Towards Rechargeable Zinc-Air Batteries with Aqueous Chloride Electrolytes

This paper presents a combined theoretical and experimental investigation of aqueous near-neutral electrolytes based on chloride salts for rechargeable zinc-air batteries (ZABs). The resilience of near-neutral chloride electrolytes in air could extend ZAB lifetime, but theory-based simulations predict that such electrolytes are vulnerable to other challenges including pH instability and the unwanted precipitation of mixed zinc hydroxide chloride products. In this work, we combine theory-based simulations with experimental methods such as full cell cycling, operando pH measurements, ex-situ XRD, SEM, and EDS characterization to investigate the performance of ZABs with aqueous chloride electrolytes. The experimental characterization of near-neutral ZAB cells observes the predicted pH instability and confirms the composition of the final discharge products. Steps to promote greater pH stability and control the precipitation of discharge products are proposed.Comment: 13 pages, 12 figure

Designing Aqueous Organic Electrolytes for Zinc-Air Batteries: Method, Simulation, and Validation

Aqueous zinc-air batteries (ZABs) are a low-cost, safe, and sustainable technology for stationary energy storage. ZABs with pH-buffered near-neutral electrolytes have the potential for longer lifetime compared to traditional alkaline ZABs due to the slower absorption of carbonates at non-alkaline pH values. However, existing near-neutral electrolytes often contain halide salts, which are corrosive and threaten the precipitation of ZnO as the dominant discharge product. This paper presents a method for designing halide-free aqueous ZAB electrolytes using thermodynamic descriptors to computationally screen components. The dynamic performance of a ZAB with one possible halide-free aqueous electrolyte based on organic salts is simulated using an advanced method of continuum modeling, and the results are validated by experiments. XRD, SEM, and EDS measurements of Zn electrodes show that ZnO is the dominant discharge product, and operando pH measurements confirm the stability of the electrolyte pH during cell cycling. Long-term full cell cycling tests are performed, and RRDE measurements elucidate the mechanism of ORR and OER. Our analysis shows that aqueous electrolytes containing organic salts could be a promising field of research for zinc-based batteries, due to their Zn$^{2+}$ chelating and pH buffering properties. We discuss the remaining challenges including the electrochemical stability of the electrolyte components.Comment: 16 pages, 12 figure

Manganese oxide catalysts for secondary zinc air batteries: from

An efficient, durable and low cost air cathode with low polarization between the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is essential for a high performance and durable secondary zinc-air battery. Different valence states and morphologies of MnxOy catalysts were synthetized via thermal treatment of EMD (generating Mn2O3 and Mn3O4) and acid digestion of synthetized Mn2O3 (producing a-MnO2) in order to develop an efficient Bifunctional Air Electrode (BAE). Change in the ratio H+ to Mn2O3 during the acid digestion affects the sample microporosity, the crystallographic plane distribution, as well as the physical and chemical adsorbed water which was related to defects, i.e. cation vacancies (Mn4+) and Mn3+. These characteristics were discussed and linked to the electrocatalytic activity. The best ORR performing catalyst was that with the higher surface water content (associated to material BET surface area) and a (310) surface as the 2nd more contributing plane (after 211). On the other hand, the catalyst with the higher structural water and with (110) and (200) crystallographic planes being the most intensity contributors (after 211) was the most OER active material. In this work, it was able to find a relationship between catalyst structure and air-efficiency through a volcano-like relationship between air-efficiency and surface water content. Air-efficiency (also take as round-efficiency discharge/charge in battery context) can be taken as a good descriptor of potentially good materials for Zn-Air secondary batteries technology. In this term, we were able to prepare a Bifunctional Air Electrode based on the selected a-MnO2 sample which demonstrated a roundefficiency of 53%, a DV around 1 V and a neglected loss of the charge potential (about 2.1 V) over the entire lifecycle test (more 200 cycles over 30 hours) with a capacity retention superior to 95%.European Commission H2020: Proyecto ZAS “Zinc Air Secondary innovative nanotech based batteries for efficient energy storage” (Grant Agreement 646186

Barriers of mental health treatment utilization among first-year college students: First cross-national results from the WHO World Mental Health International College Student Initiative.

