domain: shs sdv

text:
Gervaise Debucquet

Allan Maignant

Anne-Laure Laroche

Caroline Widehem

Philippe Morel

BRINGING NATURE INTO PRIVATE URBAN HOUSING: ENVIRONMENTAL, SOCIAL AND FOOD CONNECTIONS FOR URBAN RESILIENCE

Keywords: Urban housing, private gardening, environmental awareness, sustainable food, multi-scalar urban planning

Ongoing confinement for millions of urban citizens due to the Covid-19 pandemic has raised ecological consciousness, changed food habits and questioned the relationship urban dwellers have with nature. There is more interest in bringing plants into urban homes and in sustainable food sources, but no research have studied the relationships between food behaviours and plant-care activities. To address this gap and explore urban citizens' nature relatedness through the greening of private areas, we conducted a national survey of French, young urban citizens (n = 1,000), who are more committed to 'edible' cities than older generations but have the lowest rate of plant purchasers. A quantitative approach reveals the prevalence of aesthetic/hedonistic expectations for plants in private housing but also demonstrates contrasting perceptions of tasks for plant maintening and unequal valuation of social issues around plants. We discuss continuities between environmental awareness, commitment to sustainable food and natural/social uses of plants and argue that urban planning processes should address potential synergies for more integrative resilience. Community building around green areas, urban agriculture or collective gardens, in cities, can have ripple effects towards the greening of private housing. Lastly, the multi-disciplinary approach bridging psychosociology and urban studies can inspire multi-scalar urban planning.

Introduction

The lockdowns imposed by governments in response to the spread of Covid-19 have brought about unprecedented experiences, especially for urban citizens whose living space has been mostly limited to a matter of square meters inside buildings. During these periods, a need for a 'dose de nature' [START_REF] Jiang | A dose of nature: Tree cover, stress reduction, and gender differences[END_REF], has been observed, with growing interest in home gardening in small gardens, on balconies or simply in containers [START_REF] Sunga | The" plantito/plantita" home gardening during the pandemic[END_REF]. While some citizens have started growing food at home on a small scale [START_REF] Mullins | Home Food Gardening in Canada in Response to the COVID-19 Pandemic[END_REF], others are simply taking care of green plants or flowers.

Caring for nature often correlates with caring for people strongly disadvantaged in the Covid-19 context [START_REF] Stok | Social Inequality and Solidarity in Times of COVID-19[END_REF], thus illustrating two interlinked fronts of motivation, as advocated by Ling [START_REF] Wong | Giving multicultural community groups a voice in creating, using, and managing urban greenspace[END_REF]. Either way, those changes to confined people's everyday lives were perceived as a way of reconnecting with nature and as a psychological experience [START_REF] Sunga | The" plantito/plantita" home gardening during the pandemic[END_REF].

In fact, this crisis has revealed the importance of connections with nature for Homo urbanus [START_REF] Bourdeau-Lepage | Ville et nature, vers une nouvelle alliance[END_REF], exposed to an overload of auditory, visual and technological stimuli in urban areas. Several studies over the last decade have confirmed the benefits of exposure to natural sounds on well-being, notably stress reduction [START_REF] Alvarsson | Stress recovery during exposure to nature sound and environmental noise[END_REF][START_REF] Chalmin-Pui | It made me feel brighter in myself"-The health and well-being impacts of a residential front garden horticultural intervention[END_REF], and of gardening on healthy ageing [START_REF] Hawkins | Doing" gardening and "being" at the allotment site: exploring the benefits of allotment gardening for stress reduction and healthy aging[END_REF][START_REF] Soga | Gardening is beneficial for health: A metaanalysis[END_REF][START_REF] Wang | The benefits of gardening for older adults: a systematic review of the literature[END_REF]. Alongside the activities of individuals, social innovations like garden collectives or urban agriculture [START_REF] Aubry | The Rise of Urban Agriculture: An Epiphenomenon or a Slow Revolution?[END_REF], illustrate the increasing activity around plants and food production in the heart of cities. At the same time, stakeholders in the urban planning field have been imagining new ways to introduce nature in urban areas throughout the world, allowing citizens to experience urban green spaces [START_REF] Skandrani | Beyond green-planning political orientations: contrasted public policies and their relevance to nature perceptions in two European capitals[END_REF]. Moreover, in areas strongly modified by human activities, 'reconciliation' [START_REF] Rosenzweig | Reconciliation ecology and the future of species diversity[END_REF] between biodiversity and socio-economic life could be of benefit to both nature and human beings.

However, with the growing urban resilience agenda, scholarly attention has been drawn mainly to the environmental externalities produced through home gardening, the aim being to push citizens towards more sustainable garden practices [START_REF] Coisnon | Information on biodiversity and environmental behaviors: A European study of individual and institutional drivers to adopt sustainable gardening practices[END_REF]. At the European level, private gardens might actively contribute to ecological transition and reduction of greenhouse gas emissions [START_REF] Cleveland | The potential for urban household vegetable gardens to reduce greenhouse gas emissions[END_REF] by becoming a 'resource by small gardening actions' pointing to the positive cumulative outcome of individual, pro-environmental gardening practices [START_REF] Dewaelheyns | A toolbox for garden governance[END_REF]. However, comparison between rural and urban areas shows lower biodiversity knowledge and less sensibility to sustainable practices among urban citizens [START_REF] Coisnon | Information on biodiversity and environmental behaviors: A European study of individual and institutional drivers to adopt sustainable gardening practices[END_REF]). 1 While it has been suggested that providing more trustworthy information on biodiversity could go some way to reducing the gap, there is clearly a paradox around the increasing concern in big cities for organic food, 2 permaculture or chemical-free food production in the interstices of the building [START_REF] Aubry | The Rise of Urban Agriculture: An Epiphenomenon or a Slow Revolution?[END_REF]. While abundant nature inside cities can respond to the deep need to feel an affinity with the natural world, also called 'biophilia' [START_REF] Kellert | The biophilia hypothesis[END_REF], one can ask more specifically whether the development of 'edible cities' [START_REF] Sartison | Edible cities-An innovative nature-based solution for urban sustainability transformation? An explorative study of urban food production in German cities[END_REF] will influence the greening of private housing and plant care practices.

At industry level, the horticulture sector is increasingly developing sustainabilityrelated labelling 3 and technical innovations (ecofriendly products, renewable containers, 1 47% of urban gardeners declared being 'rather interested' in environmental labelling while only 28% were 'very interested' (French national survey, Val'hor, TNS Sofres, 2014). 2 According to French Agence Bio survey (BioEdition 2020), market penetration of organic produce is highest in Paris and surrounding suburbs, with 55% of consumers eating organic food at least once a week. More broadly, the increase in consumption of organic produce is highest in cities with 2,000 to 20,000 people and cities of more than 100,000 people (+2% between 2018 and 2019) 3 For instance, in France, the Plante bleue certification recognises environmentally friendly production methods.

etc.) to address ecological challenges and push the purchase of plants by urban dwellers [START_REF] Yue | Consumer preference for sustainable attributes in plants: Evidence from experimental auctions[END_REF]. Overall, those consumers are less likely to buy organic plants and fertilisers than organic food; the willingness to pay for eco-friendly attributes is rather low and only a small portion of consumers have an interest in locally grown plants and biodegradable/compostable containers [START_REF] Yue | Consumer preference for sustainable attributes in plants: Evidence from experimental auctions[END_REF]French Agence Bio survey, 2018). Little is known about the views of the rising urban population regarding plants in urban housing, and there has not been any research so far that bridges concerns for food and plants. While that point is of interest for all generations, it is more acute for young urban citizens, firstly due to their environmental awareness and increasing commitment to sustainable food in cities (Sessego and Hebel, 2019;[START_REF] Salladarré | Some good reasons for buying fish exclusively from community-supported fisheries: The case of Yeu Island in France[END_REF][START_REF] Aertsens | Personal determinants of organic food consumption: a review[END_REF] and, secondly, because the young generation have the lowest rates of plant purchasers. 4 Moreover, little is known about how these interelations can be used in a multi-scalar approach of urban resilience [START_REF] Flax | Greening schoolyards : An urban resilience perspective[END_REF]. Following recommendations of [START_REF] Zhang | Green building, pro-environmental behavior and well-being: Evidence from Singapore[END_REF] for a multidisciplinary approach, we bridge psychosociology and urban studies to try to fill these gaps. Using a national quantitative survey (n = 1,000), we explore French young urban citizens' perceptions of caring for plants while relating them to their food behaviours, the social aspects of bringing nature into the domestic sphere and lastly, the triggers at the city level.

There follows: (1) a literature review on the nature relatedness of citizens living in cities and the recent shift, due to the Covid-19 crisis, in their motivations for caring for plants at home and finally, background literature on young urban citizens' overall attitudes towards sustainable food; (2) our research questions and methodological choices; (3) findings from spontaneous evocations around caring for plants and from an analysis of the relationships between environmental concern, commitment to sustainable food and valuation of technical/social uses of plant care activities; (4) a discussion about the enhancement of nature relatedness through synergies between food habits and plants care activities in the context of the rise of green and 'edible' cities; and (5) a conclusion with reflections on ways for better addressing continuities between public green spaces and the greening of private housing in urban planning processes.

