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This Review presents the latest advances in designing electrochemical sensors by using carbon nanodots (CNDs), carbon dots (CDs), and graphene quantum dots (GQDs), as a new class of photoluminescent nanomaterials, which can be employed in a vast number applications, such as biomedicine, biosensing, optical sensing, catalysis, solar cells, imaging, and electrochemical sensing. Recently, the application of CDs and GQDs in electrochemical sensing has increased significantly because of their unique features such as quantum size, excellent biocompatibility, enzyme‐like activity, high active area, and abundance of hydrophilic groups, such as hydroxyl, carboxyl, or amine functionalities. The applications of CDs and GQDs for the electrochemical sensing of hydrogen peroxide, glucose, other organic molecules, as well as metal ions are presented. The utilization of CDs and GQDs for immunosensing and aptasensing is also reported. [ABSTRACT FROM AUTHOR]

BACKGROUND: In this work, a small‐scale ceramic microbial fuel cell (MFC) with a novel type of metal–carbon‐derived electrocatalyst containing iron and nicarbazin (Fe‐NCB) was developed, to enhance electricity generation from neat human urine. Substrate oxidation at the anode provides energy for the separation of ions and recovery from urine without any chemical or external power additions. RESULTS: The catalyst was shown to be effective in clear electrolyte synthesis of high pH, compared with a range of carbon‐based metal‐free materials. Polarisation curves of tested MFCs showed up to 53% improvement (44.8 W m−3) in performance with the use of Fe‐NCB catalyst. Catholyte production rate and pH directly increased with power performance while the conductivity decreased showing visually clear extracted liquid in the best‐performing MFCs. CONCLUSIONS: Iron based catalyst Fe‐NCB was shown to be a suitable electrocatalyst for the air‐breathing cathode, improving power production from urine‐fed MFCs. The results suggest electrochemical treatment through electro‐osmotic drag while the electricity is produced and not consumed. Electro‐osmotic production of clear catholyte is shown to extract water from urine against osmotic pressure. Recovering valuable resources from urine would help to transform energy intensive treatments to resource production, and will create opportunities for new technology development. © 2018 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

In recent years, the microbial fuel cell (MFC) technology has drawn the attention of the scientific community due to its ability to produce clean energy and treat different types of waste at the same time. Often, expensive catalysts are required to facilitate the oxygen reduction reaction (ORR) and this hinders their large-scale commercialisation. In this work, a novel iron-based catalyst (Fe-STR) synthesised from iron salt and streptomycin as a nitrogen-rich organic precursor was chemically, morphologically and electrochemically studied. The kinetics of Fe-STR with and without being doped with carbon nanotubes (CNT) was initially screened through rotating disk electrode (RDE) analysis. Then, the catalysts were integrated into air-breathing cathodes and placed into ceramic-type MFCs continuously fed with human urine. The half-wave potential showed the following trend Fe-STR > Fe-STR-CNT ≫ AC, indicating better kinetics towards ORR in the case of Fe-STR. In terms of MFC performance, the results showed that cathodes containing Fe-based catalyst outperformed AC-based cathodes after 3 months of operation. The long-term test reported that Fe-STR-based cathodes allow MFCs to reach a stable power output of 104.5 ± 0.0 μW cm−2, 74% higher than AC-based cathodes (60.4 ± 3.9 μW cm−2). To the best of the Authors' knowledge, this power performance is the highest recorded from ceramic-type MFCs fed with human urine. • Novel iron-streptomycin derived catalyst for ORR in MFCs fed with urine. • The half-wave potential results of Fe-STR show a better kinetic towards ORR. • Fe-STR-based cathodes allow MFCs to reach a power output 74% higher than AC. • The maximum power density output was 104.5 ± 0.0 μW cm−2 by using Fe-STR-cathodes. • MFC performance was stable for 3 months working with Fe-STR-based cathodes. [ABSTRACT FROM AUTHOR]