BACKGROUND: Although mental disorders and suicidal thoughts-behaviors (suicidal thoughts and behaviors) are common among university students, the majority of students with these problems remain untreated. It is unclear what the barriers are to these students seeking treatment. AIMS: The aim of this study is to examine the barriers to future help-seeking and the associations of clinical characteristics with these barriers in a cross-national sample of first-year college students. METHOD: As part of the World Mental Health International College Student (WMH-ICS) initiative, web-based self-report surveys were obtained from 13,984 first-year students in eight countries across the world. Clinical characteristics examined included screens for common mental disorders and reports about suicidal thoughts and behaviors. Multivariate regression models adjusted for socio-demographic, college-, and treatment-related variables were used to examine correlates of help-seeking intention and barriers to seeking treatment. RESULTS: Only 24.6% of students reported that they would definitely seek treatment if they had a future emotional problem. The most commonly reported reasons not to seek treatment among students who failed to report that they would definitely seek help were the preference to handle the problem alone (56.4%) and wanting to talk with friends or relatives instead (48.0%). Preference to handle the problem alone and feeling too embarrassed were also associated with significantly reduced odds of having at least some intention to seek help among students who failed to report that they would definitely seek help. Having 12-month major depression, alcohol use disorder, and suicidal thoughts and behaviors were also associated with significantly reduced reported odds of the latter outcome. CONCLUSIONS: The majority of first-year college students in the WMH-ICS surveys report that they would be hesitant to seek help in case of future emotional problems. Attitudinal barriers and not structural barriers were found to be the most important reported reasons for this hesitation. Experimental research is needed to determine whether intention to seek help and, more importantly, actual help-seeking behavior could be increased with the extent to which intervention strategies need to be tailored to particular student characteristics. Given that the preference to handle problems alone and stigma and appear to be critical, there could be value in determining if internet-based psychological treatments, which can be accessed privately and are often build as self-help approaches, would be more acceptable than other types of treatments to student who report hesitation about seeking treatment.status: publishe

WHO World Mental Health Surveys International College Student Project: Prevalence and Distribution of Mental Disorders

Increasingly, colleges across the world are contending with rising rates of mental disorders, and in many cases, the demand for services on campus far exceeds the available resources. The present study reports initial results from the first stage of the WHO World Mental Health International College Student project, in which a series of surveys in 19 colleges across 8 countries (Australia, Belgium, Germany, Mexico, Northern Ireland, South Africa, Spain, United States) were carried out with the aim of estimating prevalence and basic sociodemographic correlates of common mental disorders among first-year college students. Web-based self-report questionnaires administered to incoming first-year students (45.5% pooled response rate) screened for six common lifetime and 12-month DSM-IV mental disorders: major depression, mania/hypomania, generalized anxiety disorder, panic disorder, alcohol use disorder, and substance use disorder. We focus on the 13,984 respondents who were full-time students: 35% of whom screened positive for at least one of the common lifetime disorders assessed and 31% screened positive for at least one 12-month disorder. Syndromes typically had onsets in early to middle adolescence and persisted into the year of the survey. Although relatively modest, the strongest correlates of screening positive were older age, female sex, unmarried-deceased parents, no religious affiliation, nonheterosexual identification and behavior, low secondary school ranking, and extrinsic motivation for college enrollment. The weakness of these associations means that the syndromes considered are widely distributed with respect to these variables in the student population. Although the extent to which cost-effective treatment would reduce these risks is unclear, the high level of need for mental health services implied by these results represents a major challenge to institutions of higher education and governments. (PsycINFO Database Record (c) 2018 APA, all rights reserved).status: publishe

Las reacciones redox en 2º de Bachillerato a través de experiencias POE (Predecir-Observar-Explicar)