Literature review

Nature relatedness of urban citizens: a shift in post-Covid-19 cities

The Covid-19 crisis and long-term lockdowns have surfaced reflections on the reconnection of urban dwellers with nature, or what [START_REF] Nisbet | The nature relatedness scale: Linking individuals' connection with nature to environmental concern and behavior[END_REF] called 'nature relatedness'. This wider construct encompasses the connectedness and relationship with nature and allows to explain effective engagementor notin sustainable behaviour. It also allows us to capture individual differences in making sense of the diversity of forms of nature observable in big cities.

That diversity has been described by philosophers in the light of nature-culture dualism and expressed through a degree of human control over nature and/or proximity [START_REF] Hiernaux | Philosophie du végétal[END_REF][START_REF] Larrère | Penser et agir avec la nature: une enquête philosophique[END_REF][START_REF] Fleury | Le souci de la nature. Apprendre, inventer, gouverner[END_REF].

Their analyses provide an interesting frame in which to capture the cognitive and symbolic dimensions that citizens associate with diverse forms of urban nature. Research on the rural-urban fringe and gentrification shows that symbolic constructions of nature highly prefigure attitudes towards green areas and their wildness [START_REF] Phillips | People, Culture and Nature in the Rural-Urban Fringe: Impressions from a Study of Rural Gentrification in the English Midlands[END_REF].

Following the [START_REF] Hiernaux | Philosophie du végétal[END_REF] typology of nature, urban citizens in modern cities can in their everyday lives be in touch with wild nature (e.g. plant species that appear spontaneously, without human intervention, in brownfield areas) [START_REF] Mathey | Public perception of spontaneous vegetation on brownfields in urban areas: Results from surveys in Dresden and Leipzig (Germany)[END_REF], with cultivated nature (e.g. permaculture set up by citizens who want to restore natural balance in agriculture by limiting human intervention) [START_REF] Copeman | Permaculture: Design principles for urban sustainability. Steering sustainability in an urbanising world: policy, practice and performance[END_REF], with domesticated nature (e.g. areas with plants specifically selected by humans) [START_REF] Kareiva | Domesticated nature: shaping landscapes and ecosystems for human welfare[END_REF], with urbanised nature (e.g. plants in areas with high constraints on growth, such as rare water, unfit soil, etc.; plants supported by high technology, like zero-acreage farming on rooftops) [START_REF] Thomaier | Farming in and on urban buildings: Present practice and specific novelties of Zero-Acreage Farming (ZFarming)[END_REF] or with human-made nature (e.g. potted plants inside the home, produced only for aesthetic purpose) [START_REF] Risi | Generating artificial plant morphologies for function and aesthetics through evolving L-systems[END_REF]. Moreover, urban citizens can search for fantasy nature [START_REF] Jo | The Keukenhof Garden in Netherland & Canal in western Europe[END_REF], as observed through increased web searches on this topic, social networks dedicated to green art, and the success of special art exhibitions around plants and flowers (e.g. the Floralies" in France or the Keukenhof in the Netherlands). Lastly, cultural differences exist in the attachment to certain forms of nature, from heritage conceptions of gardens to the classical organisation of French gardens (echoing domesticated and human-made nature) or to the wildness of British gardens [START_REF] Weiss | Mirrors of Infinity: The French Formal Garden and 17th-Century Metaphysics[END_REF]Phillips, 2021). So far, little is known about cultural differences when it comes to potted plants, potted flowers or small vegetable gardens in urban housing.

For their part, anthropologists [START_REF] Escobar | Construction nature: Elements for a post-structuralist political ecology[END_REF] are increasingly trying to withdraw from nature-culture dualism, especially in occidental countries, to highlight the need for a paradigm shift to address ecological challenges5 ; and also to enhance urban resilience while nature-based solutions and ecoservices are still embedded in anthropocentric urban planning processes and actually don't give voice to non-human elements [START_REF] Bush | Building urban resilience with nature-based solutions: How can urban planning contribute?[END_REF]. Inspired by non-occidental countries, they claim for the removal of the concept of nature in order to consider non-human and human beings as a whole [START_REF] Latour | We have never been modern[END_REF]. In line with that view, the emerging concept of 'biophilic cities' [START_REF] Beatley | Biophilic cities and healthy societies[END_REF][START_REF] Beatley | Biophilic cities are sustainable, resilient cities[END_REF] goes beyond the urban/nature dichotomy and emphasises the embeddedness of cities within complex and interconnected ecosystems. So far, the concept of nature is still used widely6 and this can influence the attitudes of citizens, especially when it comes to sustainable gardening practices. [START_REF] Clayton | Domesticated nature: motivations for gardening and perceptions of environmental impact[END_REF] has questioned the extent to which the home landscape is considered part of nature or part of home, noting that in advertising, the front yard is presented as an extension of the home and lawns are sometimes described as being 'carpet-like'. But what about weeds and undesirable plants? When control over nature is expected, weeds or unusual flowers found in wild spaces can be not 'tolerated' in domestic spaces because they are not 'desired'. This highlights the ambiguity in the opposition between wild and domestic nature [START_REF] Clayton | Domesticated nature: motivations for gardening and perceptions of environmental impact[END_REF]. In turn, natural mechanisms associated with caring for plants (e.g. the presence of insects; the development of weeds, including in pots) may or may not be perceived as constraints to gardening, depending on how urban citizens make sense of the opposition between wildness and domestic planting. A current growing trend for rewilding private gardens has been observed and recent research has been aimed at understanding the motivational and behavioural factors [START_REF] Webb | A study protocol to understand urban rewilding behaviour in relation to adaptations to private gardens[END_REF].

If we consider density of vegetation in urban homes (including indoor and/or outdoor spaces like balconies or rooftops), this depends on external factors, such as lot sizes, socio-economic inequity and leisure time, but also on psychological factors like nature relatedness. The more urban citizens feel related to nature through their yards or visits of public green spaces, the more vegetation they will have in their private spaces, influencing in turn the benefits they will get from gardening, as suggested by [START_REF] Lin | How green is your garden?: Urban form and socio-demographic factors yard vegetation, visitation, and ecosystem service benefits[END_REF]. When only a window view of green spaces is possible, the effect on the greening of private apartments remains unknown. In the same way, little is yet known about the influence of urban Green Infrastructure (GI), increasingly developed to address sustainability challenges [START_REF] Van Der Jagt | Co-creating urban green infrastructure connecting people and nature: A guiding framework and approach[END_REF], on the motivation for having indoor plants. Lastly, in post-pandemic cities, there is an increasing need to put the contribution of indoor greening alongside public green spaces on the agenda for reconnecting humans with the more-than-human world [START_REF] Kaplan | The nature of the view from home: Psychological benefits[END_REF][START_REF] Qin | Understanding the relationship between neighbourhood green space and mental wellbeing: A case study of Beijing, China[END_REF][START_REF] Zhang | Green building, pro-environmental behavior and well-being: Evidence from Singapore[END_REF][START_REF] Dobson | Wellbeing and blue-green space in post-pandemic cities: Drivers, debates and departures[END_REF].

Psychological and relationship benefits from caring for plants: a renewed interest for confined dwellers

The diversity of gardens and of plants-related practices in urban homes expresses various relationships with non-human beings and suggests different interests in natural/social uses [START_REF] Coisnon | Information on biodiversity and environmental behaviors: A European study of individual and institutional drivers to adopt sustainable gardening practices[END_REF]. Gardening activities can contribute to food production, sustainable development, enhanced well-being and development of social relationships, or even feelings of community belonging as studied in community gardens and collaborative urban agriculture. Thus, ways of gardening can reflect personal values, but also depend on societal norms and pressures to adopt sustainable practices [START_REF] Kiesling | How green is your thumb? Environmental gardening identity and ecological gardening practices[END_REF]. Little is known about normative forces that influence the greening of private urban housing, where the range of expectations of bringing nature in can be wider (visual satisfaction, hygiene, well-being, small food production, etc.).

Especially during the Covid-19 lockdowns, the presence of plant at home or the ability to view them from a window were associated with higher well-being and lower negative psychological health outcomes such as anxiety, irritability and sleep disturbance [START_REF] Spano | Association between indoor-outdoor green features and psychological health during the COVID-19 lockdown in Italy: A cross-sectional nationwide study[END_REF]. Inhabitants confined to small homes with lower natural light and few or no plants had more negative emotional well-being [START_REF] Pérez-Urrestarazu | Particularities of having plants at home during the confinement due to the COVID-19 pandemic[END_REF]. In professional contexts, both green indoor and outdoor environments improved mental health, emotional well-being and loyalty of consumers and employees (Han and Hyun, 2019). The presence of indoor natural plants in restaurants increased experiential value and customer satisfaction [START_REF] Apaolaza | Natural plants in hospitality servicescapes: the role of perceived aesthetic value[END_REF]. Thus, as observed in residents of green buildings [START_REF] Zhang | Green building, pro-environmental behavior and well-being: Evidence from Singapore[END_REF], mere exposure to indoor plants can have positive effects on consumers, even in non-lockdown contexts.

