Romane Mettauer
email: mettauer.romane@gmail.com

Victor Baron

T Turinah

Puspita Demitria

Hans Smit

Zulkifli Alamsyah

Eric Penot

Cécile Bessou

Turinah

Bénédicte Chambon

Jean Ollivier

Alexis Thoumazeau
email: alexis.thoumazeau@cirad.fr

Investigating the links between management practices and economic performances of smallholders' oil palm plots. A case study in Jambi province, Indonesia

Investigating the links between management practices and economic performances of smallholders' oil palm

plots. A case study in Jambi province, Indonesia

Introduction

Vegetable oil consumption has continuously increased worldwide over the last decades (FAO, 2020).

Because of its high oil-to-land ratio, oil palm is a promising crop to meet the fast growing demand in vegetable oil [START_REF] Barthel | Study on the environmental impact of palm oil consumption and on existing sustainability standards -For European Commission[END_REF]. Palm oil and palm kernel oil account for more than one third of the global vegetable oil production (FAO, 2020). Yet, oil palm production in Indonesia faces deep environmental and social concerns due to conversion of tropical rainforest to oil palm monoculture and conflict between local and migrant communities [START_REF] Cramb | Characterizing Oil Palm Production in Indonesia and Malaysia[END_REF]. Recently initiatives such as the RSPO (Roundtable on Sustainable Oil Palm) certification aim at promoting optimal practices for the development of oil palm cultivation limiting its negative impacts [START_REF] Lee | Does oil palm certification create trade-offs between environment and development in Indonesia?[END_REF][START_REF] Padfield | Co-producing a Research Agenda for Sustainable Palm Oil[END_REF]. The largest producing country is Indonesia with 12 Mha planted, which account for 56% of the total planted area in 2019 (Oil World, 2019). In Indonesia, oil palm is produced by a large diversity of growers, namely: privately-owned, state-owned companies and smallholders [START_REF] Cramb | Characterizing Oil Palm Production in Indonesia and Malaysia[END_REF]. Smallholders' establishment in Indonesia followed several pathways. Some smallholders settled on their own, and they are referred to as independent smallholders, while others followed governmental programs: these are scheme smallholders or plasma smallholders. Many smallholders also owned a combination of independent and plasma plots; therefore grower types are not exclusive.

Several governmental programs were put in place in order to promote an oil palm development that would include both industries and smallholders. Historically the oil palm sector in Indonesia was dominated by state owned companies after the Indonesian Independence (1960's and 1970's). In the late 1970s, the Indonesian Government notably initiated a specific type of farming contract named "Nucleus Estate and Smallholders" (NES) or "Perkebunan Inti Rakyat" (PIR) [START_REF] Zen | The Oil Palm Complex: Smallholders, Agribuisness and the State in Indonesia and Malaysia[END_REF]. NES schemes were sometimes adopted by the local population although they were initially linked to transmigration programs, which displaced people from overpopulated Java to the outer islands of Sumatra and Borneo [START_REF] Fearnside | Transmigration in Indonesia: Lessons from Its Environmental and Social Impacts[END_REF]. Plasma smallholders under NES program were organized around a nucleus estate managed by a state-owned company [START_REF] Zen | The Oil Palm Complex: Smallholders, Agribuisness and the State in Indonesia and Malaysia[END_REF]. Private companies as nucleus estates emerged quickly, and became the prominent model from the late 1980s to the 1990s. The nucleus estate supervised the land preparation and the planting of the 2 ha-plot per smallholder family, the plot management during immature stage and the supply of inputs. The estate was also in charge of coordinating harvesting rotation, road maintenance and providing technical assistance to the smallholders [START_REF] Zen | The Oil Palm Complex: Smallholders, Agribuisness and the State in Indonesia and Malaysia[END_REF]. Usually, plasma smallholders repaid the cost of plantation after 5 to 10 years, although they remained bound to the nucleus estate for plot management and for harvesting Fresh Fruit Bunches (FFBs) until the end of the oil palm cycle [START_REF] Barlow | The Indonesian Oil Palm Industry[END_REF]. In the 1990s, a new version of the NES scheme emerged, called the KKPA (Kredit (KUD -Kooperasi Unit Desa) of around 500 farmers each, and each cooperative was divided into farmers' groups (Kelompok Tani) of about 20 members. By the end of the repayment period, smallholders under KKPA system managed their plots by themselves, sometimes with some assistance from the cooperatives services. Such farmers are referred to as "ex-plasma smallholders".

Those schemes were abandoned in the early 2000's under the reformasi era but were instrumental in developing the smallholder sector with more than 900 000 ha planted under the NES scheme until 2003 [START_REF] Zen | The Oil Palm Complex: Smallholders, Agribuisness and the State in Indonesia and Malaysia[END_REF]. After this period, smallholders increasingly developed their own independent plantation outside of agroindustrial partnership, either with scheme smallholders looking to extend their area or with newcomer looking to join the growing oil palm sector [START_REF] Feintrenie | Why do Farmers Prefer Oil Palm? Lessons Learnt from Bungo District, Indonesia[END_REF].

Both plasma and ex-plasma smallholders benefited from the planting standards adopted by the nucleus estate, using 100% selected high quality Tenera seedlings, as well as optimal planting density and maintenance during the immature stage. This initial good maintenance made it possible to fill part of the yield gap observed in smallholders' plantations [START_REF] Rhebergen | Closing yield gaps in oil palm production systems in Ghana through Best Management Practices[END_REF]. Selected planting material indeed impacts both the yield and the extraction rate, leading to a potential increase of about +40% in oil production compared to non-selected material [START_REF] Cochard | Intérêts des semences commerciales améliorées de palmier à huile (Elaeis guineensis Jacq[END_REF][START_REF] Tiemann | Chapter Four -Feeding the Palm: A Review of Oil Palm Nutrition[END_REF]. Management practices are also drivers of the performance of palms, especially fertilization and weeding practices [START_REF] Darras | Reducing Fertilizer and Avoiding Herbicides in Oil Palm Plantations-Ecological and Economic Valuations[END_REF][START_REF] Rhebergen | Closing yield gaps in oil palm production systems in Ghana through Best Management Practices[END_REF]. Yields measured in plasma and ex-plasma smallholders' plots reach an average of 17-22 tFFB ha -1 yr -1 [START_REF] Euler | Exploring yield gaps in smallholder oil palm production systems in eastern Sumatra, Indonesia[END_REF][START_REF] Lifianthi | Productivity And Income Peformance Comparison of Smallholder Oil Palm Plantation at Dry Land and Wet Land of South Sumatra Indonesia[END_REF]. This is generally higher than for other independent smallholders established on their own [START_REF] Baudoin | Linking the transformation of production structures to a multidimensional sustainability assessment grid of smallholders' oil palm plantations[END_REF], who attain yields of 10-15 tFFB ha -1 yr -1 [START_REF] Jelsma | Unpacking Indonesia's independent oil palm smallholders: An actor-disaggregated approach to identifying environmental and social performance challenges[END_REF].

Various development contexts have led to a great diversity of growers and production systems.

Smallholders optimum management practices have usually been studied in broad groups that only allow for comparison between plasma and independent [START_REF] Jelsma | Do wealthy farmers implement better agricultural practices? An assessment of implementation of Good Agricultural Practices among different types of independent oil palm smallholders in Riau, Indonesia[END_REF][START_REF] Lee | Does oil palm certification create trade-offs between environment and development in Indonesia?[END_REF][START_REF] Schoneveld | Certification, good agricultural practice and smallholder heterogeneity: Differentiated pathways for resolving compliance gaps in the Indonesian oil palm sector[END_REF][START_REF] Woittiez | FERTILISER APPLICATION PRACTICES AND NUTRIENT DEFICIENCIES IN SMALLHOLDER OIL PALM PLANTATIONS IN INDONESIA[END_REF]. Thus, the linkages between the diversity of management practices in smallholders' plots and the oil palm performance at local scale remain poorly investigated, making it uneasy to propose adapted recommendations for both plantation management and replantation programs. Proposals at regional scales were recently made, exploring differences between various smallholder organization systems [START_REF] Jelsma | Do wealthy farmers implement better agricultural practices? An assessment of implementation of Good Agricultural Practices among different types of independent oil palm smallholders in Riau, Indonesia[END_REF][START_REF] Moulin | Deciphering agricultural practices and environmental impacts in palm oil plantations in Riau and Jambi provinces, Indonesia[END_REF][START_REF] Woittiez | Nutritional imbalance in smallholder oil palm plantations in Indonesia[END_REF]. Such descriptive analysis performed at a large scale may not be locally explicit enough to feed local recommendations. In addition, it was found important to involve socioeconomic aspects when analyzing the determinism of smallholders' plots performances. Economic constraints are indeed one of the main drivers of smallholders' choices in the management of systems based on perennial cash crops [START_REF] Baudoin | Linking the transformation of production structures to a multidimensional sustainability assessment grid of smallholders' oil palm plantations[END_REF][START_REF] Feintrenie | Why do Farmers Prefer Oil Palm? Lessons Learnt from Bungo District, Indonesia[END_REF].

In the present study, we explored the diversity of management practices implemented by ex-plasma oil palm smallholders in the Jambi province, Indonesia. We presented two typologies based on agricultural practices or economic variables, in order to decipher the local diversity at the plot level.

Then, we explored the relationship between the two typologies, in order to understand better the agro-economic performances across ex-plasma smallholders' plantations. Our main hypothesis is that the diversity of practices in the study area would help to adjust a local agro-economical optimum that could further be a reference for management recommendations.

Material and Methods

Study area

The study was conducted in two neighboring villages located in the Petaling area (-1.73770°N, 103.83479°E), Sungai Gelam sub-district, Muaro Jambi regency, Jambi Province, in the Sumatra Island in Indonesia. Within Petaling, two oil palm cooperatives were studied, namely KUD Marga Jaya (in Petaling Jaya village) and KUD Makarti (in Sido Mukti village). The two cooperatives gather 461 explasma smallholders owning 610 oil palm ex-plasma plots for a total planted area rounding 1,300 ha.

KUD Marga Jaya was settled in 1984 and historically took responsibility for the development of the plasma plots in the area. Due to the increasing number of oil palm smallholders and the development of independent plots, KUD Makarti was then created in 2013 following a similar organization. The two cooperatives manage the harvesting of FFB every ten days for each plot and sell the fruits to the mill of the nucleus estate that is located at around 20 km from the study area. Besides, cooperatives retail subsidized fertilizers and pesticides to their members.

