Götz Eichhorn
email: g.eichhorn@nioo.knaw.nl

Manfred R Enstipp

Jean-Yves Georges

Dennis Hasselquist

Bart A Nolet

Resting metabolic rate in migratory and non-migratory geese following range expansion; go south, go low

Keywords: arctic, body mass, growth, latitude, life-history, metabolic rate, migration, moult, plasticity

While many species suffer from human activities, some like geese benefit and may show range expansions. In some cases geese (partially) gave up migration and started breeding at wintering and stopover grounds. Range expansion may be facilitated and accompanied by physiological changes, especially when associated with changes in migratory behaviour. Interspecific comparisons found that migratory tendency is associated with a higher basal or resting metabolic rate (RMR). We compared RMR of individuals belonging to a migratory and a sedentary colony of barnacle geese Branta leucopsis.

The migratory colony is situated in the traditional arctic breeding grounds (Russia), whereas the sedentary colony has recently been established in the now shared wintering area (the Netherlands). We measured RMR by oxygen consumption ( O2 ) during two ontogenetic phases (juvenile growth and adult wing moult). We also investigated juvenile growth rates and adult body mass dynamics.

Mass-independent O2 was 13.6% lower in goslings from the sedentary colony than in goslings from the migratory colony. Similarly, in adult geese, mass-independent O2 was 15.5% lower in sedentary than in migratory conspecifics. Goslings in the Netherlands grew 36.2% slower than goslings in Russia, while we found no differences in body dimensions in adults. Adult geese from both colonies commenced wing moult with similar body stores, but whereas Russian barnacle geese maintained this level throughout moult, body stores in geese from the Netherlands fell, being 8.5% lower half-way through the moult.

We propose that the colony differences in resting metabolic rate, growth rate and body mass dynamics during moult can be explained by environmental and behavioural differences. The less stringent time constraints combined with poorer foraging opportunities allow for a smaller 'metabolic machinery' in non-migratory geese. Our analysis suggests that range expansion may be associated with changes in physiology, especially when paired with changes in migratory tendency.

Introduction

Humans are impacting the globe to such an extent that, according to some, a new geological era has started [START_REF] Crutzen | The "Anthropocene[END_REF]. Many species with sensitive requirements suffer and show range contractions, whereas some broadly tolerant species benefit and show range expansions [START_REF] Mckinney | Biotic homogenization: a few winners replacing many losers in the next mass extinction[END_REF]. For instance, while many bird species have declined with the intensification of agriculture [START_REF] Donald | The Common Agricultural Policy, EU enlargement and the conservation of Europe's farmland birds[END_REF], some herbivores like geese profit from the abundance of highly nutritious agricultural crops [START_REF] Fox | Why geese benefit from the transition from natural vegetation to agriculture[END_REF]. Range expansions cause species to experience new abiotic and biotic environmental conditions, inducing phenotypic changes in behaviour and physiology that may arise through acclimatization, adaptation, or a combination of both [START_REF] Burton | Trade-offs and the evolution of life-histories during range expansion[END_REF][START_REF] Colautti | Evolutionary constraints on adaptive evolution during range expansion in an invasive plant[END_REF][START_REF] Hendry | Human influences on rates of phenotypic change in wild animal populations[END_REF][START_REF] Weiss-Lehman | Rapid trait evolution drives increased speed and variance in experimental range expansions[END_REF].

Large-scale latitudinal range expansions will bring about differences in seasonality such as changes in propensity to be migratory or sedentary. Animals can exploit seasonally changing environments by migrating between reproductive and survival habitats. Under these circumstances, migration enhances reproduction by exploitation of food peaks and reduces mortality through avoidance of food scarcity [START_REF] Holt | Theoretical reflections on the evolution of migration[END_REF]Fryxell 2011, Lack 1968a) and pathogens [START_REF] O'connor | The evolution of immunity in relation to colonization and migration[END_REF][START_REF] Westerdahl | Pathogens and moving hosts[END_REF]. Although beneficial, such migratory behaviour requires certain abilities (e.g., physiological, navigational) and also bears substantial costs, such as the energetic and survival costs of the migratory journey as well as its preparatory phase [START_REF] Alerstam | Long-distance migration: evolution and determinants[END_REF][START_REF] Hein | Energetic and biomechanical constraints on animal migration distance[END_REF][START_REF] Hulbert | Life and death: metabolic rate, membrane composition, and life span of animals[END_REF][START_REF] Klaassen | When and where does mortality occur in migratory birds? Direct evidence from long-term satellite tracking of raptors[END_REF]. Arctic-breeding migratory birds, for example, can benefit from the combination of long daylight hours and superior food conditions. But the short-lasting nature of the seasonal resource pulse may put them under time pressure in order to accomplish activities like reproduction, growth, moult and deposition of body stores needed for migration. This may result in increased food intake to meet the demands for faster growth and (re)fueling. To process more food animals primarily enlarge their gastrointestinal tract and accessory organs like the liver, and studies on birds including waterfowl have shown that individuals adjust the size of such organs in relation to actual needs and feeding conditions [START_REF] Battley | Adaptive interplay between feeding ecology and features of the digestive tract in birds[END_REF][START_REF] Karasov | Digestive plasticity in avian energetics and feeding ecology[END_REF][START_REF] Van Gils | Longer guts and higher food quality increase energy intake in migratory swans[END_REF]. Moreover, hypertrophy of exerciserelated organs like skeletal muscles, kidneys and heart has been observed in birds preparing for longdistance migration [START_REF] Piersma | Rapid changes in the size of different functional organ and muscle groups during refueling in a long-distance migrating shorebird[END_REF]). These food-processing and exercise-related organs have disproportionately high metabolic rates, so that maintaining them at larger size results in higher massspecific maintenance metabolism at whole-body level (Daan et al. 1990, Konarzewski and[START_REF] Konarzewski | Evolution of basal metabolic rate and organ masses in laboratory mice[END_REF]. Comparative studies of species or subspecies indeed find that migratory tendency is associated

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'This article is protected by copyright. All rights reserved.' with higher basal or resting metabolic rate as compared with a sedentary strategy [START_REF] Jetz | Environment, migratory tendency, phylogeny and basal metabolic rate in birds[END_REF][START_REF] Swanson | Seasonal metabolic variation in birds: functional and mechanistic correlates[END_REF], Wikelski et al. 2003). Because species and subspecies that occupy different ranges with no or limited gene flow are bound to differ in other respects besides migratory tendency, intra-specific comparisons could provide a more rigorous test [START_REF] Garland | Why not to do two-species comparative studies: limitations on inferring adaptation[END_REF] of whether migratory individuals have higher resting metabolic rates, but such comparisons are largely lacking.

Goose populations in Europe and North-America have increased in response to decreased hunting pressure in combination with food subsidies provided by changes in agriculture in their wintering and stopover areas [START_REF] Abraham | The dynamics of landscape change and snow geese in mid-continent North America[END_REF][START_REF] Ebbinge | Factors determining the population size of arctic-breeding geese wintering in western Europe[END_REF][START_REF] Fox | Effects of agricultural change on abundance, fitness components and distribution of two arctic-nesting goose populations[END_REF]. In many cases, these increases in population numbers have been accompanied by range expansions. Probably as a response to changing costs and benefits of migration, some populations of species like barnacle geese Branta leucopsis even partially or fully stopped migrating, and started breeding at stopover sites or even at wintering sites [START_REF] Van Der Jeugd | Keeping up with early springs: rapid range expansion in an avian herbivore incurs a mismatch between reproductive timing and food supply[END_REF]. Aside from distance to wintering grounds, the original and new breeding environments of barnacle geese are expected to differ in other biotic and abiotic characteristics, notably temperature, photoperiod and foraging conditions. Together they yield different pressures that are expected to result in seasonal or permanent adjustments of the physiological phenotype [START_REF] Wikelski | The physiology of life histories[END_REF].

We set out to compare individuals belonging to either migratory or sedentary colonies of barnacle geese. The migratory individuals spend the summer at the traditional breeding grounds in the Russian Arctic, whereas the sedentary individuals summer at the newly established breeding grounds within the shared wintering area in Western Europe. We measured resting metabolic rate during two ontogenetic phases, namely during the growth of juvenile birds and during the wing moult of adult birds.

Because of the direct relationship between metabolism and biosynthesis, we also investigated growth rates of juveniles and body mass dynamics of adults, both life-history traits with clear fitness consequences. We hypothesized that the non-migratory birds that stay for breeding in the south have a lower resting metabolic rate and slower growth rate than their Arctic migratory conspecifics.

Materials and methods

Study populations and handling of geese

Barnacle geese from the ancestral migratory population were studied in a colony at Kolokolkova Bay near the abandoned settlement of Tobseda (6835'N, 5220'E), situated along the Barents Sea in arctic Russia.

Barnacle geese from the newly established, sedentary population were studied in a colony in the Delta

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'This article is protected by copyright. All rights reserved.' area in the southwest of the Netherlands (5140'N, 414'E), within the temperate wintering range shared by the two populations. Both populations have been subject to long-term studies on various life-history aspects, and more details on study sites and populations can be found in [START_REF] Eichhorn | Skipping the Baltic: the emergence of a dichotomy of alternative spring migration strategies in Russian barnacle geese[END_REF] and [START_REF] Van Der Jeugd | Keeping up with early springs: rapid range expansion in an avian herbivore incurs a mismatch between reproductive timing and food supply[END_REF] and references therein. Juvenile (i.e., goslings) and wing-moulting adult geese were captured during their flightless phase, both in the Netherlands (between 31 May and 16 July 2012) and in Russia (between 27 July and 17 August 2014). Wing moult in geese comprises the simultaneous replacement of all primary and secondary flight feathers. Geese were driven into a catching pen. Immediately after capture, geese were transferred to small compartmented tents; goslings were housed separately from adults to avoid trampling. Ringing (with uniquely marked leg rings) and collection of morphometric measurements started approximately 30 min after capture. All geese except five goslings were sexed by cloacal examination. Adult geese selected for respirometry did not have unmoulted juvenile feathers, and hence most likely comprise mature birds in their third calendar year or older (Cramp and Simmons 1977). Birds were weighed with precision spring-scales (PESOLA Präzisionswaagen AG, Switzerland) at ±5 g and ±10 g accuracy for weights up to and above 600 g, respectively, and measured with callipers (±0.1 mm) for tarsus length, and with a ruler (±1 mm) for head length, wing length (maximum chord) and the length of the 9th primary feather (P9), counted inward out. Geese and other waterfowl shed and re-grow their flight feathers simultaneously; thus, measuring a single primary allows monitoring the progress of moult. Among the (re-)captured geese were also goslings that had been marked at hatch in that season with uniquely coded web-tags.

