High Pore Volume Research Articles

Facile Synthesis of Ordered Mesoporous Orthorhombic Niobium Oxide (T-Nb2O5) for High-Rate Li-Ion Storage with Long Cycling Stability

Herein, we describe the synthesis and evaluation of hierarchical mesoporous orthorhombic niobium oxide (T-Nb2O5) as an anode material for rechargeable lithium-ion batteries (LIB). The as-synthesized material addresses key challenges such as beneficial porous structure, poor rate capability, and cycling performance of the anode for Li-ion devices. The physicochemical characterization results reveal hierarchical porous nanostructure morphology with agglomerated particles and a 20 to 25 nm dimension range. Moreover, the sample has a high specific surface area (~65 m2 g−1) and pore volume (0.135 cm3 g−1). As for the application in Li-ion devices, the T-Nb2O5 delivered an initial discharging capacity as high as 225 mAh g−1 at 0.1 A g−1 and higher rate capability as well as remarkable cycling features (~70% capacity retention after 300 cycles at 250 mA g−1) with 98% average Coulombic efficiency (CE). Furthermore, the scan rate-dependent charge storage mechanism of the T-Nb2O5 electrode material was described, and the findings demonstrate that the electrode shows an evident and highly effective pseudocapacitive Li intercalation behaviour, which is crucial for understanding the electrode process kinetics. The origin of the improved performance of T-Nb2O5 results from the high surface area and mesoporous structure of the nanoparticles.

Hierarchical porous carbon with tunable apertures and nitrogen/oxygen heteroatoms for efficient adsorption and separation of VOCs

Porous carbon materials are good adsorbent candidates for removing volatile organic compounds (VOCs) due to their reliability and cost-effectiveness. Especially, the pore size distribution (PSD) of porous carbon materials plays a critical role in VOCs adsorption and separation. In this study, we present benzimidazole-derived porous carbons (BICs) with controllable pore size and oxygen/nitrogen heteroatoms. The pore size is controlled by adjusting the reaction degree of inorganic salt (K2CO3) and nitrogen-containing groups in the system. The tunable pore size distribution makes BICs highly efficient for the adsorption and separation of VOCs. BIC-3-900 exhibits a dominant pore size distribution of 3.0–4.0 nm and an exceptionally high total pore volume of 2.42 cm3/g, leading to ultra-high adsorption capacities for benzene and methanol (22.19 and 52.65 mmol/g, respectively) at 293 K. In addition, BIC-1-700 dominated by the micropore size is more favorable for adsorbing low-concentration small molecule VOCs (with a dynamic adsorption capacity of 4.21 mmol/g for methanol at 3800 ppm). In terms of the adsorptive separation performance, BIC-1-900 has an efficient benzene/methanol selectivity (∼30.13) due to its PSD (1.2–2.0 nm) which was more favorable for capturing low-concentration benzene. Moreover, both experimental and simulation (GCMC and DFT) results demonstrate that the abundant N/O heteroatoms doping in BIC-1-700 results in charge heterogeneity and stronger electrostatic affinity for polar VOCs molecules. Herein, our findings suggest that BICs with controllable pore size and oxygen/nitrogen heteroatoms have significant potential in practical VOCs capture and separation.

METHODS FOR TREATING WASTEWATER FROM ORGANIC POLLUTANTS

Water pollution by organic and inorganic compounds, heavy metals, dyes, radioactive and pharmaceutical compounds has a significant impact on its use. Globalization, structural changes, economic development around the world-all this contributes to the release of pollutants that cause water pollution. That is why wastewater treatment and recovery, removing pollutants from wastewater and wastewater, is one of the most important issues. This literature review examines the problems of water pollution, its sources and types of pollutants found in water, and also discusses purification methods, including methods of water purification from pollutants, in particular phenol, in the presence of nanocomposite catalysts. Recently, more and more attention has been paid to applied nanotechnology using magnetic and carbon nanocomposites due to the advantages of production cost, high surface area, pore volume and environmental sustainability. The literature review focuses on magnetic nanocomposite catalysts based on alternating and rare earth metals with advantages such as low cost of preparation for water disinfection, high surface area, pore volume and environmental resistance. Magnetic carbon nanocomposites usually demonstrate excellent adsorption characteristics of pollutants from aqueous solutions, therefore, the expansion of the use of nanotechnology in water purification is of great importance.

