Foreign aid—Economic Growth Nexus in Africa: Does Financial Development Matter?

Appiah-Otoo Isaac,Acheampong Alex O.,Song Na,Obeng Camara Kwasi,Appiah Isaac K. 한국국제경제학회 2022 International Economic Journal Vol.36 No.3

This study explored the role of financial development in foreign aid (measured by agriculture, humanitarian, health, economic infrastructure and services, and education aid) and economic growth relationship for 37 African countries spanning the 2002–2018 period. Using the instrumental variable generalized method of moments model, our findings indicated that while foreign aid impedes Africa’s growth, financial development spurs economic growth. The conditional effect analysis showed that financial development conditions foreign aid to spur economic growth. The country-specific analysis further showed that foreign aid has a higher growth elasticity in countries with relatively better financial systems, such as Mauritius, South Africa, Gabon, Tunisia, and Botswana, whilst the growth elasticity of aid is smaller in countries with a relatively weak financial system such as Malawi, Guinea Bissau, Sierra Leone, and the Democratic Republic of Congo. The study recommended the need for policymakers in Africa to implement innovative ways to improve domestic revenue mobilization. The study also recommended that policymakers in Africa should create an enabling environment that will enhance the development of Africa’s financial system to mitigate the adverse effect of aid on economic growth.

A coupled chemo-mechanical model to study the effects of adhesive strength on the electrochemical performance of silicon electrodes for advanced lithium ion batteries

Appiah, Williams Agyei,Park, Joonam,Byun, Seoungwoo,Cho, Inseong,Mozer, Attila,Ryou, Myung-Hyun,Lee, Yong Min Elsevier 2018 Journal of Power Sources Vol.407 No.-

<P><B>Abstract</B></P> <P>A coupled chemo-mechanical model which considers the contact resistance as well as the influence of the attractive forces inside the contact area between the electrode and current collector was developed to evaluate the effects of the adhesive strength of a binding material on the electrochemical performance of silicon-based lithium-ion batteries. The increase in contact resistance between the electrode and current collector was introduced as a factor that reduces the electrochemical performance of the cell. The model predictions were validated with experimental data from coin-type half-cells composed of Li metal, Si electrodes, and Cu current collectors coated with binding materials with different adhesive strengths. The contact resistance increased with an increasing number of cyclic current rate. The adhesive strength decreased with cyclic current rate. The proposed model was used to investigate the effects of adhesive strength and various cell design parameters on the specific capacity of the Si-based Li-ion cells.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A coupled chemo-mechanical model for Si-based secondary batteries. </LI> <LI> Model parameterization and model validation. </LI> <LI> Contact resistance and adhesive strength between Si composite electrode and Cu current. </LI> <LI> Optimal electrode design for specific capacities. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

A highly selective <b>SBA-15</b> supported fluorescent “turn-on” sensor for the fluoride anion

Appiah-Ntiamoah, Richard,Chung, Wook-Jin,Kim, Hern The Royal Society of Chemistry 2015 NEW JOURNAL OF CHEMISTRY Vol.39 No.7

<P>In an effort to improve the performance of organic-based F<SUP>−</SUP> receptors, organo-silica receptors are being developed taking advantage of the large surface area that mesoporous silica offers. In this work, we investigated the possibility of using a simple “piece-wise” assembly method to immobilize silyl-ether protected fluorescein isothiocyanate (FITC) molecules (receptor <B>1</B>) on the surface of 3-aminopropyltrimethoxysilane (APTES) and 3-[2-(2-aminoethylamino)ethylamino] propyltrimethoxysilane (APAEAETMS) modified <B>SBA-15</B> to form sensors <B>ASBA</B> and <B>TSBA</B>, respectively. We showed that aqueous solutions of <B>TSBA</B> (or <B>ASBA</B>) produce distinct changes in absorption and emission spectra upon F<SUP>−</SUP> addition due to the F<SUP>−</SUP> directed cleavage of Si–O bonds on receptor 1. <B>TSBA</B> (or <B>ASBA</B>) remained stable upon prolonged exposure to UV light (losing ∼0.12% of its fluorescence intensity), and was highly selective towards F<SUP>−</SUP> over other common anions (Cl<SUP>−</SUP>, Br<SUP>−</SUP>, I<SUP>−</SUP>, HPO<SUB>4</SUB><SUP>2−</SUP>, ACO<SUP>−</SUP>, and NO<SUB>3</SUB><SUP>−</SUP>). Furthermore, the sensitivity of this type of sensor architecture followed a dependence on the kind of amino-silane compound used, which opens up the possibility of synthesizing sensors with tailored detection limits. The detection limit of <B>TSBA</B> and <B>ASBA</B> was 0.02 μM and 0.5 μM, respectively.</P> <P>Graphic Abstract</P><P><B>TSBA</B> (or <B>ASBA</B>) remained stable upon prolonged exposure to UV light (losing ∼0.12% of its fluorescence intensity), and was highly selective towards F<SUP>−</SUP> over other common anions (Cl<SUP>−</SUP>, Br<SUP>−</SUP>, I<SUP>−</SUP>, HPO<SUB>4</SUB><SUP>2−</SUP>, ACO<SUP>−</SUP>, and NO<SUB>3</SUB><SUP>−</SUP>). <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c5nj00495k'> </P>

