https://repository.seku.ac.ke/handle/123456789/82 2026-07-23T23:21:23Z 2026-07-23T23:21:23Z Ngumbi, Paul K. https://repository.seku.ac.ke/handle/123456789/8405 2026-07-23T08:42:42Z 2021-01-01T00:00:00Z

Synthesis and characterization of graphene-gold nanocomposite counter electrodes for dye-sensitized solar cell application Ngumbi, Paul K. With the increasing global demand for alternative energy, photovoltaic (PV) technology has undergone remarkable evolution extending over four PV generations. Currently, dye-sensitized solar cells (DSSCs), which fall within the 3rd generation are more versatile and relatively less costly compared to the dominant crystalline silicon (Si) technologies. These DSSCs bear the hope of becoming the future solution to the energy crisis. However, the poor catalysis within the electrodes makes DSSCs relatively inefficient hence ineffective for full commercial utilization. This study investigated the use of composite counter electrodes (CEs) by incorporating graphene (Gr) and/or plasmonic gold (Au) nanoparticles (NPs) with the aim to improve the efficiency of the conventional platinum (Pt) based DSSCs. Gold nanoparticles (AuNPs) in colloidal form with an average size of ~12 nm were prepared by chemical reduction method. The size evaluation was done using the resulting localized surface plasmon resonance (LSPR) peak wavelength values (518- 520 nm) based on the UV-Vis spectra data. Graphene modules prepared by chemical vapor deposition (CVD) on a glass/Fluorine-doped tin oxide (FTO) were used as electrode substrates in this work. These modules depicted G and 2D bands at 1380 and 1700 cm-1 respectively under Raman spectroscopy, indicating low defect state of the Gr. Four different sets of composite CE modules namely FTO/Pt (CE1), FTO/Gr/Pt (CE2), FTO/Gr/AuNPs (CE3) and FTO/Gr/AuNPs/Pt (CE4) were prepared. The CEs recorded transmittance above 70% under UV-Vis spectroscopy, meeting requirements for back-electrode application in DSSCs. Photoanode (PA) modules consisting of titanium dioxide (TiO2) on the FTO/Gr substrate were prepared by the doctor-blade method. Three DSSC modules were assembled based CEs CE2, CE3 and CE4 and their PV performance compared to the reference cell based on CE1. Both Gr and AuNPs were found to possess PV properties and enhanced the resulting DSSCs by 13.4 and 10.1% respectively. The DSSCs containing Pt (CE1, CE2, and CE4) performed better in terms of power conversion efficiency (PCE) due to the increased short circuit current densities (Jsc) and fill factors (FF). However, change in the open-circuit voltage (Voc) was negligible. The CE4 based cell design presented the best PCE at 4.62%, In this composite DSSC, the Jsc (9.92 mA/cm2 ), Voc (0.68 V) and FF (0.69) values recorded representing 13.9%, 0.8% and 9.0% increase respectively compared to CE1. This represented a 24.9% PCE enhancement, which is the synergistic effect of Gr and AuNPs as a composite. This remarkable performance could be due to increased electrocatalytic activity resulting from the graphene matrix within the electrodes. It is also partly attributed to the suppressed carrier recombination resulting from the LSPR property of the Au contacts anchored on the Gr surface in the CE structure. Doctor of philosophy in physics, 2021

2021-01-01T00:00:00Z Kasyoka, Philemon https://repository.seku.ac.ke/handle/123456789/7493 2024-01-19T08:45:19Z 2022-01-01T00:00:00Z

Certificateless signcryption for Wireless Sensor Networks Kasyoka, Philemon Wireless sensor networks (WSNs) have become popular in the field of information and communications technology, they are increasingly being used in applications such as surveillance systems, patient monitoring, object tracking, forest fire detection and habitat monitoring among others. By its very nature, a WSN provides a resource constrained environment where devices used are limited in resource usage. Due to these limitations, security challenges have emerged in their applications. Hence, the need for computationally efficient but still secure cryptosystems. Traditional cryptographic primitives cannot be directly applied on WSNs due to their resource constrained nature, this has led to the challenge of achieving cryptographic security goals which are important for effective communication of information on WSNs. Recent studies have shown that it is possible to apply public key cryptography such as ECC to resource constrained devices by using the right selection of algorithms and associated parameters, optimization and low power concepts. To address security challenges on WSNs, this thesis proposed an efficient digital signature scheme, a variant of ECDSA that can be applied on WSNs to provide authentication. Further, the variant of ECDSA was used in the design of a signcryption schemes. The signcryption schemes are intended to be efficient enough for use on WSNs and for that reason the research work focused on certificateless cryptography (CLC) for the design of the signcryption scheme with a property of ciphertext authenticity. The research methodology employed was experimental. Major contributions of this research were an efficient variant of ECDSA more efficient in the signing and verification process that does not suffer from the security challenges inherent in the original ECDSA. Out of the proposed digital signature scheme a certificateless pairing free authentication scheme for wireless body area network in healthcare management system and a multi-user broadcast authentication scheme for WSNs were constructed. Three certificateless signcryption schemes were designed, two signcryption schemes were designed from the proposed ECDSA variant and one signcryption was a modification of a scheme by Wei and Ma (2019). A formal security proof for indistinguishability against adaptive chosen ciphertext attack and existential unforgeability against adaptive chosen message attack was provided for the three signcryption schemes in the random oracle model. The signcryption schemes were more efficient with respect to computational cost, communication overhead and energy consumption comparison with other existing related schemes. Doctor of Philosophy in Computer Science, 2022