Morphological measurements on goslings of known age were used to compare juvenile growth rate between colonies. Size of head and tarsus as measured in moulting adult geese during 2012-2015 seasons were used as indicators of final body size targeted by growing goslings.

Upon ringing, a subsample of geese (one to nine individuals per capture event/day) was transferred to an enclosure placed on grassland suitable for foraging, and provided with water for drinking. Geese were removed from the enclosure and kept in ventilated boxes without food and water for 2 h (juveniles) to 3 h (adults) before start of respirometry. This time was sufficient for allowing them to empty their guts, which were anyway not filled much due to restricted food intake while in captivity. All birds were released after measurements.

Respirometry

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'This article is protected by copyright. All rights reserved.'

We used an open-circuit respirometry system ('Turbofox'; Sable Systems International, Las Vegas, NV, USA) installed within walking distance from the breeding colonies to measure the oxygen consumption rates ( O2 ) and carbon dioxide production rate ( CO2 ) of geese resting in a chamber (an opaque plastic box). Depending on goose size, chambers with a volume of 11 L (29 x 22 x 18 cm, Length x Width x Height), 18 L (31 x 24 x 24 cm) or 52 L (43 x 33 x 37 cm) were used during a trial, ensuring that geese could sit comfortably. Gas-impermeable tubing was used to make connections between the various components of the respirometry system. The O 2 -analyser was calibrated before each trial using ambient air scrubbed of water vapour and CO 2 (set to 20.95% O 2 ; the zero point was fixed and not subjected to drift). The CO 2 -analyser was calibrated daily using calibration gas (0.49% CO 2 ). The humidity meter was calibrated at least weekly, but usually every other day, according to manufacturer recommendations. We used wet and dried (using magnesium perchlorate) ambient air to set the span and zero water vapour pressure reading (kPa), respectively. During trials with larger geese (>0.6 kg) the primary flow control unit of the Turbofox pulled air through the chamber at a rate of 5-6 L min -1 (automatically corrected to STP: 273 K and 101.3 kPa). A subsample (200 ml min -1 ) was passed through a humidity meter (RH-300), a CO 2 -analyser, and a fuel-cell oxygen-analyser (i.e., the analyser unit of the Turbofox). For trials with smaller geese, the subsample pump of the system was used as the main flow generator, pulling air at a rate of up to 1.5 L min -1 through the chamber, which was then passed through the analyser unit. Flowrates were chosen based on preliminary trials to ensure that O 2 concentration within the chamber remained at ~20.5% and CO 2 concentration did not exceed 0.5%. The O 2 and CO 2 concentrations within the chamber, main flow rate through the chamber, humidity of the gas sample, and barometric pressure were recorded every second onto a laptop computer using ExpeData software (1.4.15,Sable Systems). Lag time between air leaving the respiration chamber and detection by the gas analysers was found to be 25 s for the oxygen analyser and 20 s for the CO 2 analyser. These lag times were taken into account when calculating oxygen consumption rates and CO 2 production rates. The time constants of the respiration chambers were calculated to be 7.3 min for trials with small goslings (11 l chamber volume, 1.5 l min -1 flowrate) and 9.5 min for trials with large goslings and adult geese (52 l chamber volume, 5.5 l min -1 flowrate). The respirometry system was tested frequently using the N 2 dilution technique [START_REF] Fedak | One-step N 2 -dilution techni ue for calibrating open-circuit O 2 measuring systems[END_REF]. The set-up was identical in both locations with the respirometry chamber kept in a well-ventilated, separate room, to reduce disturbance from the experimenter.

After initial disturbance, birds typically calmed down quickly inside the chamber, and the recorded O 2 and CO 2 traces stabilised within ~30 min. Respirometry trials lasted for two to three hours and were conducted throughout the 24-h day. A residual analysis indicated no effect of daytime on measured O2

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'This article is protected by copyright. All rights reserved.' (Supporting Information: Fig. S1). Body mass was determined by weighing the birds at the end of each respirometry trial. Adult geese were generally in a post-absorptive state, whereas some of the goslings may have been still in an absorptive state, as gauged from the respiratory exchange ratio (RER= CO2 / O2 ; mean ± sd of 0.75±0.04 in adults and 0.81±0.06 in juveniles). Due to their carbohydrate-rich diet a relatively high RER might be expected in geese during foraging, which may take longer to decrease and stabilize after digestion. Moreover, due to gut fermentation, a truly post-absorptive (but not yet fasting) state might be generally difficult to achieve in herbivorous geese. Chamber temperatures during respirometry ranged 10-18 °C (mean=14 °C) for adult and 12-22 °C (mean=15 °C) for juvenile geese in Russia, and 15-21 °C (mean=18 °C) for adult and 15-27 °C (mean=20 °C) for juvenile geese in the Netherlands. Although thermal relationships have not been determined specifically for the barnacle goose, we are confident that our experimental temperatures were within their thermal neutral zone.

Combining t lc measurements from 33 seabird species, [START_REF] Ellis | Energetics of free-ranging seabirds[END_REF] established the allometric equation: t lc = 43.15 -6.58 log mass -0.26 latitude, where mass is in grams and latitude in degrees. Using this formula and individually measured body mass we estimate t lc =5.4±2.2 °C (mean ± sd) in the adult barnacle geese and t lc =9.7±3.6 °C in the juvenile geese. This inferred lower critical temperature for adults fits the one measured in the closely related black brant (Branta bernicla orientalis, mass=1.130 kg, t lc =6 °C) during the Alaskan summer [START_REF] Irving | The metabolism of some Alaskan animals in winter and summer[END_REF]. Moreover, when chamber temperature was included in the analysis it did not improve the model fitted for O2 . Accordingly, a residual analysis suggests that the experienced temperatures had no effect on the measured O2 (Supporting Information: Fig. S2). The measurements in Russia took place when arctic summer temperatures peaked and in fact often overlapped with ambient temperatures in the Netherlands during juvenile growth (Supporting Information: Fig. S3).

Data analysis

Respirometry data

Respirometry data were analysed using ExpeData, correcting for gas-analyser drift and lag time of the respirometry system. Also, main flow rate was corrected to STP dry (STPD) using equation 8.6 in [START_REF] Lighton | Measuring metabolic rates: a manual for scientists[END_REF]. Similarly, we did not scrub water vapour before gas analysis but corrected for this dilution effect during data analysis using equation 15.3 in [START_REF] Lighton | Measuring metabolic rates: a manual for scientists[END_REF]. O2 and CO2 were calculated using equations 11.7 and 11.8 in [START_REF] Lighton | Measuring metabolic rates: a manual for scientists[END_REF], respectively. From each respirometry trial, a 15-min data segment encompassing the lowest stable O2 values was selected to represent the resting metabolic

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'This article is protected by copyright. All rights reserved.' rate (RMR) of the bird for that trial. Oxygen consumption rate values are presented on a whole animal basis (ml min -1 ).

Statistical analysis

Resting O2 was compared between the migratory and sedentary colony while accounting for an allometric relationship of O2 with body mass (M): O2 = a Col M b e ε , where a and b represent the allometric constant and scaling exponent, respectively, Col is a dimensionless factor representing colony, ε represents the error term, and e is the base of the natural logarithm. Log-log transformation linearizes the relationship between O2 and M, so that our basic model becomes: ln( O2 ) = ln(a) + bln(M) + ln(Col) + ε.

In contrast to goslings, which usually continued foraging while held in captivity, adult geese showed a much more wary behaviour. They likely compromised foraging activity, and some may have fasted to a variable extent during captivity. Time since capture (i.e., time elapsed between capture and O2 measurement, t catch-O2 , centred to its mean) was, therefore, included in our models as controlling covariate. Additionally, capture date was included as random effect, in a more conservative approach that accounts for potential correlation among individuals that were caught and processed together. Sex, respirometry chamber temperature during O2 measurements and, for adult geese, progress of wing moult (expressed by the length of the 9th primary, P9) were added as further explanatory variables. For further details see Supporting Information: Appendix 1.

Juvenile growth and adult body mass dynamics were analysed from cross-sectional data from the same colonies and seasons, using simple linear ANCOVA models. For juvenile geese, length of wing, head, tarsus and body mass were combined in a principal component analysis (PCA) to obtain the first principal component (PC1) that was then related to gosling age and colony. For adult geese, a single sizevariable PC1 was obtained from a PCA of tarsus and head length, which was included as covariate in the model relating body mass to moult stage (length of P9) and colony. All statistical analyses were performed in IBM SPSS Statistics 24.0 (Armonk, NY: IBM Corp.).

Results

Metabolic rates

Metabolic rate was studied in a total of 62 juvenile and 39 adult barnacle geese ranging in mass 0.122-1.555 kg and 1.270-2.250 kg, respectively (Fig. 1), and was modelled separately for juvenile and adult

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'This article is protected by copyright. All rights reserved.' geese. Sample sizes and summary statistics of variables used to model oxygen consumption rates are given in Supporting Information: Tables S1-S2.

In juvenile geese, the final model of resting metabolic rate ( O2 ) retained the significant fixed effects of body mass and colony (model I in Table 1, Fig. 2). O2 scaled with body mass at an exponent of 0.68 and was 13.6% lower in goslings in the sedentary colony in the Netherlands than in goslings in the migratory colony in arctic Russia (or, vice versa, O2 in Russian goslings was 15.5% higher than in Netherlands goslings). O2 mean estimates for goslings at 1 kg body mass were 18.9 ml min -1 in Russia (RU) and 16.3 ml min -1 in the Netherlands (NL).

