Fundamentals and Principles of Solid-State Electrochemical Sensors for High Temperature Gas Detection

The rapid development of science, technology, and engineering in the 21st century has offered a remarkable rise in our living standards. However, at the same time, serious environmental issues have emerged, such as acid rain and the greenhouse effect, which are associated with the ever-increasing need for energy consumption, 85% of which comes from fossil fuels combustion. From this combustion process, except for energy, the main greenhouse gases-carbon dioxide and steam-are produced. Moreover, during industrial processes, many hazardous gases are emitted. For this reason, gas-detecting devices, such as electrochemical gas sensors able to analyze the composition of a target atmosphere in real time, are important for further improving our living quality. Such devices can help address environmental issues and inform us about the presence of dangerous gases. Furthermore, as non-renewable energy sources run out, there is a need for energy saving. By analyzing the composition of combustion emissions of automobiles or industries, combustion processes can be optimized. This review deals with electrochemical gas sensors based on solid oxide electrolytes, which are employed for the detection of hazardous gasses at high temperatures and aggressive environments. The fundamentals, the principle of operation, and the configuration of potentiometric, amperometric, combined (amperometric-potentiometric), and mixed-potential gas sensors are presented. Moreover, the results of previous studies on carbon oxides (COx), nitrogen oxides (NOx), hydrogen (H2), oxygen (O2), ammonia (NH3), and humidity (steam) electrochemical sensors are reported and discussed. Emphasis is given to sensors based on oxygen ion and proton-conducting electrolytes. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.Fotini Tzorbatzoglou, Costas Molochas, and Panagiotis Tsiakaras thankfully acknowledge co-financing from the European Union and Greek national funds through the Operational Program for Competitiveness, Entrepreneurship, and Innovation, under program RESEARCH-CREATE-INNOVATE (Project code: T1EDK-02442)

A review of the most efficient low-Pt and Pt-free electrocatalysts for hydrogen PEMFCs

The main target of the PEMFC's scientific community is to reduce the total Pt loading to ca. 150 g cm-2 MEA, maintaining simultaneously high PEMFCs performances. The last years, promising results have also been reported concerning the design, fabrication, characterization and testing of novel Pt-free anodes and cathodes for H2-PEMFCs applications. The present work aims at providing the state-of-the-art of the most efficient low-Pt and Pt-free electrocatalysts for H2-PEMFC. Copyright © 2013 Delta Energy and Environment

Electrocatalytic activity of Vulcan-XC-72 supported Pd, Rh and PdxRhy toward HOR and ORR

Carbon-supported (Vulcan XC-72) Pd, Rh, and PdxRhy (20 wt%, x:y = 1:1, 3:1, 1:3) electrocatalysts are prepared according a modified pulse-microwave assisted polyol synthesis method and their electrocatalytic activity toward hydrogen electrooxidation (HOR) and oxygen reduction (ORR) reactions is investigated. The as-prepared electrocatalysts are physicochemically characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Their electrochemical characterization is carried out by the aid of cyclic voltammetry (CV), rotating disk electrode (ROE) and chronoamperometry (CA) techniques. It is found that among the as-prepared catalysts, PdRh3 exhibits the highest HOR (i(k) = 7.6 mA cm(-2)) and ORR (i(k) = 5.20 mA cm(-2)) electrocatalytic activity. It is also found that the addition even of small amount of Rh (Pd3Rh-2.7 mu g cm(-2)) enhances both Pd's HOR and ORR electrocatalytic activity by 33% and by 53%, respectively. For the as prepared electrocatalysts, the HOR activity order, in terms of kinetic current density, is found to be as follows: PdRh3 approximate to PdRh > Rh > Pd3Rh > Pd, while a similar trend was found for ORR activity: PdRh3 > PdRh > Rh > Pd3Rh > Pd. (C) 2015 Elsevier B.V. All rights reserved

