Inspired by various bio-motors, the research on self-propulsion strategies has gained increasing attention and gradually focuses on possible applications, such as directed transportation, drug delivery, bio-mimicking, separation of special cargo, manipulation of cells, and macroscopic supramolecular assembly etc. Among them, electricity generation based on the Faraday's law of electricity induction has been an advanced progress, which converted mechanical motions to electricity through self-propulsion strategies. In this aspect, we have developed functionally cooperative smart systems with cyclic diving-surfacing motions for electricity generation when exposed to magnetic field (Figure 1). Besides, mini-generators based on the Pt-H₂O₂ reaction or the CaCO₃-HCl reaction were developed to either improve the motion efficiency or introduce bubble systems non-hazardous to smart surfaces, respectively. To handle the challenge of improving energy conversion efficiency, we learned from the working mechanism of the swim bladder by only consuming the inner energy of bubbles instead of indirectly using chemical reactions. We fabricated a smart device made of a 3D printed model fish with a superhydrophobic surface and realized diving/surfacing motions of by directly consuming the inner energy of bubbles, thus leading to improved energy conversion efficiency by 10-fold compared with previous results. When the buoyancy driven diving-surfacing motions were carried out in magnetic field by cutting the magnetic lines repeatedly, we achieved electricity generation with the as-prepared smart device, which worked as a mini-generator. Moreover, the applied ambient pressure is close to the range of human blood pressure, indicating possible application of in situ mini generator to solve the power supply problem such as for cardiac pacemaker implanted in human body.

In natural photosynthesis, green plants and algae have evolved highly delicate, complex structures to convert light to chemical energy. Inside the photosynthetic factories termed chloroplasts, a membrane-bound thylakoid is the site of the light-dependent reactions of photosynthesis. Following light harvesting by chlorophyll molecules, photoinduced electrons are passed through electron transport chain to the enzyme ferredoxin-NADP reductase, leading to the reduction of nicotinamide adenine dinucleotide phosphate (NAD(P)) to NAD(P)H, completing the storage of light energy in one kind of “energy currency” of the cells. Inspired by the photosynthesis, g-C₃N₄ was employed for NADH regeneration in the presence of [Cp*Rh(bpy)Cl]Cl as an electron and proton mediator. Inspired by the photosynthetic thylakoid membrane of chloroplasts, the rational design of mesoporous structured colloids made up of graphitic carbon nitride nanosheets were synthesized and were applied in photocatalytic NADH regeneration for sustainable enzymatic synthesis. The in situ NADH regeneration rate is high enough to reverse the biological pathway of the three consecutive dehydrogenase enzymes (formate dehydrogenase, formaldehyde dehydrogenase, and alcohol dehydrogenase, respectively), which allows for instance the successive conversion of formaldehyde to methanol and the reduction of carbon dioxide into methanol and other enzymatic reactions. The synthesis sequence developed here thereby might open further possibilities by combining with microfluidics to engineer high efficiency artificial photosynthesis systems in the future.

Nitrogen is an essential nutrient for sustaining life and molecular dinitrogen (N₂) must be fixed to bio-available ammonia (NH₃) or nitrate (NO₃^-) for metabolism. Nitrogen is “fixed” in nature by nitrogenase enzymes under ambient temperature and pressure. Currently, over half of the fixed nitrogen is supplied biologically by nitrogenase, while nearly the other half is from the widespread use of the industrial Haber-Bosch process, which operates under high temperature (400-500 °C) and high pressure (200-250 bar) in the presence of metallic iron catalyst. Photochemical conversion provides a promising approach to convert nitrogen into ammonia by using solar energy. Previous attempts used titania, Fe₂Ti₂O₇, Ru/Au/Nb-SrTiO₃, diamond, BiOBr, but these produced only impractically low efficiencies or abological. Inspired by the structure and function of MoFe protein in binding and reducing N₂ also based on our previous works, we are motivated to prepare a MoFe protein mimicked chalcogel that incorporated both FeMoco-like Mo₂Fe₆S₈ and P-cluster-like Fe₄S₄ clusters linked together with Na₄Sn₂S₆. The resultant chalcogels were performed in photocatalytic N₂ fixation and fairly higher N₂ reduction. The initial binding of N₂ with chalcogels under illumination was observed with in situ diffuse-reflectance Fourier transform infrared spectroscopy (DRIFTS). ¹⁵N₂ isotope experiments confirm that the generated NH₃ derives from N₂. Density functional theory (DFT) electronic structure calculations suggest that the N₂ binding is thermodynamically favorable only with the highly reduced active clusters.  The work should shed some light on the ongoing efforts of achieving nitrogen fixation under mild, ambient, and environmentally friendly conditions. 