The present End of Studies Project suggests an intervention proposal for the content related to oxidation-reduction reactions and electrochemistry of the chemistry subject of 2nd year of Baccalaureate. The proposal seeks to develop activities for the student to show a greater interest in the subject, to know both the industrial applications and the daily use of the theoretical contents that are worked on and to have a greater understanding of the phenomena described in the didactic unit under study. For this, the interpretive POE (Predict-Observe-Explain) experiences are used, which require an active role of the student in the teaching-learning process, as it is the student who raises and justifies a hypothesis (predict), tests the hypothesis experimentally (observe) and rationalizes the results obtained (explain). The work consists, first of all, of a bibliographic review regarding the problems related to the content to be worked on, in addition to compiling previous experiences described in the bibliography regarding these activities. Secondly, the intervention proposal of the didactic unit is detailed, where the activities and the objectives and competencies to be worked on are described, in addition to the evaluation of the didactic unit. Finally, an assessment of the intervention proposal is carried out and the conclusions, limitations and possible ways of development that can be realized as a result of the work carried out are detailed.El presente Trabajo de Fin de Estudios (TFE) plantea una propuesta de intervención para el contenido relacionado con las reacciones de oxidación-reducción y la electroquímica de la asignatura de química de 2º de Bachillerato. La propuesta busca elaborar actividades que permitan que el alumnado muestre un mayor interés por la asignatura, conozca las aplicaciones tanto industriales como de uso diario de los contenidos teóricos que se trabajan y tenga una mayor comprensión de los fenómenos descritos en la unidad didáctica objeto de estudio. Para ello, se hace uso de las experiencias interpretativas POE (Predecir-Observar-Explicar), las cuales requieren un papel activo del alumno en el proceso de enseñanza-aprendizaje, ya que es el alumnado quien plantea y justifica una hipótesis (predecir), comprueba su hipótesis experimentalmente (observar) y justifica los resultados obtenidos (explicar). El trabajo consta, en primer lugar, de una revisión bibliográfica respecto a la problemática relacionada con el contenido a trabajar, además de recopilar experiencias previas descritas en la bibliografía en torno a estas actividades. En segundo lugar, se detalla la propuesta de intervención de la unidad didáctica, donde se detallan las actividades y los objetivos y competencias que se trabajarán, además de describir la evaluación de la unidad didáctica. Por último, se realiza una valoración de la propuesta de intervención y se detallan las conclusiones, limitaciones y posibles vías de desarrollo que pueden llevarse a cabo a raíz del trabajo realizado

Desarrollo de baterías secundarias de zinc-aire con electrolitos acuosos alternativos

