Electrocatalytic water splitting using power generated from renewable energy to produce hydrogen has been considered as one of the more attractive approach to alleviate the problems of energy crisis and environmental pollution. One of the biggest challenges for the large-scale application of water electrolysis is the searching of the low cost electrocatalysts with high and stable activity toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The well-developed porous features of materials synthesized from the earth abundant elements endow them with the enhanced mass transfer and improved electronic interconnection during electrochemical reactions, resulting in the excellent electrocatalytic performance for both OER and HER. Herein, this review focuses on the recent development of innovation strategies for the fabrication of porous non-noble-metal materials including heteroatom-doped carbon-based and transition metal (mainly Co, Ni, and Fe)-based materials as efficient electrocatalysts for overall water splitting. Specially, a detailed discussion of the structure-activity correlation gives an insight on the origin of the high electrocatalytic performance of porous materials obtained from different strategies, and provides guidance for future design and preparation of highly efficient electocatalysts based on non-precious carbon or metal materials for overall water splitting.

Microbial fuel cells (MFCs) have gained remarkable attention as a novel wastewater treatment that simultaneously generates electricity. The low activity of the oxygen reduction reaction (ORR) remains one of the most critical bottlenecks limiting the development of MFCs. To date, although research on biochar as an electrocatalyst in MFCs has made tremendous progress, further improvements are needed to make it economically practical. Recently, biochars have been considered to be ORR electrocatalysts with developmental potential. In this review, the ORR mechanism and the essential requirements of ORR catalysts in MFC applications are introduced. Moreover, the focus is to highlight the material selection, properties, and preparation of biochar electrocatalysts, as well as the evaluation and measurement of biochar electrodes. Additionally, in order to provide comprehensive information on the specific applications of biochars in the field of MFCs, their applications as electrocatalysts, are then discussed in detail, including the uses of nitrogen-doped biochar and other heteroatom-doped biochars as electrocatalysts, poisoning tests for biochar catalysts, and the cost estimation of biochar catalysts. Finally, profound insights into the current challenges and clear directions for future perspectives and research are concluded.

The selective dissolution of V and Fe from spent denitrification catalyst (SDC) with oxalic acid was investigated to minimise their environmental effects. The dissolution kinetics of different elements from SDC by using 0.1-1.5 mol L^-1 oxalic acid concentration was studied at 60℃-90℃. V and Fe were preferentially released (65% and 81%) compared with Al, Ti and W within 5 min due to the redox reactions of oxalic acid. The dissolved fractions of Fe, V, Al, W and Ti increased with the increase of oxalic acid concentration and reaction temperature. The dissolution kinetic experiments were analysed and controlled diffusion with n < 0.5 according to the Avrami dissolve reaction model (R² > 0.92). The Arrhenius parameters of the E_a values of Ti, W, V, Fe and Al from SDC with oxalic acid were 30, 26, 20, 19 and 11 kJ mol^-1, respectively. The obtained Avrami equation of V and Fe was successfully used to predict their leaching behaviour in oxalic acid. Toxicity characteristic leaching procedure revealed that the toxicity risk of V and Fe metals from SDC after leaching with oxalic acid decreased to below 5 mg kg^-1 residua. Overall, the leaching residua by oxalic acid indicated its safety for the environment.

The effect of salinity on biological nitrogen and denitrifying phosphorus removal was investigated in a Modified University of Cape Town (MUCT) system. Removal rates of COD, NH₄⁺-N, NO₃^--N, NO₂^--N, phosphorus and the sludge characteristics at salt concentrations (0.0, 3.2, 6.4, 11.2 and 16.0 g L^-1) were analyzed. With the salt concentration increasing, all the COD, NH₄⁺-N, TN and TP removal rates exhibited a trend of decline, and exhibited an initial reduction and subsequent increase at every stage of salt concentration. NH₄⁺-N, TN and TP removal rates were 92.7%, 51.5% and 67.2% in 16 g L^-1 salt concentration, respectively. And they were outperformed the literature reported and acceptable in practical applications. When the salinity of wastewater changed from 0.0 to 16.0 g L^-1, the biomass yield coefficients increased from 0.0794 to 0.126 g VSS/g COD. Increased salinity had a detrimental effect on phosphorus-accumulating organisms (PAOs) and denitrifying PAOs (DPAOs) (especially DPAOs). Therefore, phosphorus removal gradually depended on PAO. The simultaneous nitrification and denitrification (SND) rate and nitrogen removal rate (including nitrification rate, denitrification rate, and total nitrogen removal rate) gradually decreased with the increased salinity.