In adult geese, the final resting metabolic rate ( O2 ) model retained the significant fixed effects of body mass, colony, a curvilinear effect of moult stage, and time since capture (model II in Table 1, Fig. 3). O2 scaled with body mass at an exponent of 0.71 and was 15.5% lower in sedentary geese in the Netherlands than in migratory conspecifics in Russia (or, vice versa, O2 in Russian geese was 18.2% higher than in Netherlands geese). O2 estimates at mean adult body mass were 24.7 ml min -1 (RU) and 20.9 ml min -1 (NL) at the start of wing moult (i.e., P9=0). It increased by 10.0% to peak with 27.2 ml min - 1 (RU) and 23.0 ml min -1 (NL) at P9=95 mm, 13 days into moult (Fig. 3). Extrapolated to 1 kg body mass, adult mean O2 is estimated at 16.6 ml min -1 (RU) and 14.0 ml min -1 (NL) at the start of moult, and expected to peak with 18.2 ml min -1 (RU) and 15.4 ml min -1 (NL) during moult.

Juvenile growth and adult body mass dynamics

There was a clear difference in growth trajectories of goslings raised in the sedentary and migratory colony (interaction colony × age, P < 0.001; Fig. 4, Supporting Information: Table S3). Over the studied body size range, which was comparable across colonies, goslings in the Netherlands grew 36.2% slower than goslings in arctic Russia. It took Netherlands' goslings 63 days to attain a body size that their Russian conspecifics attained in only 40 days. Goslings from both colonies, however, apparently reach a similar final body size at adult age, as is indicated by head and tarsus length of adult barnacle geese (n=315 in RU, n=301 in NL): estimated marginal means (±SE) accounting for sex-differences were 83.7±0.2 mm (RU) and 84.1±0.2 mm (NL) for head (mean difference n.s., P= 0.10), and 82.9±0.2 mm (RU) and 82.6±0.2 mm (NL) for tarsus length (mean difference n.s., P= 0.18).

Adult geese from both colonies commenced wing moult with similar body stores (i.e., body mass corrected for size) which were maintained by migratory geese (RU) but decreased in sedentary geese (NL) during the moult (interaction P9 × colony, P = 0.005; Fig. 5, Supporting Information: Table S4).

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'This article is protected by copyright. All rights reserved.' Estimated marginal mean body mass half-way through the moult (at P9=132 mm) amounts to 1.801±0.016 (±SE) kg in Russian barnacle geese and to 1.648±0.023 kg in their conspecifics from the Netherlands (i.e., a mass difference of 8.5%).

Discussion

Our analysis of resting metabolic rates revealed that, when corrected for body mass, barnacle geese from the newly established temperate-breeding, sedentary colony in the Netherlands exhibit lower metabolic rates than their migratory conspecifics from an arctic-breeding (Russian) colony. This was the case during post-natal growth of juveniles as well as during post-breeding wing moult of adults. The magnitude of the differences between the sedentary and migratory individuals were similar for both age groups. Moreover, juvenile growth was slower, and adult body mass loss during moult was larger in individuals from the sedentary colony in the Netherlands.

Two main, non-mutually exclusive factors may explain these patterns, namely differences in environment (often characterized by latitude) and in life-style (migratory or not). Comparing bird species from different latitudes, metabolic rates of adults were found to decrease towards lower latitudes, both in terms of maintenance metabolism (i.e., basal metabolic rate, BMR) [START_REF] Jetz | Environment, migratory tendency, phylogeny and basal metabolic rate in birds[END_REF][START_REF] Londoño | Basal metabolism in tropical birds: latitude, altitude, and the 'pace of life[END_REF][START_REF] Mcnab | Ecological factors affect the level and scaling of avian BMR[END_REF][START_REF] Wiersma | Tropical birds have a slow pace of life[END_REF]) and in total daily energy expenditure (i.e., field metabolic rate, FMR) [START_REF] Anderson | The broad-scale ecology of energy expenditure of endotherms[END_REF]Jetz 2005, Piersma et al. 2003). Similar latitudinal metabolic differences across species have been reported for hatchlings [START_REF] Klaassen | An analysis of hatchling resting metabolism: in search of ecological correlates that explain deviations from allometric relations[END_REF] and during post-natal growth [START_REF] Ton | Metabolism correlates with variation in post-natal growth rate among songbirds at three latitudes[END_REF]. Comparing subspecies from different latitudes, a lower BMR (Wikelski et al. 2003) or FMR [START_REF] Tieleman | Physiological and behavioural correlates of life-history variation: a comparison between tropical and temperate zone House Wrens[END_REF]) has been reported in tropical-sedentary versus temperate-migratory songbirds. Comparing migratory life-styles, [START_REF] Jetz | Environment, migratory tendency, phylogeny and basal metabolic rate in birds[END_REF] found a lower BMR in non-migrant than in migrant bird species, but they found an even stronger correlation of BMR with annual temperature. They suggested that the higher BMR of migratory species is, at least in part, the result of acclimatization to colder breeding areas at higher latitudes. This hypothesis of an overriding temperature effect was put to the test by comparing tropical bird species living at different altitudes (as opposed to latitudes), but no association between BMR and altitude was found [START_REF] Londoño | Basal metabolism in tropical birds: latitude, altitude, and the 'pace of life[END_REF]. Moreover, in the tropics migratory bird species have a higher BMR during overwintering than tropical sedentary bird species that share the same habitat [START_REF] Bushuev | Basal metabolic rate in free-living tropical birds: the influence of phylogenetic, behavioral, and ecological factors[END_REF]). These findings indicate that a low BMR in tropical sedentary birds is not merely the result of acclimatization to warm stable temperatures, but rather associated with the sedentary lifestyle.

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'This article is protected by copyright. All rights reserved.' Correspondingly, we believe that the differences in RMR between our study colonies were not or only to a minor extent due to acclimatization to different ambient temperatures. We measured RMR during adult wing moult and growth of young, which both take place during the warmest period of the year when ambient temperature differences between the arctic and temperate colonies are minor (Supporting Information: Fig. S3). At other times of the breeding season thermoregulatory costs may be higher in the arctic site than in the temperate site. For instance, adult geese may pay such costs temporarily early in the breeding season, and goslings during the early post-natal phase shortly after hatching. For this reason, we selected only larger/older goslings (> 2 weeks) well after they had reached thermal independence from (brooding) parents (own obs.). Outside breeding, geese from both colonies share common wintering grounds for half of the year (November to May), where they experience similar ambient temperatures including the lowest in the annual cycle. Thus, overall, rather than climate-driven thermoregulatory effects, a combination of other environmental and behavioural differences seem more plausible explanations for the lower RMR and growth rates in sedentary versus migratory geese. The less stringent time constraints for growth and lack of migratory disposition combined with poorer foraging opportunities allow for a smaller 'metabolic machinery' and hence reduction in maintenance costs in nonmigratory geese (Swanson et al. 2017, Williams andTieleman 2000).

Juvenile growth and energetics

Post-natal growth rate can influence offspring survival during pre-and post-fledging periods and, moreover, if growth rate affects final body size, it can also affect fecundity later in life [START_REF] Gebhardt-Henrich | Causes of growth variation and its consequences for fitness[END_REF]Richner 1998, Sedinger et al. 1995). In the present study, we did not link growth rate to final body size at the individual level, nor did we account for potential cohort effects. Nevertheless, our data suggest that goslings in the sedentary colony in the Netherlands grow slower but finally reach a similar adult body size as their migratory conspecifics in the Russian Arctic colony. Hence, the growth phase appears to be shorter in the Russian Arctic than in the Netherlands. This may be the effect of differences in predation pressure, season length, day length, foraging conditions and/or parasite pressure.

A flightless state and small body size makes goslings vulnerable to predation. Thus, timedependent mortality could be an important selection pressure for fast growth towards reducing the prefledging period (Lack 1968b). We did frequently observe goslings being killed in the Arctic study site by various predators (glaucous gull Larus hyperboreus, Heuglin's gull L. heuglini, arctic skua Stercorarius parasiticus, white-tailed eagle Haliaeetus albicilla, arctic fox Vulpes lagopus). In many temperate goose colonies, including our study colony in the Netherlands, these predators or their local equivalents (e.g.,

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'This article is protected by copyright. All rights reserved.' red fox V. vulpes) are either not present or have been largely reduced in number by human intervention.

Depression of natural predators (besides reduced persecution by man) has likely supported the colonization of temperate breeding areas, and may thus also have reduced the selection pressure on fast juvenile growth.

With regard to season length, [START_REF] Van Der Jeugd | Keeping up with early springs: rapid range expansion in an avian herbivore incurs a mismatch between reproductive timing and food supply[END_REF] analysed timing of breeding and postfledging survival of barnacle geese including the same study colonies as in the present study. Not only is the time window for successful breeding much wider for geese in the temperate breeding areas, their fledglings also enjoy higher survival that, importantly, shows no decline with hatch date. In contrast, survival of fledglings from the arctic-migratory birds drops steeply with date of hatch, likely because for late-born chicks, the season becomes too short to prepare for autumn migration. This suggests that both breeding adults (who determine hatch date of their offspring) as well as their developing young are under less time pressure in the sedentary colony [START_REF] Van Der Jeugd | Keeping up with early springs: rapid range expansion in an avian herbivore incurs a mismatch between reproductive timing and food supply[END_REF].

However, lower growth rate in the sedentary birds of the Netherlands may also be constrained due to poorer foraging conditions. Small avian herbivores such as geese depend on high-quality food (graminoids with low fibre and high protein content), and food abundance and food quality are known to be important determinants of gosling growth and survival as well as final body size [START_REF] Cooch | Body size variation in lesser snow geese: environmental plasticity in gosling growth rates[END_REF][START_REF] Larsson | Body size declines despite positive directional selection on heritable size traits in a barnacle goose population[END_REF][START_REF] Richman | Ecological implications of reduced forage quality on growth and survival of sympatric geese[END_REF]. During the southwest expansion of the breeding area into temperate regions, barnacle geese preferred, and probably relied upon, managed grassland. Grazing or mowing regimes and direct or indirect input of fertilizers at these sites had created foraging and broodrearing sites of improved food quality that may rival the food quality available at the arctic foraging grounds [START_REF] Van Der Jeugd | Keeping up with early springs: rapid range expansion in an avian herbivore incurs a mismatch between reproductive timing and food supply[END_REF]). However, the arctic and temperate sites differ in potential daily foraging time. We observed that broods in the Dutch colony interrupted their daytime foraging activity for a nocturnal roost of approximately 6 hours on predator-free islands without foraging opportunities, whereas broods in the Russian colony continued foraging throughout 24 hours of daylight. Accounting for differences in daylight hours experienced during growth largely reduces colony differences in growth rate (Supporting Information: Fig. S4 and Table S5).

