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Research Activities

Electrochemical devices including sensors, energy storage devices and the integrated energy autonomous systems have recently attracted significant attention due to numerous applications including pollution monitoring, food quality, healthcare, electric vehicles, robots and defence. Considering the high electrical, electrochemical and physical (including flexibility/wearability) performances, much attention is required in materials preparation and the fabrication of new electrochemical devices. Here I describe my major expertise and research interests.

 

Advanced Materials

For the development of think and thick film electrodes materials, I synthesized and characterized many novel and  composite materials including  (1) Binary metal oxide pastes (RuO2-Ta2O5, RuO2-SnO2 and RuO2-TiO2),  (2) Graphite-polyurethane resin composites (flexible supercapacitor fabrication - filed patent),  (3) Glass powder KCl composite for Ag/AgCl/KCl reference electrode , (4) Graphite: CuO Polyurethane composites (5) PEDOT:PSS doped DMSO ink in cellulose/polyester cloth (sweat based wearable supercapacitor - filed patent), (6) ABO3 perovskite-based materials, (7) CuO nanostructure based pastes (8) Al doped ZnO, Sn doped ZnO, CuAlO2 and Ga-Sn-Zn-O solutions for transparent thin films and  (9) PZT-PZN composition for electrocermics. 

Sensors

I have developed new potentiometric pH sensors for water quality and health monitoring. One of the major issue in potentiometric pH sensors are the lack of compatible thick film reference electrode (RE). For potentiometric sensors,  flexible and non flexible thick film Ag|AgCl|KCl based REs were developed by screen printing method.  For a stable potential, I prepared a new KCl -glass powder composite.  The thick film Ag|AgCl|KCl was found to attain a much longer lifetime and proper measurement stability. The thick film REs were fabricated on alumina, low temperature co-fired ceramic tape and flexible PET substrates. For online water quality monitoring,  developed potentiometric RuO2 based pH sensors ( based on binary oxides RuO2-Ta2O5, RuO2-SnO2 and RuO2-TiO2 and thick film RE) on alumina and low temperature co-fired ceramic (LTCC) substrate shows sensitivity of 55-57 mV/pH with a response time of < 15 s in the pH range 2-12. Selectivity test proved that the presence of Li+, Na+, and K+ions in a solution had no significant influence on the sensor performance. In addition to this, the same sensors received attention for meat quality analysis by an industry. Comparison of X-ray photoelectron spectra of the sensors before and after treatment in various solutions revealed some differences in surface composition dependent on pH value.

For wearable application flexible/stretchable pH sensors were developed on PET, PDMS and PVC substrates. I involved in development of stretchable wireless system for sweat pH monitoring which is able to withstand up to 53% uniaxial strain and more than 500 cycles to 30% strain.  The pH data can be wirelessly and continuously transmitted to smartphone through a stretchable RFID antenna, of which the radiating performance is stable under 20% strain, as proved by vector network analyzer measurement. To evaluate the full system, the pH value of a human sweat equivalent solution has been measured and wirelessly transmitted to a custom-developed smart phone App. In another work thorough a collaboration with University of Washington Bothell, USA, flexible IrO2 based pH sensor integrated with inductively coupled wireless transmission system for wearable applications

To solve the issues of lack of reference electrode conductimteric pH and ammonia sensors were also designed on alumina and PET substrates using interdigitated electrode (IDE) structure in the frequency range 10 Hz to 2 MHz. As a collaborative work (University of NoviSad , NorthPoint Ltd Serbia) a miniaturized  USB LCR meter developed for metal oxide conductimetric pH sensors.  In collaboration with NIT, Thrichy, India developed CuO nano-structured  pH sensors. 