The issue of 'know-how' is also of interest when looking at failures or successes in caring for plants or gardening. Lack of expertise is often given as explanation for a low prevalence of plants in urban housing or the reluctance to get plants (Végépolys' French survey, 2017). For the most expert gardeners, whose yards, balconies or rooftops provide a very public demonstration of know-how, psychological motives come into play [START_REF] Clayton | Domesticated nature: motivations for gardening and perceptions of environmental impact[END_REF]. The aim of having a nice garden or balcony is sometimes to gain higher social status, increase social interactions or fuel feeling of belongingness to a local community [START_REF] Clayton | Domesticated nature: motivations for gardening and perceptions of environmental impact[END_REF]. In this light, for some citizens, gardening is much more than a simple technical endeavor; it is also loaded with social meaning.

Environmental concern through gardening versus eating practices

Research on environmentaly responsible behaviour (ERB) has paid a lot of attention to people's connectedness with the natural worldthrough the concept of nature relatednessto explain some gaps between environmental concern and behaviour [START_REF] Nisbet | The nature relatedness scale: Linking individuals' connection with nature to environmental concern and behavior[END_REF]. Indeed, environmental concern expresses to what extent people feel an affinity with the natural world, but it does not lead systematically to ecological practices.

Food-related activities and gardening are human activities that offer ways of enhancing nature relatedness, especially for urban dwellers, but no research has yet been conducted relating to both gardening practices and food habits. Understanding consistencies and inconsistencies between food/gardening behaviours could help us to identify drivers for or barriers to caring for plants at home or to push forward greening practices in urban housing.

Before the lockdowns, a national survey7 found that gardening was among the favourite domestic tasks in French households, overtaking crafting and cooking [START_REF] Brousse | Travail professionnel, tâches domestiques, temps « libre » : quelques déterminants sociaux de la vie quotidienne[END_REF]. Although some differences can be observed according age, gender and occupation, this result is worth further study given that French identity is strongly associated with cooking and gastronomy [START_REF] Rozin | Broad themes of difference between French and Americans in attitudes to food and other life domains: personal versus communal values, quantity versus quality, and comforts versus joys[END_REF].

Furthermore, concerning sustainable gardening practices, [START_REF] Yue | Consumer preference for sustainable attributes in plants: Evidence from experimental auctions[END_REF] found that American and Canadian consumers are less enthusiastic about organic plants/fertilisers than they are about organic food. While the same results were obtained for French consumers, it is worth mentioning that organic production is very low in France and mainly dedicated to aromatic plants. 8 Above gender, age or home ownership status, income and education seem to be major drivers of engagement in practices for sustainable gardening [START_REF] Coisnon | Information on biodiversity and environmental behaviors: A European study of individual and institutional drivers to adopt sustainable gardening practices[END_REF]. Moreover, due to low environmental awareness and poor knowledge of biodiversity, urban households are less likely to adopt sustainable gardening practices [START_REF] Coisnon | Information on biodiversity and environmental behaviors: A European study of individual and institutional drivers to adopt sustainable gardening practices[END_REF] 9 . However, as advocated by [START_REF] Dewaelheyns | A toolbox for garden governance[END_REF], the accumulation of small individual actions in private gardens can have sustainability benefits, and ways to promote sustainable individual behaviours have been identified, such as bottom-up or neighbourhood diffusion of good practices [START_REF] Goddard | Why garden for wildlife? Social and ecological drivers, motivations and barriers for biodiversity management in residential landscapes[END_REF]. While these may improve behaviours, there is no empirical evidence on how urban citizens actually bridge their food habits with their plant care practices or with the density of plants they have in their homes; nor do we have evidence on the degree to which their ecological sensitivity echoes their attitudes towards plants and constraints in growing practices. Those questions, addressed in this paper, are particularly of interest in relation to young people, who are more likely to be interested and involved in alternative food procurement (permaculture, urban agriculture, community-supported agriculture, etc.) in urban areas.

Relationship between commitment to sustainable food and the greening of private housing: singularities of the younger generation

An analysis of the age effect on both commitment to sustainable food and rates of plant purchasing for private housing shows that the younger generation are more motivated regarding sustainable food than for gardening. Young people have the lowest rate of plant purchasers (see Footnote 4) despite being highly committed to sustainable food. Highlighting this apparent paradox is of interest for enriching research on the various synergies that can enhance nature relatedness in cities and increase motivation for bringing more plants into private spaces. Indeed, recent surveys on attitudes towards sustainable consumption identified that people from upper classes, with high levels of education, and from the younger generation present higher interest in sustainable practices (consuming organic food, the collaborative economy, community-supported agriculture, urban agriculture, etc.) (Sessego and Hebel, 2019;[START_REF] Salladarré | Some good reasons for buying fish exclusively from community-supported fisheries: The case of Yeu Island in France[END_REF][START_REF] Aertsens | Personal determinants of organic food consumption: a review[END_REF]. Those attitudes respond to higher ecological sensitivity but also to quest for social distinction in a context where those behaviours are socially recognised.

In another study, de Gavelle et al. ( 2019) examined self-declared food identities of French consumers. This confirmed findings on reduction of meat consumption observed in international surveys [START_REF] Pribis | Beliefs and attitudes toward vegetarian lifestyle across generations[END_REF][START_REF] De Boer | Towards a reduced meat diet: Mindset and motivation of young vegetarians, low, medium and high meat-eaters[END_REF][START_REF] Rothgerber | A comparison of attitudes toward meat and animals among strict and semi-vegetarians[END_REF]: the motivations are age-dependent, with younger adults (25-34 age group) more likely to be concerned about animal welfare (especially vegetarians), environmental issues (especially pro-flexitariansthose who consume meat/fish no more than once a week) and health-related issues (especially flexitariansthose who limit meat consumption), while older participants gave mainly health-related reasons for reducing meat consumption.

Those findings illustrate a generational shift in engagement with ecological challenges. Eating natural or organic foods and swapping meat consumption for a more plant-based diet are some ways in which young people reconnect with nature. Eating and gardening are intrinsically linked in the sense that both provide, to varying degrees, a connection with nature. As little is know on the relation young urban generation have with plants at home and no research has bridged their food motivations and gardening motivations, our research aims to fill this gap. The research is aimed at addressing three specific questions: (Q1) What are young urban people' representations associated with owning plants in private housing?, (Q2) How are plant care activities valuated versus social relationships around caring for plants?, (Q3) What is the relationship between food practicesnotably the commitment to sustainable foodand home plant-related practices?

Overall, our research is aimed at understanding how potential synergies between food concern and plant concern can enhance nature relatedness (Figure 1). At the city scale, our research can inform urban planning about the possible ripple effect from 'edible cities' to the greening of private housing. personal information (sociodemographic variables, food habits, awareness and sensitivity to environmental issues). The results presented in this paper come from a set of questions that we have selected to address our research questions (Appendix 1).

To answer our research questions, we chose to focus on young urban citizens aged 25 to 40 because: (1) the instability of life for people younger than 25 is not particulary favourable to caring for plants and (2) people younger that 40 (Generation Y) show the apparent paradox of higher commitment to sustainable food but lower interest in caring for plants. To select exclusively respondents from big agglomerations, the web-based survey institute has pre-selected a wider panel with people living in urban agglomerations with more than 100 000 inhabitants and in Paris conurbation. In the questionnaire, a control question was formulated for the location: 'Where do you live? In city centre / On close periphery of big towns / In rural areas'. People who have responded 'rural areas' were excluded. Moreover, Bilendi designed a quota sampling approach based on age, gender and occupation (for a total sample size of 1,000). In terms of location, 53.7% of respondents were in inner-city areas and 46.3% were on the very close periphery of big towns. Moreover, 38.0% lived in a house while 59.9% lived in an apartment (Table 1).

Data analysis

Responses to open questions were analysed using the textual analysis software Iramuteq 11 (version 0.7 Alpha 2). This software allows a quantitative analysis of textual responses. It separates text segments into lexical classes according to descending hierarchical classification based on co-occurrence of words (exactly, 'reduced forms' from lemmatisation process) [START_REF] Reinert | Les mondes lexicaux et leur logique[END_REF][START_REF] Illia | Applying co-occurrence text analysis with ALCESTE to studies of impression management[END_REF]. This classification of respondents' words is based on the idea that the words used by each person are part of different 'word contexts' or even person's 'frameworks of reference'. As data come from a sample of respondents around a monothematic, all text segments were aggregated in one single set of text segments [START_REF] Reinert | Les mondes lexicaux et leur logique[END_REF]. It helps understanding how mental representations are embedded in social and cultural contexts [START_REF] Debucquet | Perception of oyster-based products by French consumers. The effect of processing and role of social representations[END_REF]; in this research, it reflects young French citizens' relationship with nature.