Lower parasite pressure in arctic than in temperate breeding grounds [START_REF] Piersma | Do global patterns of habitat use and migration strategics co-evolve with relative investments in immunocompetence due to spatial variation in parasite pressure?[END_REF]) could be another factor that may explain part of the observed variation in growth rate. Parasites divert nutrients and energy away from their hosts or may incur costs through immune system activation [START_REF] Hanssen | Costs of immunity: immune responsiveness reduces survival in a vertebrate[END_REF][START_REF] Hasselquist | Physiological mechanisms mediating costs of immune responses: what can we learn from studies of birds?[END_REF][START_REF] Schmid-Hempel | Evolutionary parasitology: the integrated study of infections, immunology, ecology, and genetics[END_REF].

The lower growth rate of the goslings from the sedentary (NL) colony was associated with a lower RMR. The RMR of growing organisms reflects the metabolic turnover for biosynthesis and for

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'This article is protected by copyright. All rights reserved.' maintenance. Part of the lower RMR in the goslings from the Netherlands is likely a direct consequence of slower biosynthesis (Fig. 4). But we also suspect that they have a slower maintenance metabolism, because food-processing organs can be maintained at smaller size or used less intensively [START_REF] Mckechnie | Phenotypic flexibility in basal metabolic rate and the changing view of avian physiological diversity: a review[END_REF][START_REF] Swanson | How low can you go? An adaptive energetic framework for interpreting basal metabolic rate variation in endotherms[END_REF][START_REF] Van Leeuwen | Effects of food ration on SMR: influence of food consumption on individual variation in metabolic rate in juvenile coho salmon (Onchorhynchus kisutch)[END_REF], Williams and Tieleman 2000). The direction of control, i.e., whether metabolic rate drives growth or vice versa, is not easy or perhaps not even meaningful to tease apart as these processes are clearly intertwined [START_REF] Burton | What causes intraspecific variation in resting metabolic rate and what are its ecological consequences?[END_REF][START_REF] Glazier | Is metabolic rate a universal 'pacemaker' for biological processes?[END_REF].

Adult wing moult

We found a curvilinear pattern of RMR over the course of wing moult, with a peak at approximately 2 weeks after old flight feathers were shed, which may reflect variation in instantaneous overall moult intensity caused by partially overlapping growth periods of different feather tracts (e.g. flight feathers and wing coverts; [START_REF] Hohman | Ecology and management of postbreeding waterfowl[END_REF]. Our findings are consistent with previous findings in captive-bred barnacle geese as well as with studies of other species showing RMR to reflect moult intensity [START_REF] Lindström | The energetic cost of feather synthesis is proportional to basal metabolic rate[END_REF][START_REF] Portugal | Annual changes in body mass and resting metabolism in captive barnacle geese (Branta leucopsis): the importance of wing moult[END_REF]. Aside from varying moult intensity, a changing body composition due to the dynamic breakdown and rebuilding of breast and leg muscles may also contribute to varying RMR during wing moult in geese [START_REF] Portugal | Testing the use/disuse hypothesis: pectoral and leg muscle changes in captive barnacle geese Branta leucopsis during wing moult[END_REF].

In contrast to colony differences of juvenile growth rates observed by us, van der Jeugd et al.

(2009) reported similar growth rates of the 9 th primary suggesting similar rates of feather synthesis in moulting adult barnacle geese from arctic and temperate populations. However, the different body mass trends observed throughout moult in the present study (Fig. 5) indicate different turnover rates of other tissues. Although the relatively low sample sizes underlying the estimates of adult primary feather growth rates in van der Jeugd et al. ( 2009) may call for a re-examination of potential population differences, their conclusions are not necessarily in conflict with present results, because body mass changes and the progress of moult are regulated by different hormones [START_REF] Jenni-Eiermann | Temporal uncoupling of thyroid hormones in Red Knots: T3 peaks in cold weather, T4 during moult[END_REF]. Our present findings are in line with and extend to previous findings about the regulation of body stores during incubation (preceding wing moult): sedentary, temperate (NL) barnacle geese spent more time on the nest thereby accepting a higher body mass loss, whereas migratory, arctic (RU) breeders took more foraging recesses and conserved their lean (protein) mass [START_REF] Eichhorn | Fuelling incubation: differential use of body stores in arctic-and temperatebreeding barnacle geese (Branta leucopsis)[END_REF]. Migratory barnacle geese increase their lean mass during the pre-migratory period [START_REF] Eichhorn | Does agricultural food provide a good alternative to a natural diet for body store deposition in geese?[END_REF] presumably to strengthen their flight capabilities. Overall, we suspect that differences in body composition and size of organs involved in exercise and digestion may have contributed to the observed colony differences in whole organism metabolic rate.

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'This article is protected by copyright. All rights reserved.' Due to inferior foraging conditions (especially less foraging time, see above) temperate geese are more limited in the potential rate of tissue accretion during moult. However, due to a sedentary lifestyle that removes the pressure of depositing body stores for autumn migration, temperate geese may also simply not aim to maximize food intake. This additional explanation is supported by the finding that captive barnacle geese with ad libitum access to food showed even greater body mass loss during wing moult than the wild geese studied here [START_REF] Portugal | Annual changes in body mass and resting metabolism in captive barnacle geese (Branta leucopsis): the importance of wing moult[END_REF]. In captivity, the rate of mass loss was larger in individuals that initiated moult with larger body stores and spent more time resting and less time foraging [START_REF] Portugal | Greater energy stores enable flightless moulting geese to increase resting behaviour[END_REF]. [START_REF] Portugal | Annual changes in body mass and resting metabolism in captive barnacle geese (Branta leucopsis): the importance of wing moult[END_REF] concluded that the reduction of food intake and concomitant mass loss during the flightless period is a behavioural response of 'innate nature', with the adaptive value of reducing predation risk in the wild. Our additional findings on geese in the wild, however, suggest that foraging behaviour and mass loss during moult are plastic and depend on the environmental context such as future needs.

Finally, as evoked for growing juveniles, differences in (human-induced) predation risk and parasitemia in the temperate and arctic colony are further factors worth exploring to explain the observed differences in body mass dynamics.

Perspectives

Within thirty years after the establishment of a sedentary population of barnacle geese in the Netherlands, we found individuals from a temperate and arctic colony to differ in metabolic rate, growth rate, and adult body mass dynamics. The considerable logistic challenges of conducting respirometry trials in the (remote) field have limited our analyses to two study colonies we deem representative for the two populations. Clearly, for a solid and more general conclusion about differences between temperate and arctic breeding geese, we would need to include individuals from more than one colony at both breeding areas, or include another breeding population. We were able to investigate this with respect to gosling growth rates by combining measurements taken in the Russian and Dutch study colony over several years with measurements taken in the short-distance migratory Baltic breeding population (i.e., in the island of Gotland) which is located within the flyway at intermediate latitude (unpublished data). As would be expected from our analysis presented here, the growth rate of Baltic goslings was intermediate between that of goslings from arctic Russia and the Netherlands.

Studies of heritability in RMR and growth performance (body mass and mass gain) suggest that there is generally potential for a response to selection on these traits in birds [START_REF] Nilsson | Heritability of resting metabolic rate in a wild population of blue tits[END_REF][START_REF] Sadowska | Genetic correlations in a wild rodent: grass-eaters and fast-growers evolve high basal metabolic rates[END_REF][START_REF] Van Noordwijk | Genetic aspects of growth[END_REF]. In our study system, however, there might be Accepted Ar ticle 'This article is protected by copyright. All rights reserved.' little scope for a genetic response, apart from a founder effect perhaps, given the relatively recent split (ca. 30 years). There is also continuing exchange of individuals and genetic material between the migratory and non-migratory populations. These populations still share their wintering range, and sometimes pairs are formed between birds from either population (Jonker et al. 2013, Van der Jeugd and[START_REF] Van Der Jeugd | Travels and traditions: long-distance dispersal in the Barnacle Goose Branta leucopsis based on individual case histories[END_REF]. Thus, phenotypic plasticity may be a more likely explanation for our results, because RMR as well as growth rate are highly flexible traits, and metabolic adjustments related to migration can be short-lasting and reversible [START_REF] Mckechnie | Phenotypic flexibility in basal metabolic rate and the changing view of avian physiological diversity: a review[END_REF][START_REF] Swanson | Seasonal metabolic variation in birds: functional and mechanistic correlates[END_REF]. Also, the aforementioned intermediate growth rate of goslings from the Baltic population is more in line with predictions from a migratory or latitude driven explanation than with an explanation favouring genetic (founder) effects. As a next step, we suggest that it would also be interesting to investigate if our colony differences in RMR also persist during other life-cycle stages, in particular when individuals from both populations experience the same environment, either in a common-garden experiment or in the field on their shared wintering grounds.

Similarly, in order to investigate whether genetic differences play a role, one should try and rear goslings from eggs of both populations under the same conditions. and cross symbols mark subjects that were also measured for O2 in this study. Growth rate was 36.2% slower in the sedentary (NL) colony. See Supporting Information Table S3 for details of the statistical model. line and open symbols), respectively. Data were collected during same seasons when O2 was measured, and cross symbols mark subjects that were also measured for O2 in this study. The slope is statistically significant different from zero for NL but not for RU, and slopes differ significantly from each other (Supporting Information: Table S4).

Fig. 1 .

 1 Fig. 1. Resting rate of oxygen consumption ( O2 ) versus body mass as measured in growing juvenile (circles) and wing-moulting adult (triangles) barnacle geese from a migratory and a sedentary colony in arctic Russia (RU) and the Netherlands (NL), respectively.Accepted Ar ticle

Fig. 2 .

 2 Fig.2. Resting rate of oxygen consumption ( O2 ) versus body mass as estimated from model I (Table1) for growing juvenile barnacle geese from a migratory and a sedentary colony in arctic

'

  This article is protected by copyright. All rights reserved.'

Fig. 4 .

