Oxygen reduction kinetics on graphite cathodes in sediment microbial fuel cells

Sediment microbial fuel cells (SMFCs) have been used as renewable power sources for sensors in fresh and ocean waters. Organic compounds at the anode drive anodic reactions, while oxygen drives cathodic reactions. An understanding of oxygen reduction kinetics and the factors that determine graphite cathode performance is needed to predict cathodic current and potential losses, and eventually to estimate the power production of SMFCs. Our goals were to (1) experimentally quantify the dependence of oxygen reduction kinetics on temperature, electrode potential, and dissolved oxygen concentration for the graphite cathodes of SMFCs and (2) develop a mechanistic model. To accomplish this, we monitored current on polarized cathodes in river and ocean SMFCs. We found that (1) after oxygen reduction is initiated, the current density is linearly dependent on polarization potential for both SMFC types; (2) current density magnitude increases linearly with temperature in river SMFCs but remains constant with temperature in ocean SMFCs; (3) the standard heterogeneous rate constant controls the current density temperature dependence; (4) river and ocean SMFC graphite cathodes have large potential losses, estimated by the model to be 470 mV and 614 mV, respectively; and (5) the electrochemical potential available at the cathode is the primary factor controlling reduction kinetic rates. The mechanistic model based on thermodynamic and electrochemical principles successfully fit and predicted the data. The data, experimental system, and model can be used in future studies to guide SMFC design and deployment, assess SMFC current production, test cathode material performance, and predict cathode contamination.     

复杂阴极保护体系三维有限元建模研究

滨海电厂常以天然海水作冷却水,其碳钢水室和钛管凝汽器在实施阴极保护过程中,要求钢电位应低于-0.85V(vs.SCE),电位过正则碳钢保护不足,而钛的电位则须高于-0.75V(vs.SCE),电位过负则易使钛管发生氢脆损伤.研究牺牲阳极阴极保护,需先得出两种金属在阴极保护过程中的电位分布.本文在设计室建立碳钢水室钛管模型,根据该实验体系阴极保护过程的(稳态极化和恒电流极化)测定获得两种金属有效的边界条件,用有限元(FEM)计算在一个牺牲阳极阴极保护状态得到三维电位分布.计算结果和测量结果基本一致,为优化阴极保护设计提供了理论依据.     

加油站地下储运设备阴极防蚀之研究

加油站的地下金属结构物除地下储运设备外 (油管与油槽 ) ,地表尚有钢筋混凝土结构 ,且混凝土中的钢筋常与地下储运设备相搭接而造成彼此间的电通性 ;由于地下储运设备的包覆阻抗多大于混凝土阻抗 ,以致在进行阴极保护时 ,保护电流较易由钢筋流过 ,造成储运设备的保护电位不足 .本研究使用外加电流式阴极保护法进行加油站的阴极防蚀 ,讨论钢筋与地下储运设备搭接或未搭接时的防蚀效果 .结果显示 ,在加油站内进行阴极保护电位评估时 ,若只采用 - 85 0mV作为保护评估标准 ,可能会产生误判结果 ,应当使用电流断电器量测结构物在阴极保护下的极化量 ,方能正确判断保护效果     

Effect of gate length and dielectric thickness on ion and fluid transport in a fluidic nanochannel

We have simulated the effect of gate length and dielectric thickness on ion and fluid transport in a fluidic nanochannel with negative surface charge on its walls. A short gate is unable to induce significant cation enrichment in the nanochannel and ion current is controlled mostly by cation depletion at positive gate potentials. The cation enrichment increases with increasing gate length and/or decreasing dielectric thickness due to higher changes induced in the surface charge density and zeta-potential. Thus, long gates and thin dielectric layers are more effective in controlling ion current. The model without Navier-Stokes equations is unable to correctly predict phenomena such as cation enrichment, increase in channel conductivity, and decreasing electric field. Body force and induced fluid velocity decrease slowly and then rapidly with gate potentials. The effectiveness of ion current control by a gate reduces with increasing surface charge density due to reduced fractional change in zeta-potential.     