Adsorption of Benzene from Aqueous Solution Using Base Modified Expanded Perlite

Appiah-Ntiamoah, Richard,Mai, Xuan Thang,Momade, Francis W.Y.,Kim, Hern Trans Tech Publications, Ltd. 2012 Advanced materials research Vol.622 No.-

<P>In this study, the adsorption capacity of expanded perlite (EP) for benzene at low concentrations in water was investigated after EP was treated with sodium hydroxide (NaOH). IR spectra used to characterize the modified EP showed that there was no bonding between NaOH and the hydroxyl groups on the surface of EP. However, the NaOH provided a basic medium for negatively charged surface oxide ions (-SO-) to form on EP. This fact was corroborated by pH readings of the modification solution. This reduced in pH from 10 to 9 at the end of the reaction which indicated that the hydroxyl OH- groups on the EP underwent deprotonation and hence releases H+ into the solution, and also positive sites on EP adsorbed OH- ions from the base solution. Mahir et al. in their paper Zeta potential of unexpanded and expanded perlite samples in various electrolyte media confirmed that EP has no isoelectric point and exhibits negative zeta potential in the pH range of 2-11. The surface oxides (-SO-) were believed to have given EP it adsorptive potential. Adsorption isotherm values correlated reasonably well with the Langmuir isotherm model and it parameters (qo and K) were obtained using linear regression analysis. A maximum adsorption capacity (qo) value of 19.42 mg/g was achieved.</P>

A Physics-Based Model Capacity Fade Analysis of LiMn₂O₄/Graphite Cell at Different Temperatures

Appiah, Williams Agyei,Ryou, Myung-Hyun,Lee, Yong Min The Electrochemical Society 2019 Journal of the Electrochemical Society Vol.166 No.3

<P>The capacity fading behavior of a LiMn<SUB>2</SUB>O<SUB>4</SUB>/graphite lithium ion cells at different temperatures is analyzed using a physics-based porous composite electrode model and a parameter estimation technique. The parameter estimation technique is used to extract capacity fade dependent model parameters from experimental cycling data. Although the capacity fading mechanism of the LiMn<SUB>2</SUB>O<SUB>4</SUB>/graphite lithium ion cells are greatly influenced by temperature, major capacity fading mechanism is closely related to the trapping of Li ions into solid electrolyte interphase on the graphite negative electrode and the reduction in the volume fraction of the active material in the LiMn<SUB>2</SUB>O<SUB>4</SUB> positive electrode. At 25°C, the dominant capacity fading mechanisms is the formation of the solid electrolyte interphase while at 60°C the dominant capacity fading mechanism is the reduction in the volume fraction of the positive active material. The efficacy of the physics-based composite electrode model is validated with experimental discharge profiles obtained from cells cycled at 25 and 60°C.</P>

Design optimization of LiNi_0.6Co_0.2Mn_0.2O₂/graphite lithium-ion cells based on simulation and experimental data

Appiah, W.A.,Park, J.,Song, S.,Byun, S.,Ryou, M.H.,Lee, Y.M. Elsevier Sequoia 2016 Journal of Power Sources Vol.319 No.-