Direct alcohol fuel cells: Challenges and future trends

Among the most considered future alternative energy conversion systems are fuel cells. A fuel cell is an electrochemical device that continuously and directly converts chemical energy to electricity with the most common technologies to be the Polymer Electrolyte Membrane Fuel Cells (PEMFC) and the Solid Oxide Fuel Cells (SOFC). In such devices hydrogen is considered as the preferred fuel in virtue of its high activity and its environmental benignity.Fuel supply is an important problem to be solved for the commercial application of fuel cells' technology. Conventional fuel-cell types require hydrogen as the fuel, which has to be free of impurities when operated at temperatures below 100°C. The storage and distribution of hydrogen is still one of the open questions in the context of a customer-oriented broad commercial market. The last two decades research effort has been devoted to Direct Alcohol Fuel Cells dedicated to overcome the hydrogen specific restrictions. In this direction direct alcohol fuel cells have been extensively studied and considered as possible power production systems for portable electronic devices and vehicles in the near future. However, because of the relatively low performances and the high cost of platinum-based catalysts, a number of research groups have oriented their efforts mainly towards the development: a) of low or non platinum electrocatalysts (anodes and cathodes) and b) of nanostructured electrocatalysts based on non-noble metals. The challenges and the prospects related to the low and non platinum anodes for direct alcohol (methanol and ethanol) PEM fuel cells are presented and discussed in the present work. © 2011 INESC Coimbra

Filtration efficiency and pressure drop modelling of particulate filters with rear plug damage

A model suitable for wall-flow particulate filters with partial rear plug damage is developed and experimentally validated in this work. A ceramic filter with 16% of the rear plugs mechanically removed is tested at steady-state conditions on the engine bench and transient driving cycle conditions on the chassis dynamometer. After decanning of the monolith, destructive analysis is conducted to identify deposit loading variations and scanning electron microscopy is used to study the deposit structures in the channels. It is shown that channels without rear plugs develop distinct deposit structures in the entry zone. Hence, a local pressure loss coefficient is applied to model the effect of entrance flow constrictions, taking also into account deposit restructuring phenomena at higher flow rates. In addition, a deep-bed filtration submodel is used to capture the effect of non-uniform wall velocities on deposit accumulation in the wall. The modified model is first fitted to the engine bench data and then validated in a wider range of conditions using the driving cycle tests. With the exception of prolonged steady-state loading conditions, good pressure drop and filtration efficiency predictions are obtained throughout the tests in conjunction with correct deposit property profiles. Notably, the cold-start worldwide harmonized light vehicles test cycle shows that the current European on-board diagnosis threshold limit for particulate mass is too relaxed to trigger a malfunction indication for moderate filter faults. In conclusion, the model can be applied in damaged particulate filter studies for the assessment of leaked particulate mass, the specification of more effective legislation limits and the development of rigorous on-board diagnosis systems and algorithms. © IMechE 2020

Experimental and Computational Investigation of Particle Filtration Mechanisms in Partially Damaged DPFs

Understanding the filtration mechanisms in partially damaged Diesel Particulate Filters is very important for the design of exhaust systems with efficient On-Board Diagnosis functionality, especially as new threshold limits have been recently applied for particulate mass leakage. Two common types of DPF failure are included in this study, namely rear plug removal and internal failure due to uncontrolled regeneration with excessive deposit loading. Initially, the two respective filters were loaded on the engine bench with particle measurement upstream and downstream, and then they were disassembled and sectioned to study the deposit distribution. The analysis of the second filter revealed several modes of failure that should be expected under real-life conditions such as material accumulation in the inlet channels, substrate melting, and crosswise and diagonal crack development. Moreover, a computational model with the necessary adjustments is used to simulate the loading experiments and interpret the underlying filtration mechanisms. The processed results reveal small effects of temperature and mass flow rate on the filtration efficiency and a comparatively stronger impact of the total deposit loading. The local deposit loading is uniform in the intact segments, while it is non-uniform with a minimum value at the failure location in the unplugged and internally damaged segments. This finding is consistent with the wall flow predicted by the model, whereas some discrepancies of the model can be attributed to a secondary collection mechanism. © 2019 SAE International and © 2019 SAE Naples Section. All rights reserved

Numerical study of flow and particle deposition in wall-flow filters with intact or damaged exit