分散液膜气/液/固三相线界面区宽度在金属大气腐蚀中的作用(英文)

使用接触角测量和电化学技术研究了三相线界面区(TPB)宽度在分散液膜下金属腐蚀过程中的作用.研究发现,阴极极限电流和腐蚀电流密度随TPB宽度增加而线性增大,表明TPB宽度对气/液/固多相体系腐蚀过程具有重要影响.因此,可根据TPB宽度评价分散液膜厚度对大气腐蚀速率之影响.     

Formation of Calcareous Deposit during CathodicProtection and Its Properties

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聚硫堇半导体性质的电化学证据

聚硫堇的电导率为9.6×10－6 S•cm－1,属有机半导体.在10 ℃至60 ℃温度范围内,测定了聚硫堇的循环伏安图、恒电位下的放电曲线和交流阻抗图,并测定了电解质溶液的电导率随温度的变化.在聚硫堇的循环伏安图上有一阳极峰和一阴极峰,它们的峰电流均随温度的升高而增加;恒电位下的放电电流也随温度升高而增加;而交流阻抗的结果表明,聚硫堇的电荷传递电阻Rct随温度升高而下降,即它的交换电流i0随温度升高而增加;而溶液电阻和膜电阻也随温度的升高而下降,这同样会引起电流随温度升高而增加.所以温度对固体半导体聚硫堇的电极反应速率的影响包含了电极反应的本身和电极的电导率等因素的影响.聚硫堇的扫描电镜图证实了它的表面是一种纤维状结构.     

Electroconvection in systems with heterogeneous ion-exchange membranes after thermal modification

It is found that the variations in the structure (morphology and microrelief) and chemical composition of surface of heterogeneous ion-exchange membranes as a result of thermal modification have different effects on the current—voltage characteristics and conditions for the generation of electroconvective instability at the membrane/solution interface under intense current modes. After thermal treatment of strongly acidic sulfocation-exchange membrane, which is characterized by a low catalytic activity in the reaction of water dissociation and a high thermal stability of fixed groups, a fraction of conducting surface area increases and the membrane microrelief develops. As a result, the diffusion limiting current density increases and the length of plateau of the current—voltage curve decreases. Therewith, the thickness of the region of electroconvective instability of solution in the near-membrane region increases and the polarization of electromembrane system, at which the mode of unstable electroconvection is reached, decreases. The thermodestruction of strongly basic anion-exchange membranes, conversely, leads to suppression of electroconvection and an increase in the length of plateau of the current—voltage curve due to the formation of fixed weakly basic amino groups, which are catalytically active in the reaction of water dissociation. A linear correlation is found between the dimensions of the region of electroconvective instability and a fraction of weakly basic functional amino groups in the composition of strongly basic membranes.     

Speciation of cadmium in seawater — a direct voltammetric approach

The present report deals with differential pulse anodic stripping voltammetry (DPASV) applied for the analysis of cadmium in open ocean seawater. Evaluation of different Cd species can generate information about distribution and speciation of Cd in the open ocean. Distribution of Cd was investigated in surface waters of the Atlantic Ocean over a wide geographical range as well as in the water column. Surface water sampling on board the research vessel Polarstern was performed from the bow boom of the ship as well as with a snorkel system which allowed continuous sample-taking. Two different Cd species could be differentiated in the voltammograms. UV-irradiation experiments allowed the identification of an inorganic and organic Cd form, the latter caused by the association between Cd and organic matter as e.g. humic substances (HS). Atlantic ocean surface seawater normally contains between 2 and 4 ng organically complexed Cd/kg and no detectable inorganic Cd. Some areas however showed readings of up to 14 ng inorganic Cd/kg in addition. Water column samples exhibited an enrichment of inorganic Cd by depth. Occurrence of inorganic Cd at the surface could be related to specific oceanographical conditions. Together with analytical results of trace metal contents in the particulate phases of surface seawater, new aspects could be established about the biogeochemical cycling of Cd in the sea.     