Abstract The scalability of bioelectrochemical systems is a key parameter for their practical implementation in the real-world. Up until now, only urine-fed self-stratifying microbial fuel cells (SSM-MFCs) have been shown to be scalable in width and length with limited power density losses. For practical reasons, the present work focuses on the scalability of SSM-MFCs in the one dimension that has not yet been investigated, namely height. Three different height conditions were considered (1 cm, 2 cm and 3 cm tall electrodes). The normalised power density of the 2 cm and 3 cm conditions were similar either during the durability test under a hydraulic retention time of ≈39 h (i.e. 15.74 ± 0.99 μW.cm−3) and during the polarisation experiments (i.e. 27.79 ± 0.92 μW.cm−3). Conversely, the 1 cm condition had lower power densities of 11.23 ± 0.07 μW.cm−3 and 17.73 ± 3.94 μW.cm−3 both during the durability test and the polarisation experiment, respectively. These results confirm that SSM-MFCs can be scaled in all 3 dimensions with minimal power density losses, with a minimum height threshold for the electrode comprised between 1 cm and 2 cm. Highlights • Membraneless microbial fuel cells (SSM-MFCs) with different heights were investigated. • SSM-MFCs can be scaled-up in height without power density losses. • SSM-MFCs cannot be scaled-down below 1 cm tall electrodes without losses. • Stable operation under potentiostatic conditions achieved for 48 days. [ABSTRACT FROM AUTHOR]

Abstract A self-stratified microbial fuel cell fed with human urine with a total internal volume of 0.55 ml was investigated as an internal supercapacitor, for the first time. The internal self-stratification allowed the development of two zones within the cell volume. The oxidation reaction occurred on the bottom electrode (anode) and the reduction reaction on the top electrode (cathode). The electrodes were discharged galvanostatically at different currents and the two electrodes were able to recover their initial voltage value due to their red-ox reactions. Anode and cathode apparent capacitance was increased after introducing high surface area activated carbon embedded within the electrodes. Peak power produced was 1.20 ± 0.04 mW (2.19 ± 0.06 mW ml−1) for a pulse time of 0.01 s that decreased to 0.65 ± 0.02 mW (1.18 ± 0.04 mW ml−1) for longer pulse periods (5 s). Durability tests were conducted over 44 h with ≈2600 discharge/recharge cycles. In this relatively long-term test, the equivalent series resistance increased only by 10% and the apparent capacitance decreased by 18%. Graphical abstract Image 1 Highlights • Supercapacitive microbial fuel cells fed with human urine were investigated. • Galvanostatic discharges were studied and key parameters presented. • ESR and capacitance were improved by adding supercapacitive features. • Highest power obtained was 1.20 ± 0.04 mW (2.19 ± 0.06 mW ml−1) for a t pulse of 0.01 s. • Stability over 2600 cycles of discharge/self-recharge (44 h) was shown. [ABSTRACT FROM AUTHOR]

Abstract The anode and cathode electrodes of a microbial fuel cell (MFC) stack, composed of 28 single MFCs, were used as the negative and positive electrodes, respectively of an internal self-charged supercapacitor. Particularly, carbon veil was used as the negative electrode and activated carbon with a Fe-based catalyst as the positive electrode. The red-ox reactions on the anode and cathode, self-charged these electrodes creating an internal electrochemical double layer capacitor. Galvanostatic discharges were performed at different current and time pulses. Supercapacitive-MFC (SC-MFC) was also tested at four different solution conductivities. SC-MFC had an equivalent series resistance (ESR) decreasing from 6.00 Ω to 3.42 Ω in four solutions with conductivity between 2.5 mScm−1 and 40 mScm−1. The ohmic resistance of the positive electrode corresponded to 75–80% of the overall ESR. The highest performance was achieved with a solution conductivity of 40 mS cm−1 and this was due to the positive electrode potential enhancement for the utilization of Fe-based catalysts. Maximum power was 36.9 mW (36.9 W m−3) that decreased with increasing pulse time. SC-MFC was subjected to 4520 cycles (8 days) with a pulse time of 5 s (i pulse 55 mA) and a self-recharging time of 150 s showing robust reproducibility. Highlights • Supercapacitive features of 1 L microbial fuel cell were investigated. • Iron based catalyst was used at the cathode to increase the potential. • The positive electrode shown high ESR and high capacitance. • Maximum power output from the supercapacitive MFC was 36.9 mW (36.9 W m−3). • 4520 cycles of discharge/self-recharge were presented showing system stability. [ABSTRACT FROM AUTHOR]