All studied ex-plasma plots are homogeneous in terms of pedo-climatic conditions and historical organization. First, the area is relatively small and plots are collated (25 km²), with flat to slightly hilly topography and Acrisols soils. Second, all the studied plots were established by a nucleus estate in 1995 and 1996 using Tenera seedlings planted according to a triangular pattern (9 m x 9 m x 9 m).

The management (soil preparation, planting, management during immature phase) of plots was uniform as all plots were managed by the nucleus estate. This situation lasts until repayment of loans for plasma development. Since the beginning of the 2000s, smallholders have been managing their plots by themselves (fertilization, weeding and pruning).

Sampling method and data collection

As the historical and pedoclimatical context was rather homogeneous, we randomly selected 79 smallholders out of 461 (17%) from the cooperatives' database in September 2019. For smallholders owning several plots, only one plot was considered, in order to maximize the diversity of practices sampled. Directed interviews about management strategies and practices within each targeted plot were performed in November and December 2019 with an Indonesian expert on oil palm practices and staff members from the cooperatives. The help of local experts and some data triangulation based on cooperatives' records provided some safeguards regarding uncertainty embedded in farmers' interviews due to the lack of a systematic tracking of practices.

Smallholders' interviews and field visits

Interviews focused on agricultural practices implemented over the year preceding interviews.

Through these interviews only, we collected quantitative information at the plot scale about the dose and application frequency for each type of fertilizer and herbicide. If any changes between year 2018 and year 2019 were recorded, we collected data for both years (see Supplementary Material 1) and worked on average practices for data analysis. We also collected information about the workforce involved in each activity. Field visits mainly consisted in assessing weed control practices. We defined four distinct zones in the central area of the plot, namely: the harvesting path, the weeded circle, the windrow and the remaining zone (called "Other" zone, see Supplementary Material 2 [START_REF] Carron | Spatial heterogeneity of soil quality around mature oil palms receiving mineral fertilization[END_REF]). We could not consider plot spatial heterogeneity for weed biomass measurements in the plot. We undertook all weed biomass observations in the "Other" zone that was considered as a proxy zone to describe the diversity of weed management between the plots. First, weeds height was scored into four classes (referred as "Height" indicator). Second, the biomass of understory vegetation was scored according to an adapted Braun-Blanquet scale [START_REF] Wikum | Application of the Braun-Blanquet Cover-Abundance Scale for Vegetation Analysis in Land Development Studies[END_REF] at three locations in the plot, inside a 25 cm x 25 cm square (average score is referred as "Grass" indicator). Third, the number of woody weeds within each square was recorded (the average score is referred to as "Wood" indicator).

Complementary data

Palm tree density was checked based on satellite imagery from Bing aerial plugin on QGIS software (visited on March 10, 2020). No significant differences in tree mortality was recorded between plots.

Average plot density was 130 tree ha -1 . We collected precise data on oil palm yields from KUD Marga Jaya and KUD Makarti cooperatives for each plot (t ha -1 yr -1 ) over the January to December 2018 period.

The average selling price of FFB was estimated at 1,320 IDR kg -1 based on Governmental data communicated online (InfoSawit.com, retrieved 5 Feb. 2020) and values were crosschecked with information from experts. The cooperatives provided values for the cost of fertilizers and pesticides applied by the end of December 2019 and they provided data on irreducible costs (e.g. maintenance, loading,…). Costs and time for labor per activity were estimated with the help of the cooperatives, depending on the nature of provided services (Supplementary Material 3).

Description of input variables 2.3.1. Descriptors of agricultural practices

Descriptors of agricultural practices were divided into two categories: fertilization and weeding.

Fertilization was characterized by inputs of key elements for oil palm nutrition, namely: N, P, K, Mg and Ca in kg ha -1 yr -1 [START_REF] Dubos | HIGH SOIL CALCIUM SATURATION LIMITS USE OF LEAF POTASSIUM DIAGNOSIS WHEN KCL IS APPLIED IN OIL PALM PLANTATIONS[END_REF]Woittiez et al., 2017). The concentration in each element was recorded from fertilizer packaging. Measurements of leaf nutrient contents were performed in January and February 2021. Leaf K contents showed to be lower than results from [START_REF] Baron | Critical parameters for integrating co-composting of POME and EFB into life cycle assessment models of palm oil production[END_REF] obtained in the same pedoclimatic conditions in agroindustrial plantations. As we could not measure the K contents in EFB, we derived K EFB content for our data sample based on [START_REF] Baron | Critical parameters for integrating co-composting of POME and EFB into life cycle assessment models of palm oil production[END_REF] leaf contents results. This led to EFB content of 0.43% K in the dry matter on average.

Weeding practices were firstly characterized by the treatment frequency index (TFI). TFI estimates the number of registered doses of pesticide used per hectare and was calculated following equation 1 [START_REF] Gravesen | The Treatment Frequency Index: an indicator for pesticide use and dependency as well as overall load on the environment[END_REF][START_REF] Lechenet | Reducing pesticide use while preserving crop productivity and profitability on arable farms[END_REF].

TFI = ⅀ Wfq *Qa (equation 1)
Wfq is the frequency of chemical weeding done for a given commercial herbicide (yr -1 ), Qa is the applied quantity of active product per application for the given commercial herbicide (mg ha -1 yr -1 ), and Md stands for the minimal dose of active product ratified for the given herbicide for oil palm plantations (mg ha -1 yr -1 ).

In addition to chemical weeding, we investigated the frequency of manual and mechanical weeding (NonChWeeding; times yr -1 ).

Lastly, we generated an indicator of weed biomass observed in the plot. To do so, we performed a Principal Component Analysis (PCA) from the scorings of weed biomass observed in the field, referred as Height, Grass, Wood (see part 2.2.2; field visits), TFI and NonChWeeding (Supplementary Material 4). Height, Grass and Wood indicators were highly correlated on axis 1 and they were gathered into "Weed" indicator using individual contributions on axis 1. "Weed" score thus aggregated various components of the weed biomass observed in the field.

Indicators of economic performances

Economic performances was characterized with basic agro-economic indicators at the plot scale which included economic value of the production, gross margin and return to labor. The economic value of the production corresponds to FFB production per ha multiplied by the selling price at farm gate [START_REF] Feintrenie | Why do Farmers Prefer Oil Palm? Lessons Learnt from Bungo District, Indonesia[END_REF]. We calculated the indicator "Revenues" (IDR ha -1 yr -1 ) following equation 2:

Revenues =[Yield]*([FFB selling price] -[Compulsory cooperative's costs]) (equation 2)

Compulsory cooperatives costs were proportional to the production of FFB and corresponded to the costs for FFB harvest, weighing, loading, transportation, unloading, road maintenance and cooperative management.

The gross margin (GrossMargin; IDR ha -1 yr -1 ) was calculated following equation 3 :

GrossMargin=[Revenues]-[Variable Costs] (equation 3)
Variable costs were defined by the costs for fertilizers (CostFerti; IDR ha -1 yr -1 ), herbicides (CostHerbi;

IDR ha -1 yr -1 ), labor for fertilization (LaborFerti; IDR ha -1 yr -1 ) and weeding (LaborWeed; IDR ha -1 yr -1 ).

Costs for pruning were integrated in variable costs to calculate gross margin. However, it was not considered within economic indicator set because the service for pruning was a yearly flat fee, the same for all smallholders (500 000 IDR ha -1 yr -1 ).

For labor, we considered both hired and family labor as costs, on the basis of a 7-hour-working day.

Return to labor (ReturnToLabor, IDR day -1 ) was the amount earned per day worked in the plot and it was obtained according to equation 4 [START_REF] Feintrenie | Why do Farmers Prefer Oil Palm? Lessons Learnt from Bungo District, Indonesia[END_REF]:

ReturnToLabor=[Gross Margin]/[

Data analysis

We performed two distinct clustering, on agricultural practices on the one hand, and on economic performances on the other hand, using R Software (R Development Core Team, 2008, R 3. 2020). Chi-square value was used to explain dependency between agricultural practices and economic clusters [START_REF] Bendixen | A Practical Guide to the Use of Correspondence Analysis in Marketing Research[END_REF]. Relationships were then characterized while looking at the distance between practices and economic clusters on CA's asymmetric biplot [START_REF] Greenacre | Contribution Biplots[END_REF].

Results

Analysis of agricultural practices

3.1.1. Fertilization and weeding performed by smallholders

The quantity and types of fertilizers applied by smallholders were variable, resulting in a high variability in nutrients inputs, as presented in Table 1. The most popular mineral fertilizers were NPK, KCl, Urea, and Dolomite. Only 15% of the smallholders did apply organic fertilizers, mainly using EFB around the weeded circle in order to benefit from its high K content. A mere 5% of the smallholders did not apply any fertilizer. Average N application rate was 84 kg ha -1 yr -1 with a coefficient of variation (CV) of 67%. Average K application rate was 117 kg ha -1 yr -1 with a CV of 67%. Applications of Mg, Ca and P were even more variable, with respective CV reaching up to 86%, 93% and 92%

(resp. means: 35 kg Mg ha -1 yr -1 , 55 kg Ca ha -1 yr -1 and 21 kg P ha -1 yr -1 ).

When weeding practices were considered, 28% of the interviewed smallholders used only chemical herbicides, and 15% of them cleared weeds only manually or mechanically. The 56% remaining used both chemical and mechanical weeding. Weeding practices are detailed in Table 1. The applied doses of herbicides varied from 0 to 5 times the minimal advised dose. TFI results showed that, on average, smallholders did not exceed 2 times the minimal advised dose. Manual or mechanical weeding was performed up to 2 times per year. Weed observation in the plots highlighted different strategies, ranging from blanket weeding to less managed vegetation (Grass, Wood and Height indicators, Table 1). Grass roughly covered 50% to 75% of the surface area in the studied zone (Score 3 for Grass indicator), while woody weeds were highly controlled (small average number for Wood indicator).

The height of the understory vegetation was mainly contained under 50 cm.

Typology of agricultural practices

Figure 1 gathers the variables linked to agricultural practices. Dimensions 1, 2 and 3 were selected as they presented 63% of cumulative variance of the dataset. Fertilization practices varied towards two distinct directions. Indeed, Mg and Ca applications were highly correlated together on the first dimension and mostly contributed to this dimension (resp. 76.5% and 67.7%). Applications of N, K and P were highly correlated. They were represented on the first and third dimensions and mostly explained the construction of the third dimension. Weed and TFI indicators were negatively correlated on the second dimension, while the "NonChWeeding" indicator was not correlated to the other indicators on weeding practices and it only explained the variability within the dataset to a lesser extent.