2022-01-01T00:00:00Z Waswa, Gabriel A. https://repository.seku.ac.ke/handle/123456789/6793 2023-11-30T09:56:55Z 2022-06-02T00:00:00Z

Characterization and application of nanozeolitic materials as smart delivery systems for urea fertilizer and lambda cyhalothrin pesticide Waswa, Gabriel A. Natural zeolites could be applied as smart delivery system for controlled release of agricultural inputs resulting in enhanced productivity and reduced environmental pollution caused by excessive use of fertilizer and pesticides. This is because zeolites have nano porous voids and channels that can be loaded with quest molecules like urea fertilizer and pesticides. The formulated zeolite composites can then be applied as carrier agent for target and slow delivery of the fertilizer or pesticide to the intended part of the plants, thus improving the efficiency and effectiveness of the agricultural inputs. Besides, these zeolites being natural are meant to be more cost effective and pose less harm to the environment. This research work aimed at sampling natural zeolites from different parts in Kenya and characterizing them in comparison with the commercial zeolites applied as the standard. Kinetics studies were then conducted to determine their fertilizer and pesticide loading properties. After which formulation, modelling, and agronomic simulation studies were done using urea and lambda cyhalothrin pesticide on tomatoes and spinach. Sample collection was done, guided by Kenya’s geological and mineralogical mapping in five selected places named as Eburru volcanic crater, Lake Magadi, Lake Baringo, Ebulbul-Ngong, and Kitum caves-Mt. Elgon. The collected samples transported to the laboratory were mechanically grinded and sieved to obtain homogeneous fine particles... Doctor of Philosophy Degree in Chemistry, 2020

2022-06-02T00:00:00Z Mutwiri, Robert M. https://repository.seku.ac.ke/handle/123456789/6354 2023-11-30T09:48:28Z 2021-10-07T00:00:00Z

Statistical distributions and modelling of GPS-Telemetry elephant movement data including the effect of covariates Mutwiri, Robert M. In this thesis, I investigate the application of various statistical methods towards analysing GPS tracking data collected using GPS collars placed on large mammals in Kruger National Park, South Africa. Animal movement tracking is a rapidly advancing area of ecological research and large amount of data is being collected, with short sampling intervals between successive locations. A statistical challenge is to determine appropriate methods that capture most properties of the data is lacking despite the obvious importance of such information to understanding animal movement. The aim of this study was to investigate appropriate alternative models and compare them with the existing approaches in the literature for analysing GPS tracking data and establish appropriate statistical approaches for interpreting large scale mega-herbivore movements patterns. The focus was on which methods are the most appropriate for the linear metrics (step length and movement speed) and circular metrics (turn angles) for these animals and the comparison of the movement patterns across herds with covariate. A four parameter family of stable distributions was found to better describe the animal movement linear metrics as it captured both skewness and heavy tail properties of the data. The stable model performed favourably better than normal, Student’s t and skewed Student’s t models in an ARMA-GARCH modelling set-up. The flexibility of the stable distribution was further demonstrated in a regression model and compared with the heavy tailed t regression model. We also explore the application circular linear regression model in analysing animal turn angle data with covariate. A regression model assuming Von Mises distributed turn angles was shown to fit the data well and further areas of model development highlighted. A couple of methods for testing the uniformity hypothesis of turn angles are presented. Finally, we model the linear metrics assuming the error terms are stable distributed and the turn angles assuming the error terms are von Mises distributed are recommended for analysing animal movement data with covariate. Doctor of Philosophy in Statistics

2021-10-07T00:00:00Z