Les bateries secundàries de zinc-aire són una tecnologia prometedora degut la seva elevada capacitat, cost reduït i baixa toxicitat. No obstant, aquesta tecnologia presenta algunes limitacions que han de ser superades per al seu desenvolupament i comercialització. La major part d’aquestes limitacions estan lligades a la naturalesa aquosa del sistema electròlit. És per això que, recentment, s’ha proposat la substitució del tradicional electròlit aquós alcalí per sistemes electrolítics aquosos “quasi-neutres” (pH comprès entre 4 i 8), basats en ZnCl2, NH4Cl i NH4OH. Degut la manca de publicacions sobre aquesta novedosa estratègia, l’objectiu principal d’aquesta Tesi s’ha centrat en establir les bases d’aquesta proposta, a través de l’estudi de cadascun dels components de les bateries de zinc-aire: el sistema electròlit, l’elèctrode bifuncional d´aire i l’ànode de zinc. En primer lloc es va dur a terme un estudi sistemàtic de diferentes formulacions electrolítiques, tot determinant la influencia de cadascun dels components de l’electròlit sobre propietats, tals com la solubilitat, la conductivitat iònica o la naturalesa de les espècies precipitades durant la descàrrega de la bateria. Tenint en compte el conjunt d’aquestes propietats, es van seleccionar dues formulacions electrolítiques per a el subsegüent desenvolupament dels elèctrodes de la bateria de zinc-aire: una descrita a l’estat de l’art (a pH 4) i l’altra (a pH 8), analitzada experimentalment per primera vegada en aquest treball, i que havia estat proposada teòricament a la literatura. L’elèctrode bifuncional d’aire va ser desenvolupat i optimitzat a través de la caracterització fisicoquímica i electroquímica de diferents agents conductors, basats en materials carbonosos, i catalitzadors bifuncionals d’aire, basats en òxids de manganés. Els resultats mostren un baix sobrepotencial de la bateria de zinc-aire, mitjançant l’ús de nanotubs de carboni (CNT) com a agent conductor per a els dos sistemes electrolítics, conseqüència de la seva àrea superficial més gran i porositat observades a l’anàlisi fisicoquímica. Un cop seleccionat l’agent conductor més adient (els CNT), es va avaluar l’activitat catalítica de l’elèctrode amb diferents catalitzadors (α-MnO2, γ-MnO2 y Mn2O3), tot comprovant que el òxids de manganés estudiats no contribuïen a l’activitat catalítica a pH 4, essent l’agent conductor (CNT) el que actuava com a catalitzador. Per contra, a l’electròlit de pH 8, la incorporació d’un 20 % de α-MnO2 va millorar les propietats catalítiques de l’elèctrode. Per aquest motiu, aquesta darrera formulació va ser seleccionada per al desenvolupament posterior. Les cel·les de zinc-aire analitzades amb l’elèctrode optimitzat van demostrar per una part, una estabilitat superior a 400 h per ambdós electròlits i, per l’altra, més avantatges en fer servir l’electròlit a pH 8: un sobrepotencial més baix, la possibilitat d’evitar la dissolució del manganés i l’estabilitat del pH durant el funcionament de la bateria. La utilització de materials de zinc en pols permet de preparar ànodes tridimensionals que faciliten l’accessibilitat de l’electròlit a la seva estructura i, a més a més, seleccionar el material actiu de zinc que mostri millors prestacions. En aquest context, es van analitzar diferents aliatges de zinc y es va triar el material de corrosió més baixa y eficiència més alta en els electròlits pH 4 i 8, per a preparar pastilles de zinc poroses. La pastilla de zinc anomenada GR, amb un 30 % d’agent porogen, va permetre reduir la corrosió anòdica i mantenir un baix sobrepotencial de la bateria a densitats de corrent més grans. Finalment, els tres components de la ce·la optimitzats en aquesta Tesi es van incorporar en una cel·la completa de zinc-aire, aconseguint més de 40 Wh kg-1EBA+Zn (10 Wh kg-1Cel·laCompleta), cosa que estableix les bases per a un desenvolupament de bateries secundàries de zinc-aire amb electròlits aquosos “quasi-neutres”.Las baterías secundarias de zinc-aire son una tecnología prometedora debido a su elevada capacidad, reducido coste y baja toxicidad. Sin embargo, esta tecnología presenta ciertas limitaciones que deben ser superadas para su desarrollo y comercialización. Gran parte de estas limitaciones están ligadas a la naturaleza acuosa alcalina del sistema electrolítico. Por ello, recientemente, se ha propuesto la sustitución del electrolito acuoso alcalino tradicional por sistemas electrolíticos acuosos “”casi neutros”” (pH comprendido entre 4 y 8) basados en ZnCl2, NH4Cl y NH4OH. Debido a la escasez de publicaciones relativas a esta novedosa estrategia, el objetivo principal de esta Tesis se ha centrado en sentar las bases de esta propuesta a través del estudio de cada uno de los componentes de las baterías de zinc-aire: el sistema electrolítico, el electrodo bifuncional de aire y el ánodo de zinc. En primer lugar, se llevó a cabo un estudio sistemático de distintas formulaciones electrolíticas determinando la influencia de cada componente del electrolito en propiedades como la solubilidad, la conductividad iónica o la naturaleza de las especies precipitadas durante la descarga de la batería. Teniendo en cuenta el conjunto de esas propiedades, se seleccionaron dos formulaciones electrolíticas para el subsiguiente desarrollo de los electrodos para la batería de zinc-aire: una descrita en el estado del arte (a pH 4) y otra (a pH 8) analizada experimentalmente por primera vez en este trabajo, y que había sido propuesta teóricamente en la literatura. El electrodo bifuncional de aire fue desarrollado y optimizado a través de una caracterización fisicoquímica y electroquímica realizada a distintos agentes conductores basados en materiales carbonosos y a distintos catalizadores bifuncionales de