The AHC highlighted four clusters of smallholders' practices (Table 2). The first cluster, called A1, consisted of 16 smallholders representing 22% of the total sample. This cluster was characterized by the lowest inputs in N and K fertilizers (resp. 55 kg N ha -1 yr -1 and 61 kg K ha -1 yr-1 ). Chemical weeding was also the lowest, with an average TFI of 0.6, indicating that A1 smallholders poorly managed weeds in their plot.

The second cluster, called A2, consisted of 18 smallholders representing 25% of the total sample.

Fertilization practices of this cluster were similar to the ones found in cluster A1. The A2 cluster was mainly characterized by the use of chemical weeding with the highest average TFI (2.7). On the contrary, NonChWeeding and Weed indicators were found to be low, with average values of 0.3 times yr -1 and -1.0 times yr -1 respectively.

The third cluster, called A3, consisted of 24 smallholders representing 33% of the total sample. It differed from the other clusters by the high levels of Mg and Ca applied in the plots (resp. 66 kg Mg ha -1 yr -1 and 112 kg Ca ha -1 yr -1 ). Inputs in N, P and K were statistically not different from the ones found in clusters A1 and A2. Smallholders from cluster A3 did highly control weeds through chemical and nonchemical weeding, resulting in low weed biomass in the plot.

The fourth and last cluster, called A4, consisted of 15 smallholders representing 20% of the total sample. This cluster differed from the others by the high levels of N and K applied in the plots (resp.

142 kg N ha -1 yr -1 and 211 kg K ha -1 yr -1 ). Mg and Ca inputs were similar to the ones found in clusters A1 and A2. Weeding practices were similar to A3, mainly driven by nonchemical weeding (1.1 times yr -1 ) and a rather low TFI (1.4).

Oil palm yields are presented in Figure 2. Overall average yield was of 18 tFFB.ha -1 .yr -1 . A significant difference was observed between clusters A1 (mean = 18 t ha -1 yr -1 , CV = 24%) and A4 (mean = 20 t ha -1 yr -1 , CV = 19%). Clusters A2 (mean = 19 t ha -1 yr -1 ) and A3 (mean = 19 t ha -1 yr -1 ) had quite variable responses to yields with respective CVs being equal to 32% and 27%.

Analysis of economic performances

The global economic performances

Results on economic indicators are presented in Table 3. On average, smallholders' revenues reached 20M IDR yr -1 ha -1 (eq. 1 209 € yr -1 ha -1 in 2020 1 ). Fertilizers were the major source of expenses mobilizing around 15% of the revenues from FFB sales (Table 3). Wages both for weeding and fertilizer application represented the second main cost (3% of revenues). Expenditure in herbicides accounted only for 0.5% of revenues. Mean gross margin was 16 M IDR yr -1 (eq. 967 € yr -1 ha -1 in 2020), i.e. expenses for inputs and wages represented 20% of total revenues. Smallholders' family labor and hired workers together worked in total around 30 days.yr -1 for 1 hectare of oil palm.

Thus, smallholders earned on average 564,000 IDR day -1 ha -1 (eq. 34 € day -1 ha -1 in 2020).

Typology on economic performances

Figure 3 gathers the indicators on economic performances. The first and second dimensions were used, as they represented 67% of cumulative variance of the dataset. First dimension was mainly explained by "Revenues", "GrossMargin" and "ReturnToLabor" indicators. The second dimension was mainly explained by the costs for fertilization (CostFerti and LaborFerti). Costs for weeding (CostHerbi and LaborWeeding) did not explain much the variability from the dataset.

Three clusters of smallholders with similar economic performances were built after AHC. (Table 4).

The first cluster, called E1, consisted of 33 smallholders representing 45% of the total sample. This cluster was characterized by the lowest revenues across of the studied population, which were mainly limited by low yields (15 t ha -1 yr -1 on average). Input and labor costs for fertilization and weeding were the lowest and they little impacted the gross margin and return to labor, which were also the lowest compared to other groups.

The second cluster, called E2, consisted of 22 smallholders representing 30% of the total sample.

Revenue was the highest in this population, as reflected by high yields (23 t ha -1 yr -1 on average).

Costs for fertilizer inputs and costs for labor for weeding were similar to E1 but costs for labor for fertilization were found to be 1.7 times higher. GrossMargin and ReturnToLabor were overall the highest and respectively averaged 20.7 M IDR yr -1 ha -1 (eq. 1,251 € yr -1 ha -1 in 2020) and 775,882 IDR day -1 ha -1 (eq. 47 € yr -1 ha -1 in 2020), indicating the group with highest profitability at plot level.

The third and last cluster, called E3, consisted of 18 smallholders representing 25% of the total sample. Revenues were similar to E2 with similar yields results (22 t ha -1 yr -1 on average). Costs for fertilization input were 1.8 times higher than E2 cluster. Costs for labor for fertilization and weeding were high. These costs had a high impact on gross margin and return to labor results: gross margin was 20% lower than for E2 smallholders and return to labor was similar to E1 smallholders (around 16.5 M IDR yr -1 ha -1 ; eq. 997 € yr -1 ha -1 in 2020).

Relations between agricultural practices and economic performances

The correspondence analysis between agricultural and economic clusters was significant (pvalue < 0.01; chi-square = 21.2). This result indicated a strong link between the two clustering approaches as illustrated in Figure 4. The first dimension represented 86.6% of the global variance between the clusters. We mainly focused on the correlation found between A and E clusters on this dimension (Figure 4).

Cluster A1 was correlated with E1 cluster (distance=0.06). Low fertilization and low-intensity weeding practices (A1) were associated with economic performances that were limited by yields (E1).

A2 cluster was associated with E2 cluster (distance=0.43) and E1 cluster (distance=0.57). Indeed, 44%

of the smallholders implementing chemical weed control but applying few nutrients (A2) were associated with the most profitable group (E2), 44% of them were associated with poor yield-limited economic performances (E1).

Cluster A3 was the cluster on agricultural practices that was the most correlated with E2 cluster (distance=0.04). The highest plot's profitability (E2) was related to medium use of chemical weeding and the application of Mg and Ca.

A4 cluster was associated with both clusters E2 (distance=0.55) and E3 (distance=0.75). 53% of A4 smallholders had economic performances that were limited by the costs for fertilization (E3). Then, 33% of smallholders had high profitability for the studied zone (E2).

Discussion

4.1. Oil palm ex-plasma plots are productive and profitable for smallholders

Our study focused on management practices implemented by smallholders in ex-plasma oil palm plantations. We observed relatively low "yield taking" gaps [START_REF] Rhebergen | Closing yield gaps in oil palm production systems in Ghana through Best Management Practices[END_REF] in the area.

Indeed, all plots were established by the nucleus estate following similar practices for planting and immature stage. Planting design, plant material, field access and harvest frequency are known to be key factors explaining variability in oil palm production (Woittiez et al., 2017). The studied plots were around 25 years old and average production was 19 t ha -1 yr -1 . This result is far above the production recorded in areas cultivated by fully independent farmers [START_REF] Euler | Exploring yield gaps in smallholder oil palm production systems in eastern Sumatra, Indonesia[END_REF][START_REF] Jelsma | Do wealthy farmers implement better agricultural practices? An assessment of implementation of Good Agricultural Practices among different types of independent oil palm smallholders in Riau, Indonesia[END_REF][START_REF] Lee | Oil palm smallholder yields and incomes constrained by harvesting practices and type of smallholder management in Indonesia[END_REF][START_REF] Lifianthi | Productivity And Income Peformance Comparison of Smallholder Oil Palm Plantation at Dry Land and Wet Land of South Sumatra Indonesia[END_REF]. Some plots even showed yields comparable with those from agroindustrial plantations producing around 26 t ha -1 yr -1 , a trend that was already highlighted in similar areas by [START_REF] Moulin | Deciphering agricultural practices and environmental impacts in palm oil plantations in Riau and Jambi provinces, Indonesia[END_REF].

Ex-plasma organization with cooperatives might have helped in limiting costs for inputs and intermediaries, and in pooling smallholders' economic efficiency. This led to high revenues, with an average gross margin of 16M IDR ha -1 yr -1 (eq. 967 € ha -1 yr -1 in 2020) for only 30 working days per year which is very low for labor needs. The return to labor of such systems is thus 3 times higher (average of 564 000 IDR day -1 ; eq. 34 € ha -1 day -1 in 2020) compared to the one obtained in independent contexts (192 800 IDR day -1 ; eq. 12 € ha -1 day -1 in 2020) [START_REF] Euler | Exploring yield gaps in smallholder oil palm production systems in eastern Sumatra, Indonesia[END_REF]. This revenue level makes oil palm productivity very competitive when compared to other agricultural commodities in Indonesia, like rubber monoculture or agroforests [START_REF] Feintrenie | Why do Farmers Prefer Oil Palm? Lessons Learnt from Bungo District, Indonesia[END_REF].

Ex-plasma thus seemed to be an efficient system in terms of productivity and revenues for farmers (in terms of both margin and return to labor). However, trade-offs dealing with social and environmental aspects were not integrated in the present study. For example, smallholders are contractually bound to a specific mill to sell their production, and these bonds might increase vulnerability, such as price volatility [START_REF] Cahyadi | Contract Farming and Vulnerability to Poverty among Oil Palm Smallholders in Indonesia[END_REF][START_REF] Gatto | Oil Palm Boom, Contract Farming, and Rural Economic Development: Village-Level Evidence from Indonesia[END_REF]. Contracts also makes it impossible for smallholders to take benefit from market competition and to negotiate for advantages such as the access to organic matter from EFB to be used for fertilization.

From initial homogeneity to current diversity

Despite the homogeneity of the studied zone in terms of both pedoclimatic conditions and management of immature stage, our study highlighted a large diversity of agricultural practices implemented by smallholders. Based on fertilization and weeding practices, we were able to identify four groups of smallholders. A clear gradient in management intensity was observed. A1 smallholders were found to have lower management of their plots compared to other, especially concerning weeds management leading to a relatively high weed biomass in the plots. A2 and A3 were intermediary groups, with trends for an increase of fertilization and weed maintenance. On the opposite extreme of the gradient, the A4 smallholders were mostly different to other groups in terms of N and K fertilization. They represented around 20% of the smallholders of the area. Management practices in group A4 are the closest to conventional practices applied in agroindustry [START_REF] Darras | Reducing Fertilizer and Avoiding Herbicides in Oil Palm Plantations-Ecological and Economic Valuations[END_REF][START_REF] Pardon | Key unknowns in nitrogen budget for oil palm plantations. A review[END_REF] and recommended by best management practices [START_REF] Rhebergen | Closing yield gaps in oil palm production systems in Ghana through Best Management Practices[END_REF]Woittiez et al., 2017) to optimize oil palm yield.