In natural photosynthesis, green plants and algae have evolved highly delicate, complex structures to convert light to chemical energy. Inside the photosynthetic factories termed chloroplasts, a membrane-bound thylakoid is the site of the light-dependent reactions of photosynthesis. Following light harvesting by chlorophyll molecules, photoinduced electrons are passed through electron transport chain to the enzyme ferredoxin-NADP reductase, leading to the reduction of nicotinamide adenine dinucleotide phosphate (NAD(P)) to NAD(P)H, completing the storage of light energy in one kind of “energy currency” of the cells. Inspired by the photosynthesis, g-C₃N₄ was employed for NADH regeneration in the presence of [Cp*Rh(bpy)Cl]Cl as an electron and proton mediator. Inspired by the photosynthetic thylakoid membrane of chloroplasts, the rational design of mesoporous structured colloids made up of graphitic carbon nitride nanosheets were synthesized and were applied in photocatalytic NADH regeneration for sustainable enzymatic synthesis. The in situ NADH regeneration rate is high enough to reverse the biological pathway of the three consecutive dehydrogenase enzymes (formate dehydrogenase, formaldehyde dehydrogenase, and alcohol dehydrogenase, respectively), which allows for instance the successive conversion of formaldehyde to methanol and the reduction of carbon dioxide into methanol and other enzymatic reactions. The synthesis sequence developed here thereby might open further possibilities by combining with microfluidics to engineer high efficiency artificial photosynthesis systems in the future.

Nitrogen is an essential nutrient for sustaining life and molecular dinitrogen (N₂) must be fixed to bio-available ammonia (NH₃) or nitrate (NO₃^-) for metabolism. Nitrogen is “fixed” in nature by nitrogenase enzymes under ambient temperature and pressure. Currently, over half of the fixed nitrogen is supplied biologically by nitrogenase, while nearly the other half is from the widespread use of the industrial Haber-Bosch process, which operates under high temperature (400-500 °C) and high pressure (200-250 bar) in the presence of metallic iron catalyst. Photochemical conversion provides a promising approach to convert nitrogen into ammonia by using solar energy. Previous attempts used titania, Fe₂Ti₂O₇, Ru/Au/Nb-SrTiO₃, diamond, BiOBr, but these produced only impractically low efficiencies or abological. Inspired by the structure and function of MoFe protein in binding and reducing N₂ also based on our previous works, we are motivated to prepare a MoFe protein mimicked chalcogel that incorporated both FeMoco-like Mo₂Fe₆S₈ and P-cluster-like Fe₄S₄ clusters linked together with Na₄Sn₂S₆. The resultant chalcogels were performed in photocatalytic N₂ fixation and fairly higher N₂ reduction. The initial binding of N₂ with chalcogels under illumination was observed with in situ diffuse-reflectance Fourier transform infrared spectroscopy (DRIFTS). ¹⁵N₂ isotope experiments confirm that the generated NH₃ derives from N₂. Density functional theory (DFT) electronic structure calculations suggest that the N₂ binding is thermodynamically favorable only with the highly reduced active clusters.  The work should shed some light on the ongoing efforts of achieving nitrogen fixation under mild, ambient, and environmentally friendly conditions. 

Graphene oxide (GO) is a semiconductor that can absorb light to generate electron-hole pairs for photocatalytic reactions. GO is a suitable medium for photocatalytic reactions in an aqueous solution because it is highly dispersed in water. Our study tuned the electronic properties of GO by varying its size and the embedded functionalities. As-received GO was a p-type semiconductor. We doped nitrogen into the GO sample by ammonia treatment and converted the conductivity type. We also reduced the size of the GO sample to increase the surface area. Size modulation along with chemical modification represent a means to tune the photocatalytic activity of GO. We obtained GO quantum dots (GOQDs) that exhibited size-dependent photoluminescence emissions. Presence of nitrogen functionalities in GOQDs eliminated vacancy defect states to suppress charge recombination and resulted in the conjugation of nitrogen lone-pair electrons with the aromatic p orbitals. The Pt-deposited nitrogen-doped GOQDs effectively catalyzed H₂ evolution from a triethanolamine aqueous solution. The quantum yield for H₂ evolution reached 20 % under monochromatic irradiation at 420 nm.