aire basados en óxidos de manganeso. Los resultados muestran un reducido sobrepotencial de la batería de zinc-aire mediante el uso de nanotubos de carbono (CNT) como agente conductor para los dos sistemas electrolíticos, consecuencia de la mayor área superficial y porosidad observadas en el análisis fisicoquímico realizado. Seleccionado el agente conductor adecuado (los CNT), se evaluó la actividad catalítica del electrodo con distintos catalizadores (α-MnO2, γ-MnO2 y Mn2O3), comprobando que los óxidos de manganeso estudiados no contribuyen en la actividad catalítica del electrolito pH4, siendo el agente conductor (CNT) el que actúa como catalizador. En cambio, en el electrolito pH8, la incorporación de un 20% de α-MnO2 mejoró las propiedades catalíticas del electrodo, por lo que dicha formulación fue seleccionada para el subsiguiente desarrollo. Las celdas de zinc-aire analizadas con el electrodo optimizado demostraron, por un lado, una estabilidad superior a 400 horas para ambos electrolitos y, por otro, diversas ventajas al utilizar el electrolito pH8: un menor sobrepotencial, la posibilidad de evitar la disolución de manganeso y la estabilidad del pH durante el funcionamiento de la batería. El uso de materiales de zinc en polvo permite preparar ánodos tridimensionales que faciliten la accesibilidad del electrolito en su estructura y, además, seleccionar el material activo de zinc que mejores prestaciones presente. En este contexto, se analizaron distintas aleaciones de zinc y se seleccionó el material con menor corrosión y mayor eficiencia en los electrolitos pH4 y pH8 para llevar a cabo la preparación de pastillas porosas de zinc. La pastilla de Zn denominada GR con un 30% de agente porógeno permitió reducir la corrosión anódica y mantener un bajo sobrepotencial de la batería a mayores densidades de corriente. Finalmente, se incorporaron los tres componentes de la celda optimizados en esta Tesis en una celda completa de zinc-aire, consiguiendo más de 40 Wh kg-1EBA+Zn (10 Wh kg-1CeldaCompleta), lo cual sienta las bases para el desarrollo de baterías secundarias de zinc-aire con electrolitos acuosos “casi neutros”.Secondary zinc-air batteries are a promising technology due to their high capacity, reduced cost and low toxicity. However, this technology faces some problems that need to be overcome for their development and commercialization. Many of these limitations are related to the aqueous alkaline nature of the electrolyte system. Thus, the substitution of the traditional aqueous alkaline electrolyte with “near neutral” aqueous electrolytes (pH between 4 and 8) based on ZnCl2, NH4Cl and NH4OH has been recently proposed. Due to the scarcity of publications regarding this novel approach, the main aim of this Thesis is based on laying the foundation for this proposal by studying each of the components of a zinc-air battery: the electrolyte system, the bifunctional air electrode and the zinc anode. First, a systematic study was carried out for different electrolyte formulations, where it could be observed the influence of each electrolyte component in different properties such as the solubility, ionic conductivity or the nature of the precipitated species formed during the discharge of the battery. Taking into account an overview of the properties, two different electrolyte formulations were selected for the subsequent electrode development for the zinc-air battery: a formulation described in the state-of-the-art (at pH 4) and another formulation (at pH8) experimentally analyzed for the first time in this work that was theoretically proposed in the literature. The bifunctional air electrode was developed and optimized by a physicochemical and electrochemical characterization carried out to different conductive agents based on carbonaceous materials and to different manganese oxide-based bifunctional air catalysts. The obtained results showed a reduced overpotential of the zinc-air battery by using carbon nanotubes (CNT) as the conductive agent for the studied electrolyte systems, due to the higher surface area and porosity observed in the physiochemical analysis. Once the appropriate conductive agent (CNT) was selected, the catalytic activity of the electrode was evaluated with different catalysts (α-MnO2, γ-MnO2 y Mn2O3), proving that the studied manganese oxides do not contribute to the catalytic activity in pH4 electrolyte, being the conductive agent (CNT) the one acting as a catalyst. In contrast, in pH8 electrolyte, the incorporation of 20% of α-MnO2 enhanced the catalytic properties of the electrode, and was selected for the subsequent development. The zinc-air cells with the optimized electrode showed not only a high stability for more than 400 hours in both electrolytes, but also many advantages when using pH8 electrolyte: a lower overpotential, the possibility to avoid manganese dissolution, and high pH stability during battery operation. The use of zinc powder materials allows preparing tridimensional anodes that promote the accessibility of the electrolyte on its structure and, moreover, selecting a zinc active material with enhanced properties. In this context, different zinc alloys were studied and the one with lower corrosion and higher efficiency for pH4 and pH8 electrolytes was selected for preparing porous zinc pellets. Zn pellets named as GR with 30% of porogen showed a reduced anodic corrosion and a low overpotential of the battery at higher current densities. Finally, the optimized cell components in this Thesis were incorporated in a full-cell zinc-air battery, obtaining more than 40 Wh kg-1EBA+Zn (10 Wh kg-1FullCell). These results lay the foundation for the development of secondary zinc-air batteries with “near neutral” aqueous electrolytes