For air pollution monitoring I partially involved in fabrication and characterization of impedancemetric and potentiometric gas sensors (NO and Oxygen) by using perovskite based materials. A solid-state planar impedancemetric NO sensors was fabricated using yttria stabilized zirconia (YSZ) for a solid electrolyte substrate and a perovskite-type oxides Sm0.9Sr0.1CoO3-δ and Nd0.9Sr0.1CoO3-δ for thick film sensing electrodes. In collaboration with NIT, Thrichy, I developed a flexible room temperature operating ammonia gas sensor. The colleagues in NIT prepared CuO nanorectangle and Nanoflowers by hydrothermal systhesis method and I developed IDE based flexible sensor. The sensor exhibited significant response down to 5 ppm of ammonia with a quick response time of 90 s and recovered to baseline within 120 s.

For wearable pressure sensor, I partially involved in fabrication and characterization of Glycine–Chitosan-Based Flexible Biodegradable Piezoelectric Pressure Sensor. The measured capacitance of the β-glycine/chitosan film is in the range from 0.26 to 0.12 nF at a frequency range from 100 Hz to 1 MHz, and its dielectric constant and loss factor are 7.7 and 0.18, respectively, in the high impedance range under ambient conditions.

Energy Storage

Flexible energy storage holds the key for advances in several emerging fields which include wearable systems, robotics, and autonomous vehicle. I have developed new flexible supercapacitor (SC) for operating sensors, actuators and motors.  One of the SC was developed using  3D graphene foam (GFSC) and Ag conductive epoxy. The GFSC exhibits excellent electrochemical and supercapacitive performance. At a current density of 0.67 mA cm−2, the GFSCs show excellent performance with areal capacitance (38 mF cm−2) about three times higher than the values reported for flexible carbon-based SCs. The observed energy and power densities (3.4 µW h cm−2 and 0.27 mW cm−2 respectively) are better than the values reported for carbon-based SCs. Analyzed under static and dynamic bending conditions, the GFSCs are stable with up to 68% capacitance retention after 25000 charge–discharge cycles. 

Energy Autonomous System​

For self-powered sensors for healthcare applications, I carried out development of fully flexible self-charging power pack (FSPP). GFSC integrated with flexible solar cell and flexible chemi-resistive CuO nanorod based pH sensor for sweat monitoring. The GFSC in the FSPP is fully charged through solar cell under 1 sun illumination (3.8 mW cm−2, Isc = 26.6 mA, Voc = 4.7 V), using a voltage divider to limit the charging current and voltage up to 0.5 mA and 0.8 V, respectively. For powering the motors of prosthetic/robotic hand , the GPU based SC were integrated with solar cell. This work demonstrate for the first time the performance energy autonomy of prosthetic hand by using solar powered SCs for e skin applications. 

For energy autonomous water quality monitoring, the conductivity (RuO2 based sensor) and temperature (graphite based sensor) sensors were powered by using GPU SC and solar cell pack.  The operating range of 10-1000 μS/cm of the conductivity sensor shows its applicability in portable, tap and river water quality monitoring. The sensitivity of the temperature sensor in the range of 25-45°C is found to be 0.280/°C.

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In another work, to obtain high energy density and operating potential I developed a new ​graphene- graphite polyurethane (GPU) based SC.  The  SCs have maximum energy and power densities of 10.22 μWh/cm2 and 11.15 mW/cm2 respectively at a current density of 10 mA/cm2 and operating voltage of 2.25 V (considering the IR drop).  The fabricated SCs show stable response for more than 15000 charging/discharging cycles at current densities of 10 mA/cm2 and operating voltage of 2.5 V (without considering the IR drop). The developed SCs have been tested as energy storage devices for wide applications, namely: (a) solar-powered energy-packs to operate 84 LEDs for more than a minute and to drive the actuators of a prosthetic limb; (b) powering high-torque motors; and (c) wristband for wearable sensors.  