In this 21^st century, the exploration of renewable energy to replace the traditional fossil fuels is of scientific significance. Amid the available renewable energy techniques, the utilization of clean solar energy for artificial photosynthesis is regarded as one of the auspicious strategies to combat the onslaught of climate change and surmount the contemporary descending resources of fossil fuels. In view of the massive potential of “solar + water/CO₂ → energy fuels”, photocatalysis has been categorized as the “Holy Grail” of modern photochemistry, which has underpinned incessant research fascination for the past few decades. Recently, graphitic carbon nitride (g-C₃N₄) has spurred a renaissance of interest in the realm of energy production due to its compelling properties such as its earth-abundant nature, non-toxicity and moderate band gap of around 2.7 eV with visible light activation.^1,2 In this talk, the state-of-the-art research advancement in our laboratory toward effective photocatalytic solar energy conversion (i.e. CO₂ reduction and H₂O splitting) using g-C₃N₄-based nanocomposites will be systematically presented. Various modification methods of pristine g-C₃N₄ will be discussed. This includes: the incorporation of metal-free carbonaceous nanomaterials (e.g. two-dimensional (2D) graphene^3,4 and zero-dimensional (0D) carbon nanodots⁵), the hybridization of another semiconductor photocatalyst,⁶ the coupling of transition metal phosphides (e.g. Ni₂P, Ni₁₂P₅ and Co₂P) and so forth. In short, the present studies open up a new invigorating prospect toward intelligent design of g-C₃N₄-based nanocomposites with ameliorated charge separation and migration in CO₂ reduction and water splitting, which can be extended to various energy-related applications such as solar cells, photovoltaics and electrocatalysis.

Hematite (α-Fe₂O₃) has been extensively suggested as a superior photoanode for the photoelectrochemical (PEC) water splitting, owing to its natural abundance, high chemical stability, and theoretical solar-to-hydrogen (STH) efficiency (16.8%). Nevertheless, the recalcitrant electron−hole recombination resulting in poor charge separation and injection efficiency limits its PEC performance. Herein, a simple hydrothermal/atomic layer deposition (ALD) process was used to fabricate two types of HfO₂ functionalized hematite (i.e., HfO₂ overlayer and nanoparticles modified α-Fe₂O₃, HfFe-L and HfFe-P for short) photoanodes. It was revealed that HfO₂ overlayer and nanoparticles could successfully passiviate the surface trap states of hematite, resulted in enhanced PEC performances for both HfFe-L and HfFe-P. More surprisingly, the photocurrent density of the HfFe-P reached as high as 1.21 mA cm-2 at 1.23 _vs. RHE, with ~4.8 and ~3.3 fold enhancement as compared to that of the bare hematite and even HfFe-L, respectively. Such great PEC performance enhancement in HfFe-P was revealed to be attributed to the multifunction of HfO₂ nanoparticles distributed on the surface of hematite nanorod arrays, which not only passivated surface defects to suppress the surface charge recombination by eliminating the surface trapping states, but can also facilitated the hole extraction from bulk of hematite to hematite/electrolyte interface.

Intensive effort has been made to develop photocatalytic processes to convert solar energy to clean fuels. So far, however, energy conversion efficiency for the visible-light photocatalytic splitting of water to hydrogen is less than 1%. In this talk, Prof. Hu will highlight the recent findings for visible light photocatalysis in his group, including: (1) demonstrating that surface-diffuse-reflected-light can be 2 orders of magnitude more efficient than incident light for photocatalysis, (2) revealing that the inefficiency of absorbed visible light for the photocatalytic H₂ production from water with a sacrificial agent is due to its kinetic limitation, (3) inventing a novel temperature-induced visible light photocatalytic H₂ production from water steam with a sacrificial agent to reach a high photohydrogen yield of 497 mmol/h/gcat with a large apparent quantum efficiency (QE) of 65.7% for entire visible light range, and (4) creating an efficient catalyst system for photocatalytic CO₂ reforming of methane (PCRM) with a high hydrogen yield that is 3 orders of magnitude larger than the reported values.

Photocatalysis has been extensively investigated for energy and environmental applications. As the key role in photocatalysis, a great variety of photocatalysts have been developed towards low cost, low toxicity, and high activity. Titanium dioxide has been regarded as the most popular photocatalyst, yet cannot harvest visible light, the bigger energy contribution than UV from sunlight. Recently, graphitic carbon nitride (g-C₃N₄) was discovered as a polymeric photocatalysis with the promising nature of metal-free and visible light response. Because of the powerful reduction ability, g-C₃N₄ has been readily for water splitting and CO₂ reduction. This talk, however, focuses on its photocatalytic oxidation ability. To this end, a number of modification methods were introduced. Particularly for maintaining the metal-free nature, different nanocarbons were used to fabricate completely metal-free, g-C₃N₄-based photocatalysts for degradation of organic pollutants in water.

Microfluidic electrochemical system precisely manipulate fluid flow in microstructure to generate and transform energy. Among various microfluidic devices, microfluidic biofuel cells combine the microfluidic technology with bio-catalysts (e.g. enzyme, bacteria) and represent a novel carbon-neutral power generator. This paper gives a brief review of the recent accomplishments in microfluidic biofuel cells including fundamental working principles, state-of-the-art devices and key challenges. To be amenable to other micro systems, issues of energy output, scaling-up and functional-architecture design are especially proposed. The potential application opportunities in niche power source, sensor and analysis system are also discussed.