After discussions with smallholders during interviews, it seems that A3 smallholders purchase fertilizers with high Mg and Ca contents for three main reasons: (i) Mg and Ca inputs are cheaper than NPK fertilizers; (ii) Mg and Ca inputs are more available at cooperatives' shops than other types of fertilizers; (iii) most smallholders believe they have acid soil and put Mg inputs to correct soil pH.

4.3. Linking practices to economic performances, where is the optimum?

We collected a large set of data on the economic performances at plot level, which were consistent with previously published studies [START_REF] Euler | Exploring yield gaps in smallholder oil palm production systems in eastern Sumatra, Indonesia[END_REF][START_REF] Feintrenie | Why do Farmers Prefer Oil Palm? Lessons Learnt from Bungo District, Indonesia[END_REF][START_REF] Jelsma | Unpacking Indonesia's independent oil palm smallholders: An actor-disaggregated approach to identifying environmental and social performance challenges[END_REF][START_REF] Lifianthi | Productivity And Income Peformance Comparison of Smallholder Oil Palm Plantation at Dry Land and Wet Land of South Sumatra Indonesia[END_REF]. We identified three economic clusters covering three different strategies in terms of revenues vs expenses balance. E1 groups showed the lowest revenues in the ex-plasma area under study. Such results were still better than those observed for independent smallholders by [START_REF] Euler | Exploring yield gaps in smallholder oil palm production systems in eastern Sumatra, Indonesia[END_REF]. Clusters E2 and E3 accounted for about 55% of the smallholders of the area, and got much higher revenues, due to high oil palm yields (+8M IDR ha -1 yr -1 compared to E1). In E2, the costs of fertilization and labor were similar to the ones incurred by E1 smallholders, whereas in E3

these costs are about two times higher (mainly because of fertilization costs, Table 4). The E2 group corresponds to the economic optimum found in the studied area.

It is worth to note that we could not cover practices and economic performances temporal dynamics and history. We based our analysis on actual plots performances. Crossing economic optimum with the analysis of the clusters on management practices stressed some optimal practices to meet economic efficiency at plot's level. On one side, A1 was close to E1 group due to poor yields that limit the gain-cost balance. Farmers from A1 group implemented a low plot maintenance (e.g. 31% of smallholders did not fertilize their plot). However, the study did not highlight any significant difference neither on N, P and K fertilization between A1, A2 and A3 groups nor on fertilization costs between E1 and E2 clusters. Fertilization may thus not be the only factor explaining the low production of A1 plots. For example, other possible co-factors such as plots management history could not be integrated in this study and may have interfered our causal link approach. Further experiments may be of interest if we want to better understand the production gap evidenced in A1 group. While not being at the very top, those farmers benefit from a sizeable gross margin from their oil palm plots reaching 12M IDR yr -1 ha -1 (±725 €).

On the other side, most of A4-type smallholders were close to those from the E3 group. The A4 smallholders tended to implement practices that were too costly when compared to benefits gained from harvesting. This can be linked to the large use of KCl fertilizer that are more costly than other. In this case, intensive management practices (A4) were costly for smallholders and less profitable.

Within the midrange, E2 group was at the economic optimum, with high revenues and relatively low costs for fertilization. A2 and A3 management practices were the closest to E2. In a context of relatively low palm oil prices, A2 and A3 farmers were at the same revenue range and the same main costs (N, P, K fertilization); these are optimal to provide a sufficient yield together with manageable costs. Practices of N, P and K fertilization for A2 and A3 corresponds to doses of fertilizer that were considered lower than best management practices [START_REF] Jelsma | Do wealthy farmers implement better agricultural practices? An assessment of implementation of Good Agricultural Practices among different types of independent oil palm smallholders in Riau, Indonesia[END_REF][START_REF] Woittiez | FERTILISER APPLICATION PRACTICES AND NUTRIENT DEFICIENCIES IN SMALLHOLDER OIL PALM PLANTATIONS IN INDONESIA[END_REF]. In best management practices guidelines, fertilizer recommendations are often derived from mineral balance models, in which the amount of nutrient to be supplied is theoretically estimated. However experimental optimisation of marginal economic gains through leaf sampling and yield analysis frequently leads to lower recommendations in terms of nutrient supply [START_REF] Dubos | Precision agriculture in oil palm plantations: diagnostic tools for sustainable N and K nutrient supply[END_REF]. Based on our results, it is difficult to provide recommendation in terms of both weeding practices and Ca -Mg fertilisation. Indeed, their costs are relatively low compared to the other fertilization costs and no literature data could highlight any linkage between these practices with productivity in a similar soil context. However, indirect antagonisms between Ca-Mg fertilization and K nutrients have recently been observed, and excessive fertilization may limit K availability for oil palms (Dubos et al., 2017[START_REF] Dubos | Potassium uptake and storage in oil palm organs: the role of chlorine and the influence of soil characteristics in the Magdalena valley, Colombia[END_REF]. Furthermore, we analysed the mineral content of the fertilizers used by farmers in October 2020. Result especially evidenced a lower content in Ca and Mg than indicated on packages for some fertilizers. This makes conclusions on the linkages between Ca -Mg applications and productivity even more hazardous. High chemical weeding in A2 group and high Ca-Mg fertilization in A3 group may also be detrimental to the environment.

Our results highlighted that technical studies aiming at filling yield gaps and spreading best management practices [START_REF] Rhebergen | Closing yield gaps in oil palm production systems in Ghana through Best Management Practices[END_REF]Woittiez et al., 2017) may not be sufficient. Indeed, the integration of economic parameters enabled us to show that smallholders might not have any economic interest in maximizing their production (reaching A4 group). In addition to technical evidence, we need to integrate socio-economic bottlenecks for a better understanding of farmers' performance, as shown by [START_REF] Woittiez | FERTILISER APPLICATION PRACTICES AND NUTRIENT DEFICIENCIES IN SMALLHOLDER OIL PALM PLANTATIONS IN INDONESIA[END_REF].

Toward generic recommendations

Our study helped in providing recommendations to smallholders, aimed at optimising their management practices through the integration of economic aspects. We noticed that only 29% of the smallholders adopted the management practices we described as economically optimal (A3). A sizeable portion (50%) of the smallholders under study could reduce their herbicide use. The strong link between A4 and E3 smallholders stressed that reducing A4 farmers' inputs in fertilizers towards a more balanced fertilization might lead to reduced cost without affecting yields and then increase economic benefits. Moreover, results showed that an important proportion of smallholders is over fertilizing in Mg and under fertilizing in K. Thus, 30% of the interviewed smallholders could reduce or re-adapt their fertilization balance strategy towards more optimal outcomes. NES ex-plasma smallholders might benefit from the cooperatives' organizational capacity in improving their practices. There is thus a need for the dissemination of optimal practices at the cooperatives' level.

Our results can help in fine-tuning some of Principles and Criteria governing certification, especially within current RSPO (Roundtable on Sustainable Palm Oil) certification dynamics that aims at integrating more smallholders practices [START_REF] Apriani | Non-state certification of smallholders for sustainable palm oil in Sumatra, Indonesia[END_REF]. However, one must bear in mind that results from our study are site-specific and adapted to ex-plasma smallholders. It is rather risky to extrapolate our recommendations to other regions and other types of smallholders (e.g. independent smallholders). In addition, we need to understand local contexts and factors governing smallholders' organization at a larger scale before drawing up general outcomes from such a sitespecific study. Our research focused on the plot level, but complementary plots and or activities will affect farm's performance.

To provide the full understanding of the agroecosystem performance that is needed to design recommendations, we need to complement this economic study with data measurements of the environmental component. For instance, nitrogen fertilization may rather be a driver of N loss than a driver of production. In addition, herbicide use and understory management are generally intensive, with no evidence of competition between weeds and oil palms. A full multicriteria study of various smallholders' management strategies, integrating environmental aspects would be of paramount interest [START_REF] Darras | Reducing Fertilizer and Avoiding Herbicides in Oil Palm Plantations-Ecological and Economic Valuations[END_REF].

Limitations of the study

Our study aimed at investigating the diversity of agricultural practices and related economic performances of ex-plasma smallholders settled in the same historical and pedoclimatic context and gathered into two neighboring cooperatives. Working with data collected during interviews from smallholders is known to be at the origin of uncertainties. Smallholders do not maintain proper records of inputs, applications rates or time needed to perform activities (for detailed labor costs calculation) in the plots, nor the exact person who performed each activity [START_REF] Jelsma | Do wealthy farmers implement better agricultural practices? An assessment of implementation of Good Agricultural Practices among different types of independent oil palm smallholders in Riau, Indonesia[END_REF]. In our context, the strict organization of the 2ha plots into farmers' group and cooperatives as well as crosschecks with Indonesian oil palm experts, staff members of the cooperatives and plots observation helped us to limit these uncertainties. Specific precautions were also taken with farmers owning several plots that mostly managed all their plots identically and thus avoid possible economy of scale.

Farmers may adapt supply in fertilizers and pesticides to prices and climate conditions like drought or outbreak of pests. The period considered did not include any extreme events or pest outbreak.

Moreover, as we investigated practices in old plantations (end of first cycle), we could assume that plantations were in a rather stable state embodying long-term hysteresis effect. However, it is important to recall that as a perennial crop, oil palm performances may be affected by practices with some delay [START_REF] Combres | Simulation of inflorescence dynamics in oil palm and estimation of environment-sensitive phenological phases: a model based analysis[END_REF][START_REF] Fairhurst | Oil palm: management for large and sustainable yields. Oil palm: management for large and sustainable yields[END_REF]. A more complex panorama of practices all over the perennial cycle would be necessary to have a mechanistic assessment of all potential impacts of practices.

Finally, we did not investigate the reasons of divergence in practices nor the role of socio-economic context in the adoption of certain practices by the studied ex-plasma smallholders. That information would give some clue to better understand links between agricultural practices and economic performances.

Conclusion

The present study showed that ex-plasma smallholders implement a large range of agricultural practices within homogeneous contexts. The overall economic performances of oil palm plantations is globally very profitable. The gradient of management was not fully linked to economic performances. Integrating management costs proved that the practices linked to optimal management did not fit with optimal economic performance at the plot level. It was found more profitable for smallholders to reduce the level of inputs in order to increase plot profitability. This result opens up room to explore more in depth the present paradigm of yield optimisation for oil palm in the light of optimal ex-plasma smallholders' revenues. Further investigation is also needed to decipher the links between profitable practices and environmental impact, in order to fine tune recommendations to farmers.