多单元气相缓蚀剂的合成,气相缓蚀能力及电化学研究

以吗啉 ,甲醛和环己胺作原料 ,合成N ,N_二 (4_吗啉甲基 )_环己胺 (BMMCH) ,应用红外光谱和氢核磁共振谱表征其结构 ,并由气相防锈甄别实验和气相防锈能力试验考察其气相防锈性能 ;采用密闭空间挥发减量实验 ,比较其气化挥发能力 .结果表明 ,BMMCH对碳钢具有较好的防锈效果 ;其挥发能力较亚硝酸二环己胺弱 .另通过碳钢在模拟大气腐蚀水中的极化曲线测试 ,发现BMMCH的存在导致碳钢电极的腐蚀电位负移 ,对阴极过程能起抑制作用 ,同时降低了阳极钝化区的电流密度 .电化学阻抗谱研究表明 ,BMMCH对碳钢具有较好成膜稳定性     

Effect of Cathodic and Anodic Polarization in a Lithium Sulfate Solution on the Electrical Conductivity of Carbonized Fibrous Carbon Materials

The effect of cathodic and anodic polarization of carbonized fibrous carbon materials of the KNM and NT-1 types in a Li₂SO₄ solution on their properties: electrical conductivity, steady-state electrode potential, electrical conductivity profile across the electrode thickness in relation to the electrolysis time, current density, Li₂SO₄ concentration, electrode thickness, and current reversal, was studied. The stability of the resulting electrical conductivity profile across the electrode thickness with time and in the course of the subsequent electrolysis was assessed. The reduction of Fe³⁺ in the ferri-ferrocyanide system on electrically iso- and nonisoconducting fibrous carbon electrode was considered.     

Current Distribution in Porous Electrodes Whose Solid Phase's Conductivity Increases from the Rear Current Lead

The distribution of the current and the efficiency of porous electrodes (PE) of different types are studied theoretically and experimentally. The conductivity of the solid phase of PE under study is the same or either decreases or increases from the rear current lead. As compared with a homogeneous porous matrix, decreasing the solid-phase conductivity deep into PE improves the PE efficiency by approximately 2.5 times. Conversely, increasing the conductivity deep into PE dramatically degrades the PE efficiency. Simultaneously, the current concentrates near edges of PE and an anodic zone emerges inside PE. As a result, dependences of the effective working thickness of PE and the current efficiency for the target reaction on the total current density acquire a characteristic form, with a maximum and a subsequent decay to zero, even up to a change in the sign. Qualitatively similar form of such dependences is observed experimentally during cathodic reduction of K₃Fe(CN)₆on PE comprising three layers of carbon felts with gradually increasing conductivities.     

Electrokinetics of a poly(N-isopropylacrylamid-co-carboxyacrylamid) soft thin film: evidence of diffuse segment distribution in the swollen state