Lastly, for data gathered from closed questions, we performed a multiple correspondence analysis (MCA) using XLSTAT Statistics software. MCA provides a 11 Iramuteq is free open-source software inspired by ALCESTE (analysis of co-occurrent lexemes in simple wordings of a text), using the Reinert (1993)'s method. For another illustration of textual data analysis with Iramuteq, see [START_REF] Idoiaga | Exploring children's social and emotional representations of the Covid-19 pandemic[END_REF].

structuring framework that allows the graphical interpretation of associations between different variables of large categorical data sets, by reducing a data matrix within a few dimensions or factors [START_REF] Hoffmann | Correspondence analysis: graphical of categorical data in marketing research[END_REF]. Two to four principal components are usually considered. Principal components can be understood as the latent or projected axes, which are constructed in such a way that the largest data variance is explained (Le [START_REF] Roux | Multiple Correspondence Analysis[END_REF]. The MCA method was used to assess: (1) how different concerns for environmental issues and different levels of commitment to sustainable food are related to each other and (2) related to different valuations of social issues around caring for plants and of diverse plant care activities. The MCA was performed on a large set of variables including demographics, current versus desired number of plants owned, environmental awareness, food profiles focusing on commitment to sustainable food, concern for social issues around plants and perceptions of plant care activities. Table 2 presents the variables and modalities.

Results

Among respondents in our sample, 26.8% report having no plants (inside or outside the home), while 54.1% report having a few and 19.1%, a lot. When it comes to do-ityourself, 9.2% very often carry out activities like seeding, potting, rooting of stem cuttings and 43.8% do so sometimes. In the following sections, we first relate results from the lexical analysis of young urban citizens' representations of benefits/constraints associated with plants in the domestic sphere. We then present the results from the MCA exploring the relationships between environmental concern, plant/food practices and valuation of technical/social issues around plants. The major category, Class 1 (38.7% of all text segments categorised), expresses the psychological impact of owning plants in urban housing. The contact with 'nature' provides a feeling of 'well-being' and of having a 'calm', 'relaxing' and 'zen' moment.

Lexical analysis of cultural representations of benefits/constraints associated with plants in the domestic sphere

Class 2 (30.1%) focuses on the symbolic dimension of caring for plants, described as 'living' and 'natural'. Their presence appears 'essential', 'indispensable' or 'vital' for human beings. Items in Class 3 (20.9%) characterise an esthetic and hedonistic relationship with plants, the latter bringing 'colour', 'smell', 'fragrance' and 'beauty' into urban housing and, in turn, 'pleasure', 'happiness' and a good 'mood'. The smallest category, Class 4 (10.2%), reveals physiological and ecological aspects of caring for plants: plants contribute to improved 'air' quality, that is more 'pure' and 'fresh', and better 'breathing'. In sum, plants in urban housing help citizens to reconnect physically and symbolically with nature, benefitting mental and physical health. Surprisingly, there was no reference to opportunities for gaining knowledge and understanding about plant physiology and ecology: in response to a closed question 52% said they did seek technical information before purchasing plants, whereas 48% said they did not. These findings suggest that urban dwellers relate to plants emotionally and practical concerns are of secondary importance. That might explain the negative representations of home gardening (Figure 3). Lexical analysis of negative words covered 906 text segments (73.3% were classified) together with 633 different words/lexemes. The descendent hierarchical classification revealed three lexical classes (Figure 3).

The major category, Class 1 (43.1%), identifies 'maintenance', 'watering', 'repotting' and 'pruning' as constraints to plant care/gardening. Toxicity for animals and children is also mentioned. Class 2 (31.3%) includes the nuisances associated with plants: 'insects', 'soil', 'smell', 'dirt', 'humidity' and 'carbon dioxide'. Thus, what is often perceived positively outsidethat is, nature and its componentscan be perceived negatively when bringing plants inside. Lastly, Class 3 (25.6%) covers the allocation of 'space' and 'time' for plant care. This class includes the failure of those who do not have a 'green thumb'. In sum, these three classes reveal the gap between the emotional, symbolic relationship with plants and the actual requirements for plant care in urban settings. An additional closed question confirms that the 'ideal plant' is expected to be convenient: inside urban housing the main expectations were that plants are 'easy to maintain' (62.1%), 'resistant' (37.5%) and 'creative/esthetic' (35.4%) while for outside housing, 'easy to maintain' (50.5%), 'adapted to extreme conditions (lack/excess of water, sun)' (42.6%) and 'good for the environment (birds, shadow)' (38.9%).

Multiple correspondence analysis of relationships between environmental, food and home plant issues

The MCA results reveal some relationships between environmental concern, commitment to sustainable food and social/technical valuation of home plant-care activities (Figure 4). Figure 4 shows a good degree of scattering, which indicates that the chosen sets of variables have low similarity. The test values indicate significant spread from the centre of the plot (p<0.05) for all modalities of variables related to plants (except for 'low number of desired plants' on factor 1) (Supplementary material 2), of variables related to environmental awareness (except for 'low/high interest in permaculture' on factor 1/factor 2, respectively) and those related to sustainable food commitment (except for 'high willingness to food change' on factor 2 and 'flexitarian / not flexitarian' on factor 2) (Supplementary material 3). On the contrary, most modalities of the sociodemographic variables show no significant spread from the origin (Supplementary material 4), thus they appeared around the centre of the plot (Figure 4).

The first two dimensions are able to explain 20.19% of the total data variance. The contributions of variables/modalities to the construction of the principal component axes are provided in Supplementary materials 2, 3 and 4. With regard to the contributions to factor 1 and factor 2, we note that the highest contributions to both factors (Ctr from 5.71% to 11.14% to F1/F2) are from, in decreasing order of contributions, variables related to species loss concern, lifestyle changing, changing food habits, purchasing organic food and, lastly, those related to conviviality around plants, the number of plants owned and the desired number of plants. Moreover, other high contributions to a single factor come from variables related to social sharing around plants (Ctr 6.60%, F1), interest in permaculture (Ctr 10.02%, F1), feeling of belongingness through plant care activities (Ctr 10.82%, F2) and chatting about plants (Ctr 8.56%, F2). Those results show the importance of sociability around plants in the definition of factors. The lowest contributions to both axes come from sociodemographics variables (Supplementary material 4), so age, gender, location of housing and professional occupation are not key factors explaining concerns for the environment, sustainable food and for caring for plants.

The relative positions of modalities of variables allow for the visual interpretation of degrees of association. Higher association is indicated by higher proximity, as is the case for high environmental concern, high sustainable food commiment, high valuation of sociability around plants and of plant care activities, and high number of plants owned/desired (see the left side of Figure 4). Those strong associations concerning only a part of our sample of young urban dwellers show the interrelationship between concern for environmental issues, commitment to sustainable food and positive attitudes towards caring for plants in private housing. In contrast, the right-hand side of Figure 4 shows the modalities of variables related to no environmental concern, no sustainable food awareness, no concern for sociability around plants, no valuation of plant care, no plants owned and no desire have any. The larger graphical distance between plant issues and environmental/food issues suggests that those issues are not really associated for a part of our sample of young urban dwellers, or even connected in their minds.

Finally, those results demonstrate that the more young urban citizens are engaged in sustainable food practices and have environmental concern, the more they value plant care tasks such as potting, cutting and watering and sociability around caring for plants, thus suggesting the existence of some continuities between 'outdoors' and 'indoors' or even internal consistencies in behaviours. Concerning the valuation of social exchange around plants, it is worth noting that feelings of belongingness to a community through chatting obtained the lowest rate of agreement (57.40%), meaning that when engaging with others around plants, online interaction is not as meaningful as that with friends or neighbourhood acquaintances (Supplementary material 2).

Discussion

In this section, we discuss our results regarding the relationship that young French urban citizens have with plant care and gardening; most notably the gap between ecological challenges and actual expectations/practices regarding plants at home. Then, we envision the role of social triggers, at the level of close neighborhood and of urban planning, to enhance the greening of private housing. Finally, we suggest ways to benefit from continuities -which could be spurred within edible cities -in young people's attitudes towards caring for plants and their food profiles.

Feelings and experiences with caring for plants at home: limits on and ways to enhance nature relatedness

The gap between environmental concern and actual ecological behaviour has been highlighted through the nature relatedness concept [START_REF] Nisbet | The nature relatedness scale: Linking individuals' connection with nature to environmental concern and behavior[END_REF]. Individual differences in nature relatedness are rather stable over time, but environmental education or positive emotions from experiences in nature can change feelings about the naturehuman relationship [START_REF] Pooley | Environmental education and attitudes: Emotions and beliefs are what is needed[END_REF]. While research on nature relatedness has concentrated mainly on the impact of outdoor activities in contact with nature [START_REF] Lin | How green is your garden?: Urban form and socio-demographic factors yard vegetation, visitation, and ecosystem service benefits[END_REF], little is known about indoor / at home everyday experiences with plants, especially for urban dwellers. We adressed that gap by focusing on young French urban people, who are the group least likely to purchase plants.

By analysing spontaneous feelings associated with plant care at home, we avoid desirability bias about ecological practices in domestic settings [START_REF] Uren | Exploring social and cultural norms to promote ecologically sensitive residential garden design[END_REF]. A lexical analysis of young urban dwellers' survey responses revealed a relationship with plant care based less on ecological benefits than on individual convenience or humancentred concerns such as psychological well-being, aesthetic benefits and the effect on physical health of improved air quality. Moreover, all aspects of maintaining plants were described rather in negative terms, with opinion divided as to whether those tasks are a 'constraint' or a 'pleasure'. As the nature relatedness concept embraces both positive and negative feelings for nature [START_REF] Nisbet | The nature relatedness scale: Linking individuals' connection with nature to environmental concern and behavior[END_REF], we can ask how negative aspects of caring for plants in private urban housing can be associated with more positive emotions and, in turn, how we can help people to become more connected to nature. That could then lead urban dwellers to derive more from innate biophilic tendencies [START_REF] Kellert | The biophilia hypothesis[END_REF][START_REF] Zelenski | Happiness and feeling connected: The distinct role of nature relatedness[END_REF].

