Membranes for sustainable heat-to-electricity conversion

The loss of a large fraction of generated energy as low-grade waste heat (≤ 100 °C), coupled with the rapid rise in global energy needs, underscores the urgent need for scalable energy-conversion technologies capable of recovering this underutilised resource. To be viable for widespread deployment, such systems must also be sustainable, relying on abundant, non-toxic, and biodegradable materials that minimise environmental impact while ensuring long-term scalability. Ionic thermoelectric devices (i-TE), which exploit the Soret effect (ion diffusion under a thermal gradient), have recently emerged as promising alternatives to conventional electronic thermoelectrics for harvesting such waste heat. Performance of ionic thermoelectric devices is governed by the ionic transport through the membrane, which is in turn controlled by membrane dimensions and surface charge, creating unique opportunities to design membrane-based materials that are both functional and environmentally benign. In this context, cellulose membranes offer a highly sustainable, abundant and biodegradable platform with rich surface chemistry and hierarchical fibrillar architecture. The overall aim of this thesis is to establish functionalised cellulose membranes as sustainable nanofluidic systems for ion-selective transport and ionic thermoelectric energy conversion. To achieve this, in this thesis work, I have systematically engineered surface charge and structural organisation of cellulose membranes to enhance ionic selectivity, conductivity, and ionic thermovoltage output/apparent ionic Seebeck coefficient, employing scalable, green fabrication routes. This thesis presents tailored surface chemistries on cellulose for surface charge engineering and the effect of surface charge and membrane dimension, and architecture on ionic transport and apparent ionic Seebeck coefficient using a range of cellulose membranes, from commercially available regenerated cellulose to wood-derived nanofluidic membranes. Chapter 1 introduces and reviews different cellulose-based membranes, covering the fundamentals of nanofluidic ion transport, electrokinetic phenomena, and state-of-the-art ionic thermoelectric (i-TE) materials. It highlights how fixed surface charge density on membrane walls and electrolyte confinement enable selective ion transport and enhanced ionic Seebeck coefficient. In Chapter 2, commercial regenerated cellulose (RC) membranes with different pore sizes were modified with positive and negatively charged moieties. These modifications increased surface charge density and yielded up to 950-fold higher ionic conductivity compared to pristine RC. For example, positively charged CHMAC-functionalised RC exhibited an apparent ionic Seebeck coefficient of +6.1 mV K-1 in dilute HCl, compared to -0.6 mV K-1 for pristine RC, demonstrating the direct correlation between surface charge density, ion selectivity, and thermovoltage output. Building on these results, Chapter 3 reports sustainable cellulose membranes extracted from waste wood chips and functionalisation with tricarboxylate silane (TMSDA). Silanisation enhanced carboxylate density by 5.4-fold, resulting in high cation-selective transport in neutral KCl electrolyte. TMSDA functionalised membranes delivered an apparent ionic Seebeck coefficient of −12.8 mV K-1 and a power factor of 24.7 μW m-1 K-2, representing a ~3.2-fold improvement over unmodified cellulose. Notably, compared to the surface-modified commercial RC membranes from Chapter 2, the lab-fabricated waste wood-derived TMSDA-functionalised cellulose membrane exhibited approximately a twofold improvement in apparent ionic Seebeck coefficient, underscoring the synergistic effect of sustainable, nano-structural cellulose source, and high fixed charge density on ionic thermoelectric performance. Chapter 4 further advances this functionalisation approach by employing dual-silane functionalisation of naturally aligned wood-derived cellulose nanochannel membranes using dicarboxylate and tricarboxylate silanes. This dual-silane functionalisation strategy systematically tuned surface charge density, amplifying nanofluidic conduction and enabling ionic gating behaviour similar to that of nanofluidic transistors. Under dilute NaOH, tricarboxylate membranes achieved an apparent ionic Seebeck coefficient of ~14.5 mV K-1 and a power factor of ~77 μW m-1 K-2, highlighting the synergistic effect of enhanced fixed charge with dual-silane and aligned nanofibrillar channels in wood-derived cellulose. To show the versatility of the functionalisation approach for surface charge tuning, Chapter 5 presents complementary polycarbonate (PC) membranes functionalised with amine and glycine groups, and the estimation of effective surface charge densities. This chapter provides a non-cellulosic benchmark for achievable charge densities and contextualises the cellulose results. Thus, by leveraging surface-charge engineering, silane chemistry, and natural fibre alignment, this thesis demonstrates scalable improvements in ionic conductivity and apparent ionic Seebeck coefficient under benign aqueous electrolytes. The findings provide mechanistic insight and practical design strategies toward green, high-performance ionic thermoelectrics, laying the foundation for future applications in wearable electronics, distributed sensing, and large-scale waste-heat recovery.

Grantor dc:publisher

University College Cork

Year dc:date.issued

2025

Author dc:creator

• Ashokan, Anjali

Advisors dc:contributor.advisor

• Holmes, Justin
• Biswas, Subhajit
• Razeeb Mahmood, Kafil

• Cellulose membranes
• Ionic thermoelectrics
• Nanofluidics
• Ion-selective transport
• Surface charge engineering
• Sustainable materials
• Waste heat recovery
• Energy harvesting
• Ionic conductivity
• Electrokinetic phenomena

Statement dc:rights

• © 2025, Anjali Ashokan.

Licence dc:rights.uri

• https://creativecommons.org/licenses/by-nc-nd/4.0/

Language dc:language.iso

en

Handle dc:identifier.uri

https://hdl.handle.net/10468/18944

OAI identifier oai:identifier

oai:cora.ucc.ie:10468/18944