Desarrollo de baterías secundarias de zinc-aire con electrolitos acuosos alternativos /

Departament responsable de la tesi: Departament de Química.Les bateries secundàries de zinc-aire són una tecnologia prometedora degut la seva elevada capacitat, cost reduït i baixa toxicitat. No obstant, aquesta tecnologia presenta algunes limitacions que han de ser superades per al seu desenvolupament i comercialització. La major part d'aquestes limitacions estan lligades a la naturalesa aquosa del sistema electròlit. És per això que, recentment, s'ha proposat la substitució del tradicional electròlit aquós alcalí per sistemes electrolítics aquosos "quasi-neutres" (pH comprès entre 4 i 8), basats en ZnCl2, NH4Cl i NH4OH. Degut la manca de publicacions sobre aquesta novedosa estratègia, l'objectiu principal d'aquesta Tesi s'ha centrat en establir les bases d'aquesta proposta, a través de l'estudi de cadascun dels components de les bateries de zinc-aire: el sistema electròlit, l'elèctrode bifuncional d´aire i l'ànode de zinc. En primer lloc es va dur a terme un estudi sistemàtic de diferentes formulacions electrolítiques, tot determinant la influencia de cadascun dels components de l'electròlit sobre propietats, tals com la solubilitat, la conductivitat iònica o la naturalesa de les espècies precipitades durant la descàrrega de la bateria. Tenint en compte el conjunt d'aquestes propietats, es van seleccionar dues formulacions electrolítiques per a el subsegüent desenvolupament dels elèctrodes de la bateria de zinc-aire: una descrita a l'estat de l'art (a pH 4) i l'altra (a pH 8), analitzada experimentalment per primera vegada en aquest treball, i que havia estat proposada teòricament a la literatura. L'elèctrode bifuncional d'aire va ser desenvolupat i optimitzat a través de la caracterització fisicoquímica i electroquímica de diferents agents conductors, basats en materials carbonosos, i catalitzadors bifuncionals d'aire, basats en òxids de manganés. Els resultats mostren un baix sobrepotencial de la bateria de zinc-aire, mitjançant l'ús de nanotubs de carboni (CNT) com a agent conductor per a els dos sistemes electrolítics, conseqüència de la seva àrea superficial més gran i porositat observades a l'anàlisi fisicoquímica. Un cop seleccionat l'agent conductor més adient (els CNT), es va avaluar l'activitat catalítica de l'elèctrode amb diferents catalitzadors (α-MnO2, γ-MnO2 y Mn2O3), tot comprovant que el òxids de manganés estudiats no contribuïen a l'activitat catalítica a pH 4, essent l'agent conductor (CNT) el que actuava com a catalitzador. Per contra, a l'electròlit de pH 8, la incorporació d'un 20 % de α-MnO2 va millorar les propietats catalítiques de l'elèctrode. Per aquest motiu, aquesta darrera formulació va ser seleccionada per al desenvolupament posterior. Les cel·les de zinc-aire analitzades amb l'elèctrode optimitzat van demostrar per una part, una estabilitat superior a 400 h per ambdós electròlits i, per l'altra, més avantatges en fer servir l'electròlit a pH 8: un sobrepotencial més baix, la possibilitat d'evitar la dissolució del manganés i l'estabilitat del pH durant el funcionament de la bateria. La utilització de materials de zinc en pols permet de preparar ànodes tridimensionals que faciliten l'accessibilitat de l'electròlit a la seva estructura i, a més a més, seleccionar el material actiu de zinc que mostri millors prestacions. En aquest context, es van analitzar diferents aliatges de zinc y es va triar el material de corrosió més baixa y eficiència més alta en els electròlits pH 4 i 8, per a preparar pastilles de zinc poroses. La pastilla de zinc anomenada GR, amb un 30 % d'agent porogen, va permetre reduir la corrosió anòdica i mantenir un baix sobrepotencial de la bateria a densitats de corrent més grans. Finalment, els tres components de la ce·la optimitzats en aquesta Tesi es van incorporar en una cel·la completa de zinc-aire, aconseguint més de 40 Wh kg-1EBA+Zn (10 Wh kg-1Cel·laCompleta), cosa que estableix les bases per a un desenvolupament de bateries secundàries de zinc-aire amb electròlits aquosos "quasi-neutres".Las baterías secundarias de zinc-aire son una tecnología prometedora debido a su elevada capacidad, reducido coste y baja toxicidad. Sin embargo, esta tecnología presenta ciertas limitaciones que deben ser superadas para su desarrollo y comercialización. Gran parte de estas limitaciones están ligadas a la naturaleza acuosa alcalina del sistema electrolítico. Por ello, recientemente, se ha propuesto la sustitución del electrolito acuoso alcalino tradicional por sistemas electrolíticos acuosos ""casi neutros"" (pH comprendido entre 4 y 8) basados en ZnCl2, NH4Cl y NH4OH. Debido a la escasez de publicaciones relativas a esta novedosa estrategia, el objetivo principal de esta Tesis se ha centrado en sentar las bases de esta propuesta a través del estudio de cada uno de los componentes de las baterías de zinc-aire: el sistema electrolítico, el electrodo bifuncional de aire y el ánodo de zinc. En primer lugar, se llevó a cabo un estudio sistemático de distintas formulaciones electrolíticas determinando la influencia de cada componente del electrolito en propiedades como la solubilidad, la conductividad iónica o la naturaleza de las especies precipitadas durante la descarga de la batería. Teniendo en cuenta el conjunto de esas propiedades, se seleccionaron dos formulaciones electrolíticas para el subsiguiente desarrollo de los electrodos para la batería de zinc-aire: una descrita en el estado del arte (a pH 4) y otra (a pH 8) analizada experimentalmente por primera vez en este trabajo, y que había sido propuesta teóricamente en la literatura. El electrodo bifuncional de aire fue desarrollado y optimizado a través de una caracterización fisicoquímica y electroquímica realizada a distintos agentes conductores basados