Streaming current measurements were performed on poly(N-isopropylacrylamid-co-carboxyacrylamid) (PNiPAAM-co-carboxyAAM) soft thin films over a broad range of pH and salt concentration (pH 2.5-10, 0.1-10 mM KCl) at a constant temperature of 22 °C. The films are negatively charged because of the ionization of the carboxylic acid groups in the repeat unit of the copolymer. For a given salt concentration, the absolute value of the streaming current exhibits an unconventional, nonmonotonous dependence on pH with the presence of a maximum at pH ～6.4. This maximum is most pronounced at low electrolyte concentration and gradually disappears with increasing salinity. Complementary ellipsometry data further reveal that the average film thickness increases by a factor of ～2.2 with increasing pH over the whole range of salt concentration examined. The larger the solution salt concentration, the lower the pH value where expansion of the hydrogel layer starts to take place. The dependence of film thickness on pH and electrolyte concentration remarkably follows that obtained for surface conductivity. The streaming current and surface conductivity results could be consistently interpreted on a quantitative basis using the theory we previously derived for the electrokinetics of charged diffuse (heterogeneous) soft thin films complemented here by the derivation of a general expression for the surface conductivity of such systems. In particular, the maximum in streaming current versus pH is unambiguously attributed to the presence of an interphasial gradient in polymer segment density following the heterogeneous expansion of the chains within the film upon swelling with increasing pH. A quantitative inspection of the data further suggests that pK values of ionogenic groups in the film as derived from the streaming current and surface conductivity data differ by ～0.9 pH unit. Such a difference is attributed to the impact of position-dependent hydrophobicity across the film on the degree of ionization of carboxylic sites.     

Improving Performance of MFC by Design Alteration and Adding Cathodic Electrolytes

Performance of two microbial fuel cells (MFCs) was investigated under batch and continuous mode of operation using different cathodic electrolyte. The wastewater was supplied from the bottom port provided to the anode chamber in both the MFCs and the effluent left the anode chamber from the top port in MFC-1, whereas in MFC-2, the effluent exit was provided close to membrane. Stainless steel (SS) mesh anode was used in both the MFCs with surface area of 167 and 100 cm(2) in MFC-1 and MFC-2, respectively. Under batch mode and continuous mode of operation, these MFCs gave chemical oxygen demand removal efficiency more than 85% and about 68%, respectively. Under batch mode of operation, maximum power density of 39.95 and 56.87 mW/m(2) and maximum current density of 180.83 and 295 mA/m(2) were obtained in MFC-1 and MFC-2, respectively. Under continuous mode of operation, a reduction in power and current density was observed. Even with less surface area of the anode, MFC-2 produced more current (1.77 mA) than MFC-1 (1.40 mA). Among the cathodic electrolyte tested, these can be listed in decreasing order of power density as aerated KMnO(4) solution > KMnO(4) solution without aeration > aerated tap water > aerated tap water with NaCl.     

Microrelief of Copper Deposit Surfaces: Effect of the Substrate Surface Microirregularities and Electrolysis Conditions

The growth of copper electrodeposits on glassy-carbon and copper electrodes from acidic sulfate electrolytes are investigated using statistical analysis of their surface microrelief. The regularities of variations in the root-mean-square deviation and the mean angle of microirregularities, the excess and the skewness of microrelief of the deposit surface during deposit growth are determined for various electrolysis conditions. At the initial stages of growth of deposits, their roughness substantially depends on the cathodic current density, concentration of copper ions, and crystalline heterogeneity of the cathode surface. An analysis of dependences of the mean angle of microirregularities, the excess, and the skewness of surface microrelief on the deposit thickness showed that the variations in the deposit structure and morphology become more pronounced with increasing cathodic current density and decreasing concentration of copper ions.     

Effects of Cathode and Anode on Deposition of Trimethylsilane in Glow Discharge

DC cathodic polymerization of trimethylsilane (TMS) was carried out in plasma reactors with and without using anode assembly. In DC cathodic polymerization, the TMS plasma polymers are mainly deposited on the cathode (substrate) surface. As a result, fast deposition of TMS plasma polymers was easily achieved in DC cathodic polymerization as compared with AF or RF plasma polymerization. DC cathodic polymerization without using anode assembly has its advantageous features that the size and number of substrates (as cathodes) are not restricted by the size and the location of anode assembly. It was found that the maximum deposition rate on the cathode surfaces was obtained without anode assembly. The DC cathodic polymerization of TMS was conducted also in a large volume reactor with multiple cathodes (substrates). The same deposition mechanisms for DC cathodic polymerization with a single cathode also apply to the multiple cathodes. Uniform deposition on each cathode could be obtained with appropriate spacing of multiple cathodes and by adjusting the operational parameters, which are based on the current density and the system pressure.     