The Application of Biochar for CO2 Capture: Influence of Biochar Preparation and CO2 Capture Reactors.

This work investigates three types of biochar (bamboo charcoal, wood pellet, and coconut shell) for postcombustion carbon capture. Each biochar is structurally modified through physical (H2O, CO2) and chemical (ZnCl2, KOH, H3PO4) activation to improve carbon capture performance. Three methods (CO2 adsorption isotherms, CO2 fixed-bed adsorption, and thermogravimetric analysis) are used to determine the CO2 adsorption capacity. The results show that a more than 2.35 mmol·g-1 (1 bar, 298 K) CO2 capture capacity was achieved using the activated biochar samples. It is also demonstrated that the CO2 capture performance by biochar depends on multiple surface and textural properties. A high surface area and pore volume of biochar resulted in an enhanced CO2 capture capacity. Furthermore, the O*/C ratio and pore width show a negative correlation with the CO2 capture capacity of biochars.

Long-Term Behavior Related to Water Ingress in Mortars Which Combine Expanded and Natural Cork Lightweight Aggregates and Eco-Friendly Cements

The water ingress plays an important role in building materials’ degradation. The use of lightweight aggregates is interesting in terms of sustainability, because they reduce the density of cement-based materials, among other advantages. The development and use of new lightweight aggregates, such as cork granulates, is a current research topic. In the present work, water ingress performance of sustainable mortars which combined expanded and natural cork aggregates and cements with slag, fly ash and limestone has been studied. Mortars produced with sand and expanded clay were also prepared. Bulk density, water absorption, drying capacity and gel and capillary pores were studied. Tests were carried out at 28 days and 1 year. A good behavior has been generally observed when an addition was incorporated to the binder, especially slag or fly ash. Regarding the new non-standardized lightweight cork aggregates, mortars with natural cork showed lower water absorption and lower volume of permeable pore space in the long term than mortars with expanded cork. At one year, natural cork mortars had an adequate water absorption performance compared to those with expanded clay, which may be due to the high volume of small capillary pores (100 nm–1 µm) in natural cork mortars.

The Impact of Mesoporous Silica Nanoparticles on Electrochemical Performance

Mesoporous silica nanoparticles were synthesized via sol–gel method to produce uniform size nanoparticles using n-Octadecyl-trimethoxy silane which gives a good dispersion of silica nanoparticles in hydrophobic mediums. Scanning electron microscopy (SEM), infrared spectroscopy, X-ray diffraction (XRD), thermal gravimetric analysis, and nitrogen adsorption-desorption tests were used to thoroughly investigate the nanocomposites' morphology and structure. BET results show a high surface are of 760 m2/g and specific high pore size (30Ȧ) and pore volume (0.336 cm3/g). The SEM results present that the mesoporous silica nanoparticles possess a well dispersed and uniform particle morphology and FTIR interpenetrating the well-prepared silica nanoparticles which possess Si-O-Si and Si-O bond. The XRD analysis confirmed the amorphous nature silica nanoparticles. The electrochemical properties of silica nanoparticles were evaluated in a potassium chloride solution. With the advantages of a large specific surface area and a suitable pore size distribution, a pair of broad and symmetric redox peaks centred at -0.15 V and 0.6 V appears. Mesoporous silica with a large effective specific surface area demonstrated excellent electrochemical performance, making them excellent candidates for supercapacitors and fuel cells.

Amine functionalized lignin-based mesoporous cellular carbons for CO2 capture

Amine-impregnated mesoporous carbon sorbents have been considered one of the most promising sorbents for CO2 capture from streams with low CO2 concentrations. In this work, a series of novel solid amine adsorbents were prepared by impregnating polyethyleneimine (PEI) on mesoporous carbons using low-cost bio-waste material “lignin” as a carbon precursor via a facile templating method. Our results demonstrated that the mesoporous carbon with 3D interconnected porous structure and large pore size and pore volume exhibited excellent CO2 adsorption capture of 2.90–3.13 mmol/g at a temperature operating window of 75–90 °C under CO2 partial pressure of 0.15 bar, being significantly higher than PEI impregnated sorbents prepared by using mesoporous carbon with 2D porous structures and also many amine-impregnated carbon-based sorbents reported in previous studies. The well-developed 3D interconnected mesoporous structure, high pore volume (up to 1.80 cm3/g), and large pore size permit the facile dispersion and immobilization of PEI within their pores and high availability of amine groups, which significantly improves both the CO2 adsorption capacity and kinetics. In addition, the cyclic adsorption–desorption test showed that the PEI-impregnated sorbents exhibit superior thermo-stability. These results indicate that the PEI-impregnated mesoporous adsorbents are promising ideal candidates for post-combustion CO2 capture. This work provide a potential strategy to prepare advancved amine impregnanted mesoporous carbons from lignin for efficient carbon capture.