en materiales carbonosos y a distintos catalizadores bifuncionales de aire basados en óxidos de manganeso. Los resultados muestran un reducido sobrepotencial de la batería de zinc-aire mediante el uso de nanotubos de carbono (CNT) como agente conductor para los dos sistemas electrolíticos, consecuencia de la mayor área superficial y porosidad observadas en el análisis fisicoquímico realizado. Seleccionado el agente conductor adecuado (los CNT), se evaluó la actividad catalítica del electrodo con distintos catalizadores (α-MnO2, γ-MnO2 y Mn2O3), comprobando que los óxidos de manganeso estudiados no contribuyen en la actividad catalítica del electrolito pH4, siendo el agente conductor (CNT) el que actúa como catalizador. En cambio, en el electrolito pH8, la incorporación de un 20% de α-MnO2 mejoró las propiedades catalíticas del electrodo, por lo que dicha formulación fue seleccionada para el subsiguiente desarrollo. Las celdas de zinc-aire analizadas con el electrodo optimizado demostraron, por un lado, una estabilidad superior a 400 horas para ambos electrolitos y, por otro, diversas ventajas al utilizar el electrolito pH8: un menor sobrepotencial, la posibilidad de evitar la disolución de manganeso y la estabilidad del pH durante el funcionamiento de la batería. El uso de materiales de zinc en polvo permite preparar ánodos tridimensionales que faciliten la accesibilidad del electrolito en su estructura y, además, seleccionar el material activo de zinc que mejores prestaciones presente. En este contexto, se analizaron distintas aleaciones de zinc y se seleccionó el material con menor corrosión y mayor eficiencia en los electrolitos pH4 y pH8 para llevar a cabo la preparación de pastillas porosas de zinc. La pastilla de Zn denominada GR con un 30% de agente porógeno permitió reducir la corrosión anódica y mantener un bajo sobrepotencial de la batería a mayores densidades de corriente. Finalmente, se incorporaron los tres componentes de la celda optimizados en esta Tesis en una celda completa de zinc-aire, consiguiendo más de 40 Wh kg-1EBA+Zn (10 Wh kg-1CeldaCompleta), lo cual sienta las bases para el desarrollo de baterías secundarias de zinc-aire con electrolitos acuosos "casi neutros".Secondary zinc-air batteries are a promising technology due to their high capacity, reduced cost and low toxicity. However, this technology faces some problems that need to be overcome for their development and commercialization. Many of these limitations are related to the aqueous alkaline nature of the electrolyte system. Thus, the substitution of the traditional aqueous alkaline electrolyte with "near neutral" aqueous electrolytes (pH between 4 and 8) based on ZnCl2, NH4Cl and NH4OH has been recently proposed. Due to the scarcity of publications regarding this novel approach, the main aim of this Thesis is based on laying the foundation for this proposal by studying each of the components of a zinc-air battery: the electrolyte system, the bifunctional air electrode and the zinc anode. First, a systematic study was carried out for different electrolyte formulations, where it could be observed the influence of each electrolyte component in different properties such as the solubility, ionic conductivity or the nature of the precipitated species formed during the discharge of the battery. Taking into account an overview of the properties, two different electrolyte formulations were selected for the subsequent electrode development for the zinc-air battery: a formulation described in the state-of-the-art (at pH 4) and another formulation (at pH8) experimentally analyzed for the first time in this work that was theoretically proposed in the literature. The bifunctional air electrode was developed and optimized by a physicochemical and electrochemical characterization carried out to different conductive agents based on carbonaceous materials and to different manganese oxide-based bifunctional air catalysts. The obtained results showed a reduced overpotential of the zinc-air battery by using carbon nanotubes (CNT) as the conductive agent for the studied electrolyte systems, due to the higher surface area and porosity observed in the physiochemical analysis. Once the appropriate conductive agent (CNT) was selected, the catalytic activity of the electrode was evaluated with different catalysts (α-MnO2, γ-MnO2 y Mn2O3), proving that the studied manganese oxides do not contribute to the catalytic activity in pH4 electrolyte, being the conductive agent (CNT) the one acting as a catalyst. In contrast, in pH8 electrolyte, the incorporation of 20% of α-MnO2 enhanced the catalytic properties of the electrode, and was selected for the subsequent development. The zinc-air cells with the optimized electrode showed not only a high stability for more than 400 hours in both electrolytes, but also many advantages when using pH8 electrolyte: a lower overpotential, the possibility to avoid manganese dissolution, and high pH stability during battery operation. The use of zinc powder materials allows preparing tridimensional anodes that promote the accessibility of the electrolyte on its structure and, moreover, selecting a zinc active material with enhanced properties. In this context, different zinc alloys were studied and the one with lower corrosion and higher efficiency for pH4 and pH8 electrolytes was selected for preparing porous zinc pellets. Zn pellets named as GR with 30% of porogen showed a reduced anodic corrosion and a low overpotential of the battery at higher current densities. Finally, the optimized cell components in this Thesis were incorporated in a full-cell zinc-air battery, obtaining more than 40 Wh kg-1EBA+Zn (10 Wh kg-1FullCell). These results lay the foundation for the development of secondary zinc-air batteries with "near neutral" aqueous electrolytes