Textile Based Energy Storage and Sensors- Sustainable Electronics

To avoid the electronic waste containing hazardous and toxic substance, I developed new textile based energy storage device. For wearable systems, smart textiles hold great potential for patient monitoring and personal healthcare. For operating the sensors and electronics, I new sweat electrolyte based textile SC were developed. With PEDOT: PSS coated onto cellulose/polyester cloth, the SC shows specific capacitance of 8.94 F g−1 (10 mF cm−2) at 1 mV s−1. With artificial sweat, the energy and power densities of the SC are 1.36 Wh kg−1 and 329.70 W kg−1, respectively for 1.31 V and its specific capacitance is 5.65 F g−1. With real human sweat the observed energy and power densities are 0.25 Wh kg−1, and 30.62 W kg−1, respectively. The SC performance is evaluated with different volumes of sweat (20, 50, and 100 µL), bending radii (10, 15, 20 mm), charging/discharging stability (4000 cycles), and washability. With successful on‐body testing, the first demonstration of the suitability of a sweat‐based SC for self‐powered cloth‐based sensors to monitor sweat salinity is presented. With attractive performance and the use of body fluids, the presented approach is a safe and sustainable route to meet the power requirements in wearable systems. 

In another textile based SC, I involved in characterization of metal coated fabrics as the active material and current collector with nontoxic polyvinyl alcohol (PVA)‐KCl gel electrolyte for wearable SCs. A newly developed metal free graphite printed textile (cellulose‐polyester) based SC were fabricated and used for wearable temperature sensing applications. 

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Textile based sensors have huge impact in wearable system especially for disposable sensors. I developed potentiometric  pH sensor on cloth  by printing method. Sensitive (thick film graphite composite) and reference electrodes (Ag/AgCl) are printed on cellulose-polyester blend cloth. The sensor could be potentially used wearable sweat monitoring.  For sweat monitoring, I also developed salinity sensor on cloth.

Circular economy focusing on the reuse and recycling of materials is gaining significant interest these days as the concern for environment sustainability is increasing. As part of circular economy project, I developed a temperature sensors (30°C/Ω in the temperature range of 25-60°C) using cellulose  and PEDOT:PSS as active electrode. In another work I supported  a master degree student in fabrication glucose sensor on cloth. Currently my research is focusing on the design of jute fibre based SC and sensors for food quality monitoring. 

Transparent conductive oxide (TCO) film for sensors and optoelectronics

I prepared  solution for transparent p-type (CuAlO2), n-type (Al doped ZnO, Zinc Tin Oxide) electrode. Aluminium doped ZnO (AZO) thin films were prepared by both dip and spin coating method. A single phase delafossite CuAlO2 TCO (resistivity 10 Ωcm) thin films were successfully prepared by dip coating technique. The structural properties of the films have been characterized by XRD, AFM, SEM. Transmittance and optical band gap energy of the films were investigated by using UV-visible spectroscopy. Electrical properties of the TCO films were measured by Hall Effect measurement system. TCO based thin film transistor (TFT) fabrication: Ga-Sn-Zn-O (GTZO) TCO thin film for Thin Film Transistor was prepared by spin-coating technique at much lower annealing temperatures, below 350°C. This work carried for short period at CENIMAT University of Lisbon Portugal. 

​Currently I focusing on the development of TCO film based electrochemical sensors.

Electro-ceramics: Piezoelectric Multilayer Actuators  

Piezoelectric based Bimorph Mirror (PBM) based Actuator and Multilayer actuator (MLA) fabricated based on standard solid state ceramic method, tape casting and screen printing. In PBM it was found that, for a given PZT thickness, the radii of curvature increase with increased fused silica thickness owing to increase in stiffness of the mirror, also for a given fused silica thickness, the radii of curvature decrease with increased PZT thickness owing to increase in the cross sectional area leading to increase in generated force. It was also found that the radius of curvature is not a simple function of total thickness square but depends on the PZT/fused silica thickness ratio. For MLA design, carried out synthesis of new low temperature co-fired ceramic (LTCC) materials based on PZT-PZN, investigated the influence of binder burnout processes in in LTCC tape. Prepared LTCC tape for actuator fabrication. Bimorph actuators were tested for energy harvesting application (support for bachelor technologies students project).

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