Exfoliating kaolin to ultrathin nanosheets with high aspect ratio and pore volume: A comparative study of three kaolin clays in China

A revised intercalation-assisted liquid exfoliation method is proposed to exfoliate three typical kaolin clays (Beihai, Longyan and Suzhou) in China. By combining intercalation, ball milling, ultrasound and freeze-drying technologies, the ultrathin kaolin nanosheets with large lamellar size (1–2 μm), low thickness (∼10 nm), high aspect ratio (∼130) and large pore volume (∼0.3 cm3/g) were obtained. A large number of kaolinite monolayer structures were observed in AFM images, indicating high exfoliation efficiency. The morphology, crystal structure and microstructures of kaolin samples during the intercalation and exfoliation processes are studied in detail by high resolution XRD, SEM, BET and AFM analysis. The mechanism for effective intercalation, exfoliation and stabilization of kaolin clays is clarified. The ultrathin kaolinite nanosheets slightly curved after exfoliation process were found to form some porous structures between the kaolin nanolayers, providing plenty of interlayer spacing for functional loading or interfacial binding. These ultrathin kaolin nanosheets might have the advantages to improve the adhesion ability, loading capacity, and mechanical properties of kaolin-based composite materials.

Phenothiazine-functionalized rGO for Electrochemical Capacitor

The functionalization arose as a technique to improve the physicochemical properties of the reduced graphene oxide (rGO) and consequently enhance the supercapacitor performance. The functionalization compound, phenothiazine (PTZ) introduces nitrogen and sulfur heteroatoms into rGO via the one-pot hydrothermal method at different mass ratio produced PTZ-rGO 5 and PTZ-rGO 10. Incorporation of PTZ on the rGO sheets in PTZ-rGO 5 contributes to the high surface area (163.49 m2 g−1) and pore volume (0.3187 cm3 g−1) properties. Contradictory, overloaded PTZ not only shows a lower reduction effect but also reduces the amount of PTZ functionalized in the PTZ-rGO 10 and consequently shows lower electrochemical performance. The excellent properties enable PTZ-rGO 5 enable it to achieve 119.5 F g−1 at 0.5 A g−1 for its specific capacitance and drive it to be the promising electrode material for supercapacitors.

Synthesis and characterization of novel denosumab/magnesium-based metal organic frameworks nanocomposite prepared by ultrasonic route as drug delivery system for the treatment of osteoporosis.

Introduction: The metal-organic frameworks (MOF) have shown fascinating possibilities in biomedical applications, and designing a drug delivery system (DDS) based on the MOF is important. This work aimed at developing a suitable DDS based on Denosumab-loaded Metal Organic Framework/Magnesium (DSB@MOF (Mg)) for attenuating osteoarthritis. Materials and Methods: The MOF (Mg) (Mg3(BPT)2(H2O)4) was synthesized using a sonochemical protocol. The efficiency of MOF (Mg) as a DDS was evaluated by loading and releasing DSB as a drug. In addition, the performance of MOF (Mg) was evaluated by releasing Mg ions for bone formation. The MOF (Mg) and DSB@MOF (Mg) cytotoxicity towards the MG63 cells were explored by MTT assay. Results: MOF (Mg) characterized by using XRD, SEM, EDX, TGA, and BET. Drug loading, and releasing experiments proved that DSB was loaded on the MOF (Mg) and approximately 72% DSB was released from it after 8h. The characterization techniques showed that MOF (Mg) was successfully synthesized with good crystal structure and thermal stability. The result of BET showed that MOF (Mg) had high surface areas and pore volume. This is the reason why its 25.73% DSB was loaded in the subsequent drug-loading experiment. Drug release and ion release experiments indicated DSB@MOF (Mg) had a good controlled release of DSB and Mg ions in solution. Cytotoxicity assay confirmed that the optimum dose of it had excellent biocompatibility and could stimulate the proliferation of MG63 cells as time went on. Conclusion: Due to the high loading amount of DSB and releasing time, DSB@MOF (Mg) can be promising as a suitable candidate for relieving bone pain caused by osteoporosis, with ossification-reinforcing functions.