 4 Fig. 4. Growth of body size (PC1 of wing, head and tarsus length, and body mass) in juvenile barnacle geese from a migratory and a sedentary colony in arctic Russia (RU, grey line and filled symbols, slope=0.086) and the Netherlands (NL, black line and open symbols, slope=0.055), respectively. Data were collected during same seasons when O2 was measured,

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  This article is protected by copyright. All rights reserved.'

Fig. 5 .

 5 Fig. 5. Body mass (corrected for size, PC1 of head and tarsus length) versus length of the 9th primary feather in wing-moulting adult barnacle geese from a migratory and a sedentary colony in arctic Russia (RU, grey dashed line and filled symbols) and the Netherlands (NL, black solid

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  This article is protected by copyright. All rights reserved.'

Table 1 )

 1 for growing juvenile barnacle geese from a migratory and a sedentary colony in arctic Russia (RU, grey lines) and the Netherlands (NL, black lines), respectively; dashed lines are 95% CIs about mean estimates shown as solid lines. The original data values are plotted as filled and open circles.Fig. 3. Resting rate of oxygen consumption ( O2 ) versus body mass (a), and versus the length of the 9th primary feather (b), as estimated from model II (Table1) for wing-moulting adult barnacle geese from arctic Russia (grey lines) and the Netherlands (black lines), respectively; dashed lines are 95% CIs about mean estimates shown as solid lines. For the illustration of the instantaneous effect of body mass on O2 in panel (a) P9 was held at zero, and time since capture was held at its mean. In panel (b) both time since capture and body mass were held at their mean values for modelling the instantaneous effect of P9 on O2 . The original O2 data were corrected accordingly, and are plotted as filled (Russia) and open triangles (Netherlands).Flight feathers are moulted simultaneously, and it takes approximately 5 weeks to grow P9 to a full length of 264 mm (average of females and males; own obs.)[START_REF] Van Der Jeugd | Keeping up with early springs: rapid range expansion in an avian herbivore incurs a mismatch between reproductive timing and food supply[END_REF]).

	Accepted Ar ticle	V̇O 2 (ml min -1 )	0 5 10 15 20 25 30	0.0	0.2	0.4	0.6 Body mass (kg) 0.8 1.0	1.2	1.4 juvenile RU juvenile NL	1.6

'This article is protected by copyright. All rights reserved.'

Table Legends Table 1 .

 Legends1 Parameter estimates of fixed effects in final models of resting rate of oxygen consumption ln( O2 ) in juvenile (model I) and adult (model II) barnacle geese from a migratory and a sedentary colony (Col) in arctic Russia (reference level, included in estimate for the intercept) and the Netherlands, respectively. The variance estimate for the random effect of capture date was 0.007±0.003 (Wald Z= 2.363, P= 0.018) in juveniles, and 0.001±0.001 (Wald Z= 1.217, P= 0.224) in adults. Mbody mass [kg]; P9 (moult stage) -9th primary feather length [mm] indicating the progress of moult; t catch-O2time since capture [hours]; SE -standard error; ± 95% CIlower and upper bound of 95% confidence intervals.

	Parameter I. Juvenile geese Intercept ln(Col) ln(M) II. Adult geese Intercept ln(Col) ln(M) P9 (moult stage) P9 2 t catch-O2 Accepted Ar ticle	Estimate 2.935 -0.145 0.680 2.804 -0.168 0.705 0.0020 -0.000010 -0.0050	SE 0.039 0.046 0.024 0.071 0.035 0.096 0.0008 0.000004 0.0022	P 0.005 <0.001 <0.001 <0.001 0.013 0.023 0.028	-95% CI 2.853 -0.241 0.632 2.660 -0.243 0.510 0.0004 -0.000019 -0.0094	+95% CI 3.017 -0.048 0.729 2.948 -0.093 0.899 0.0035 -0.000002 -0.0006

Acknowledgements -We are grateful to a large flock of fieldworkers that helped with the catching of geese. Family de Leeuw and 'Staatsbosbeheer' are thanked for permission to access their land. We thank J.S. Shonkwiler for discussions on problems of allometric analysis, and two anonymous reviewers for their valuable suggestions on an earlier version of this manuscript.

Funding -xxxx.

Authors' contributions -xxxx.

Funding

-GE was funded by a Marie Curie Intra-European Fellowship (Project No: 301108), and DH was supported by the Swedish Research Council (grant 621-2013-4357 and 2016-04391), the Linnaeus excellent research environment CAnMove funded by the Swedish Research Council (349-2007-8690) and Lund University.

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'This article is protected by copyright. All rights reserved.'

Conflicts of interest

The following Supporting Information is available for this article online: Appendix 1: Statistical method details; Supporting Tables: Table S1 -Table S5; Supporting Figures: Fig. S1 -Fig. S4.

Accepted Ar ticle


Yasmina Bencheikh

Maxime Harnois

Roxana Jijie

Ahmed Addad

Pascal Roussel

Sabine Szunerits

Toufik Hadjersi

Sidik El Hak Abaidia

Rabah Boukherroub
email: rabah.boukherroub@iemn.univ-lille1.fr

High Performance Silicon Nanowires/Ruthenium Nanoparticles Micro-Supercapacitors

Keywords: Micro-supercapacitors, Silicon nanowires, Ruthenium nanoparticles, Solid-state devices

The continuous increase of small electronic devices calls for small energy storage components, commonly known as micro-supercapacitors, that can ensure autonomous operation of these devices. In this work, we propose a simple and straightforward method to achieve high energy and power densities of a silicon-based micro-supercapacitor, consisting of silicon nanowires decorated with ruthenium nanoparticles (Ru/Si NWs). The Si NWs are obtained through the common vaporliquid-solid (VLS) growth mechanism, while a simple electroless process is used to deposit Ru nanoparticles. While silicon nanostructuration allows to increase the surface area, coating with Ru NPs introduces a pseudocapacitance necessary to attain high energy and power densities. The Ru/Si NWs micro-supercapacitor exhibits a specific capacitance of 36.25 mF cm -2 at a current density of 1 mA cm -2 in a neutral Na 2 SO 4 electrolyte and a high stability over 25000 cycles under galvanostatic charge-discharge at 1 mA cm -2 . A solid state supercapacitor is then fabricated with symmetric electrodes separated by a polyvinyl alcohol/sulfuric acid electrolyte. The device displays a specific capacitance of ~18 mF cm -2 at a current density of 1 mA cm -2 and a specific power density 0.5 mW cm -2 . This solid-state nanowire device also exhibits a good stability over 10000 cycles of charge-discharge cycles.

Introduction

Supercapacitors, also known as ultracapacitors or electrochemical capacitors, have attracted great attention for energy storage owing to their high power density, superior rate capability, rapid charging/discharging rate, long cycle life, and safe operation [START_REF] Halper | Supercapacitors: A brief overview[END_REF]. Supercapacitors, depending on the charge storage mechanism, can be classified in two types: Electric double layer capacitors (EDLCs) and pseudocapacitors (or redox supercapacitors). While EDLCs store electric charges at the electrode surface-electrolyte interface, in pseudocapacitors, the capacitance arises from rapid redox reaction at the electrode surface [START_REF] Halper | Supercapacitors: A brief overview[END_REF].

Recent technological progress towards small scale and portable electronic devices calls for the development of miniaturized energy storage sources able to provide these devices with necessary energy to operate autonomously [START_REF] Beidaghi | Capacitive energy storage in micro-scale devices: recent advances in design and fabrication of microsupercapacitors[END_REF][START_REF] Kyeremateng | Microsupercapacitors as miniaturized energy storage components for on-chip electronics[END_REF]. It is thus desirable to design micro-supercapacitors that can be easily integrated with electronics on the same chip.

Silicon is the basic semiconductor in microelectronics and thus represents an essential electrode material for the fabrication of micro-supercapacitors. The last decades have witnessed a huge body of work devoted to improve the performance of EDLCs using silicon based materials.

One important aspect of this work was through the formation of silicon nanostructures to increase the surface area. Initial studies in this area focused on the preparation of silicon nanowires (Si NWs) using the vapor-liquid-solid (VLS) growth mechanism of different morphologies and doping type and level [START_REF] Thissandier | Highly doped silicon nanowires based electrodes for micro-electrochemical capacitor applications[END_REF][START_REF] Thissandier | Ultra-dense and highly doped SiNWs for micro-supercapacitors electrodes[END_REF][START_REF] Thissandier | Are tomorrow's microsupercapacitors hidden in a forest of silicon nanotrees?[END_REF][START_REF] Thissandier | Tuning siliconnanowires doping level and morphology for highly efficient micro-supercapacitors[END_REF][START_REF] Aradilla | High performance of symmetric micro-supercapacitors based on silicon nanowires using N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide as electrolyte[END_REF][START_REF] Thissandier | Micro-ultracapacitors with highly doped silicon nanowires electrodes[END_REF][START_REF] Gaboriau | Silicon nanowires and nanotrees: elaboration and optimization of new 3D architectures for high performance on-chip supercapacitors[END_REF]. To minimize surface oxidation and achieve stable performance, organic solvents or ionic liquids were used to operate the developed Si NWs-based microsupercapacitors.

Following this work, another strategy was proposed to ensure stable performance of Si NWs micro-supercapacitors, prepared using VLS or metal-assisted chemical etching (MACE), in aqueous media through coating with silicon carbide [START_REF] Alper | Silicon carbide coated silicon nanowires as robust electrode material for aqueous micro-supercapacitor[END_REF], diamond [START_REF] Aradilla | A step forward into hierarchically nanostructured materials for high performance micro-supercapacitors: Diamond-coated SiNW electrodes in protic ionic liquid electrolyte[END_REF][START_REF] Aradilla | Powering electrodes for high performance aqueous micro-supercapacitors: Diamond-coated silicon nanowires operating at a wide cell voltage of 3 V[END_REF], organic thin films [START_REF] Devarapalli | Glucose-Derived Porous Carbon-Coated Silicon Nanowires as Efficient Electrodes for Aqueous Micro-Supercapacitors[END_REF][START_REF] Kumar | Sucrose derived carbon coated silicon nanowires for supercapacitor application[END_REF][START_REF] Sun | Bi-functional electrode for UV detector and supercapacitor[END_REF][START_REF] Alper | Selective Ultrathin Carbon Sheath on Porous Silicon Nanowires: Materials for Extremely High Energy Density Planar Micro-Supercapacitors[END_REF], alumina layer [START_REF] Gaboriau | Atomic Layer Deposition Alumina-Passivated Silicon Nanowires: Probing the Transition from Electrochemical Double-Layer Capacitor to Electrolytic Capacitor[END_REF], TiO 2 [START_REF] Konstantinou | TiO 2 coated Si nanowire electrodes for electrochemical double layer capacitors in room temperature ionic liquid[END_REF], and graphene [START_REF] Soam | Performance enhancement of microsupercapacitor by coating of graphene on silicon nanowires at room temperature[END_REF]. Even though Si NWs coating enhanced

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significantly the stability of the electrode material, it was not possible to enhance the intrinsic capacitance of the electrode using this approach.