LiNi<SUB>0.6</SUB>Co<SUB>0.2</SUB>Mn<SUB>0.2</SUB>O<SUB>2</SUB> cathodes of different thicknesses and porosities are prepared and tested, in order to optimize the design of lithium-ion cells. A mathematical model for simulating multiple types of particles with different contact resistances in a single electrode is adopted to study the effects of the different cathode thicknesses and porosities on lithium-ion transport using the nonlinear least squares technique. The model is used to optimize the design of LiNi<SUB>0.6</SUB>Co<SUB>0.2</SUB>Mn<SUB>0.2</SUB>O<SUB>2</SUB>/graphite lithium-ion cells by employing it to generate a number of Ragone plots. The cells are optimized for cathode porosity and thickness, while the anode porosity, anode-to-cathode capacity ratio, thickness and porosity of separator, and electrolyte salt concentration are held constant. Optimization is performed for discharge times ranging from 10 h to 5 min. Using the Levenberg-Marquardt method as a fitting technique, accounting for multiple particles with different contact resistances, and employing a rate-dependent solid-phase diffusion coefficient results in there being good agreement between the simulated and experimentally determined discharge curves. The optimized parameters obtained from this study should serve as a guide for the battery industry as well as for researchers for determining the optimal cell design for different applications.

Compact design of high voltage switch for pulsed power applications

Appiah, G. N.,Jang, S. R.,Bae, J. S.,Cho, C. G.,Song, S. H.,Ryoo, H.J. Institute of Electrical and Electronic Engineers 2017 IEEE transactions on dielectrics and electrical in Vol.24 No.4

<P>Generally, for pulsed power applications with low jitter and high repetition rates, the use of semiconductor switches have replaced the traditional gap switches and thyratron circuits due to the limited lifetime of the thyratron and gap switches under such operating conditions. However, the limited ratings of these semiconductor switches require series and/or parallel stacking in order to handle high voltages and current levels associated with the discharge energy. Described in this paper is the design of a high voltage switch that uses a simple and reliable gate driver circuit for pulsed power application. In this design, the switch module is made up of insulated-gate bipolar transistor (IGBTs) and their gate driver circuits compactly fitted onto a single module. The ease by which the switch modules can be configured (series stacking and/or parallel stacking) to meet future load variations allows for flexible operation of this design. In addition, the detailed implementation of the gate driver circuit for reliable and easier synchronization of the switches, the simultaneous transfer of both signal and power for current boost are also described in this paper. A 120 nF capacitor bank energy was discharged using a configuration of the developed high voltage switch and a developed 15 kV, 1.5 kJ/s peak power capacitor charger, and by experimental results, the operation of the proposed circuit was verified to be effectively used as a switch for pulse discharging.</P>

A Mathematical Model for Cyclic Aging of Spinel LiMn₂O₄/Graphite Lithium-Ion Cells

Appiah, Williams Agyei,Park, Joonam,Byun, Seoungwoo,Ryou, Myung-Hyun,Lee, Yong Min The Electrochemical Society 2016 Journal of the Electrochemical Society Vol.163 No.13

A Comprehensive Capacity Fade Model for Spinel LiMn₂O₄/Graphite Lithium Ion Cell

( Appiah Williams Agyei ),박주남,변승우,유명현,이용민 한국공업화학회 2016 한국공업화학회 연구논문 초록집 Vol.2016 No.1

To fully understand the capacity fade mechanism in lithium ion batteries composed of a spinel-based cathode and an artificial graphite, a comprehensive mathematical model describing the cycling performance of LiMn₂O₄/graphite lithium ion cell is developed in this work. The developed model is incorporated into the Newman`s Porous Composite Electrode framework (PCE) and implemented in the battery module of COMSOL Multiphysics. The proposed model is used to investigate the effects of variations in the ambient temperature and of voltage range of cycling on the capacity fade. The effect of changes in the volume fraction of cathode active material, resistance in the cell and state of charge are also studied.

Design of High Voltage Switch for Pulse Discharging

Appiah Gideon Nimo(아피아 기드온 니모),Sung-Roc Jang(장성록),Hong-Je Ryoo(류홍제) 전력전자학회 2016 전력전자학술대회 논문집 Vol.2016 No.7