Connections between 'indoor' and 'outdoor' experiences could contribute to developing an 'intimate sensitiveness' [START_REF] Gorz | Green indoor and outdoor environment as nature-based solution and its role in increasing customer/employee mental health, well-being, and loyalty[END_REF] with nature which is essential in facing ecological challenges [START_REF] Latour | We have never been modern[END_REF]. From that perspective, 'landsenses ecology' [START_REF] Zheng | Visitors' perception based on five physical senses on ecosystem services of urban parks from the perspective of landsenses ecology[END_REF] has highlighted the adaption of natural environments to suit people; for example using sensory stimuli (seeing plants or water; the feel of sunlight; hearing animals) in the design of urban parks so that they appeal more to people. Green places in cities can also contribute to developing people's latent capacity to make sense of the various functions of urban nature (benefits for non-human beings, animals, other plants, etc.) [START_REF] Ng | Recognising the edible urban commons: Cultivating latent capacities for transformative governance in Singapore[END_REF]. When it comes to caring for plants at home, we can hypothesis that those initiatives would allow urban dwellers to envision differently the diverse requirements of natural beings, to position their plants in the overall urban ecosystem and to view plantcare activities as green citizenship.

Furthermore, to enhance nature relatedness and ecological practices in urban housing, emotions and experiences in urban green spaces could play more significant roles than knowledge. It has been known for some time that knowledge of environmental issues does not necessarily predict pro-environmental behaviour [START_REF] Nisbet | The nature relatedness scale: Linking individuals' connection with nature to environmental concern and behavior[END_REF].

Spaces for plants care are obviously less available in urban housing but surprisingly, concern for environmental issues (protection of local species, selection of plants that are good for birds or insects, use of domestic compost, seasonality, etc.) and willingness to learn more about non-human living beings' requirements were near absent in our results.

Finally, from that perspective, young urban dwellers are no different from the overall population [START_REF] Beumer | Show me your garden and I will tell you how sustainable you are: Dutch citizens' perspectives on conserving biodiversity and promoting a sustainable urban living environment through domestic gardening[END_REF][START_REF] Coisnon | Information on biodiversity and environmental behaviors: A European study of individual and institutional drivers to adopt sustainable gardening practices[END_REF]. To push forward the greening of urban housing and more sustainable gardening practices, we look next to social relationships around plants.

Social ties around caring for plants: A trigger for building community and green identity in urban setting

By demonstrating the link between higher environmental awareness and more positive evaluation of technical/social issues around caring for plants at home, our results reveal two different urban gardener profiles, in line with [START_REF] Clayton | Domesticated nature: motivations for gardening and perceptions of environmental impact[END_REF]: urban gardeners with an ecological ethic, caring for plants because of the benefits for nature and for social interaction and, 'utilitarian' urban gardeners who look for convenience and ease of maintenance. Marketing plants on the basis of their resilience to urban constraints is one response to the expectations of 'utilitarian' urban gardeners, but social triggers might also been considered to help them to engage with ecological arguments for keeping plants.

Indeed, as [START_REF] Coisnon | Information on biodiversity and environmental behaviors: A European study of individual and institutional drivers to adopt sustainable gardening practices[END_REF] wrote, 'apart from their own environmental consciousness, individuals are also known to be strongly influenced by imitation behaviours and social norms […] and more particularly in the case of outdoors spaces exposed to neighbours'. Moreover, social interactions among residents within green buildings can provide a supportive and educational environment to enhance environmental awareness and pro-environmental behaviours [START_REF] Zhang | Green building, pro-environmental behavior and well-being: Evidence from Singapore[END_REF].

We suggest that social influences in the context of urban housing and/or green places in city centers can help to drive the greening of private urban housing. Following [START_REF] Zhang | Green building, pro-environmental behavior and well-being: Evidence from Singapore[END_REF], urban planners could assess more formally the overall psychosocial benefits of green buidings. Moreover, local authorities and private entrepreneurs (e.g. in urban agriculture) can participate actively to enhance social interaction around plants in cities. In turn, those social interactions contribute to building community around shared values and to the enhancement of 'green identity' [START_REF] Bell | Emerging Community Food Production And Pathways For Urban Landscape Transitions[END_REF] or 'ecological identity'; the latter having direct and indirect influences on various environmental behaviours through social values and world views [START_REF] Conn | Living in the earth: Ecopsychology, health and psychotherapy[END_REF][START_REF] Walton | Ecological identity: The development and assessment of a measurement scale[END_REF]. Ultimately, this could encourage presence of plants in private housing, though further longitudinal studies are needed to evaluate the actual porosity between public spaces and private settings.

Sustainable food choices and greening of individual urban housing: expanded scope of edible cities

With increasing concern in all occidental countries for sustainable food sources, our study explored the relationship between food profiles (willingness to change food habits due to climate change, flexitarian profile, purchasing organic foods, purchase via short channels) and attitudes towards caring for plants. The population of young French urban dwellers was of interest because they are more committed to sustainable food than older generations, but they have the lowest rate of plant purchasers. However, we found that the more young urban citizens are engaged in food transition and sustainable food, the more they value plant care tasks and the social aspects of gardening. The reverse was also observed. These continuities between commitment to sustainable food habits and attaching value to caring for plants at home have not been demonstrated previously.

Recent research on the motivations of commitment to urban short channels, notably community-supported agriculture, shows that consumers look for fresh and local products but also for reconnection with nature [START_REF] Salladarré | Some good reasons for buying fish exclusively from community-supported fisheries: The case of Yeu Island in France[END_REF]. Moreover, sense of belongingness is highly developed through social exchanges during the picking of vegetable baskets and around food preparation [START_REF] Debucquet | Sense of belonging and commitment to a community-supported fishery. The case of Yeu Island, France[END_REF]. Based on our results, young urban dwellers committed to more sustainable food sources seem be predisposed to bringing more plants into their homes, to valuing social ties around plant care and to cultivating a sense of belongingness. The greening of their living environment is actually a logical continuation of their food habits. Synergic effects could ultimately enhance nature relatedness, especially when alternative, local ways of food purchase are offered to urban dwellers, helping them to engage concretely with nature and to make sense of plants' and other non-human beings' specific requirements. Further research could explore those synergic effects in more depth and examine how they can contribute to strengthening both food identity and green identity. As [START_REF] Walton | Ecological identity: The development and assessment of a measurement scale[END_REF] mentioned, one aspect of ecological identity deals with beliefs about shared-group membership with nature and the direct individual experiences one has with it. In 'edible cities', ecological identity forged around food communities could stimulate community building around plant care in private housing.

Conclusion

Through a national survey with young urban dwellers in France, our quantitative study explored attitudes and practices towards plant care in private housing (in apartments, on balconies, in small yards). As this specific gardening context has not been explored to date, we provide useful insights not only for the horticultural sector but also for the local governance of cities, as reflecting upon the porosity between public and private spaces can lead to higher inclusion of natural elements and to open new ways to support them in urban planning processes.

While better alignment of plant selection (ease of maintenance, resistance, adaptation to urban constraints, aesthetic criteria, etc.) could meet the expectations of 'utilitarian' or aesthetic-and hedonistic-oriented urban citizens, other triggers for bringing more plants into private housing are worth considering. For instance, in line with the emergent concept of 'edible cities' [START_REF] Sartison | Edible cities-An innovative nature-based solution for urban sustainability transformation? An explorative study of urban food production in German cities[END_REF], making food production closer to urban dwellers may be one way forward. Also, recognising the importance of nature-based solutions for food security and urban resilience to climate change, bringing more urban gardens or entrepreneurial urban agriculture into the heart of cities might help young urban dwellers to reconnect with nature and food production.

That can also promote inclusivity [START_REF] Cobbinah | Accommodating green spaces in cities: Perceptions and attitudes in slums[END_REF] and community building, especially through volunteer engagement in agricultural tasks [START_REF] Sartison | Edible cities-An innovative nature-based solution for urban sustainability transformation? An explorative study of urban food production in German cities[END_REF][START_REF] Hou | Governing urban gardens for resilient cities: Examining the 'Garden City Initiative'in[END_REF]; similar effects were observed through the greening of schoolyards around a community participatory approach [START_REF] Flax | Greening schoolyards : An urban resilience perspective[END_REF]. For urban planning to effectively address potential continuities between dwellers' attitudes towards public green areas and caring of plants in private housing, there is a need to consider the entanglement of ecological and social uses of nature. We advocate that favouring social ties around food production or public green areas, leading to more civic commitment in urban projets [START_REF] Brazeau-Béliveau | Citizen participation at the micro-community level: The case of the green alley projects in Quebec City[END_REF] and helping non-expert citizens to gain knowledge about plants and their benefits for the ecosystem, can push forward the greening of private housing. Greening the city could have a ripple effect on the greening of private housing, thus enhancing nature relatedness. In that perspective, indoor plants in individual dwellings might be considered 'proxies' for nature. It echoes with the multiscalar approach of urban planning, spatial (from city to close neigbourhood), temporal (from young to elder people) and social (from family to local community), as suggested by [START_REF] Flax | Greening schoolyards : An urban resilience perspective[END_REF] to build more integrative resilience.