With the aim to improve the energy and power densities, recent studies focused on the use of hybrid materials through the introduction of NiO [START_REF] Lu | Electrochemical properties of high-power supercapacitors using ordered NiO coated Si nanowire array electrodes[END_REF], MnO x [START_REF] Ortaboy | MnOxdecorated carbonized porous silicon nanowire electrodes for high performance supercapacitors[END_REF] MnO 2 [START_REF] Dubal | 3D hierarchical assembly of ultrathin MnO 2 nanoflakes on silicon nanowires for high performance micro-supercapacitors in Li-doped ionic liquid[END_REF], VO x [START_REF] Li | Hierarchical Branched Vanadium Oxide Nanorod@Si Nanowire Architecture for High Performance Supercapacitors[END_REF], and PEDOT [START_REF] Aradilla | Designing 3D Multihierarchical Heteronanostructures for High-Performance On-Chip Hybrid Supercapacitors: Poly(3,4-(ethylenedioxy)thiophene)-Coated Diamond/Silicon Nanowire Electrodes in an Aprotic Ionic Liquid[END_REF]. Particularly, transition metal oxides, known as pseudocapacitive materials, are very promising for capacitance increase. The charge storage in these pseudocapacitive materials stems from their redox reactions, allowing rapid and reversible Faradaic surface reactions along with high charge-discharge rate.

Ru-based electrode materials represent an interesting category for designing supercapacitive electrodes owing to their excellent electrochemical properties, including high surface capacitance, rapid reversible redox process, and good cycling stability [START_REF] Li | Ruthenium based materials as electrode materials for supercapacitors[END_REF]. Silicon nanowires decorated with ruthenium nanoparticles (Ru/Si NWs) substrates have been reported in a few reports. For example, O'Brien et al. [START_REF] O'brien | Photomethanation of Gaseous CO 2 over Ru/Silicon Nanowire Catalysts with Visible and Near-Infrared Photons[END_REF] prepared Ru/Si NWs by sputtering Ru NPs onto black silicon, obtained using the metal-assisted chemical etching (MaCE) technique, and successfully applied it for gas-phase CO 2 photomethanation. Ru/Si NWs, obtained through electroless deposition of Ru NPs from RuCl 3 precursor on hydrogenated Si NWs, were recently used for electrochemical hydrogen evolution reaction (HER) [START_REF] Zhu | Ru-modified silicon nanowires as electrocatalysts for hydrogen evolution reaction[END_REF].

To the best of our knowledge, there is only one report on the preparation of ruthenium oxide/Si NWs electrode material for solid-state on chip supercapacitors [START_REF] Zheng | High-performance solid-state on-chip supercapacitors based on Si nanowires coated with ruthenium oxide via atomic layer deposition[END_REF]. In this work, the Si NWs were obtained through metal-assisted anodic etching, while a thin layer of ruthenium oxide was deposited using atomic layer deposition (ALD) at 290°C. A specific capacitance of 19 mFcm -

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In this study, we adopted the electroless deposition technique to decorate Si NWs, produced through the VLS growth mechanism, with Ru nanoparticles at room temperature. This approach is suitable to enhance energy storage capability by taking benefit of the high surface area of Si NWs and the pseudocapacitive properties of Ru nanoparticles. The resulting electrode exhibited a specific capacitance of 36.25 mF cm -2 and a retention higher than 87% upon 25000 galvanostatic charge-discharge cycles at 1 mA/cm 2 in aqueous Na 2 SO 4 electrolyte. The advantage of the fabricated electrode is its high performance and stability. Additionally, the room temperature process used in this work is compatible with the microelectronic fabrication processes (CMOS technology).

Experimental Part

Materials and Methods

All chemicals and reagents were of analytical grade and used without any purification. Ruthenium chloride hydrate (RuCl 3 ×H 2 O), polyvinyl alcohol (PVA) powder, sodium sulfate (Na 2 SO 4 ), hydrogen peroxide (H 2 O 2 , 30%), hydrofluoric acid (HF, 48%), nitric acid (HNO 3 ), and sulfuric acid (H 2 SO 4 ) were obtained from Sigma-Aldrich (France).

The water used throughout the experiments was purified with a Milli-Q system from Millipore Co. (resistivity = 18 MΩ.cm).

Preparation of Silicon Nanowires (Si NWs)

Single-side polished silicon (100) oriented n-type wafers (Siltronix, France) (arsenic-doped, 0.001-0.005 Ohm-cm -1 resistivity) were used as substrate. The substrate was degreased in acetone and isopropyl alcohol, rinsed with Milli-Q water, and cleaned in a piranha solution (3:1 concentrated H 2 SO 4 /30% H 2 O 2 ) for 15 min at 80 °C followed by copious rinsing with Milli-Q water. The surface was further dried under a stream of nitrogen. Silicon nanowires (Si NWs) used in this work were prepared using the vapor-liquid-solid (VLS) mechanism [START_REF] Verplanck | Reversible Electrowetting on Superhydrophobic Silicon Nanowires[END_REF]. The fundamental process is based on metal-catalyst-directed chemical vapor deposition of silicon. First, a thin film of gold (4 nm thick) was evaporated on the clean Si substrate. Gold nanoparticles with a wide size distribution were obtained as a result of metal dewetting on the surface. Exposure of the gold-coated surface to silane gas at a pressure of 0.4 T (Q=40 sccm) at 500 °C for 60 min led to Si NWs growth.

Safety Considerations:

The mixture H 2 SO 4 /H 2 O 2 (piranha) solution is a strong oxidant. It reacts violently with organic materials. It can cause severe skin burns. It must be handled with extreme care in a well-ventilated fume hood while wearing appropriate chemical safety protection. HF is a hazardous acid which can result in serious tissue damage if burns were not appropriately treated.

Preparation of Silicon Nanowires-Ruthenium Nanoparticles (Ru/Si NWs)

The ruthenium-coated Si NWs were obtained by a simple electroless method [START_REF] Megouda | Photocatalytic activity of silicon nanowires under UV and visible light irradiation[END_REF][START_REF] Fellahi | Silicon nanowire arrays-induced graphene oxide reduction under UV irradiation[END_REF] using ruthenium salt. Briefly, the Si NWs substrate was immersed into an aqueous solution of RuCl 3 ×H 2 O (1 mM)/HF (0.15 M) for different times [START_REF] Verplanck | Reversible Electrowetting on Superhydrophobic Silicon Nanowires[END_REF]45, 60 and 75 min) at room temperature.

The resulting interface was rinsed with water and dried under a gentle stream of nitrogen.

Solid State Supercapacitor Fabrication

To prepare the solid PVA/H 2 SO 4 electrolyte, 0.5 g of PVA powder was slowly added into 5 mL of deionized water under magnetic stirring to completely dissolve the PVA. 281 µL of H 2 SO 4 were added into the solution and heated in an oil bath at 85°C until a homogeneous solution was obtained and then cooled down to room temperature.

The polymer-based electrolyte was drop casted on the nanowire electrodes (1 cm × 0.5 cm) and baked 10 min at 45 °C and kept in air to solidify the electrolyte.

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Characterization

Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) images were obtained using an electron microscope ULTRA 55 (Zeiss) equipped with a thermal field emission emitter and three different detectors (EsB detector with filter grid, high-efficiency In-lens SE detector, Everhart-Thornley secondary electron detector).

X-ray Diffraction (XRD)

X-ray diffraction (XRD) patterns were collected on a high flux Rigaku Smartlab rotated anode, working with a copper Kα radiation (1.5418 ang) with an applied voltage of 45 kV and an anode current of 200 mA in the 2θ range of 25-90°. The beam was made parallel through a multilayer mirror and a reflexion geometry was used. To avoid any saturation of the detector when it passes through the (004) peak of the substrate, the sample was 2° misoriented.

Transmission Electron Microscopy (TEM)

TEM analysis was performed on a TECNAT-G2 working at 200 kV, equipped with an EDS spectrometer. The Ru/SiNW composites was dispersed on a thin carbon film of a 3 mm diameter copper grid (See Fig. S1).

X-ray Photoelectron Spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) measurements were performed with an ESCALAB 220 XL spectrometer from Vacuum Generators featuring a monochromatic Al Kα X-ray source (1486.6 eV) and a spherical energy analyzer operated in the CAE (constant analyzer energy) mode (CAE=100 eV for survey spectra and CAE=40 eV for high-resolution spectra), using the electromagnetic lens mode. No flood gun source was needed due to the conducting character of the substrates. The angle between the incident X-rays and the analyzer is 58°. The detection angle of the photoelectrons is 30°.

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Electrochemical studies

Electrochemical studies were carried out using a potentiostat (Autolab, model 2.01, Methrom, France). Cyclic Voltammetry (CV), and galvanostatic charge-discharge measurements were conducted from -0.6V to +0.2V. Electrochemical Impedance Spectroscopy (EIS) was performed across the frequency range 1 MHz and 0.03 Hz with AC amplitude of 10 mV. All electrochemical tests were performed in 1 M Na 2 SO 4 electrolyte aqueous solution using a typical three-electrode electrochemical cell configuration. Ruthenium-coated SiNWs substrate was used as the working electrode, Pt foil served as the counter electrode, and Ag/AgCl as the reference electrode. All the calculated values were normalized to the surface of the active electrode material (0.29 cm 2 ).