Coke oven gas (COG) is one of the most important by-products in steel industry, and the conversion of COG to value-added products has attracted much attention from both economic and environmental views. In this work, we use the chemical looping reforming technology to produce pure H₂ from COG. A series of La_1-xSr_xFeO₃ (x=0, 0.2, 0.3, 0.4, 0.5, 0.6) perovskite oxides were prepared as oxygen carriers for this purpose. The reduction behaviors of La_1-xSr_xFeO₃ perovskite by different reducing gases (H₂, CO, CH₄ and the mixed gases) are investigated to discuss the competition effect of different components in COG for reacting with the oxygen carriers. The results show that reduction temperatures of H₂ and CO are much lower than that of CH₄, and high temperatures (> 800℃) are requested for selective oxidation of methane to syngas. The co-existence of CO and H₂ shows weak effect on the equilibrium of methane conversion at high temperatures, but the oxidation of methane to syngas can inhibit the consumption of CO and H₂. The doping of suitable amounts of Sr in LaFeO₃ perovskite (e.g., La_0.5Sr_0.5FeO₃) significantly promotes the activity for selective oxidation of methane to syngas and inhibits the formation of carbon deposition, obtaining both high methane conversion in the COG oxidation step and high hydrogen yield in the water splitting step. The La_0.5Sr_0.5FeO₃ shows the highest methane conversion (67.82%), hydrogen yield (3.34 mmol g^-1) and hydrogen purity (99.85%). The hydrogen yield in water splitting step is treble as high as the hydrogen consumption in reduction step. These results reveal that chemical looping reforming of COG to produce pure H₂ is feasible, and an O₂-assistant chemical looping reforming process can further improves the redox stability of oxygen carrier.

It is rather essential to design glorious system with high CO₂ adsorption capacity and electron migration efficiency for improving selective and effective CO₂ reduction into solar fuels. Here, as-synthesized phenolic resin spheres via suspension polymerization were carbonized and activated by water vapor to obtain activated carbon spheres (ACSs). Subsequently, Bi₂MoO₆/ACSs were prepared via hydrothermal-impregnated method. The systematical characterizations of samples, including XRD, XPS, SEM, EDX, DRS, BET, PL, CO₂ adsorption isotherm, EIS and transient photocurrent, were analyzed. The results clearly demonstrated that Bi₂MoO₆ with suitable oxidation reduction potentials and bandgap and ACSs with admirable CO₂ adsorption and electrical conductivity not only enhanced separation efficiency of photoindued electron-hole pair, but also displayed as 1.8 times CO₂ reduction activity to CO as single Bi₂MoO₆ sample under Xe-lamp irradiation. Finally, a concerned photocatalytic CO₂ reduction mechanism was proposed and investigated. Our findings should provide innovative guidance for designing a series of photocatalytic CO₂ reduction materials with highly efficient and selective ability.

The plasmonic photocatalyst of Pd supported on graphitic carbon nitride (Pd/g-C₃N₄) exhibits excellent catalytic activity in photo-induced hydrogenation of biomass-based aldehydes with environmental benign reagents of formic acid (HCOOH) as proton source and triethylamine (TEA) as sacrificial electron donator. The chemical and configurational properties of the Pd/g-C₃N₄ were systematically analyzed with XRD, TEM and XPS. Under optimized conditions, 27% yield of furfuryl alcohol with the corresponding turnover frequency (TOF) around 3.72 h^-1 were obtained from furfural and TEA-HCOOH under visible-light irradiation by using Pd/g-C₃N₄. Our research additionally reveals that Pd atom is the true catalytic active site for the hydrogenation and the photo-promoted reduction mainly occurs through noble metal nanoparticles (NPs)-induced effect of surface plasmon resonance (SPR). The photo-catalytic system of Pd/g-C₃N₄ thus demonstrates a green and effective method for the hydrogenation of biomass-based aldehydes with sustainable solar energy as a driven force.

