Synergy of slippery surface and pulse flow: An anti-scaling solution for direct contact membrane distillation

Recent progress on mitigating scaling on hydrophobic membrane distillation (MD) membrane focuses on the design of superhydrophobic/omniphobic surface and process optimization. However, the rationale for scaling resistance is not yet complete. We attempted in this work to unravel the correlation of scaling resistance based on the synergy of slippery surface (via chem-physical engineering) and pulse flow (process engineering). Superhydrophobic micro-pillared polyvinylidene fluoride (MP-PVDF) and CF4 plasma modified MP-PVDF (CF4-MP-PVDF) were utilized as the model membranes. We proposed rheometry as a simple quantitative measure for the wetting state in a hydrodynamic environment. Results showed that MP-PVDF possessed pinned wetting and prone to scaling (2000 mg/L CaSO4 solution) in both steady and pulse flow. In contrast, the CF4-MP-PVDF showed suspended wetting and excellent scaling resistance (at water recovery of 60%, the CF4-MP-PVDF surface was still clean without any crystals) under pulse flow, but not at steady flow. At steady flow, feed over-pressure changes the suspended wetting to pinned wetting by pushing the water-gas interface into the pillars, thereby resulting in scaling for CF4-MP-PVDF. At pulse flow, rhythmic fluctuation in the water-gas interface for CF4-MP-PVDF led to sustained scaling resistance. For the first time, we experimentally demonstrated a scaling resistance in DCMD via engineering surface wetting state and process. We envision that this rationale would pave the forward-looking strategy for a robust stable MD process in the near future

Direct Laser Writing of Graphene Made from Chemical Vapor Deposition for Flexible, Integratable Micro-Supercapacitors with Ultrahigh Power Output

High‐performance yet flexible micro‐supercapacitors (MSCs) hold great promise as miniaturized power sources for increasing demand of integrated electronic devices. Herein, this study demonstrates a scalable fabrication of multilayered graphene‐based MSCs (MG‐MSCs), by direct laser writing (DLW) of stacked graphene films made from industry‐scale chemical vapor deposition (CVD). Combining the dry transfer of multilayered CVD graphene films, DLW allows a highly efficient fabrication of large‐areal MSCs with exceptional flexibility, diverse planar geometry, and capability of customer‐designed integration. The MG‐MSCs exhibit simultaneously ultrahigh energy density of 23 mWh cm−3 and power density of 1860 W cm−3 in an ionogel electrolyte. Notably, such MG‐MSCs demonstrate an outstanding flexible alternating current line‐filtering performance in poly(vinyl alcohol) (PVA)/H2SO4 hydrogel electrolyte, indicated by a phase angle of −76.2° at 120 Hz and a resistance–capacitance constant of 0.54 ms, due to the efficient ion transport coupled with the excellent electric conductance of the planar MG microelectrodes. MG–polyaniline (MG‐PANI) hybrid MSCs fabricated by DLW of MG‐PANI hybrid films show an optimized capacitance of 3.8 mF cm−2 in PVA/H2SO4 hydrogel electrolyte; an integrated device comprising MG‐MSCs line filtering, MG‐PANI MSCs, and pressure/gas sensors is demonstrated

Slippery for scaling resistance in membrane distillation: a novel porous micropillared superhydrophobic surface

Scaling in membrane distillation (MD) is a key issue in desalination of concentrated saline water, where the interface property between the membrane and the feed become critical. In this paper, a slippery mechanism was explored as an innovative concept to understand the scaling behavior in membrane distillation for a soluble salt, NaCl. The investigation was based on a novel design of a superhydrophobic polyvinylidene fluoride (PVDF) membrane with micro-pillar arrays (MP-PVDF) using a micromolding phase separation (μPS) method. The membrane showed a contact angle of 166.0 ± 2.3° and the sliding angle of 15.8 ± 3.3°. After CF4 plasma treatment, the resultant membrane (CF4-MP-PVDF) showed a reduced sliding angle of 3.0o. In direct contact membrane distillation (DCMD), the CF4-MP-PVDF membrane illustrated excellent anti-scaling in concentrating saturated NaCl feed. Characterization of the used membranes showed that aggregation of NaCl crystals occurred on the control PVDF and MP-PVDF membranes, but not on the CF4-MP-PVDF membrane. To understand this phenomenon, a “slippery” theory was introduced and correlated the sliding angle to the slippery surface of CF4-MP-PVDF and its anti-scaling property. This work proposed a well-defined physical and theoretical platform for investigating scaling problems in membrane distillation and beyond

Unprecedented scaling/fouling resistance of omniphobic polyvinylidene fluoride membrane with silica nanoparticle coated micropillars in direct contact membrane distillation