Structure and Void Connectivity in Nanocolumnar Thin Films Grown by Magnetron Sputtering at Oblique Angles

The morphology and void connectivity of thin films grown by a magnetron sputtering deposition technique at oblique geometries were studied in this paper. A well-tested thin film growth model was employed to assess the features of these layers along with experimental data taken from the literature. A strong variation in the film morphology and pore topology was found as a function of the growth conditions, which have been linked to the different collisional transport of sputtered species in the plasma gas. Four different characteristic film morphologies were identified, such as (i) highly dense and compact, (ii) compact with large, tilted mesopores, (iii) nanocolumns separated by large mesopores, and (iv) vertically aligned sponge-like coalescent nanostructures. Attending to the topology and connectivity of the voids in the film, the nanocolumnar morphology was shown to present a high pore volume and area connected with the outside by means of mesopores, with a diameter above 2 nm, while the sponge-like nanostructure presented a high pore volume and area, as well as a dense network connectivity by means of micropores, with a diameter below 2 nm. The obtained results describe the different features of the porous network in these films and explain the different performances as gas or liquid sensors in electrochromic applications or for infiltration with nanoparticles or large molecules.

Effect of the Structure of Highly Porous Silica Extracted from Sugarcane Bagasse Fly Ash on Aflatoxin B1 Adsorption.

Sugarcane bagasse fly ash is industrial waste produced by incinerating biomass to generate power and steam. The fly ash contains SiO2 and Al2O3, which can be used to prepare aluminosilicate. This latter material exhibits high potential as an adsorbent in various applications, including the livestock industry where issues related to contamination of aflatoxins in animal feeds need to be addressed; addition of adsorbents can help decrease the concentration of aflatoxins during feed digestion. In this study, the effect of the structure of silica prepared from sugarcane bagasse fly ash on physicochemical properties and aflatoxin B1 (AFB1) adsorption capability compared with that of bentonite was investigated. BPS-5, Xerogel-5, MCM-41, and SBA-15 mesoporous silica supports were synthesized using sodium silicate hydrate (Na2SiO3) from sugarcane bagasse fly ash as a silica source. BPS-5, Xerogel-5, MCM-41, and SBA-15 exhibited amorphous structures, while sodium silicate possessed a crystalline structure. BPS-5 possessed larger pore size, pore volume, and pore size distribution with a bimodal mesoporous structure, while Xerogel-5 exhibited lower pore size and pore size distribution with a unimodal mesoporous structure. BPS-5 with a negatively charged surface exhibited the highest AFB1 adsorption capability compared with other porous silica. However, the AFB1 adsorption capability of bentonite was superior to those of all porous silica. Sufficient pore diameter with high total pore volume as well as high intensity of acid sites and negative charge on the surface of the adsorbent is required to increase AFB1 adsorption in the in vitro gastrointestinal tract of animals.

Investigation on Regulating Porous Polymers with Suitable Porosities and/or N-Doping for Enhanced CO2 Capture

Various cost-effective polycyclic aromatic hydrocarbons (PAHs) were used to fabricate hyper-cross-linked polymers (HCLPs) via an external cross-linker knitting method (ECLKM) followed by N-source impregnation modification. Multiple characterization techniques and thorough tests confirmed that the resultant thermally stable materials featured large specific surface areas (up to 2870 m2 g–1), narrow pore distributions (<0.70 nm), and high pore volumes (up to 1.09 cm3/g). Effects of porosities and N-doping modification ways on HCLPs’ CO2 capture capacities have been fully studied via controlling experiments. Remarkably, HCLP 1 prepared using naphthalene without further modification possessed the highest CO2 uptake capacity (up to 3.8 mmol g–1), indicating that porosities dominated the CO2 uptake process of HCLPs compared with N-doping modification. This study offers a facile strategy to construct HCLPs with great CO2 capture performances using PAHs via a simple one-pot ECLKM.

