Photobiological hydrogen production and artificial photosynthesis for clean energy: from bio to nanotechnologies

An exploratory diagnosis and proposed index of technological change and sustainable industrial development in selected OECD member countries

Industrial development has resulted in economic progress and the well-being of the society. At the same time, the impact of the industrial complex has disrupted the environment and resulted in climate change related impacts. The purpose of this study was to carry out an exploratory diagnosis and propose a technological change and sustainable industrial development index at the international level. Therefore, a network study was conducted to identify the main nodes and thematic clusters associated with cleaner production. A patent analysis was applied to technologies related three selected/relevant areas of cleaner production, i.e. carbon footprint, wastewater treatment, and renewable energy. Additionally, based on factor analysis, an index including different indicators related to scientific, technological, economic, environmental, and social issues was developed and proposed in this study.

Mitochondria and chloroplasts function in microalgae energy production

Microalgae are organisms that have the ability to perform photosynthesis, capturing CO₂ from the atmosphere to produce different metabolites such as vitamins, sugars, lipids, among others, many of them with different biotechnological applications. Recently, these microorganisms have been widely studied due to their possible use to obtain clean energy. It has been postulated that the growth of microalgae and the production of high-energy metabolites depend on the correct function of cellular organelles such as mitochondria and chloroplasts. Thus, the development of different genetic tools to improve the function of these organelles is of high scientific and technological interest. In this paper we review the recent advances in microalgae engineering and the role of cellular organelles in order to increase cell productivity and biomass.

Water oxidation reaction in the presence of a dinuclear Mn(II)-semicarbohydrazone coordination compound

Water splitting, producing of oxygen, and hydrogen molecules, is an essential reaction for clean energy resources and is one of the challenging reactions for artificial photosynthesis. The Mn₄Ca cluster in photosystem II (PS-II) is responsible for water oxidation in natural photosynthesis. Due to this, water oxidation reaction by Mn coordination compounds is vital for mimicking the active core of the oxygen-evolving complex in PS-II. Here, a new dinuclear Mn(II)-semicarbohydrazone coordination compound, [Mn(HL)(µ-N₃)Cl]₂ (1), was synthesized and characterized by various methods. The structure of compound 1 was determined by single crystal X-ray analysis, which revealed the Mn(II) ions have distorted octahedral geometry as (MnN₄OCl). This geometry is created by coordinating of oxygen and two nitrogen donor atoms from semicarbohydrazone ligand, two nitrogen atoms from azide bridges, and chloride anion. Compound 1 was used as a catalyst for electrochemical water oxidation, and the surface of the electrode after the reaction was investigated by scanning electron microscopy, energy dispersive spectrometry, and powder X-ray diffraction analyses. Linear sweep voltammetry (LSV) experiments revealed that the electrode containing 1 shows high activity for chemical water oxidation with an electrochemical overpotential as low as 377 mV. Although our findings showed that the carbon paste electrode in the presence of 1 is an efficient electrode for water oxidation, it could not withstand water oxidation catalysis under bulk electrolysis and finally converted to Mn oxide nanoparticles which were active for water oxidation along with compound 1.

ElectroCatalytic Activity of Nickel Foam with Co, Mo, and Ni Phosphide Nanostructures

In this study, the electrocatalytic activity of nickel foam, which is activated by cobalt, molybdenum, and nickel phosphide nanostructures, is prepared by the plasma hydrothermal method for use in the release of hydrogen and oxygen. The morphology and crystallographic structure of the synthesized phosphide specimens were examined by means of scanning electron microscopy, Fourier-transform infrared spectroscopy, and X-ray diffraction. Moreover, the electrolysis activity for these sets of specimens was investigated using the Tafel polarization curve or linear sweep voltammetry, cyclic voltammetry, as well as by means of the electrochemical impedance spectroscopy technique. Preliminary results show that nickel phosphide presents the highest electrocatalytic activity than the other phosphides developed in this research. In this regard, it presents an electrocatalytic activity to release hydrogen and oxygen of around −1.7 and 0.82 mV, which is measured at a current density of 100 mA·cm−2, respectively.