Along with the extensive application of energy storage devices, the spent lithium-ion batteries (LIBs) are unquestionably classified into the secondary resources due to its high content of several valuable metals. However, current recycling methods have the main drawback to their tedious process, especially the purification and separation process. Herein, we propose a simplified process to recycle both cathode (LiCoO₂) and anode (graphite) in the spent LIBs and regenerate newly high-performance anode material, CoO/CoFe₂O₄/expanded graphite (EG). This process not only has the advantages of succinct procedure and easy control of reaction conditions, but also effectively separates and recycles lithium from transition metals. The 98.43% of lithium is recovered from leachate when the solid product CoO/CoFe₂O₄/EG is synthesized as anode material for LIBs. And the product exhibits improved cyclic stability (890 mAh g^-1 at 1 A g^-1 after 700 cycles) and superior rate capability (208 mAh g^-1 at 5 A g^-1). The merit of this delicate recycling design can be summarized as three aspects:the utilization of Fe impurity in waste LiCoO₂, the transformation of waste graphite to EG, and the regeneration of anode material. This approach properly recycles the valuable components of spent LIBs, which introduces an insight into the future recycling.

Up to now, facile and pollution-free routes for catalyst preparation are in high demand. In this study, a green and cost-effective strategy was successfully developed to construct platinum/graphene aerogel (Pt/GA) nanocomposites by the co-reduction of graphene oxides (GO) and chloroplatinic acid (H₂PtCl₆·6H₂O) with the assists of γ-ray irradiation in the absence of any other reductants. Characterization studies indicated that the energy of γ-ray irradiation and the hole scavenger isopropanol (IPA) played a vital role in forming small Pt nanoparticles with uniform size of ∼3 nm on the surface of graphene aerogel (GA). Furthermore, Pt/GA synthesized with a mass ratio of 2:1 (Pt/GA-2) exhibited a lowest activation energy value and outstanding catalytic properties for the reduction of 4-nitrophenol (4-NP). The excellent catalytic and cycling performance suggest that Pt/GA-2 catalyst has a promising prospect for the reduction of nitroaromatic compounds in wastewater treatment and other industrial applications.

The semi-interpenetrating network anion exchange membranes (AEMs) based on quaternized polyvinyl alcohol (QPVA) and poly(diallyldimethylammonium chloride) (PDDA) are synthesized. The chemical cross-linking structure is formed between hydroxyl groups of QPVA and aldehyde groups of glutaraldehyde (GA), which makes PDDA more stable embed in the QPVA matrix and also improves the mechanical properties and dimensional stability of AEMs. Due to the phase separation phenomenon of AEMs swelling in water, a microporous structure may be formed in the membrane, which reduces the transmission resistance of hydroxide ions and provides a larger space for the transfer of hydroxide ions, thus improving the conductivity. The ring structure of PDDA is introduced as a cationic group to transfer hydroxide ions, and shields the nucleophilic attack of the hydroxide ions through the steric hindrance effect, which improves alkaline stability. The hydroxide conductivity of semi-interpenetrating network membrane (QPVA/PDDA_0.5-GA) is 36.5 mS cm^-1 at 60℃. And the membrane of QPVA/PDDA_0.5-GA exhibits excellent mechanical property with maximum tensile strength of 19.6 MPa. After immersing into hot 3 mol L^-1 NaOH solutions at 60℃ for 300 h, the OH^- conductivity remains 78% of its initial value. The semi-interpenetrating network AEMs with microporous structure exhibit good ionic conductivity, mechanical strength and alkaline durability.