Recent development of omniphobic membranes shows promise in scaling/fouling mitigation in membrane distillation (MD), however, the fundamental understanding is still under dispute. In this paper, we report a novel omniphobic micropillared membrane coated by silica nanoparticles (SiNPs) (SiNPs-MP-PVDF) with dual-scale roughness prepared by a micromolding phase separation (μPS) and electrostatic attraction. This membrane was used as a model for analysis of scaling behavior by calcium sulfate (CaSO4) scaling and fouling behavior by protein casein in comparison with commercial (C-PVDF) and micropillared (MP-PVDF) membranes. Unprecedented scaling/fouling resistance to CaSO4 and casein was observed in direct contact membrane distillation (DCMD) for SiNPs-MP-PVDF membrane. Similar scaling and fouling occurred for commercial PVDF and micropillared PVDF membranes. The observation corresponds well to the wetting state of all membranes as SiNPs-MP-PVDF shows suspended wetting, but MP-PVDF shows pinned wetting. From a hydrodynamic view, the suspended wetting attributes a slippery surface which reduces the direct contact of foulants to solid membrane part and leads to significantly reduced fouling and scaling. However, a pinned (or metastable) wetting state leads to a stagnant interfacial layer that is prone to severe fouling and scaling. This work highlights that both scaling and fouling resistance are indeed of suspended wetting state and slippage origin

Influence of nitrogen on corrosion behaviour of high nitrogen martensitic stainless steels manufactured by pressurized metallurgy

Effect of nitrogen on microstructure and corrosion behaviour of high nitrogen martensitic stainless steels manufactured by pressurized metallurgy was investigated by microscopy, electrochemical and spectroscopy analyses. Results indicated that increasing nitrogen content significantly enhanced the corrosion properties of martensitic stainless steels, while excess nitrogen deteriorated the corrosion resistance. The impacts of increased nitrogen content could be summarized as three aspects: the change of precipitation content and conversion of main precipitates from MC to MN; the enhanced protection performance of passive film by enrichment of Cr, especially CrO and CrN; the improved repassivation ability by increased nitrogen content in solid solution

Low sample volume origami-paper-based graphene-modified aptasensors for label-free electrochemical detection of cancer biomarker-EGFR

In this work, an electrochemical paper-based aptasensor was fabricated for label-free and ultrasensitive detection of epidermal growth factor receptor (EGFR) by employing anti-EGFR aptamers as the bio-recognition element. The device used the concept of paper-folding, or origami, to serve as a valve between sample introduction and detection, so reducing sampling volumes and improving operation convenience. Amino-functionalized graphene (NH2-GO)/thionine (THI)/gold particle (AuNP) nanocomposites were used to modify the working electrode not only to generate the electrochemical signals, but also to provide an environment conducive to aptamer immobilization. Electrochemical characterization revealed that the formation of an insulating aptamer–antigen immunocomplex would hinder electron transfer from the sample medium to the working electrode, thus resulting in a lower signal. The experimental results showed that the proposed aptasensor exhibited a linear range from 0.05 to 200 ngmL−1 (R2 = 0.989) and a detection limit of 5 pgmL−1 for EGFR. The analytical reliability of the proposed paper-based aptasensor was further investigated by analyzing serum samples, showing good agreement with the gold-standard enzyme-linked immunosorbent assa

Ex Situ Reconstruction-Shaped Ir/CoO/Perovskite Heterojunction for Boosted Water Oxidation Reaction

The oxygen evolution reaction (OER) is the performance-limiting step in the process of water splitting. In situ electrochemical conditioning could induce surface reconstruction of various OER electrocatalysts, forming reactive sites dynamically but at the expense of fast cation leaching. Therefore, achieving simultaneous improvement in catalytic activity and stability remains a significant challenge. Herein, we used a scalable cation deficiency-driven exsolution approach to ex situ reconstruct a homogeneous-doped cobaltate precursor into an Ir/CoO/perovskite heterojunction (SCI-350), which served as an active and stable OER electrode. The SCI-350 catalyst exhibited a low overpotential of 240 mV at 10 mA cm-2 in 1 M KOH and superior durability in practical electrolysis for over 150 h. The outstanding activity is preliminarily attributed to the exponentially enlarged electrochemical surface area for charge accumulation, increasing from 3.3 to 175.5 mF cm-2. Moreover, density functional theory calculations combined with advanced spectroscopy and 18O isotope-labeling experiments evidenced the tripled oxygen exchange kinetics, strengthened metal-oxygen hybridization, and engaged lattice oxygen oxidation for O-O coupling on SCI-350. This work presents a promising and feasible strategy for constructing highly active oxide OER electrocatalysts without sacrificing durability