We examine the time-dependent three-dimensional gas-particle flow in an intact wall-flow filter consisting of channels alternatively plugged at each end and a partially damaged filter in which the rear plugs are removed. Our focus is placed on highlighting the differences in the flow pattern and the deposition process between the two geometries. The Navier-Stokes equations are solved for the fluid flow coupled with a Brinkman/Forchheimmer model in order to simulate the flow in the porous walls and plugs. Discrete particle simulation is utilized to determine the nanoparticle trajectories. Using this scheme, we are able to characterize the main features of the flow fields developing in the intact and damaged filters with respect to the Reynolds number and identify those affecting the transport and deposition of particles that have three representative response times. We present fluid velocity iso-contours, which describe the flow regimes inside the channels, as well as in regions upstream and downstream of them. We provide evidence of local recirculating bubbles at the entrance of the channels and after their exit, whereas back-flow occurs in front of the rear plugs of the intact channels. We show that the flow leaves the channels as strong jets that may break up for certain flow parameters, leading to turbulence with features that depend on the presence of the rear plugs. The removal of the rear plugs affects the flow distribution, which, in turn influences the flow rates along the channels and through the walls. We describe the particle trajectories and the topology of deposited particles and show that particles follow closely the streamlines, which may cross the surface of permeable walls for both flow configurations. The distribution of deposited particles resembles the spatial variation of the through-wall flow rate, exhibiting two peak values at both ends of the intact filter channel, and one local maximum near the entrance of the damaged filter channel that is diminished at the exit. We also investigate in detail the particle deposition on the frontal face and indicate that particle accumulation at the edges of the entrance is favored for particles with low response times in flows with high fluid mass rates for both intact and damaged filters. Finally, we examine the filtration efficiency for the defective channels without rear plugs and show that fewer particles are captured as the Reynolds number is increased. A smaller reduction of the filtration efficiency is also predicted with increasing particle size. © 2019 by the authors

Very low Pt-Pd based electrocatalysts for oxygen reduction reaction

In the present manuscript very low-platinum electrocatalysts, Pd97Pt3/C, Pd98Pt2/C, Pd99Pt1/C and pure Pd/C are prepared and examined for oxygen reduction reactions (ORR) with the rotating electrode technique. From the analysis of the results the addition of Pt to the pure Pd enhances significantly the ORR activity. More specifically, the Pd98Pt2/C electrocatalyst exhibits the highest intrinsic activity, with ca. 0.75mAcm-2 exchange current density. However, further increment of the Pt loading against Pd loading, (Pd97Pt3/C) reduces dramatically the intrinsic catalytic activity from 0.75mAcm-2 to 0.2mAcm-2. The Tafel slopes at low potential values are calculated 127, 128, 156 and 94 mV per decade for the Pd97Pt3/C, Pd98Pt2/C, Pd99Pt1/C and Pd/C, respectively, indicating one electron transfer at the rate determining step. While the calculated Tafel slope at high potential values indicates two electrons transfer at the rate determining step. The lower Tafel slope of the Pd/C shows that some OHads intermediates were involved

Conductivity of Gd-doped BaCeO3 protonic conductor in H2-H2O-O2 atmospheres

This work proposes a radically new simple method of evaluation of the proton conductivity based on measurement of the conductivity of ceramic materials depending on the oxygen partial pressure (pO2) at a gradual transition from the atmosphere of pure oxygen to one of wet hydrogen: O2 → O2 (H2O) → H2o (O2) → H2o → H2o (H2) → H2 (H2O). Experimental data for the solid electrolyte BaCe0.9Gd0.1O3-δhave been fitted in accordance with the defect chemistry model. Experimental and theoretical relationships correlate well with each other, which indicate the success of the proposed model description. Analysis of the relationships allowed us to estimate the level of oxygen-ion, proton and electron components and their contribution to the overall conductivity at elevated temperature. Copyright © 2013 Delta Energy and Environment

Preparation of graphitic mesoporous carbon for the simultaneous detection of hydroquinone and catechol

A graphitic mesoporous carbon (GMC) had been successfully synthesized using Ni-Fe layered double hydroxide (LDH) as both template and catalyst under a relatively low pyrolysis temperature. The techniques of X-ray diffraction, transmission electron microscopy, Raman spectrum and N2 adsorption/desorption were used to characterize the physico-chemical properties of the as-prepared GMC. Meanwhile, the voltammetric behaviors of hydroquinone (HQ) and catechol (CC) were studied at the GMC modified glassy carbon electrode (GMC/GCE). The separation of the oxidation and reduction peak (ΔEp) for HQ and CC were decreased from 369 to 42mV and from 365 to 52mV, respectively, and the anodic peak currents for the oxidation of both HQ and CC were also remarkably increased at the GMC/GCE. Furthermore, at the GMC/GCE, the two components could be entirely separated with a large oxidation peak potential separation between HQ and CC. Under the optimized condition, the peak currents of HQ and CC increased linearly with increasing HQ and CC contents. The detection limit for HQ and CC was 3.7×10-7 and 3.1×10-7molL-1, respectively. © 2012 Elsevier B.V