Enabling Efficient Charge Separation for Optoelectronic Conversion via an Energy-Dependent Z-Scheme n-Semiconductor-Metal-p-Semiconductor Schottky Heterojunction

Achieving good charge separation while maintaining energetic electronic states in heterostructures is a challenge in designing efficient photocatalyst materials. Using first-principles calculations, we propose a Z-scheme S_n-m-S_p (n-semiconductor-metal-p-semiconductor) heterojunction as a viable avenue for achieving broad-spectrum sunlight absorption and, importantly, energy-dependent charge separation. As a proof-of-concept investigation, we investigated two ternary heterostructures, CdS-Au-PdO and SnO₂-W-Ag₂O, in which the electronic Fermi levels line up by virtue of the presence of an intermediate metal layer. A cascade of work functions in the relative order W_n < W_m < W_p drives electrons flowing from S_n to m and from m to S_p. The inner electric fields established at the S_n-m and m-S_p Schottky junctions selectively guide low-energy photoexcited electrons from S_n (CdS/SnO₂) and low-energy holes from S_p (PdO/Ag₂O) to the interposing Au or W metal, respectively. Importantly, relatively low Schottky barriers enforce charge separation by constraining high-energy photogenerated charges to the individual semiconductor layers. Operating together, these two mechanisms enable the achievement of highly efficient optoelectronic conversion.

Evaluation of hydrogen yield potential from Chlorella by photo-fermentation under diverse substrate concentration and enzyme loading

Chlorella is widely distributed, can be cultured in waste water and had short growth cycle. The high carbohydrate composition shows great potential for bioenergy output. In this work, concentrated Chlorella solution was adopted as raw material. Reducing sugar concentration, pH, and cumulative bio-hydrogen yield were taken as indexes, the effects of substrate concentration and enzyme (cellulase or neutral protease) load on photo-fermentation bio-hydrogen production process from microalgae biomass were investigated. Results showed that highest cumulative hydrogen yield was obtained at the optimal substrate concentration of 25 g/L, when the load of cellulase and protease are both 15%, the effect is the best which were 16.65 mL, 29.44 mL, and 43.62 mL, respectively. Results fitted well to the Gompertz model, indicating the feasibility of photo-fermentative bio-hydrogen production from concentrated Chlorella.

One step electrochemical route to the fabrication of highly ordered array of cylindrical nano porous structure and its electrocatalytic performance toward efficient hydrogen evolution

An efficient and non-precious metal catalyst is a key factor for hydrogen evolution reaction (HER). Here we report that the fabrication of highly ordered porous arrays of Cu-Zn-Ni alloy has been carried out in a one-step electrochemical route at a constant apparent current density of -3 A·cm^-2. The optimum film composition and reactivity of the electrodes for catalytic hydrogen evolution reaction were analyzed by using different current densities, deposition time and bath concentration. For this purpose, onset potentials in linear sweep voltammograms (LSV) were compared. The structure and morphology of nanoporous Cu-Zn-Ni and Cu-Zn alloy were characterized by SEM and energy dispersive X-ray (EDS) analysis. The experimental results on the behavior of electrocatalytic activity of prepared alloys showed that the addition of nickel to the alloys improves of the electrocatalytic performance of the electrodes toward HER. In addition, enhancement of electrochemical activity toward hydrogen evolution can be attributed to the large electrochemical active surface area and porous structure of Cu-Zn-Ni alloy. In order to improvement of reaction kinetics, Tafel plots were derived from LSV voltammograms, and the exchange current densities for HER on synthesized electrodes (Cu-Zn and Cu-Zn-Ni alloys) were calculated about 3.2 × 10^-5 and 2.1 × 10^-3 mA·cm^-2, respectively.