Results and discussion

The silicon nanowires (Si NWs) investigated in this study were produced by the well-known vapor-liquid-solid (VLS) growth mechanism [START_REF] Verplanck | Reversible Electrowetting on Superhydrophobic Silicon Nanowires[END_REF]. In this growth process, Au nanoparticles, obtained through thin Au film (4 nm) dewetting on SiO 2 /Si surface, act as catalytic sites for Si NWs growth. Under controlled conditions, the size of the Si NWs is determined by the size of the Au NPs and the length is defined by the growth time. silicon substrates has been studied in many reports in the literature [START_REF] Zhu | Ru-modified silicon nanowires as electrocatalysts for hydrogen evolution reaction[END_REF][START_REF] Tsuboi | Metal deposition into a porous silicon layer by immersion plating: Influence of halogen ions[END_REF][START_REF] Fellahi | Reduction of Cr(VI) to Cr(III) using silicon nanowire arrays under visible light irradiation[END_REF][START_REF] Megouda | Photocatalytic activity of silicon nanowires under UV and visible light irradiation[END_REF]. In the electroless mechanism, metal ions with redox potential greater than hydrogen are reduced by electrons from the Si atoms to metal atoms, Eq. 1.

Scanning electron microscopy (SEM) images, depicted in

Ru 3+ + 3 e - → Ru(s) (1)
The deposited metal atoms first form nuclei, then nanoclusters and finally nanoparticles. During Ru deposition, the silicon surface is oxidized according to Eq. 2:

Si + 2H 2 O → SiO 2 + 4H + + 4e - ) 2 (
The role of HF is to dissolve the oxide layer and generate a fresh hydrogenated Si surface. Indeed, when the electroless process was performed in absence of HF, no particle deposition occurred after 30 min immersion, as evidenced from SEM imaging (data not shown). This is mainly due to electron transfer hindrance by the silicon dioxide barrier on the Si NWs surface.

To assess the influence of deposition time on the specific capacitance, the electroless deposition was performed in RuCl 3 (1 mM)/HF (0.15 M) aqueous solution at room temperature for various times [START_REF] Verplanck | Reversible Electrowetting on Superhydrophobic Silicon Nanowires[END_REF]45, 60 and 75 min). SEM imaging showed a continuous increase of the nanoparticles density on the Si NWs substrate upon increasing deposition time (Fig. 2). Up to 60 min, it is obvious that some of the nanowires are not coated with Ru NPs. A full coverage of the nanowires was attained after 75 min. This sample gave also the best specific capacitance (as discussed later), so additional characterizations were carried out on this sample. Also, the color of the samples turned from light gold (30 min) to dark (75 min), indicating a change of the optical properties of the samples (Insets in Fig. 2).

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The XRD pattern of the initial Si NWs substrate comprises diffraction peaks at 2θ of 28.44°, 47.22° and 56.11° due to (111), ( 220) and (311) silicon planes, respectively (Fig. 3). The peaks at 38.22° and 44.44° are ascribed to Au terminating the nanowires, according to the VLS growth mechanism. The results are in accordance with data reported in the literature [START_REF] Lu | Silicon quantum-wires arrays synthesized by chemical vapor deposition and its micro-structural properties[END_REF][START_REF] Jian | Growth and Morphology Modulation of Needle-like Silicon Nanowires for SERS Application[END_REF]. After, loading Ru NPs on the Si NWs via electroless deposition, the XRD pattern exhibited additional broad XRD diffraction peaks at 2θ of 38.4° and 43.88° and other minor peaks, characteristic of Ru, along with the diffraction peaks due to Si and Au. All reflection peaks of the Ru/Si NWs can be easily indexed as pure structure of Ru 0 , which is in good agreement with the literature data (JCPDS card 70-0274). Though the diffraction peaks of (100) and ( 101) are overlapped with (002) diffraction due to widening, the product should be the hexagonal phase [START_REF] Garcia-Pena | Solventless synthesis of ruthenium nanoparticles[END_REF]. According to Scherrer's equation (d = kλ/β cos θ), the mean size of particles is about 3-5 nm.

The Ru/Si NWs substrate was further characterized by transmission electron microscopy (TEM). Figure 4a Where j is the current density (A cm -2 ), V is the potential window, r is the scan rate, and C is the specific capacitance; scan rates used for the measurements are 10, 20, 50 and 100 mV s -1 .

In addition, specific capacitance of the samples was calculated by using the following formula (4):

The energy density (E) and power density (P) were determined by using equations ( 5) and ( 6), respectively:

(5) and [START_REF] Thissandier | Are tomorrow's microsupercapacitors hidden in a forest of silicon nanotrees?[END_REF] where dV is the potential range, j is the current density and dt is the total time of discharge.

Figure 5a depicts the CV of Si NWs substrate recorded before and after decoration with Ru

NPs for various deposition times (30, 45, 60 and 75 min) in 1 M Na 2 SO 4 at a scan rate of 1 mA cm -2 in the potential window of -0.6 to + 0.2 V. The capacitance behavior of the Ru/Si NW electrodes is obvious from the rectangular shape of the CV curves with no apparent contribution from redox processes (Faradaic pseudocapacitance), suggesting that the specific capacitance is dominated by the electrochemical double layer capacitance. This result is in accordance with that recorded for Ru nanoparticles-reduced graphene oxide [START_REF] Hassan | Ruthenium nanoparticles-modified reduced graphene prepared by a green method for highperformance supercapacitor application in neutral electrolyte[END_REF]. The specific capacitance values, deduced from the CV curves using Eq. 1, increased from 12.5 to 36.25 mF cm -2 upon increasing the deposition time from 30 to 75 min, respectively (Table 1). This can be explained by the increase of the Ru NPs density, as observed in SEM analysis. shape, associated with adsorption-desorption of ions on the electrode surface, even at higher scan rates, indicating a good stability of the electrode material.

The specific capacitance values of Ru/Si NWs (75 min) at different scan rates, calculated from the CV plots, are exhibited in Fig. 5c and Table 2. It is evident that the specific capacitance decreases upon increasing the scan rate, because of insufficient ion transport and adsorption.

Additionally, the specific capacitance of Ru/Si NWs (75 min) electrode was determined using the galvanostatic charge-discharge curves at different current densities in 1 M Na 2 SO 4 (Fig. 6).

The specific capacitance values are summarized in Table 3. Upon increasing the current density, the specific capacitance value decreases as follows: 112.62, 81.5, 49.37, 36.25, 31.87, 27.5 and 26.25 mF cm -2 at a current density of 0.1, 0.2, 0.5, 1, 1.5, 2 and 3 mA cm -2 , respectively.

The stability of an electrode material for supercapacitors is of prime importance to ensure its performance and long-term utilization. Figure7 depicts the stability test of the Ru/Si NWs electrode after 25000 cycles. The electrode retains about 80% of its initial specific capacitance after 25000 cycles at 1 mA cm -2 in the potential window between -0.6 and +0.2 V in 1 M Na 2 SO 4 , indicating a good stability of the Ru/Si NWs electrode. The stability of Ru/Si NWs electrode after 25000 cycles was additionally confirmed using CV (Inset Fig. 7) and SEM imaging (Fig. S5).

Indeed, both the shape of the CV and the surface morphology were not altered significantly after the cycling experiment.

Electrochemical Impedance Spectroscopy (EIS) is a valuable tool to evaluate the electrodeelectrolyte interface and understand the electronic transport of the Ru/Si NWs electrode. Figure 8 exhibits the Nyquist plot within a frequency range from 1 MHz to 0.1 Hz at open circuit potential.

The Nyquist plot was fitted using the Randles equivalent circuit (inset Fig. 8). In the high frequency region, a semi-circle was observed due to the charge transfer resistance (R CT ). In the low frequency region, a vertical plot was seen, suggesting a capacitive behavior of the electrode.

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The Bod plot in the inset, generated at lower frequency of 100 mHz, allows to determine a phase angle of 68°, which is lower than the value expected for an ideal capacitor (90°).

A symmetric solid state supercapacitor was fabricated from the Ru/Si NWs electrode according to the procedure described in the experimental section. Figure 9a depicts the CV measurements carried out between -0.4 V and +0.4 V, a potential window where the device displayed rectangular CV curves, at various scan rates (5 mV s -1 to 50 mV s -1 ). The results indicate good electrochemical performance and fast redox reaction of Ru/Si NWs electrode with PVA/H 2 SO 4 gel electrolyte. The specific capacitance values extracted from the CV curves for each scan rate are plotted in Figure 9b. A small decrease of the specific capacitance was observed upon increasing the scan rate up to 50 mV s -1 with a significant decrease at the higher scan rate tested (Table 4).

The solid-state symmetric supercapacitor was also tested in galvanostatic charge-discharge experiments at various current densities between 0 V and +0.8 V, as shown in Figure 9c. The curves display a triangular shape with linear charge and discharge half cycles, typical of supercapacitors. The energy efficiency can be easily estimated from the capacity ratio between discharge and charge processes and is over 90.45% at 0.5 mA cm -2 current density. Similarly, the specific capacitance was calculated the galvanostatic curves for different current densities (Figure 9d). A similar trend was observed as previously with a continuous decrease of the specific capacitance with increasing the current density (Table 5).

The cycle stability of the device fabricated using Ru/Si NWs, obtained after 75 min of electroless deposition, was assessed by continuous charge-discharge measurements for a large number of cycles at a discharge current of 1 mA cm -2 . According to Figure 10, the initial maximum specific capacity value of 17.5 mF cm -2 only decreased slightly to 16.88 mF cm -2 after

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of our symmetric solid state capacitor; the repetitive charge-discharge do not induce noticeable degradation of the hybrid Ruthenium-nanowire silicon array structure.