Figure 1 :

 1 Figure 1: Nature relatedness among urban dwellers: exploring the relationships between environmental concern, food behaviour and attitudes towards caring for plants

  Spontaneous representations associated with plants in urban housing were captured through open questions designed to elicit positive and negative words associated with plants kept inside the home, on balconies or on terraces. The lexical analysis of positive words covered 963 text segments (91.7% were classified), together with 672 different words/lexemes. The descendent hierarchical classification revealed four lexical classes (Figure 2).

Figure 2 .

 2 Figure 2. 'Owning plants inside/on balcony/on terrace': Positive spontaneous representations.

Figure 3 .

 3 Figure 3. 'Owning plants inside/on balcony/on terrace': Negative spontaneous representations.

Figure 4 :

 4 Figure 4: Multiple Correspondence Analysis -First and second factorial axes (n = 1,000 respondents).

Table 1 . Profile of national sample (n = 1,000).

 1

Table 2 : Set of active variables used for MCA: wording of questions, answer categories and Likert scales

 2

Appendix 1. Categories assessed and formulation of the questions Category Formulation of the questions

  Representations of benefits/constraints associated with owning plants  'Owning plants inside / on balcony / on terrace': what are the positive/negative words that come spontaneously to your mind? Please give three to five words for positive/negative aspects. (OQ) Behaviour related to plants at home  Which statement applies to you? I have a lot of indoor/outdoor plants. I have few indoor/outdoor plants. I have no plants / only artificial plants. (UC)  Ideally, how many plants would you like at home? A lot of indoor/outdoor plants. A few indoor/outdoor plants. No plants / only artificial plants. (UC) Valuation of plant care activities  What is your perception of these tasks? For you, potting plants is rather a constraint/pleasure. (UC) For you, watering is rather a constraint/pleasure. (UC) For you, cutting is rather a constraint/pleasure. (UC) Valuation of social aspects around caring for plants at home  Having plants at home can lead to exchanges with others. To what extend do you agree with following statements? Caring for other people's plants is a way of caring for those people. (L4) Talking about plants with other people gives a feeling of belonging to a community. (L4) Giving/receiving cuttings, plants or seeds allows you to share with others. (L4) Talking about plants is a friendly moment. (L4) Talking about plants / taking part in an online forum means being part of a community. (L4) Environmental concern  Are you interested in permaculture? Not at all / Not really / Yes (UC)  Do you feel concerned about species loss? (Be honest!) Not at all / Not really / Yes (UC)  Are you willing to change your lifestyle, notably your travel habits? (Be honest!) Not at all / Not really / Yes, absolutely (UC) Food profile  Are you willing to change your food habits due to climate change? Not at all / Not really / Yes, absolutely (UC)  Are you flexitarian? No/Yes (UC)  Do you purchase organic food? Never/Sometimes/Regularly/Exclusively (UC)  Do you purchase some food in short channels? Never/Sometimes/Regularly/Exclusively (UC) Note: OQ = Open question; L4 = Likert scale with four points; UC = Unique choice

Data from the French National Kantar Survey for Val'hor/FranceAgriMer on plant, tree and flower purchasing behaviour (April 2021). People aged 15 to 44 represent 43% of the population and 28% of the buyers, with those aged over 44 representing

57% of the population and 72% of the buyers. https://www.valhor.fr/fileadmin/A-Valhor/Valhor_PDF/Etudes_BilanAchatsFrancaisVegetauxDonnees2020Complet_Kantar.pdf

The philosopher Michael Marder also suggests adopting a 'phytocentric' paradigm (see Marder, inHiernaux and Timmermans (eds), 2018: 115-132).

Notably, in the advertising and messages dispatched by the horticultural sector (e.g. the event 'Flower week for bees' by Val'hor, French umbrella association of plants companies.

'Time schedule and decisions in couples' (survey from Insee, the French National Institute forStatistics and Economic Research, 2009-2010).

The Plante bleue label relative to sustainable production practices is used for the busines-to-business market. For the consumer market, attention is at the present time more focused on Fleurs de France (Flowers from France).

See also for younger dwellers a national survey by the French ministry for ecological transition (2020): https://www.statistiques.developpement-durable.gouv.fr/les-francais-et-la-nature-frequentationrepresentations-et-opinions

This research was supported by UMT STRATège, whose partners are ASTREDHOR Val'hor.

Data collection was carried out just before periods of lockdown in France due to the Covid-19 pandemic. Longitudinal research could help capture ongoing changes in attitudes. In addition, ethnographic methods could help to reflect on some new forms of urban governance addressing the continuities between public green areas and private housing. This could serve an expanded conception of ecology as explained by urban planner Felix Guattari (1989), who stated that ecology has three dimensions: environmental ecology (human-nature connections), social ecology (direct human relationships) and mental ecology (new way of thinking, desiring and dreaming).


M Lamard

B Auvity

P Buttin

S Rosini

C Retière

Multiscale Study of PEMFC in Marine Environment: Impact of a NaCl Spray on Durability

The present work investigates the effects of a salt spray injected into the airflow of Proton Exchange Membrane Fuel Cells (PEMFCs) at various time scales and contamination levels, with the final aim of identifying the appropriate protection for maritime applications. A dedicated experimental setup generates a sodium chloride (NaCl) mist in the cathode airflow to contaminate single cells and stacks. Constant current density tests led to lifetime shortening with partial recovery after cathode rinsing. Then, Membrane Electrode Assemblies (MEAs) post-mortem characterizations revealed that NaCl was present in the cathode gas diffusion layer, and chlorine reached the catalyst layer for high concentrations. Furthermore, chlorides enhanced current collectors and bipolar plates degradation as corrosion products were identified in MEAs and stack effluents. Overall, this study reports that NaCl effects on PEMFCs not only depend on the concentration but also on the MEA type, plate material, and startup/shutdown procedure.

Introduction

In the context of global warming, there is an urgent need to find solutions to reduce air contamination by replacing combustion engines. For that, technologies as Proton Exchange Membrane Fuel Cells (PEMFCs) can be used, as they produce electricity thanks to air and hydrogen by only rejecting pure water and heat. Thus, obtaining hydrogen from a renewable source reduces greenhouse gas emissions. This technology was first introduced almost two centuries ago, but there are still some issues concerning its durability. More precisely, the impact of an aggressive environment such as marine atmosphere can reduce the fuel cell (FC) performance. Indeed, even if industrial companies are interested in PEMFCs for maritime transportation, various air pollutants are found in harbors like nitrogen oxides or sulfur dioxide, but also specific contaminants such as sea salt. This last one is mostly composed of sodium chloride (NaCl) and can reach the membrane electrode assembly (MEA) when the cells are fed with marine air. However, few studies have addressed the problem of NaCl contamination. As reported by Mikkola et al. (1), chlorides did not affect as much as expected the MEA of a single cell whereas an increase in the high-frequency resistance was detected and explained by the presence of sodium ions in the membrane. Nine years later, Sasank et al. (2) revealed that seawater droplets reduced significantly the performance of an open-cathode PEMFC stack after 48 hours of contamination with partial recovery after clean water rinsing. In line with these results, Uemura et al. (3) observed a degradation after NaCl particles injection at the cathode side of a single cell, which decreased the cathode active area. It should be noted that the degradation mechanisms require clarification, and the NaCl amount reaching the FC was very high compared to marine environments (1) or not even specified. Therefore, the objective of this experimental work is to verify and explain the trend of performance degradation for both single cells and stacks when exposed to a NaCl mist, with the final aim to predict the adaptations needed for PEMFC deployment in marine environment. Close attention is paid to the NaCl quantity injected into the airflow thanks to a dedicated experimental setup. Then, single cells and stacks contamination tests were carried out for various NaCl concentrations and duration times, at constant current density. In our approach, two types of MEAs and current collectors were used and compared for single cells. Finally, electrochemical tests and post-mortem analyses helped to evaluate the mechanisms that might be implied in the degradation. MEAs from a ship propulsion stack exposed to a real marine environment were additionally observed, which deterioration reinforced the pertinence of this study.

Materials and Methods

Contamination Setup

This subsection describes the setup used for polluting PEMFC single cells and stacks, in order to study the impact of a NaCl mist injected into the feeding airflow. Although in many ways similar to Uemura et al. setup (3), adjustments were made to inject a mist instead of solid particles, but also to be easily connected to the test benches when needed. Moreover, most of the couplings and tubes were chosen with chemically stable materials, to prevent the injection of corroded metallic impurities. An ultrasonic vibrator creates a NaCl mist in a buffer tank flushed by the airflow, which carries the salt spray. The droplets diameter is about 5 µm, as specified by the manufacturer. NaCl solutions to be sprayed were prepared with deionized water at several concentrations, which were linked to the final content of NaCl in the airflow thanks to qualification tests carried out.