An overview of the reaction conditions for an efficient photoconversion of CO2

Carbon dioxide (CO2) emission is one of the well-known causes of global warming. Photoconversion of CO2 to useful chemical compounds using solar energy is an attractive approach as it reduces the major greenhouse gas and promises a sustainable energy source. This method involves radical-chain reactions that form cation and anion radicals generated as a result of the reaction with photogenerated electrons (e−) and holes (h+) between metal oxide photocatalyst and the reactants. Therefore, the product distribution of a modified photocatalyst even under specific reaction conditions is difficult to predict. The CO2 photocatalytic reduction process is controlled by several conditions such as reactor configuration, photocatalyst type, and nature of the reducing agents. Here, we review the parameters such as temperature, pH, CO2 pressure, type of reductant, role of co-catalysts, dopants, and type of photocatalysts that influence the end products of the photocatalytic reduction of CO2. In this review, the different modifications recommended for the photocatalysts to improve CO2 reduction and receive maximum valuable end product (methane, ethanol, methanol, hydrogen, and carbon monoxide) have been listed. The discussion also includes specific behaviors of photocatalysts which lead to different product distribution. It has been noted that different metal and nonmetal dopants improve the activity of a photocatalyst and influence the end product distribution by altering the active species. Similarly, the key factors, i.e. size, morphology and doping, which have been ruling the photocatalytic activity of CO2 reduction under UV or visible light irradiation have been identified.

Nano-sized Mn oxide/agglomerated silsesquioxane composite as a good catalyst for water oxidation

Water splitting to hydrogen and oxygen is an important reaction to store sustainable energies, and water oxidation is identified as the bottleneck for water splitting because it requires the high activation energy to perform. Herein a nano-sized Mn oxide/agglomerated silsesquioxane composite was used to synthesize an efficient catalyst for water oxidation. The composite was synthesized by a straightforward and simple procedure and characterized by scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, dynamic light scattering, X-ray diffraction spectrometry, and electrochemical methods. Silsesquioxane causes good dispersion of Mn in the composite. The water-oxidizing activity of this composite was studied in the presence of cerium(IV) ammonium nitrate. The composite at the best calcination temperature (300 °C) shows a turnover frequency 0.3 (mmol O₂/mol Mn.s). Regarding the low-cost, environmentally friendly precursors, simple synthesis, and efficiency for water oxidation, the composite is a promising catalyst that can be used in artificial photosynthetic systems for water splitting. We used Agglomerated silsesquioxane as a support for nano-sized Mn oxide to synthesize a good water-oxidizing catalyst.

Hydrogen photoproduction in green algae Chlamydomonas reinhardtii sustainable over 2 weeks with the original cell culture without supply of fresh cells nor exchange of the whole culture medium

Unicellular green algae Chlamydomonas reinhardtii are known to make hydrogen photoproduction under the anaerobic condition with water molecules as the hydrogen source. Since the hydrogen photoproduction occurs for a cell to circumvent crisis of its survival, it is only temporary. It is a challenge to realize persistent hydrogen production because the cells must withstand stressful conditions to survive with alternation of generations in the cell culture. In this paper, we have found a simple and cost-effective method to sustain the hydrogen production over 14 days in the original culture, without supply of fresh cells nor exchange of the culture medium. This is achieved for the cells under hydrogen production in a sulfur-deprived culture solution on the {anaerobic, intense light} condition in a desiccator, by periodically providing a short period of the recovery time (2 h) with a small amount of TAP(+S) supplied outside of the desiccator. As this operation is repeated, the response time of transition into hydrogen production (preparation time) is shortened and the rate of hydrogen production (build up time) is increased. The optimum states of these properties favorable to the hydrogen production are attained in a few days and stably sustained for more than 10 days. Since generations are alternated during this consecutive hydrogen production experiment, it is suggested that the improved hydrogen production properties are inherited to next generations without genetic mutation. The properties are reset only when the cells are placed on the {sulfur-sufficient, aerobic, moderate light} conditions for a long time (more than 1 day at least).