Figure 1 :

 1 Figure 1: Principal Component Analysis performed on indicators relative to agricultural practices for the 73 studied plots. (1.a) is the graph of variables on dimensions 1 and 2. (1.b) is the graph of variables on dimensions 1 and 3. The variable Yield is not used for the construction of the PCA but is given as a supplementary variable.

Figure 2 :Figure 3 :

 23 Figure 2: Yields per clusters of agricultural practices (A1; A2; A3; A4). The significance was obtained with Anova. Letters indicate significant differences between clusters based on a Tukey post-hoc test.

Figure 4 :

 4 Figure4: Results of the Correspondence Analysis between the clusters on agricultural practices and economic performances. Only projection on dimension 1 is presented for illustration as it explains a high percentage of inertia (86.6%). Biplot with dimensions 1 and 2 is available in Supplementary Material 5.

Table 1 :

 1 Overview of the results obtained for agricultural practices for the 73 studied plots. The Weeds indicator combines Grass, Wood and Height indicators as presented in Supplementary Material 4.

	Variable	N	P	K	Mg	Ca	TFI	NonChWeeding	Weed	Grass	Wood	Height
	Unit			kg ha -1 yr -1			score	Times yr -1	score	score	individual	cm
	Min	0	0	0	0	0	0.0	0.0	-2.1	1.0	0.0	0.0
	Median	78	18	112	30	52	1.2	1.0	-0.3	3.3	2.3	30.0
	Mean	84	21	117	35	55	1.6	0.8	0.0	3.2	4.1	45.2
	Max	209	67	323	119	199	5.0	2.0	3.6	4.7	22.0	150.0
	CV (%)	67%	92%	67%	86%	93%	87%	89%	-	29%	122%	96%

Table 2 :

 2 Characterization of the clusters built for agricultural practices (A1; A2; A3; A4). The significances were obtained with Anova or Kruskal-Wallis( 1 ) tests and are indicated as following: *p-value<0.05 ; **p-value < 0.01 ; ***p-value < 0.001. Letters indicate significative differences between clusters based on post-hoc tests.

	Variable	N	P 1	K	Mg 1	Ca 1	TFI	NonChWeeding 1	Weed 1
	Unit			kg ha -1 yr -1			score	times yr -1	score
	A1 (n=16)
		55b (±53) 16 (±18)	61b (±49)	16bc (±20) 27b (±28) 0.6c (±0.4)	0.8a (±0.7)	1.9a (±1.1)
	A2 (n=18)
	mean 65b (±37) 15 (±11) 101b (±67)	14c (±14)	20b (±27) 2.7a (±1.5)	0.3b (±0.6)	-1.0c (±0.8)
	A3 (n=24)
	mean 80b (±48) 26 (±23) 107b (±56) 66a (±25)	112a (±37)	1.6ab (±1.2)	0.9a (±0.6)	-0.3b (±0.8)
	A4 (n=15)
	mean 142a (±48) 27 (±64) 211a (±64) 33b (±28) 34b (±31) 1.4bc (±1.3)	1.1a (±0.6)	-0.4bc (±0.7)
	p-value	***	ns	***	***	***	***	**	***

Table 3 :

 3 Overview of the results obtained on the indicators on economic performances for the 73 studied plots

Mean exchange rate for Indonesian rupiah (IDR)

estimated at : 1 € = 16,549 IDR (after data from XE online, (2020))

Acknowledgements

This study was implemented under the framework of the "Solidarity Sourcing Berbak Landscape" project funded by L'Oréal Company, through a trilateral partnership between L'Oréal, SNV and Cirad.

The authors thank L'Oréal for support and Nisrine Zaaraoui, Alice Lemont and Stéphanie Ringeissen for efficient project management. We acknowledge Dani Hidayat, SNV, ProSympac mill, local farmers, KUD Marga Jaya and KUD Makarti cooperatives for managing our access to the fields and their valuable help in data collection. We also thank Amrina for her help in farmers' survey, Marine Dromard (Cirad) for her appreciated skills in project management, Albert Flori (Cirad) for expert statistical analysis and Alain Rival and Xavier Bonneau (Cirad) for their relevant comments on the study results and text proofreading. Finally, we would like to warmly thank the two anonymous reviewers and the editor who provided useful comments to improve the quality of the paper.

Variable

Revenues GrossMargin ReturnToLabor CostHerbi CostFerti LaborWeeding LaborFerti Unit 000 IDR yr -1 ha -1 000 IDR yr -1 ha -1 000 IDR day -1 ha -1 000 IDR yr


Yasmina Bencheikh

Ahmed Addad

Yannick Coffinier

Umesh Kumar

Pascal Roussel

Sabine Szunerits

Toufik Hadjersi

Mohammed A Amin

Seddik El

Hak Abaidia

Rabah Boukherroub
email: rabah.boukherroub@univ-lille.fr

Sidik El Hak Abaidia

Silicon nanowire-hydrogenated TiO2 core-shell arrays for stable electrochemical micro-capacitors

Keywords: Silicon nanowires, TiO2 thin film, hydrogenation, micro-supercapacitor, high stability

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Today, interest in energy saving, electric and hybrid vehicles and the quest for efficiency and multi-functionality in micro-systems make the research on electrochemical energy storage a field in full expansion [START_REF] Gür | Review of electrical energy storage technologies, Materials and Systems: challenges and prospects for large-scale grid storage[END_REF][START_REF] Wang | Electrochemical capacitors: mechanism, material, systems, characterization and applications[END_REF].

In particular, miniaturized versions of electrochemical supercapacitors (called microsupercapacitors) that can be easily integrated into microelectronic circuits attracted more attention in recent years [START_REF] Zhang | Recent advances in microsupercapacitors[END_REF][START_REF] Lethien | Challenges and prospects of 3D micro-supercapacitors for powering the internet of things[END_REF][START_REF] Shen | A review of on-chip micro supercapacitors for integrated self-powering systems[END_REF]. Increasing the specific surface area and chemical stability of electrodes is the current research strategy to improve their performances in terms of storage capacity, life cycle, energy and power densities [START_REF] Beidaghi | Capacitive energy storage in micro-scale devices: recent advances in design and fabrication of micro-supercapacitors[END_REF][START_REF] Arico | Nanostructured materials for advanced energy conversion and storage devices[END_REF]. In this case, it appears that nanostructured materials and in particular nanowires are the most suitable for the supercapacitor electrode realization, due to their high developed surface area and quasi-ideal electric double layer [START_REF] Devarapalli | Glucose-Derived Porous Carbon-Coated Silicon Nanowires as Efficient Electrodes for Aqueous Micro-Supercapacitors[END_REF][START_REF] Thissandier | Are Tomorrow's Micro-Supercapacitors Hidden in Forest of Silicon Nanotrees?[END_REF][START_REF] Aradilla | High Performance of Symmetric Micro-Supercapacitors Based on Silicon Nanowires UsingN-Methyl-N-propylpyrrolidiniumbis -(trifluoromethylsulfonyl)imide as Electrolyte[END_REF][START_REF] Berton | Wide-Voltage-Window Silicon Nanowire Electrodes for Micro-Supercapacitors via Electrochemical Surface Oxidation in IonicLiquid Electrolyte[END_REF].

Silicon nanowires (SiNW) are one of these semiconductor nanostructures investigated as potential electrode material in energy storage/release devices like electrochemical supercapacitors [START_REF] Mai | Nanowire Electrodes for Electrochemical Energy Storage Devices[END_REF] and batteries [START_REF] Chan | High performance lithium battery anodes using silicon nanowires[END_REF][START_REF] Cui | Crystalline-amorphous core-shell silicon nanowires for high capacity and high current battery electrodes[END_REF][START_REF] Su | Silicon-based nanomaterials for lithium-ion Batteries: A review[END_REF]. The focus on Si-based electrodes is essentially governed by the well-established microfabrication technology, which holds great promise in the area of integrated storage devices. In addition, large surface areas can be achieved through nanostructuration of bulk Si using different routes e.g. reactive ion etching (RIE), chemical vapor deposition growth, and metal-assisted chemical etching (MACE) [START_REF] Chan | High performance lithium battery anodes using silicon nanowires[END_REF][START_REF] Su | Silicon-based nanomaterials for lithium-ion Batteries: A review[END_REF]. The latter approach, used in our work, is a simple and effective wet chemical approach to produce a nanowire array from bulk Si avoiding the use of vacuum environment.

Even though, SiNW arrays feature high developed surface area, their direct utilization as electrodes is restricted by their chemical instability in aqueous and basic solutions [START_REF] Thissandier | Are Tomorrow's Micro-Supercapacitors Hidden in Forest of Silicon Nanotrees?[END_REF][START_REF] Berton | Wide-Voltage-Window Silicon Nanowire Electrodes for Micro-Supercapacitors via Electrochemical Surface Oxidation in IonicLiquid Electrolyte[END_REF]. In fact, oxide formation and dissolution limit their long-term operation and real-world potential application. Therefore, coating of these nanowires with organic and non-organic materials was commonly applied to overcome this hurdle. In this context, coating SiNW arrays with transition metal oxides (NiO, MnO 2 , ZnO/Al 2 O 3 , and TiO 2 ) not only enhances the stability, but it also increases the specific capacity since they are battery-type or pseudo-capacitive materials [START_REF] Lu | Electrochemical properties of high-power supercapacitors using ordered NiO coated Si nanowire array electrodes[END_REF][START_REF] Dubal | 3D hierarchical assembly of ultrathin MnO2 nanoflakes on silicon nanowires for high performance micro-supercapacitors in Li-doped ionic liquid[END_REF][START_REF] Han | High k nanophase zinc oxide on biomimetic silicon nanotip array as supercapacitors[END_REF]. Other coatings based on thin diamond [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][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] and carbon [START_REF] Devarapalli | Glucose-Derived Porous Carbon-Coated Silicon Nanowires as Efficient Electrodes for Aqueous Micro-Supercapacitors[END_REF][START_REF] Alper | Silicon carbide coated silicon nanowires as robust electrode material for aqueous micro-supercapacitor[END_REF] films were also applied to enhance the capacitive performance of the SiNW electrodes. TiO2, one of these transition metal oxides, is the subject to of extensive academic and technological research, owing to its specific properties: non-toxic, abundant, inexpensive and chemically stable [START_REF] Chen | Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications[END_REF]. Despite its capacity is lower than that of other capacitive materials like RuO2, NiO or conductive polymer nanostructures, it was broadly investigated as electrode material for electrochemical supercapacitors [START_REF] Lu | Hydrogenated TiO2 nanotube arrays for supercapacitors[END_REF]. Investigation of the capacity of TiO2 nanotubes revealed significantly high values (911 μF cm -1 at 1 mV s -1 ) [START_REF] Salari | Enhancement of the capacitance in TiO2 nanotubes through controlled introduction of oxygen vacancies[END_REF], and this value was improved upon hydrogenation treatment [START_REF] Li | Electrochemical doping of anatase TiO2 in organic electrolytes for highperformance supercapacitors and photocatalysts[END_REF][START_REF] Lee | One-Dimensional Titanium Dioxide Nanomaterials: Nanotubes[END_REF]. Also, TiO2 as a transition metal oxide has been applied in perovskite devices [START_REF] Trifiletti | Engineering TiO2/perovskite planar heterojunction for hysteresis-less solar cells[END_REF][START_REF] Wang | Challenges and strategies relating to device function layers and their integration toward high-performance inorganic perovskite solar cells[END_REF].