To maximize the potential of monolayer molybdenum disulfide (MoS₂) sheet in the disposal of heavy metal ions in wastewater, we compared the adsorption of several common heavy metal ions (including Cr³⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, and Pb²⁺) in wastewater on the monolayer MoS₂ sheet through first-principles calculation. Our simulation results show that the monolayer MoS₂ sheet is a potential heavy metal adsorption material because of the attractive interaction between them. The most negative adsorption energy determines that the T_Mo site is the most stable adsorption site for the heavy metal ions. The attractive interaction is considered as chemical adsorption, and it is closely related to charge transfer. The orbital hybridization between S p and heavy metal ions p and d states electrons contributes to the adsorption, except the orbital hybridization between S p and Pb p states electrons contributes to the Pb²⁺ adsorption. All the results show that the monolayer MoS₂ sheet is most suitable for removing Ni²⁺ and Cr³⁺ ions from wastewater, followed by Cu²⁺ and Pb²⁺. For the ions Cd²⁺, Zn²⁺, and Hg²⁺, its adsorption strength remains to be improved.

Aiming to efficiently capture the formaldehyde (HCHO) with low content in the air exceeding the standard, 31,399 hydrophobic metal-organic frameworks (MOFs) were first selected from 137,953 hypothetical MOFs to calculate their formaldehyde adsorption performance, namely, adsorption capacity (N_HCHO) and selectivity (S_HCHO=N2+O2) by molecular simulation and machine learning (ML). To combine the S_HCHO=N2+O2 and N_HCHO, a new performance metric, the tradeoff between selectivity and capacity (TSC) was proposed to identify more reasonably the top-performing MOFs. The MOFs were divided into three datasets (i.e., all of the MOFs (AM), MOFs with top 5% of S_{HCHO=N2+O2HCHO=N2}+(P_S) and MOFs with top 5% of N_HCHO (P_N)) to scrutinize and explore the characteristics of different materials capturing formaldehyde from the air (N₂ and O₂). Furthermore, after four ML algorithms (the back propagation neural network (BPNN), support vector machine (SVM), extreme learning machine (ELM), and random forest (RF)) are applied to quantitatively assess the prediction effects of performance indexes in different datasets, RF algorithm with the most accurate prediction revealed that the TSC has strong correlations with the MOF descriptors in P_S dataset. In view of 14.10% of the promising MOFs occupied P_N, the design paths of excellent adsorbents for six MOF descriptors were quantitatively determined, especially for the Henry's coefficient (K_HCHO) and heat of adsorption of formaldehyde (Q_st⁰). Their probabilities of obtaining excellent MOFs could reach 100% and 77.42%, respectively, and both the relative importance and the trends of univariate analysis coherently confirm the important positions of K_HCHO and Q_st⁰. Finally, 20 best MOFs were identified for the single-step separation of formaldehyde with low concentration. The microscopic insights and structure-performance relationship predictions from this computational and ML study are useful toward the development of new MOFs for the capture of formaldehyde from air.

Environmentally friendly and energy saving treatment of black liquor (BL), a massively produced waste in Kraft papermaking process, still remains a big challenge. Here, by adopting a NiCaOCa₁₂Al₁₄O₃₃ bifunctional catalyst derived from hydrotalcite-like materials, we demonstrate the feasibility of producing high-purity H₂ (∼96%) with 0.9 mol H₂ mol^-1 C yield via the sorption enhanced steam reforming (SESR) of BL. The SESRBL performance in terms of H₂ production maintained stable for 5 cycles, but declined from the 6th cycle. XRD, Raman spectroscopy, elemental analysis and energy dispersive techniques were employed to rationalize the deactivation of the catalyst. It was revealed that gradual sintering and agglomeration of Ni and CaO and associated coking played important roles in catalyst deactivation and performance degradation of SESRBL, while deposition of Na and K from the BL might also be responsible for the declined performance. On the other hand, it was demonstrated that the SESRBL process could effectively reduce the emission of sulfur species by storing it as CaSO₃. Our results highlight a promising alternative for BL treatment and H₂ production, thereby being beneficial for pollution control and environment governance in the context of mitigation of climate change.