Dye-sensitized solar cells (DSSCs), typically composed of a dye-sensitized TiO₂ photoanode, an I⁻/I₃⁻-based electrolyte and a photocathode, also called counter electrode (CE), convert solar energy into electricity directly, which attracted considerable attention due to the high theoretical power conversion efficiency (PCE) and environmental friendliness. CEs are equally important to other components and determine in part the efficiency and cost of DSSCs. Ideal CEs should possess high electrocatalytic activity for I₃⁻ reduction reaction (IRR) to I⁻, superior charge transfer capability and stability. On the contrary to intensive research on other components, CEs were often overlooked in the past and platinum (Pt) is state-of-the-art CE in DSSCs, which however suffers from its scarcity and poor durability (decomposing to PtI₄ in electrolyte). Until recently, alternative Pt-free CEs have been actively sought to heighten the cost competitiveness of DSSCs. To date, several kinds of carbons, e.g., carbon black (CB), carbon nanotubes (CNTs) and graphene, were investigated as CEs for DSSCs due to their high conductivity and large surface areas. Nonetheless, the performance of DSSCs with carbon-based CEs was usually much inferior to Pt. Functionalization, doping and morphology control were exploited as effective approaches to improve the activity of carbons. However, the complex synthesis processes and the poor stability of the functional groups may impede their practical applications. Alternatively, metal sulphides, selenides and oxides are also explored as Pt-free CEs in DSSCs. Among them, although metal sulphides and selenides displayed high activity for IRR, the inefficient charge transfer rate of metal sulphides, selenides originated from their band structure and the complex synthesis procedures largely impede their further applications. On the other hand, metal oxides are of particular interest due to high activity, low cost, robust stability, abundant in variety and facile in preparation. Oxygen vacancies and low metal valences in metal oxides play important roles in IRR and oxygen vacancies-contained SnO₂ and WO₃ were proved to be active CEs for DSSCs. However, due to the simple atomic environment, the oxygen vacancy in simple oxides is very limited, restricting further performance enhancement. Complex oxides such as perovskites (ABO₃) are more attractive for their better chemical flexibilities. More oxygen vacancies can be generated by tailoring the compositions of the perovskite oxides. In addition, perovskites with easily produced redox couples in B-site are expected to have high IRR activity. However, up to now, few reports on the application of perovskites as CEs in DSSCs are available, due likely to their relatively low conductivity and small surface areas. In this study, for the first time, we report chlorine (Cl)-doped perovskite oxide with an orthorhombic structure that shows high activity and durability for IRR in DSSCs. By optimization of the Cl doping amount, LaFeO_2.942-δCl_0.058 showed much higher IRR activity relative to its parent compound, LaFeO₃ (LF), due to the increased concentration of oxygen vacancies (active sites), enriched lower B-site metal valance and the reduction of the bond energies of iodine with the electrocatalyst. DSSC with LaFeO_2.942-δCl_0.058 CE shows a PCE of 8.20%, outperforming Pt CE (7.11%) and being much superior to that of LF (3.10%). By applying hierarchical TiO₂ microsphere as an advanced photoanode and LaFeO_2.942-δCl_0.058 as CE, DSSC demonstrated a highly attractive PCE of 10.2% under simulated sunlight irradiation (AM 1.5G), which was 25% higher than a reference DSSC with Pt CE (8.11%). Furthermore, LaFeO_2.942-δCl_0.058 showed a much superior stability to Pt and the pristine LF for IRR and DSSC with LaFeO_2.942-δCl_0.058 CE operated very stably for 28 days. In addition, in this study, we report the development of strongly coupled LaNiO₃ perovskite/carbon composites as new Pt-free CEs for DSSCs showing excellent electrocatalytic activity and stability for IRR. To prepare such composites, a facile mechano-chemical method based on high-energy ball milling was applied, where abundant oxygen vacancies, rich mixed B-site metal valences and a strong coupling effect between carbon and LaNiO₃ were created. DSSC with LaNiO₃/CB CE exhibited comparable PCE to that of Pt CE (reaching 98%). Hybriding LaNiO₃ with multi-walled carbon nanotubes (MWCNTs) obtained a CE with even better performance than Pt or LaNiO₃/CB CEs due to enhanced electron transfer capability and improved surface area; a more attractive PCE of 9.81% was achieved by DSSC with hierarchical TiO₂ microsphere photoanode. Furthermore, LaNiO₃/carbon composites afforded superior stability to Pt, making them highly promising CEs for DSSCs. The PCEs and the performance enhancements comparing with Pt that obtained by the perovskite-based photocathodes in this study were superior to most of the state-of-the-art highly efficient Pt-free photocathodes in DSSCs. The present work provides two simple and efficient strategies to develop perovskite-based highly-efficient and stable photocathodes for Pt replacement in DSSCs. This study also highlights the extended applications of perovskites in other I₃⁻/I⁻-mediated PVs or electrochromic, batteries.