Experimental Setup and Fuel Cells Specifications

Single Cells Setup. Single cells selected for the tests have an active area of 11.76 cm² (19.6 x 0.6 cm) and were mounted on the setup presented in Chevalier et al. (4), without the segmented channels. Two types of MEA were studied, from different suppliers, to compare their durability when exposed to NaCl. Type 1 has a platinum (Pt) loading of 0.5 mg/cm² for both electrodes whereas for type 2, the Pt amount is 0.4 mg/cm² for the cathode and 0.1 mg/cm² for the anode. It should be noted that MEAs type 1 were stored for four years without specific measures, which could have caused some damage and a limited reproducibility of performance. Oppositely, MEAs type 2 were fresh and specially ordered for this study. Copper current collectors coated with a gold thin film were used, with two straight channels for the gases. Then, all of the experiments were performed with a Biologic/Paxitech instrument driving the FC (FCT-50S). In order to preserve the equipment from corrosion during contamination tests, the air outlet containing NaCl traces was not connected back to the test station. It offered a means of collecting cell effluents, even though it did not allow tests under pressure.

Stacks Setup. Stacks were constituted of five cells (MEAs type 2) with an active area of 220 cm². The geometry was different from the single cell setup, but the MEA composition was the same. Bipolar plates (BPs) were made of 316L stainless steel partially covered with graphite to improve the conductivity. The experiments were conducted with a homemade test bench at CEA Liten, commonly chosen for single cells and low-power stacks. Inlets and outlets were localized on the anode end plates side, closest to the MEA called cell 1. As for the single cells, effluents were collected at the air outlet to prevent the test bench from deteriorating.

Testing Procedures

Constant Current Testing. Constant current density tests were made at 0.5 A/cm² when polluting the airflow. After a conditioning step provided by the supplier, the FC was operated for approximately 20h at 0.5 A/cm². The operating conditions were a temperature of 60°C (for both gas lines and cell), relative humidity (RH) of 75% (single cells) and 50% (stacks), stoichiometry H 2 /O 2 of 1.5/5 (single cells) and 1.5/2 (stacks), and minimum flow rates of 100 and 500 mL/min respectively for single cells. Then, the contamination setup filled with deionized water was connected to the cathode for acquiring a baseline. Finally, NaCl solutions were prepared at concentrations between 0.1 and 50 g/L for contamination tests at 0.5 A/cm², carried out under the same experimental conditions. More precisely, this corresponds respectively to 0.01 and 5 g/m 3 NaCl in the airflow, meaning 200 to 10 6 times higher than a natural marine environment (1). Postprocessing included smoothing of the data collected to remove voltage drops due to the recording procedure and the occasional tube clogging. Furthermore, this treatment improved graph reading without cutting essential data. Electrochemical Characterizations. To highlight the NaCl impact, several electrochemical tests were combined for various contamination stages. Polarization curves were plotted to track the overall variation in performance. Additionally, electrochemical impedance spectroscopy (EIS) spectra were obtained at 0.05 A/cm² and 0.4 A/cm² to study kinetics and mass transport phenomena. The frequency range was 2000 to 0.350 Hz with a signal amplitude of 10%. Cyclic voltammetry (CV) was completed at the cathode for the stack scale study only, because of the setup configuration. The voltage sweep range was 60 mV to 0.7 V at scan rates of 50 mV/s, 100 mV/s, and 200 mV/s. Three scans were saved for each cell at the scanning speeds studied. Concerning the equipment, the FCT-50S was directly driving the characterization procedures and recordings for single cells, with clean water humidification to avoid FC degradation during the tests. It was recognized that inevitable condensation droplets can momentarily clog the tube at the cathode entrance because of the low flow rate, resulting in a slightly noisy signal for EIS at 0.05 A/cm². Fortunately, this phenomenon is acceptable when studying the NaCl contamination itself or the FC global performance. On another note, procedures were controlled via an Autolab potentiostat to achieve EIS and CV on stacks. As flow rates were higher than for single cells, clogging did not occur at this scale.

Post-Mortem Analysis. Scanning Electron Microscopy (SEM) analyses coupled with Energy Dispersive X-Ray Spectroscopy (EDS) were carried out on MEAs and effluents. MEAs samples were coated and polished, thus obtaining a precise cross-sectional view to look for potential delaminations. Then, samples were peeled to observe rigorously each layer. For single cells, both extremities were observed, whereas entrance, middle, and air outlet were studied for stacks. Concerning the stack effluents collected, they were put into a drying oven to obtain a powder for SEM-EDS characterization.

Results and Discussion

This section describes the single cells and stacks contaminations realized thanks to the dedicated setup mentioned earlier. Then, both scales results are compared to provide a solution for discriminating the direct and indirect effects of NaCl on PEMFCs, which will be explored in this paper and developed in future work.

Single Cells

As described above, two types of MEAs were studied for various time scales and contamination rates thanks to a specific setup injecting NaCl mist in the cathode airflow. The most relevant results are detailed in this paragraph. Short duration contaminations. 30 minutes exploratory tests were carried out on MEA type 1, with an approximate NaCl amount in the airflow of 0.01 and 5 g/m 3 . When the cell performance loss was about 0.1 V from the baseline, the contamination test was stopped and the cathode was humidified with deionized water again. As shown in Figure 1, 10 mg/m 3 NaCl during 30 minutes did not trigger perceptible loss of performance. It should be noted that the baseline signal fluctuates. As explained before, this is due to a condensation issue in the tubes and does not affect the global performance. Furthermore, the performance obtained during the baseline recording was not as high as expected for PEMFCs at 0.5 A/cm². This was probably caused by the use of an old MEA, as described earlier.

Conversely, 5 g/m 3 NaCl provoked a sudden and severe drop in voltage from 0.36 to 0.1 V. Then, signal stabilization was observed a few minutes after the end of nebulization. Finally, deionized water humidification allowed a voltage recovery of 84%. Unexpectedly, after a week of washing procedures (conditioning cycles with changes in gas flow rate, pressure, and RH), the single cell recovered fully its initial performance, reaching 0.45 V at 0.5 A/cm². This value is higher than the baseline one, presumably because the conditioning time was too short. As expected, the voltage drop recorded when PEMFCs were exposed to NaCl is consistent with the literature (2, 3). Additionally, partial regeneration was also reported in Li et al. work (5), even if the pollutant introduced was hydrochloric acid. Nevertheless, initial performance recovery had never been obtained after NaCl contamination to our knowledge. In future, these short contamination tests will be replicated with MEA type 2, which baseline is about 0.65 V at 0.5 A/cm². Indeed, Baturina et al. (6) noticed that a higher cell voltage might increase chloride coverage of the catalyst.

Long contaminations. After the exploratory study, longer contamination tests were undertaken at 10 mg/m 3 NaCl air content on both MEA types, as illustrated in Figure 2. MEA type 1 was polluted for 24h, with night breaks. The decrease in voltage was about 24 mV/h, followed by a 30 min nitrogen (N 2 ) cathode rinsing step triggering 68% of performance recovery. Unpredictable increases or decreases in performance were observed when restarting the FC, which will be explored in further investigations.

Concerning MEA type 2, breaks were done only every weekend after the first 90h of contamination, to save time. As for type 1, partial recovery was visible after restarts, except for the first weekend (29h) where membrane dehydration occurred, presumably not linked to the NaCl presence. After 265h of contamination with an average voltage decrease of 3.3 mV/h, the FC was run with deionized water for 30h. The voltage drop was lower but present, which means that, contrary to high concentrated and short duration experiments, this recovery method is unlikely to be efficient. Perhaps NaCl remained in the MEA because of stronger interaction due to a longer contamination time, or a degradation process was triggered and continued even under clean water operation. However, as for the type 1 MEA, 30 min of N 2 rinsing helped to recover about 80% of initial performance. Characterization tests were carried out for both MEAs although they did not yield additional information. Even if it was difficult to compare the results as the initial performance and contamination time were different, the degradation rate of MEA type 1 was about 8 times higher, supporting the idea that type 2 was likely to be more robust. Future tests with CV measurements might help to figure out if the catalyst poisoning rate increases with contamination time.

Post-mortem analyses. The single cell contaminated during the exploratory study was characterized by SEM-EDS. As illustrated in Figure 3.a), MEA sectional view shows delamination between the gas diffusion layer (GDL) and the cathode catalyst layer (CCL). NaCl was identified on GDL and chlorine significantly reached the CCL. Surprisingly, various metallic species including gold were detected on the GDL surface, as well as deposits containing Ni, Fe, Cr, and Mn, which are visible in Figure 3.b). These last impurities could come from corroded stainless steel connectors or test station parts. Furthermore, copper was strikingly found in the CCL. Most probably, copper current collectors were corroded by chlorides. Indeed, they were coated with a thin gold film deposited on a nickel diffusion barrier, which explains the presence of these elements. Thus, the sudden loss of performance can be due to sodium ions blocking the ionomer sulfonate sites (7) or/and chlorides directly contaminating, more or less reversibly, the platinum catalyst (6). However, current collectors and stainless steel corrosion enhanced by the presence of chlorides should be considered too, as the cations that can deteriorate the electrolyte membrane (8,9).