SiNW coated with TiO2 (SiNW/TiO2) were described in some reports. Konstantinou et al. used three processes to coat SiNW electrodes with TiO2 thin film: TiCl4 treatment method, dip coating path and hydrothermal method [START_REF] Konstantinou | TiO2 coated Si nanowire electrodes for electrochemical double layer capacitors in room temperature ionic liquid[END_REF]. The highest surface capacity (386 μF cm -2 ) was achieved based on the galvanostatic charge-discharge (GCD) technique at 3.7 × 10 -5 A in [Bmim][NTf2] ionic liquid [START_REF] Konstantinou | TiO2 coated Si nanowire electrodes for electrochemical double layer capacitors in room temperature ionic liquid[END_REF]. Besides, spin coating of TiO2 on SiNW attained a specific capacity of 3.55 mF cm -2 at 0.1 mA cm -2 with a retention of 68.8% after 1,000 galvanostatic charge-discharge cycles [START_REF] Zhang | Enhanced micro-supercapacitors in aqueous electrolyte based on Si nanowires coated with TiO2[END_REF].

Atomic layer deposition (ALD) is a technique able to afford compliant coatings on various

substrates with a very precise control over the coating thickness [START_REF] Meng | Emerging applications of atomic layer deposition for lithium-ion battery studies[END_REF]. Memarz et al. demonstrated that SiNW battery anode coated with TiO2 by ALD featured a remarkable improvement in its performances, as compared to the non-coated one [START_REF] Memarz | ALD TiO2 coated silicon nanowires for lithium ion battery anodes with enhanced cycling stability and coulombic efficiency[END_REF]. Although, TiO2coated electrodes achieved enhanced capacitive properties, their specific capacitance values were significantly lower than those recorded using other metal oxides [START_REF] Salari | Enhancement of the capacitance in TiO2 nanotubes through controlled introduction of oxygen vacancies[END_REF][START_REF] Salari | A highly ordered titania nanotube array as a supercapacitor electrode[END_REF][START_REF] Kim | Controlled TiO2 Nanotube Arrays as an Active Material for High Power Energy-Storage Devices[END_REF], most likely due to the limited electrochemical performance and reduced electrical conductivity of TiO2 [START_REF] Wang | Pseudocapacitive contributions to electrochemical energy storage in TiO2 (anatase) nanoparticles[END_REF][START_REF] Fabregat-Santiago | High carrier density and capacitance in TiO2 nanotube arrays induced by electrochemical doping[END_REF].

One way to address these limitations is through hydrogenation, which is capable to modify effectively the material's properties. Indeed, it has been shown its great ability in TiO2 nanocrystals for introducing significant structural changes and improvement in their optical, electronic, photocatalytic, lithium-ion storage capability and field emission [START_REF] Xia | Hydrogenated TiO2 nanocrystals: a novel microwave absorbing material[END_REF].

Based on above results, we applied the ALD method to coat a thin TiO2 layer on SiNW, prepared by Ag-assisted chemical etching, and subsequent thermal annealing at 400 ℃ for 4 h in hydrogen atmosphere. The effect of the H-TiO2 coating on the capacitive behavior of the SiNW electrode was studied in 1M Na2SO4 aqueous electrolyte. The specific capacitance of H-TiO2/SiNW was enhanced six times as compared to non-annealed one and 20-fold compared to a reference SiNW (4.5 µm) electrode under the same operating conditions. Furthermore, the hydrogenation of TiO2 thin film deposited on SiNW led to good reversibility and stability over 30,000 galvanostatic charge-discharge cycles.

Experimental section

Preparation of TiO2/SiNW

TiO2 thin coatings were produced by ALD using TiCl4 (titanium source), H2O vapor (oxygen source), and nitrogen as the purge gas. The deposition was conducted at 260°C and 2.8 mbars using a pulse duration of 250 ms for both precursors separated by 2 s of N2 purge. The thickness of TiO2 is function of the number of cycles and was determined by ellipsometry on flat Si surface (placed alongside with SiNW substrates in the reaction chamber). We set all experimental ALD parameters to obtain TiO2 thickness of 20 nm.

Hydrogenation process

The TiO2/SiNW were subsequently annealed under hydrogen atmosphere (named here after as H-TiO2/SiNW) and the other in the open air (designed as air-TiO2/SiNW) both at 400 °C for 4 h. The hydrogenation time at the maximum temperature was 4 h per sample and the hydrogen flow was started 30 min before the thermal programming and continued until the sample attained room temperature. The total duration of the hydrogen flow was about 10 h.

Results and discussion

Figure S1 illustrates the scanning electron microscopy (SEM) micrographs of SiNW arrays produced by silicon dissolution in an aqueous solution of HF/AgNO3 at 55°C for 30 min. The nanowires network forms bundles on the top surface due to inhomogeneous dissolution caused by Ag particles random distribution or van der Waals forces between the nanowires during the process of drying [START_REF] Fellahi | The antimicrobial effectof silicon nanowires decorated with silver and copper nanoparticles[END_REF]. The cross-section observations revealed that a large quantity of nanowires well-aligned and perpendicular to the surface were obtained [START_REF] Hamdi | Hydrothermal preparation of MoS2/TiO2/Si nanowires composite with enhanced photocatalytic performance under visible light[END_REF].

As it can be seen in Figure S1b, the wires are about 4.5 μm in length with different diameters. SEM images of a SiNW substrate ALD-coated with a TiO2 thin film are depicted in Figure 1. Fig. 1a andb show that a thin film of TiO2 was coated conformally on SiNW. The top view SEM images (Fig. 1c,d) confirmed that the TiO2 layer was deposited in a uniform and conformal manner on the silicon nanowires. A TiO2 film thickness of 20 nm was optimal to obtain high specific capacity and high cycling lifetime To check if there is a change in the crystalline phase of TiO2 after thermal hydrogenation, X-ray diffraction (XRD) patterns of TiO2/SiNW and H-TiO2/SiNW (400°C) were recorded. 8 reference [START_REF] Mehta | Hydrogen treated anatase TiO2: a new experimental approach and further insights from theory[END_REF]. Note that the peak at 33° is due to the lambda-half contamination of the (400) (200) diffraction plan of the silicon substrate. To study in more detail the effect of the hydrogenation process on the phase structure, the TiO2/SiNW and H-TiO2/SiNW samples were characterized by Raman spectroscopy. The Raman plots are depicted in Figure 3 and both comprise Raman bands at 397 cm -1 (B1g) and 640 cm -1 (Eg), characteristic of TiO2 anatase. The width at mid-height of these two peaks were 36.05, 33.69 cm -1 and 33.47, 27.87 cm -1 respectively for the TiO2/SiNW and H-TiO2/SiNW samples. This means that the anatase phase is maintained after hydrogenation treatment, in accordance with XRD results. The Raman peaks of the H-TiO2/SiNW sample are negatively offset and widen as compared to those of TiO2/SiNW, which suggests an increase in the amount of vacant oxygen [START_REF] Song | Fast Growth of Highly Ordered TiO2 Nanotube Arrays on Si Substrate under High-Field Anodization[END_REF]. The feature at 520 cm -1 (*) is ascribed to the silicon substrate, its high intensity dominated the expected peak at 515 cm -1 (Eg), another characteristic peak of TiO2 anatase which was not visible [START_REF] Chen | Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals[END_REF]. surface hydroxyl groups [START_REF] Lu | Hydrogenated TiO2 nanotube arrays for supercapacitors[END_REF]. The O peak remains the same meaning that the TiO2 structure was not altered [START_REF] Chen | Enhanced photoelectric performance of PbS/CdS quantum dot co-sensitized solar cells via hydrogenated TiO2 nanorod arrays[END_REF][START_REF] Raghunath | Quantum chemical elucidation of the mechanism for hydrogenation of TiO2 anatase crystals[END_REF][START_REF] Naldoni | Effect of Nature and Location of Defects on Band gap Narrowing in Black TiO2 Nanoparticles[END_REF]. The result correlates well with the literature data [START_REF] Lu | Hydrogenated TiO2 nanotube arrays for supercapacitors[END_REF], suggesting that hydrogen reacts with oxygen in TiO2, resulting in defect formation in the system [START_REF] Naldoni | Effect of Nature and Location of Defects on Band gap Narrowing in Black TiO2 Nanoparticles[END_REF].

The valence band (VB) edge positions were determined to study the impact of the hydrogenation process on TiO2 electron band structure. The VB positions of TiO2/SiNW and H-TiO2/SiNW were deduced from the XPS spectra (Figure S4). The comparison between Figure S4a and b reveals a blue-shift of the VB edge from 1.27 to 1.12 eV after hydrogenation treatment in accordance with the literature [START_REF] Mehta | Hydrogen treated anatase TiO2: a new experimental approach and further insights from theory[END_REF][START_REF] Wang | Hydrogen-treated TiO2 nanowire arrays for photo-electrochemical water splitting[END_REF][START_REF] Chen | Enhanced photoelectric performance of PbS/CdS quantum dot co-sensitized solar cells via hydrogenated TiO2 nanorod arrays[END_REF][START_REF] Naldoni | Effect of Nature and Location of Defects on Band gap Narrowing in Black TiO2 Nanoparticles[END_REF]; this could be related to hydrogen reaction with oxygen on TiO2 surface in accordance with XPS results quoted above. Optical band gap was also estimated by using the Kubelka-Munk [K-M or F(R)] method (&SI for details). The determined values from diffuse reflectance spectra revealed that the band gap was blue shifted from 3.03 eV to 2.82 eV when TiO2/SiNW sample was hydrogenated (Figure S5). A comparable shift was found after hydrogenation of TiO2 [START_REF] Mehta | Hydrogen treated anatase TiO2: a new experimental approach and further insights from theory[END_REF][START_REF] Wang | Hydrogen-treated TiO2 nanowire arrays for photo-electrochemical water splitting[END_REF][START_REF] Chen | Enhanced photoelectric performance of PbS/CdS quantum dot co-sensitized solar cells via hydrogenated TiO2 nanorod arrays[END_REF][START_REF] Raghunath | Quantum chemical elucidation of the mechanism for hydrogenation of TiO2 anatase crystals[END_REF][START_REF] Naldoni | Effect of Nature and Location of Defects on Band gap Narrowing in Black TiO2 Nanoparticles[END_REF] and was assigned to the formation of more hydroxyl groups on the H-TiO2 surface.