The high areal capacitances determined for the Ru/Si NW symmetric supercapacitor are also superior to those found for most of other recently reported silicon nanowires-based symmetric micro-supercapacitors [START_REF] Gao | Diamond-coated silicon wires for supercapacitor applications in ionic liquids[END_REF][START_REF] Aradilla | Novel hybrid micro-supercapacitor based on conducting polymer coated silicon nanowires for electrochemical energy storage[END_REF]: (1.9 mF cm -2 at 2 mA cm -2 ) for MSC based on diamond-coated Si NW electrodes in ionic liquid Et 3 NH-TFSI electrolyte [START_REF] Aradilla | Designing 3D Multihierarchical Heteronanostructures for High-Performance On-Chip Hybrid Supercapacitors: Poly(3,4-(ethylenedioxy)thiophene)-Coated Diamond/Silicon Nanowire Electrodes in an Aprotic Ionic Liquid[END_REF], (14 mF cm -2 at 0.4 mA cm -2 ) for MnO 2 @SiNWs hybrid symmetric micro-supercapacitors [START_REF] Dubal | 3D hierarchical assembly of ultrathin MnO 2 nanoflakes on silicon nanowires for high performance micro-supercapacitors in Li-doped ionic liquid[END_REF] and 2.3 mF cm -2 at 0.4 mA cm -2 for Si NW@RuO x electrode based solid-state device with ALD deposition [START_REF] Zheng | High-performance solid-state on-chip supercapacitors based on Si nanowires coated with ruthenium oxide via atomic layer deposition[END_REF].

Figure 11 depicts the Ragone plot i.e. power density (P/mW cm -2 ) vs. energy density (E/mJ cm -2 ) with comparison to other reported values. The electrode material exhibits power and energy densities ranging from 0.5 to 8.5 mWh cm -2 and from 1.5×10 -2 to 3.5×10 -2 mW cm -2 , respectively.

Conclusion

This paper describes a simple method for the fabrication of a symmetric solid-state microsupercapacitor using silicon nanowire substrates decorated with ruthenium nanoparticles (Ru/Si NWs). The Ru nanoparticles are easily decorated on the Si NWs using electroless deposition technique in a hydrofluoric acid aqueous solution containing Ru ions at room temperature. We have investigated the effect of deposition time on the specific capacitance of the Ru/Si NWs electrode. Under optimized conditions, the Ru/Si NWs displayed a specific capacity as high as 112 mF cm -2 at an applied current density of 0.1 mA cm -2 in 1 M Na 2 SO 4 . The performance of a micro-supercapacitor, consisting of two identical Ru/Si NWs electrodes, was investigated electrochemically in 0.5 M H 2 SO 4 /PVA solid electrolyte. A specific capacitance of ~28 mF cm -2 and a specific power density 0.5 mW cm -2 were obtained. This solid-state nanowire device also exhibited a good stability over 10000 cycles of charge-discharge cycles. The technique reported in this work is simple and can be implemented for other metal ions to develop various micro-supercapacitors. Additionally, the process takes place at room temperature which is compatible with microelectronic processing technology. • The device exhibits a good stability over 10000 cycles of charge-discharge cycles

Fig. 1 ,

 1 revealed a high density of nanowires with an average diameter in the range of 30-80 nm. The large distribution of the nanowires diameter is due to the size variation of the gold nanoparticles serving as catalysts for Si NWs growth. Ruthenium nanoparticles (Ru NPs) were deposited on Si NWs through a simple electroless process at room temperature. The Si NWs substrates were first treated with 10% HF aqueous solution to remove the native silicon oxide layer and generate a hydrogen termination. Subsequent immersion in RuCl 3 aqueous solution for various times (30, 45, 60, 75 min) allowed Ru NPs loading on the Si NWs. Electroless deposition of metal nanoparticles on Si NWs and porous M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

  depicts a scanning transmission electron microscopy (STEM), in bright field, of a single Si NW decorated with Ru NPs. The diameter of a single Si NW is about 50 nm and the Ru NPs exhibit a small diameter between 3 and 6 nm, in accordance with XRD analysis. STEM characterization allows chemical contrast investigation. Elemental mapping of the Ru/Si NW clearly shows the presence of Si, Ru and O, in accordance with the chemical composition of the material. The presence of oxygen is most likely due to partial oxidation of silicon during exposure to ambient. Indeed, elemental mapping of a single Ru nanoparticle reveals a very small amount of oxygen, ruling out surface oxidation of the Ru NPs (Fig.S2).

Figure 4b displays

  Figure 4b displays the TEM image of the Ru NPs on the silicon surface consisting of small

  photoelectron spectroscopy (XPS) was carried out to examine the chemical composition of the Si NWs substrate before and after loading Ru NPs. FigureS3depicts the XPS survey spectrum of Ru/Si NWs. It comprises peaks due to Ru, Si, C and O elements. While Ru and Si are expected to originate from Ru/Si NWs, oxygen may be due to partial oxidation of the silicon core during sample handling and exposure to ambient. Part of the oxygen signal may also arise from surface contamination, as commonly encountered in XPS analysis. The presence of carbon is most likely due to surface contamination during sample handling and storage.The high resolution XPS spectrum of the Ru 3p exhibits two symmetric doublets at 461.2 and 483.5 eV assigned to metallic Ru 3p3/2 and Ru 3p1/2 , respectively with a spin orbit splitting of 22.3 eV characteristic of Ru in its metallic form[START_REF] Zheng | High-performance solid-state on-chip supercapacitors based on Si nanowires coated with ruthenium oxide via atomic layer deposition[END_REF][START_REF] Morgan | Resolving ruthenium: XPS studies of common ruthenium materials[END_REF], Fig.S4a. The absence of components at higher binding energies suggests that Ru is mainly in its metallic form, as expected for electroless deposition in RuCl 3 /HF solution. The results are well in accordance with XRD and TEM measurements. The high resolution XPS spectrum of the O 1s can be deconvoluted into three peaks at 530.7, 531.7 and 533.5 eV ascribed to SiO 2 , O 2 - /Si-O and OH/adsorbed water, respectively (Fig.S4b).The electrical double layer capacitance of the resulting electrodes was analyzed utilizing cyclic voltammetry measurements in a 3 electrodes configuration consisting of a platinum counter electrode, an Ag/AgCl wire reference electrode and Ru/Si NW working electrode. Electrical connection was made directly to the back side of the sample. Scans were recorded in the potential window of -0.6 to +0.2 V vs. Ag/AgCl electrode using 1 M Na 2 SO 4 aqueous electrolyte. The specific capacitance was calculated from Equation (3):

Figure 5b

 5b Figure 5b displays the CV curves of Ru/Si NWs (75 min) at different scan rates (5, 10, 30, 50, 100 mV s -1 ) recorded in 1 M Na 2 SO 4 aqueous solution at room temperature in the potential

Figure 1 :

 1 Figure captions

Figure 2 .

 2 Figure 2. SEM images of Ru/Si NWs for different deposition times in RuCl 3 (1 mM)/HF (0.15

Figure 3 .

 3 Figure 3. X-ray diffraction (XRD) patterns of Si NWs (a) and Ru/Si NWs (b).

Figure 4 .

 4 Figure 4. (a) Scanning transmission electron microscopy (STEM) and elemental mapping, (b, c)

Figure 5 .

 5 Figure 5. (a) Cyclic voltammetry curves of Ru@SiNW electrode (75 min) recorded at a scan rate of 100 mA cm -2 in 1 M Na 2 SO 4 aqueous solution for different deposition times, (b) C(V)responses at different scan rates (5 to 100 mV s -1 ), (c) Effect of scan rate on specific capacitance.

Figure 6 .

 6 Figure 6. Specific capacitance of Ru@SiNW electrode (75 min) obtained from galvanostatic

Figure 7 .

 7 Figure 7. Lifetime testing of the device performed using 25 000 complete charge-discharge cycles at a current density of 1 mA cm -2 between -0.6 and +0.2 V. Inset shows the CV curves of the device before and after the cycling test at a scan rate of 100 V s -1 .

Figure 8 .

 8 Figure 8. Nyquist plot in the frequency range from 1 MHz to 0.1 Hz. Inset shows the Bode angle

Figure 9 .

 9 Figure 9. (a) Cyclic voltammetry curves of symmetric solid state supercapacitor, consisting of

Figure 10 .

 10 Figure10. Lifetime testing of the symmetric solid state supercapacitor device performed using 10 000 complete charge-discharge cycles at a current density of 1 mA cm -2 between 0 and +0.8

Figure 11 :

 11 Figure 11: Ragone plot of the symmetric solid state supercapacitor at different current densities and a comparison to other reported Si NWs-based supercapacitors.

•

  Figure 1

Table 1 .

 1 Specific capacitance of Ru/Si NW electrode for different deposition times obtained from CV curves at 1 mA cm -2 in 1 M Na 2 SO 4 .

	Deposition time	30	45	60	75
	(t/min)
	IR (V)	0.1	0.05	0.03	0.03
	Specific	12.5	22.66	23.37	36.25
	capacitance
	(C/mF cm -2 )

Table 2 .

 2 Specific capacitance values of Ru/Si NW (75 min) electrode obtained from CV curves at different scan rates 1 M Na 2 SO 4 .

	Scan rate	5	10	30	50	100
	(Sr/mV s -1 )
	Specific	44.75	37.81	30.50	27.65	23.82
	capacitance
	(C/mF cm -2 )

Table 3 .

 3 Galvanostatic charge-discharge plots at different applied current densities for Ru/Si NW in 1 M Na 2 SO 4 .

Table 4 .

 4 Specific capacitance values of the symmetric solid-state supercapacitor at different scan rates.

	Scan rate	5	10	30	50	100
	(Sr/mV s -1 )
	Specific	22.94	22.41	19.34	19.06	14.84
	capacitance
	(C/mF cm -²)

Table 5 .

 5 Specific capacitance values of the symmetric solid-state supercapacitor at different

			current densities.
	Current	0.1	0.2	0.5	1	1.5
	density
	(j/mA cm -²)
	Specific	27.75	25.5	21.25	17.5	14.125
	capacitance
	(C/mF cm -²)

was recorded using cyclic voltammetry in Na 2 SO 4 electrolyte at 5 mV s -1 . A solid state symmetric supercapacitor based on the modified electrode displayed a specific capacitance of 6.5 mF cm -2 at 2 mV s -1 with a good stability over 10000 cycles[START_REF] Zheng | High-performance solid-state on-chip supercapacitors based on Si nanowires coated with ruthenium oxide via atomic layer deposition[END_REF].

Acknowledgement

Financial support from the Centre National de la Recherche Scientifique (CNRS), the University of Lille, and the Hauts-de-France region is gratefully acknowledged. The silicon nanowires have been fabricated using equipments of the NanoRennes platform.