Stacks

For studying NaCl impact on more powerful systems, comparable experiments were carried out on stacks, as detailed in this section. Investigations with high NaCl concentrations were conducted, although deeper experiments are planned for the future.

Contaminations. A reference stack was operated for 360h under pure water humidification at 0.5 A/cm², showing stable performance with a negligible voltage drop of 0.012 mV/h. Then, a stack with fresh MEAs was contaminated by a 5 g/m 3 NaCl mist. Contrary to the baseline stack behavior, inspection of Figure 4.a) indicates that cell voltages oscillated after no later than 3h of contamination meanwhile outgoing effluents started getting orange. The stack became unusable after 8h, because of cell 4 sudden voltage drop. A characterization cycle, meaning CV, EIS, and polarization curves measurements under pure water helped recover 95% of performance.

Another new stack was polluted with 0.5 g/m 3 NaCl. As pointed out in Figure 4.b), the first contamination step resulted in unstable voltage after 10h, followed by a sharp voltage drop for cell 5. As for the previous experiments, this degradation was accelerated during polarization curves. Then, an excellent performance recovery of 96% was obtained after characterization cycles and 13h of operation under pure water mist. When the effluents became again colorless, a second contamination was investigated. In this case, the voltage dropped faster because of cells 1 and 5 failures, with once more an overwhelming regeneration of 95%. Most relevant polarization curves and EIS spectra of the stack contaminated with 0.5 g/m 3 NaCl can be found in Figure 5. Polarization curves highlight the excellent recovery obtained after both contamination steps, which plots are overlapped. As evident when comparing EIS spectra, the mass transfer semi-circle was extremely large after the first contamination and decreased after the recovery step. This trend seems to suggest that mass transfer limitation was dominant, meaning the contact between air and triple points was reduced because of NaCl presence, but fortunately recovered after rinsing. Concerning the CV tests, the results were somewhat unexpected. Indeed, the electrochemical surface area (ECSA), which is proportional to the charge values given in Figure 6, was not reduced with NaCl contaminations. Surprisingly, for the most degraded cells, meaning 1 and 5, the charge slightly increased after NaCl exposure and decreased under clean mist operation, from 12.0 to 10.6 C for cell 5 first contamination/recovery step. This trend is inconsistent with literature (3,10), where authors usually reported a decrease in ECSA when MEAs were exposed to chlorides, because of their adsorption on the platinum catalyst. To explain this somewhat problematic mismatch, deeper investigations are needed, including longer contamination tests.

Post-mortem analyses. Observations were undergone on collected effluents and cell 4, which was the most degraded of the 5 g/m 3 NaCl polluted stack. As illustrated in Figure 7, the BPs were corroded, especially close to the air inlet. SEM coupled with EDS confirmed the predicted link between these corrosion marks and the effluents impurities, which were both identified as NaCl and stainless steel corrosion products (Fe, Cr, and Ni). As evident from the MEA cross-sectional view, GDL and CCL were strongly delaminated. Moreover, particles composed of Na, Cl, Fe, and Cr were found on the cathode GDL and micro-porous layer surfaces at the air inlet. As for single cells, chlorides reached the CCL. However, impurities were only present on the cathode GDL surface in the middle of the cell, and neither element was encountered at the air outlet. Likewise, all the MEA portions studied were clean on the anode side. These results are consistent with single cells post-mortem analyses, consolidating the possibility of BPs corrosion enhancement when exposed to chlorides. Corroded particles were largely evacuated in effluents, but some of them could clog the GDL pores during the contamination, explaining the mass transport limitation observed. Then, a great part of impurities was probably released with rinsing, explaining the satisfactory regeneration also evidenced by ultimately colorless effluents. Furthermore, chlorides contaminated the CCL, even if the stack was polluted no later than 8h followed by a clean mist operation. Further work will help to know if delaminations are specific to NaCl exposure, and if corrosion particles and chlorine remain in the MEA after longer rinsing steps.

Beyond the scope of this study, a 35 kW PEMFC stack used for ship propulsion 10 years ago was analyzed. Even if performance did not decrease after 50h of operation in a marine environment, post-mortem characterizations revealed surprising observations. Indeed, as for laboratory tests, corrosion marks from BPs were visible on the MEAs (Figure 8) and identified as Fe, Cr, Ni, Mo, and Cl in the cathode GDL. This result provides the first evidence for PEMFC degradation when deployed for a maritime application. The next stage is to find out if longer sea operations result in chlorides reaching the CCL, with the final consequence to reduce the available power.

Single cells with graphite current collectors

Both scales experiments highlighted a decrease in PEMFC performance with partial recovery after clean mist rinsing. Chlorides corroded the metallic plates, which products could limit air transportation through the GDL, and even contaminate the membrane. Therefore, a slightly different single cell setup was realized to discriminate the NaCl direct impact on the MEA and the indirect corrosion particle effects. Indeed, the cathode plate was replaced by a graphite one, which is more adapted to severe environments. The copper current collector was kept on the anode side, as it was not directly exposed to NaCl spray.

Contamination tests were carried out in this configuration with 10 mg/m 3 NaCl. The evolution of the cell voltage as a function of time is shown in Figure 9 for the entire contamination test. In accordance with the previous results, there was a significant decrease in performance when the single cell was exposed to NaCl mist. The polarization curves recorded at various contamination times confirmed that no more than 50h of continuous pollution provoked a drastic performance loss. These results corroborate the direct impact of NaCl on PEMFC. In the future, postmortem analyses will reveal if metallic elements are still found in the MEAs with the use of a graphite current collector at the cathode side. Moreover, SEM-EDS observations are needed as well on the membrane to determine if sodium or iron could be a source of damage.

Conclusion

To our knowledge, this study has contributed to the literature in many ways:

(1) First, it confirmed the significant performance degradation of PEMFC single cells and stacks when exposed to NaCl at various concentrations, for two MEA types. (2) An excellent regeneration was obtained for short contaminations.

(3) Even if CCL irreversible degradation was not proven, they contained chlorides for high NaCl concentrations, which remained after a rinsing step. (4) Chlorides corroded gas plates for both scales, which led to the formation of metallic impurities reaching the MEA and limiting the mass transportation of gases to the triple point boundaries. (5) A real case study acknowledged the impact of NaCl on PEMFC exposed to a marine environment. Further study will help to determine if sodium is present in the membrane or not, and lower concentrations will be studied to find out an impact threshold. Moreover, CV tests on single cells will help to understand the unclear stacks ECSA observations.

Figure 1 .

 1 Figure 1. Single cell voltage time evolution depending on NaCl concentration (0.5 A/cm², 60°C, 75% RH, stoichio. 1.5/5 H 2 /O 2 with minimum flow rates of 100 and 500 mL/min).

Figure 2 .

 2 Figure 2. (a) Single cell voltage time evolution with a 10 mg/m 3 NaCl contamination for MEA type 1 and (b) for MEA type 2. Operating conditions: 0.5 A/cm², 60°C, 75% RH, stoichio. 1.5/5 H 2 /O 2 with minimum flow rates of 100 and 500 mL/min.

Figure 3 .

 3 Figure 3. SEM images of the MEA contaminated with NaCl during the exploratory study, from (a) a cross-sectional view and (b) a view of the cathode GDL surface.

Figure 4 .

 4 Figure 4. (a) Cell voltage time dependences for a 5 mg/m 3 NaCl contamination and (b) for a 0.5 mg/m 3 NaCl contamination (0.5 A/cm², 60°C, 50% RH, stoichio. 1.5/2 H 2 /O 2 ).

Figure 5 .

 5 Figure 5. (a) Polarization curves of the stack contaminated by 0.5 mg/m 3 NaCl. (b) EIS spectra recorded at 0.4 A/cm², at the beginning of recovery 1 (40h) and (c) at the end of recovery 1 (51h) (60°C, 50% RH, stoichio. 1.5/2 H 2 /O 2 ).

Figure 6 .

 6 Figure 6. Charge obtained with cathode CV measurements during NaCl contamination (third scan, 50 mV/s, 0.5 g/m 3 NaCl, 60°C, 50% RH, stoich. 1.5/2 H 2 /N 2 ).

Figure 7 .

 7 Figure 7. (a) Picture of cathode MEA and BP contaminated with 5 g/m 3 NaCl. (b) SEM observations of a cross-section and (c) cathode micro-porous layer close to the air inlet.

Figure 8 .

 8 Figure 8. Picture of a cathode MEA contaminated with 5 g/m 3 NaCl (on the left) and another one from a stack operated 50h in a marine environment (on the right), both showing corrosion marks close to the air inlet.

Figure 9 .

 9 Figure 9. (a) Contamination time dependence on the voltage at 0.5 A/cm² and (b) polarization curves associated for a single cell assembled with a graphite current collector at the cathode (10 mg/m 3 NaCl, 60°C, 75% RH, stoichio. 1.5/5 H 2 /O 2 ).

Acknowledgments

The present work was financially supported by the Pays de la Loire Region. The authors would like to acknowledge the ERIMAT-Capacités team for post-mortem analyses.