The structure of H-TiO2/SiNW was studied by transmission electron microscopy (TEM),

Disorder

image b, where a spacing of 3.5 Å between adjacent planes, assigned to anatase TiO2 (101), is clearly visible. This value correlates well with the expected TiO2 interlayer distance [START_REF] Hamdi | Hydrothermal preparation of MoS2/TiO2/Si nanowires composite with enhanced photocatalytic performance under visible light[END_REF]. In addition, a thin amorphous layer near the crystalline surface is also evident, which is in accordance with previous reports on hydrogenated TiO2 [START_REF] Xia | Hydrogenated TiO2 nanocrystals: a novel microwave absorbing material[END_REF][START_REF] Tian | Broad range energy absorption enabled by hydrogenated TiO2 nanosheets: from optical to infrared and microwave[END_REF].

We can see also, in supplementary information (Figure S6), a chemical analysis of the image area of 20 nm TiO2 layer, the concentration is very close to TiO2 (27% Ti -70% O). comprises hydroxyl species ascribed to diffusion of H atoms into the anatase TiO2 [START_REF] Lu | Self-hydrogenated shell promoting photocatalytic H2 evolution on anatase TiO2[END_REF]; this was not evidenced on the non-hydrogenated TiO2/SiNW surface in Figure 7 where the EELS spectra state the same in the shell (Figure 7b) and bulk (Figure 7c). The specific capacitance was determined from the CV plots using equation (1):

= 2 × × (1)
C is the areal capacitance, j denotes the current density, V corresponds to the potential window, r represents the scan rate.

Additionally, the specific capacitance of the samples was determined from the GCD plots using equation ( 2): The specific capacitance values are given in Table 1. The SiNW, TiO2/SiNW and air-TiO2/SiNW electrodes achieved specific capacities of 0.186, 0.489 and 0.265 mF cm -2 , respectively at 0.035 mA cm -2 . These values are comparatively smaller than that recorded for H-TiO2/SiNW electrodes (3.36 mF cm -2 ) under otherwise identical experimental conditions.

The effect of annealing under hydrogen atmosphere is highlighted by the 6-fold enhancement of the capacitance value, in comparison to that of the sample annealed in air at the same temperature i.e. air-TiO2/SiNW electrode. This improvement in energy storage capacity could be ascribed to improved conductivity, increased hydroxyl groups on the TiO2 surface, and/or generation of oxygen vacancies. Comparable results have been reported by several research groups for TiO2 nanotubes thermally annealed under hydrogen atmosphere [START_REF] Lu | Hydrogenated TiO2 nanotube arrays for supercapacitors[END_REF], thermally annealed in air followed by chemical reduction using NaBH4 [START_REF] Zhang | Remarkable supercapacitive performance of TiO2 nanotube arrays by introduction of oxygen vacancies[END_REF], or through electrochemical hydrogen doping by water electrolysis [START_REF] Wu | High-performance and renewable supercapacitors based on TiO2 nanotube array electrodes treated by an electrochemical doping approach[END_REF]. This areal capacitance is much larger than the literature data recorded for rGO-SiNW (240 μF cm -2 at 0.01 mA cm -2 ) [START_REF] Soam | Performance enhancement of microsupercapacitor by coating of graphene on silicon nanowires at room temperature[END_REF], HNO3-oxidized p-type SiNWs (404 μF cm -2 at 50 mV s -1 ) [START_REF] Qiao | Oxide-coated silicon nanowire array capacitor electrodes in room temperature ionic liquid[END_REF] and the best areal capacitance of 386 µF cm -2 recorded for TiO2-coated SiNW array electrode in [Bmim][NTF2] at 3.7 × 10 -5 A [START_REF] Konstantinou | TiO2 coated Si nanowire electrodes for electrochemical double layer capacitors in room temperature ionic liquid[END_REF]. Our result is also better than that recorded for carboncoated SiNW electrode (105 μF cm -2 at 1 V s -1 using cyclic voltammetry) [START_REF] Gao | Diamond-coated silicon wires for supercapacitor applications in ionic liquids[END_REF], Si-NR/TiN electrode (1.08 mF cm -2 at 100 mV cm -2 ) [START_REF] Lu | Nano fabricated silicon nanorod array with titanium nitride coating for onchip supercapacitors[END_REF], and diamond-coated SiNW (1.55 mF cm -2 ) using a cell voltage of 4 V [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]. The value is comparable to that reported for TiO2-coated SiNW using spin coating [START_REF] Zhang | Enhanced micro-supercapacitors in aqueous electrolyte based on Si nanowires coated with TiO2[END_REF]. A comparison table between the works already performed on TiO2 electrode is given in the supplementary information.

The CV profiles of H-TiO2/SiNW electrodes acquired at various scan rates showed nearly rectangular shape (Figure 9a). The improved capacitive behavior and high-rate capability of H-TiO2/SiNW electrodes were proved by the unchanged shape upon changing the scan rate from 20 to 500 mV s -1 . The GCD plots of H-TiO2/SiNW electrodes are depicted in Figure 9b.

All curves displayed an almost ideal triangular shape, suggesting a good capacitive behavior in full accordance with the CV measurements. The areal capacitance values of untreated SiNW, TiO2/SiNW, air-TiO2/SiNW and H-TiO2/SiNW electrodes recorded at different scan rates and current densities are compared respectively in Figure 9c andd. Upon increasing the scan rate and current density, a decline in the capacitance was evident; this behavior might be ascribed to restricted accessibility of parts of the surface to the electrolyte at high scan and charging-discharging rates. In fact, electrolyte ions diffusion into the electrode internal structure and pores is hindered at high scan rate and the interaction between the electrode surface and electrolyte becomes ineffective, thereby, the specific capacitance declines. This contrasts with low scan rates, where ions have sufficient time to diffuse deeply into pores, leading to enhanced specific capacitance. In addition, it is evident that the H-TiO2/SiNW electrode features the highest capacitance followed by TiO2/SiNW and air-TiO2/SiNW; such a behavior can be related to the enhancement of TiO2 electrical conductivity upon hydrogenation [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]. The calculated areal capacitance of H-TiO2/SiNW at a high scan rate (500 mV s -1 ) was 1.56 mF cm -2 , which correlates with a capacitance retention of almost 61% as compared to the value recorded at 20 mV s -1 . The power loss was most likely due to reduced electrical conductivity (0.009-0.01 Ω cm) of the doped silicon substrate particularly at high scan rate [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].

Ag/AgCl

Time (s) 0.035 mA cm -2 0.07 mA cm -2 0.17 mA cm -2 0.35 mA cm -2 0.7 mA cm -2 1.7 mA cm -2 (b) [START_REF] Han | High k nanophase zinc oxide on biomimetic silicon nanotip array as supercapacitors[END_REF] EIS was applied to examine the electrode/electrolyte interface and gain a better understanding on the electrochemical behavior and electrode kinetics. Furthermore, EIS was also commonly applied to assess the capacitive behavior of electrochemical micro-supercapacitors. Using equation ( 3), the imaginary part of the complex capacitance versus frequency can be determined [START_REF] Aradilla | High Performance of Symmetric Micro-Supercapacitors Based on Silicon Nanowires UsingN-Methyl-N-propylpyrrolidiniumbis -(trifluoromethylsulfonyl)imide as Electrolyte[END_REF]:

′′ ( ) = Z ′ (ω) ω|Z(ω)| (3)
where C″(ω) denotes the imaginary part of the capacitance, Z' (ω) corresponds to the real part of the impedance, Z(ω) is the impedance and ω=2πf (f is the frequency).

Figure S7

exhibits the plot of C″ versus frequency for the H-TiO2/SiNW electrode. In addition, the relaxation time constant (τ0), defined as the minimum time necessary to discharge all the energy from a supercapacitor with at least 50% efficiency, can be derived from this spectrum [START_REF] Aradilla | High Performance of Symmetric Micro-Supercapacitors Based on Silicon Nanowires UsingN-Methyl-N-propylpyrrolidiniumbis -(trifluoromethylsulfonyl)imide as Electrolyte[END_REF]. From the curve in Figure S7, a time constant of 2.05 ms was determined using the relation (τ0=1/f0). The rapid frequency response of 0.5 Hz was ascribed to the porous architecture of the electrode, which provides full access to the electrolyte ions, SEM imaging of the H-TiO2/SiNW after the stability test revealed a similar morphology, testifying the high stability of the electrode material under our experimental conditions (data not shown). Also, this could be a good indication that the storage is mainly by EDL mechanism since in this case during the charging/discharging cycles, there is not an alteration of the structure of the electrode at the interface.

Symmetric electrochemical supercapacitor devices

Using a classical 2-electrode symmetric configuration cell (Figure S8), we tested our electrode for its potential application in energy storage devices [START_REF] Balducci | Perspective-A guideline for reporting performance metrics with electrochemical capacitors: From electrode materials to full devices[END_REF]. Both the cathode and anode consisted of H-TiO2/SiNW core-shell electrodes and 1 M Na2SO4 aqueous electrolyte.

Interestingly, the device exhibited nearly rectangular CV plots (Figure 12a) and symmetrical triangular GCD profiles (Figure 12b), indicating the good capacitive performance. Upon increasing the scan rate from 0.02 to 0.1 mV s -1 , the cell capacitance remained at 76.5% (decrease from 2.89 to 2.14 mF cm -2 ), Figure 12c. The specific capacitance was determined from the CV plots using Eq.1 multiplied by a factor 2 (the factor 2 is due to the two symmetric electrodes construction) [START_REF] Chen | High-energydensity nonaqueous MnO2@nanoporous gold based supercapacitors[END_REF]. In the same configuration, as in the case of CV measurements using 1 M Na2SO4, GCD experiments carried out at 0.5 mA cm -2 allowed to determine a specific capacitance of 2.08 mF cm -2 . The energy density was calculated from a two-electrode cell as an operating device using = . . ( ) ,where C and V correspond respectively the specific capacitance and potential window. Further, energy and power densities values of 0.179 µWhcm -2 and 2.13 mWcm -2 were determined from the GCD plots at 0.5 mAcm -2 , respectively.