Investigation of a fiber optic surface plasmon spectroscopy in conjunction with conductivity as an in situ method for simultaneously monitoring changes in dissolved organic carbon and salinity in coastal waters

A combination surface plasmon resonance (SPR) and conductivity sensor array was developed and implemented to demonstrate the ability to differentiate among changes in dissolved organic carbon (DOC) and salinity in coastal water. The array is capable of achieving sufficient spatial and temporal data density to better understand the cycling and fate of terrestrial DOC in coastal areas. DOC is the second largest source of bioreactive carbon in the environment and plays a key role in mediating microbial activity and generation of atmospheric CO(2). In the coastal areas, the salinity is also an important property in many applications, such as leak detection for landfill liners, saltwater intrusion to drinking water, marine environment monitoring, and seasonal climate prediction. Conductivity sensors are the industry standard for determining salinity in ocean systems. However, both conductivity and refractive index sensors, such as SPR spectroscopy based sensors, respond to salinity and DOC levels. To demonstrate the capability of the SPR sensor and a conductivity sensor to collect complimentary data useful in discrimination of salinity and DOC in coastal zone water, conductivity, SPR, and temperature data were collected during passage from the Juan de Fuca ridge area returning to the University of Washington docks.     

电流密度对锌电积用Pb-Ag平板阳极电化学行为的影响(英文)

研究了在不同电流密度下进行长时间极化后Pb-Ag(0.8%(质量分数,w))平板阳极的阳极电位、腐蚀率及阳极钝化膜.同时,也研究了该阳极在ZnSO4-MnSO4-H2SO4电解液中的阴极电流效率和阴极锌品质.阳极钝化膜的表面形貌用扫描电镜(SEM)进行观测.实验结果表明,不管电解液中是否存在Mn2+,电流密度对阳极和阴极的电化学行为都产生了显著的影响.随着电流密度的升高,阳极电位、腐蚀率、阴极电流效率和阳极泥生成量也增加,而阴极锌中的Pb含量则减少.当电流密度从500A·m-2降到200A·m-2时,阳极在ZnSO4-MnSO4-H2SO4电解液中的稳定电位和腐蚀率分别减少64mV和40%.此外,在比较低的电流密度下,阳极电位更容易稳定,阳极表面生成的钝化膜更加致密并与基体结合牢固,这些都有利于降低阳极腐蚀率.为了降低阳极电位、减小阳极腐蚀率及阳极泥生成量并提高阴极电流效率和阳极锌品质,锌电积的理想工作条件是较低的阳极电流密度和较高的阴极电流密度.     

Preparation of Ni/poly(1,5-diaminonaphthalene)-modified carbon paste electrode; application in electrocatalytic oxidation of formaldehyde for fuel cells

This paper deals with electrochemical oxidation of formaldehyde in alkaline solution with a new electrocatalytic system composed of carbon paste electrode coated with poly(1,5-diaminonaphthalene) (P-1,5-DAN) film containing incorporated Ni(II)/Ni(III) redox ions. The modifier layer of (P-1,5-DAN-Ni)(OH)₂ at the electrode surface acts as a catalyst for the oxidation of formaldehyde in 0.1-M NaOH solution. Cyclic voltammetric and chronoamperometric experiments showed that the formaldehyde can be oxidized at the surface of Ni/P-1,5-DAN-modified carbon paste electrode. In cyclic voltammetry studies, the peak current of the oxidation of nickel hydroxide in the presence of formaldehyde increases and is followed by a decrease in the corresponding cathodic current. The rate constant (k) for the chemical reaction between the formaldehyde and nickel hydroxide has been evaluated by chronoamperometry method. This polymeric-modified electrode can oxidize the formaldehyde with high current density (over 7 mA cm⁻²). Thus, it can be a candidate as an anode for fuel cell applications.