Construction Archimedean tiling patterns based on soft materials from block copolymers and covalent organic frameworks

Soft materials based on block copolymers or covalent organic frameworks (COFs) that exhibit Archimedean tiling patterns have attracted significant interest due to their potential applications in nanopatterning, nanocomposites, and shape selectivity. Additionally, researchers have widely investigated the corresponding microporous or mesoporous materials with Archimedean tiling from COF or block copolymers as templates with high surface area and pore volume, or tunable porosity with different length scales, in separation, energy storage, drug delivery, photo-catalysis, photovoltaic solar cells, and chemical sensing. This review article highlights recent progress in constructing Archimedean tiling patterns based on the creation of ordered structures from block copolymers by self-assembly and the direct synthesis of COF materials with various topologies.

Synthesis, characterization and thermo-kinetic analysis of sawdust biochar for adsorbent and combustion drives

Biochar is synthesized using two wood species as Dalbergia sissoo wood (DSW) and Mangifera indica wood (MIW), employing pyrolysis of sawdust on a lab-scale. Chemical analysis of these biochars is carried out using elemental analyzer, BET, FTIR, FE-SEM-EDX, and TGA-DTG-DTA analyses. Results showed that high surface area (87.12 and 88.04 m2/g), pore volume (0.0439 and 0.0443 cm3/g), and gross calorific value (28.19 and 28.00 MJ/kg) of DSW and MIW biochar indicate their suitability as sorbent and energy source. Thermogravimetric modeling, based on Coats-Redfern method, is applied on combustion data of biochar samples obtained at 10 and 20 °C/min. Activation energy of DSW- and MIW-biochar are estimated as 2.61–80.49 kJ/mol and 2.58–77.90 kJ/mol, respectively. Statistical analysis of results (R2 > 0.90) indicated the adequacy of proposed kinetic modeling in reproducing the experimental combustion behavior. The propitious outcome of this study account for favoring sawdust-biochar as a good adsorbent and combustion feedstock.

Facile synthesis of nanoscale tantalum powder by electro-deoxidation of Ta2O5

We report a facile approach for the synthesis of nanoscale tantalum powder through the electro-deoxidation of Ta2O5 cathode in molten Na3AlF6-K3AlF6-AlF3. The cathodic process begins with a solid-liquid reaction between Ta2O5 and the electrolyte, resulting in the formation of MxTaO2+xF1-x (M = Na, K) intermediates, which are then electrochemically reduced to tantalum directly. We developed a Ta2O5 cathode with a new porous structure, which contains interconnected microporous, mesoporous and macroporous structures, possessing properties such as high surface area, high pore volume ratios, and excellent accessibility to active sites. This novel cathodic pore structure material enabled the formation of uniform-sized MxTaO2+xF1-x particles with a regular cubic morphology by providing abundant nucleation sites and promoting uniform growth of intermediate. The electrochemical decomposition of these particles and the nucleation, growth mechanisms of tantalum product were also investigated. Our study demonstrates the rational design and synthesis of hierarchically porous cathode materials can facilitate the controllable synthesis of nanoscale metal powders by the FFC process, and may open up new opportunities for the development of other materials with desired morphology and particle size.

A three-dimensional network modifier (dimethyldiethoxysilane) makes ZrO2-SiO2 aerogel with excellent thermal insulation performance and high-temperature stability

Dimethyl-diethoxysilane-modified ZrO2-SiO2 aerogels (DDS/ZSAs) have been prepared by using diethoxydimethylsilane as a co-precursor in the sol-gel process. Dimethyldiethoxysilane acts as a modifier for the three-dimensional structure and composition of aerogels. The pore volume increased from 1.65 cm3/g to 3.21 cm3/g, and the thermal conductivity decreased from 0.03013 Wm−1K−1 to 0.02332 Wm−1K−1 at room temperature. The number of hydroxyl groups on the aerogel surface is effectively reduced, and the particle growth caused by hydroxyl condensation was inhibited during firing. The increase in the number of Si-O-Zr bonds inhibited the growth of t-ZrO2, the phase transition temperature of the aerogel was significantly improved, and the crystallinity and crystal plane types were significantly reduced. Therefore, the porous microstructure of the modified aerogel remains good at 1000 °C. After heat treatment at 1000 °C, it still has a high specific surface area (259.6 m2/g), a high pore volume (1.51 cm3/g), and a low thermal conductivity (0.04206 Wm−1K−1). The thermal insulation performance and temperature resistance of ZrO2-SiO2 aerogels were double improved. In addition, DDS/ZSAs have high hydrophobicity and can be used in harsh wet and high-temperature environments. The research work is instructive for the design of superthermal insulation materials with high thermal stability and low thermal conductivity.

