between TO-CNF and the peptide were studied by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The mechanical properties of the composites were characterized by tensile tests and dynamic mechanical analysis (DMA). With 6.3 wt.% adhesive peptide adsorbed onto TO-CNF, the composite showed a modulus of 12.5 ± 1.4 GPa, a tensile strength of 344.5 ± (15.3) MPa, and a strain to failure of 7.8 ± 0.4%, which are 34.4%, 48.8%, and 23.8% higher than those for neat TO-CNF, respectively. This resulted in significantly improved toughness (work to fracture) for the composite, 77% higher than that for the neat TO-CNF.

Identifying surface active intermediate species is essential to reveal the catalytic mechanism of water oxidation by metal-oxides-based catalysts and to develop more efficient catalysts for oxygen-oxygen bond formation. Here we report, through electrochemical methods and ex situ infrared spectroscopy, the identification of a MnVII = O intermediate during catalytic water oxidation by a c-disordered δ-MnOx with an onset-potential-dependent reduction peak at 0.93 V and an infrared peak at 912 cm−1. This intermediate is proved to be highly reactive and much more oxidative than permanganate ion. Therefore, we propose a new catalytic mechanism for water oxidation catalyzed by Mn oxides, with involvement of the MnVII = O intermediate in a resting state and the MnIV−O−MnVII = O as a real active species for oxygen-oxygen bond formation. Inorganic Chemistry; Surface Science; Energy Materials; Electrocatalysis.

Advanced surface and interface engineering has been demonstrated to be of critical importance for the development of photoanodes for highly efficient photoelectrochemical (PEC) water oxidation. In this study, cobalt-sites-rich aza-fused conjugated microporous polymer was integrated on the nanoporous bismuth vanadate electrode. The hybrid BiVO4/eCMP-Co photoanode exhibited a high photocurrent density of 4.3 mA cm-2 at 1.23 VRHE and a very low onset potential of ~ 0.2 VRHE with an applied bias photon to current efficiency of 1.62% at around 0.6 VRHE. Moreover, studies on charge carrier kinetics showed that eCMP-Co can not only accelerate water oxidation kinetics but also significantly suppress surface recombination, thereby dramatically increasing charge transfer efficiency. These results demonstrate the great potential of conjugated polymers combined with metal coordination as heterogenous catalysts on photoelectrodes in PEC devices.