For comparison, Figure 12d summarizes the literature data on cell energy density and power density. Figure 12e depicts the cyclability test carried out at 0.5 mA cm -2 up to 10,000 cycles, which showed retention of ~84.6% of the initial capacitance. Thus, the results reveal excellent capacitive feature of the H-TiO2/SiNW in this 2-electrode device configuration.

Conclusion

In this paper, we compared electrochemical properties of silicon nanowires (SiNW) covered with a TiO2 thin film. The TiO2 layer was coated on the SiNW using atomic layer deposition (ALD), some samples of TiO2/SiNW were subjected to a hydrogen atmosphere annealing (H-TiO2/ SiNW) and the others in the open air (air-TiO2/SiNW) both at 400 °C for 4 h. The effect of hydrogenation treatment on the specific capacitance of the TiO2/SiNW electrodes was investigated. Our results revealed that hydrogenation improved substantially the energy storage ability of H-TiO2/SiNW as electrode material for electrochemical supercapacitors. Indeed, after hydrogenation, H-TiO2/SiNW featured a specific capacity of 3.36 mF cm -2 at 0.035 mA cm -2 in 1 M Na2SO4, a value 6 times larger than that recorded for air-TiO2/SiNW electrode. The electrode maintained ~81% of its original specific capacitance after complete 30,000 cycles at 0.1 mA cm -2 (potential window: -0.4 -+0.3 V) in 1 M Na2SO4. Furthermore, H-TiO2/SiNW//H-TiO2/SiNW symmetric supercapacitor device was assembled and exhibited energy and power densities of about 0.179 µWhcm -2 and 2.13 mWcm -2 , respectively, determined from the galvanostatic charge/discharge plots at 0.5 mA cm -2 in 1 M Na2SO4 with 84.6% retention of the original capacitance value after 10,000 cycles. The capacitance improvement of H-TiO2/SiNW can be ascribed to an increase of the density of hydroxyl groups and carrier density on TiO2 surface. The results obtained in this work hold great promise for the application of ALD technique and hydrogenation for the design of high-performance electrode materials for energy storage.

Figure 1 :

 1 Figure 1: Cross-section (a,b) and top-view (c,d) SEM micrographs of SiNW arrays coated with 20 nm of TiO2 by ALD.

Figure 2

 2 Figure2displays the two diffractograms that present the same peaks at 2θ angles of 25.3°, 37.84°, 48.07°, 53.95° and 55.10° corresponding respectively to the (101), (044), (200), (105) and (211) diffraction planes. This indicates that the deposited TiO2 phase is anatase (JCPDF No.21-1272) and that the thermal hydrogenation did not impact the TiO2 crystalline structure[START_REF] Lu | Hydrogenated TiO2 nanotube arrays for supercapacitors[END_REF][START_REF] Mehta | Hydrogen treated anatase TiO2: a new experimental approach and further insights from theory[END_REF]. In addition to a slight decrease of the TiO2 peak intensity after hydrogenation at high temperature, a small increase of the Full Width at High Half Maximum (FWHM) was observed, due most likely to increased defect density in TiO2 structure, in accordance with

Figure 2 .

 2 Figure 2. X-ray diffractograms of TiO2/SiNW and H-TiO2/SiNW samples.

Figure 3 .

 3 Figure 3. Raman plots of TiO2/SiNW (blue) and H-TiO2/SiNW (red) samples.

Figure

  Figure S3displays the Ti2p wide XPS scans of TiO2/SiNW and H-TiO2/SiNW. Both comprise peaks at 458.5 and 464.5 eV assigned respectively to Ti2p3/2 and Ti2p1/2, which are consistent with the typical values for TiO2[START_REF] Chen | Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals[END_REF][START_REF] Lazarus | X-ray photoelectron spectroscopy (XPS) studies of hydrogen reduced rutile (TiO2-x) surfaces[END_REF][START_REF] Wang | Hydrogen-treated TiO2 nanowire arrays for photo-electrochemical water splitting[END_REF]. Figure 4a and b depicts the O1s plots of TiO2/SiNW and H-TiO2/SiNW. Both spectra can be deconvoluted with two bands at around 530 eV (Ti-O-Ti species) and 531 eV (-OH groups). The -OH peak intensity in the O1s XPS spectrum increased after hydrogenation treatment, indicating the TiO2 surface generated more

Figure 4 .

 4 Figure 4. O1s XPS high-resolution profiles of (a) TiO2/SiNW and (b) H-TiO2/SiNW surfaces.

Figure 5a .

 5a Figure 5a. An individual silicon nanowire uniformly coated by TiO2 thin film with a diameter of about 130 nm could be evidenced. In Figure5cis exhibited a zoom of selected area of the

Figure 5 .

 5 Figure 5. (a) Scanning transmission electron microscopy (STEM) of an individual H-TiO2/SiNW; HR-TEM micrographs at low (b) and high (c) resolution. The thickness of TiO2 is 20 nm.

Figure 6

 6 Figure 6 depicts a STEM and the corresponding electron energy loss spectroscopy (EELS) plots recorded from the surface of hydrogenated TiO2/SiNW; obvious changes in shape and position, shifting to lower energies, were observed as compared to the EELS acquired in the bulk TiO2 in Figure 6c. This indicates that the surface shell around TiO2

Figure 6 .Figure 7 .

 67 Figure 6. (a) STEM-HAADF image of TiO2/SiNW surface; EELS spectra of (b) the extreme surface (less than 3 nm) and (c) 15 nm below the surface of TiO2.

Figure 8 .

 8 Figure 8. (a) CV plots of different electrodes collected at 100 mV s -1 in a potential range from -0.4 to +0.3V, (b) GCD profiles recorded at 0.035 mA cm -2 , (c) Areal capacitance of different electrodes acquired at 100 mV s -1 .

  dt denotes the time of discharge, represents the potential range, j corresponds to the current density.

Figure 9 .

 9 Figure 9. (a) CV plots of H-TiO2/SiNW acquired at different scan rates (potential range: -0.4 to +0.3 V), (b) GCD plots recorded for H-TiO2/SiNW at different current densities, (c) Areal capacitance of all electrodes versus scan rate, (d) Areal capacitance of all electrodes versus current density.

Figure 10

 10 depicts the Nyquist curves of H-TiO2/SiNW and TiO2/SiNW electrodes in 1M Na2SO4 within 100 mHz -100 kHz frequency range at open circuit potential. The inset corresponds to the enlarged high frequency region for H-TiO2/SiNW electrode. The plot comprises two main features: a single semicircular arc in the high frequency domain (inset) and a straight line in the low frequency region. The diameter of the semicircular in the middle section of Z' axis corresponds to the charge transfer resistance (Rct) of the electrode. Moreover, the straight line (low frequency domain) infers that the charge storage capability of H-TiO2/SiNW electrode is governed by rapid ion diffusion pathway. In addition, EIS analysis allowed to assess the evolution of the resistance before and after hydrogenation; the equivalent circuit is depicted in the inset of Figure 10 (Rct, Rs, Cdl,Wo, and Cl are the charge-transfer resistance, internal resistance, double-layer capacitance, Warburg element, and limit capacitance, respectively). The Rs and Rct values are dramatically reduced from initial 12.21 and 625.82 Ω to 10.47 and 90.13 Ω after hydrogenation, respectively, owing to improved electrical conductivity along with decreased ion diffusion resistance, as evidenced by the steeper line at low frequencies. Nyquist curves of TiO2 and H-TiO2 nanotube electrodes in [56] present the same decreasing trend after hydrogenation. Also, in our results, the high value of Rct for the non-hydrogenated TiO2/SiNW suggests a poor electrical conductivity of TiO2 deposited thin film.

Figure 10 .

 10 Figure 10. Nyquist plot in the 0.1 Hz to 5×10 5 Hz frequency range. Inset displays the enlarged region at high frequency for H-TiO2/SiNW electrode.

11 .

 11 rate of diffusion. This value is lower than that reported for electrochemical micro-supercapacitors based on SiNW electrodes (e.g. values 3.51 ms)[START_REF] Aradilla | High Performance of Symmetric Micro-Supercapacitors Based on Silicon Nanowires UsingN-Methyl-N-propylpyrrolidiniumbis -(trifluoromethylsulfonyl)imide as Electrolyte[END_REF].High-performance electrochemical supercapacitors are commonly characterized by enhanced cycling stability. The H-TiO2/SiNW stability was assessed by GCD for 30,000 cycles. The percentage of capacitance retention versus cycle number is exhibited in Figure The electrode retained ~81% of its original specific capacitance after complete 30,000 cycles at 0.1 mA cm -2 in the -0.4 to +0.3 V potential window in 1 M Na2SO4. The result proved the good stability of the electrode after 30,000 cycles.

Figure 11 .

 11 Figure 11. Life time testing of the H-TiO2/SiNW electrode in an aqueous 1M Na2SO4 electrolyte.

Figure 12 .

 12 Figure 12. (a) CV curves of H-TiO2 /SiNW symmetric electrochemical cell collected at various scan rates (potential window: -0.4 to +0.3 V), (b) GCD profiles of the device charged at 0.5 mA cm -2 , (c) specific capacitance versus scan rate, (d) Ragone plot, (e) Capacitance retention percentage of the H-TiO2 /SiNW symmetric electrochemical supercapacitor for 10,000 cycles at 0.5 mA cm -2 .

Table 1 :

 1 Areal capacitance of the prepared working electrodes in 1 M Na2SO4 deduced from the CV and GCD measurements.

		Technique used to calculate Areal capacitance (mFcm -2 )
	Working electrode	CV	GCD
		Scan rate=100 mVs -1	J= 0.035 mA cm -2
	SiNW	0.137	0.186
	TiO2/SiNW	0.434	0.489
	Air-TiO2/SiNW	0.227	0.265
	H-TiO2/SiNW	2.76	3.36

Acknowledgements

The Centre National de la Recherche Scientifique (CNRS), the University of Lille, and the Hauts-de-France region are acknowledged for financial support. The authors also thank the Taif University Researchers Supporting Project number (TURSP-2020/03), Taif University, Taif, Saudi Arabia.