Removal of chloroquine from the aqueous solution by adsorption onto açaí-based biochars: Kinetics, thermodynamics, and phytotoxicity

This study aimed at emerging contaminant chloroquine (CQN) removal, widely used in the COVID-19 pandemic through adsorption and employing a low-cost activated biochar from açai fruit endocarp. Two different adsorbents from the same precursor were applied. The first (CAA) was activated at a high temperature using ZnCl2, and the second (CA) was obtained by physical activation. The adsorbents were characterized through BET, FTIR, DRX, TG/DTG, and SEM. The results showed that zinc chloride activation furnished a material with a high specific surface area (SBET) and pore volume of 762 m2 g−1 and 0.098 cm3 g−1, respectively. Adsorption kinetics and isotherm were best adjusted through the pseudo-second-order (PSO) and Freundlich for both biochars. The process was thermodynamically favorable, occurring spontaneously without energy request. Additionally, the maximum adsorption capacity for CQN was 15.56 and 40.31 mg g−1 for CA and CAA, respectively, in pH 6.84, at a temperature of 25 °C, 50 mL solution and with 0.05 and 0.02 g of adsorbent. Those results are congruent with the literature showing the versatility of the material and the efficiency of the applied adsorption process.

Chemical-free fabrication of carbon fiber aerogels from egg boxes for the removal of pharmaceutically active compounds in aqueous solution

The presence of pharmaceuticals and personal care products (PPCPs) in aquatic environments extremely concerns to human health and the ecosystem; thus, their removal is essential. This work produced carbon fiber aerogels from egg box waste using a chemical-free fabrication process. The pyrolyzed egg box aerogel (PEBA) exhibited three-dimensional interconnected carbon nanofibers with high surface area and pore volume. The adsorption capacity and removal efficiency of four PPCPs, namely, diclofenac (DIC), caffeine (CF), tetracycline (TC), and ciprofloxacin (CIP), from water were high and comparable to other carbon-based adsorbents, and the adsorption time was much faster (within 20 min). The Redlich-Peterson and the pseudo-second-order models were the best-fitted isotherm and kinetic models, respectively, which imply multilayer adsorption at high concentrations and the chemisorption process. Furthermore, mechanisms responsible for the adsorption of all PPCPs were elucidated. PEBA was applied in the fixed-bed column experiment to mimic the continuous adsorption process. In addition, PEBA was recyclable after low-temperature heat treatment. The adsorption capacity (8.1 mg/g) and removal efficiency (94.03 %) for TC was still high after three cycles. Metabolomics analysis revealed that no secondary pollution is released into water after thermal treatment. Therefore, PEBA has the potential as an efficient adsorbent for removing PPCPs from water.

Biomass derived metal free hierarchical porous activated carbon for efficient oxygen evolution reaction

ABSTRACT The development of high electro-efficient and sustainable metal-free catalysts to replace noble materials is essential for the development of prospective renewable energy conversion and storage systems. Due to its ability to provide clean energy, the oxygen evolution reaction (OER) recently received a lot of interest. Here, we provide an effective “waste to clean energy” concept-based porous-activated carbon from waste rice husk with hierarchical pore architecture for OER. The physicochemical properties of the synthesized activated carbon material were examined by XRD, FESEM, and BET analysis. Characterization of the synthesized activated carbon derived from the rice husk shows a high BET surface area (755 m2/g) with high pore volume. Electro-catalytic performance of the prepared porous-activated carbon has been studied, viz. linear sweep voltammetry, Tafel slope, electrochemical impedance spectroscopy, and cyclic voltammetry in 1 M KOH solution. As an electrocatalytic performance in alkaline media, it shows low-onset potential (1.56 V vs. reversible hydrogen electrode), with a small Tafel slope (61 mV/dec), an overpotential (η10) of 0.47 V, low Rct (212 Ω), and good stability toward OER performance even after 500 cycles. Here, we have also reported the effect of electrochemically active surface area and mass activity effect on the OER performance.