Fundamental relations for rational catalyst design in oxygen electrocatalysis

The rapid growth of economy brings about serious environmental and resources problems, which force human search for more efficient and environmental benign technologies for energy conversion. Among the various technologies, fuel cells and soar energy are most attractive. The wide application of the clean energy technologies heavily relies on the efficiency electrocatalytic reactions involved. However, the sluggish kinetics of oxygen electrocatalysis, including oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), seriously limits the efficiency of the clean energy technologies. The kinetics can’t satisfy practical requirements even on the best catalysts of oxygen electrocatalysis, making them highly irreversible reactions. What’s worse, the best catalysts are mainly consist of precious materials and their long-term stability is far below practical requirements, such as RuO2 and IrO2 in OER, Pt and Pd in ORR. On the other hand, earth abundant elements such as Fe, Co, Ni are more stable in oxygen electrocatalysis, but their activity is much lower than the precious catalysts. Therefore, detailed elementary processes in oxygen electrocatalysis and their origin should be identified so that one can rationally design new catalysts to fulfill the critical requirements from activity, stability and cost. A number of important reactions such as the oxygen evolution reaction (OER) are catalyzed by transition metal oxides (TMOs), whereas surface reactivity of which is rather elusive. Therefore, rationally tailoring adsorption energy of intermediates on TMOs to achieve desirable catalytic performances still remains a great challenge. My first research work is the identification of a general and tunable surface structure, coordinatively unsaturated metal cation (MCUS), as a good surface reactivity descriptor for TMOs in OER. Surface reactivity of a given TMO increases monotonically with the density of MCUS, thus increase in MCUS improves the catalytic activity for weak-binding TMOs but impairs that for strong-binding ones. Then, I continued to identify the fundamental relations for catalyst design. The most challenging but critical research in the field of catalysis is to identify the rate determining step and associated with elementary thermodynamic origin. However, sophistication of electrified liquid/solid interface and complexity of catalyst’s structure and composition make it incredibly difficult to derive the surface thermodynamics. Here for the first time, we developed a new kinetic model to give a quantitative description of the electrochemical kinetics of oxygen electrocatalysis with elementary surface thermodynamics. Based on the distinctive features in the kinetics for different surface thermodynamics, a straightforward methodology is developed to identify surface thermodynamics from simple electrochemical tests. Our results show that the mechanistic information derived from one reaction is a critical complement to the other, whereas individual study of either reaction could only provide incomplete mechanistic information. The predictive power of our method in developing better catalysts was successfully demonstrated on α-MnO2. Based on our model, we further answered several questions in oxygen electrocatalysis. For example, what’s the origin of the inconsistence between exchange current density with overall catalytic activity? Exchange current density has been used to represent activity in hydrogen electrocatalysis. However, in oxygen electrocatalysis, exchange current density usually does not correlate with catalytic activity. Through comparison with kinetic behaviour of hydrogen electrocatalysis we prove that kinetics of oxygen electrocatalysis and other highly irreversible reactions are predominantly dependent on Tafel slope, instead of exchange current density. Low Tafel slope of good catalysts originates from the collective contribution from RDS and pre-adsorbed intermediates prior to RDS, which also causes orders decrease in exchange current density predicted from Tafel plots.Doctor of Philosophy (SCBE

Towards the Rational Design of Stable Electrocatalysts for Green Hydrogen Production

Now, it is time to set up reliable water electrolysis stacks with active and robust electrocatalysts to produce green hydrogen. Compared with catalytic kinetics, much less attention has been paid to catalyst stability, and the weak understanding of the catalyst deactivation mechanism restricts the design of robust electrocatalysts. Herein, we discuss the issues of catalysts’ stability evaluation and characterization, and the degradation mechanism. The systematic understanding of the degradation mechanism would help us to formulate principles for the design of stable catalysts. Particularly, we found that the dissolution rate for different 3d transition metals differed greatly: Fe dissolves 114 and 84 times faster than Co and Ni. Based on this trend, we designed Fe@Ni and FeNi@Ni core-shell structures to achieve excellent stability in a 1 A cm−2 current density, as well as good catalytic activity at the same time

Scaling resistance by fluoro-treatments: the importance of wetting states

Membrane distillation is a thermally driven separation process using hydrophobic, porous membranes. Among various problems faced by membrane distillation, scaling remains an unresolved challenge in treating streams of high salinity. Development of superhydrophobic membranes has been a central approach to address this, with CF4 plasma treatment or fluorochemical modification commonly used. However, contradictory observations often occur where some membranes are scaling resistant, but others are not. For the first time, we examine this issue by systematic comparison of the impacts of commonly used fluoro-treatments on scaling resistance. A state-of-the-art surface patterned micro-pillared poly (vinylidene fluoride) membrane (MP-PVDF) was used and both CF4 plasma and fluorosilane reagents were utilized to enhance membrane hydrophobicity. The resulting membranes CF4-MP-PVDF (by CF4 plasma) and FAS-MP-PVDF (via fluorosilane) were systematically characterized and their anti-scaling performance was evaluated using a supersaturated CaSO4 solution. Although both modified membranes showed an increased water contact angle, reduced sliding angle and surface energy, CF4-MP-PVDF demonstrated better scaling resistance than FAS-MP-PVDF. Conventional thermodynamic nucleation models dictate similar nucleation energy barriers for both, in discrepancy to experimental observations. Instead, the wetting states and hydraulic surface slippage were identified as the determinant factors. The CF4-MP-PVDF in a suspended-wetting state with slippage resisted scaling robustly, while FAS-MP-PVDF in an unstable transition state and pristine MP-PVDF in a pinned state were suspectable to scaling. These results unravel, for the first time, the fundamental mechanism behind the differences in scaling resistance by CF4 plasma treatment and fluorosilane surface modification