Designing Aqueous Organic Electrolytes for Near-Neutral Zinc-Air Batteries: Method, Simulation, and Validation

Aqueous zinc–air batteries (ZABs) are a low‐cost, safe, and sustainable technology for stationary energy storage. ZABs with pH‐buffered near‐neutral electrolytes have the potential for longer lifetime compared to traditional alkaline ZABs due to the slower absorption of carbonates at nonalkaline pH values. However, existing near‐neutral electrolytes often contain halide salts, which are corrosive and threaten the precipitation of ZnO as the dominant discharge product. This paper presents a method for designing halide‐free aqueous ZAB electrolytes using thermodynamic descriptors to computationally screen components. The dynamic performance of a ZAB with one possible halide‐free aqueous electrolyte based on organic salts is simulated using an advanced method of continuum modeling, and the results are validated by experiments. X‐ray diffraction, scanning electron microscopy, and energy dispersive X‐ray spectroscopy measurements of Zn electrodes show that ZnO is the dominant discharge product, and operando pH measurements confirm the stability of the electrolyte pH during cell cycling. Long‐term full cell cycling tests are performed, and rotating ring disk electrode measurements elucidate the mechanism of oxygen reduction reaction and oxygen evolution reaction. The analysis shows that aqueous electrolytes containing organic salts could be a promising field of research for zinc‐based batteries, due to their Zn2+ chelating and pH buffering properties. The remaining challenges including the electrochemical stability of the electrolyte components are discussed