Mathieu Etienne
email: mathieu.etienne@univ-lorraine.fr

Jose F Vivo-Vilches

Ivan Vakulko

Claire Genois

Liang Liu

Michel Perdicakis

Rolf Hempelmann

Alain Walcarius

Layer-by-Layer modification of graphite felt with MWCNT for vanadium redox flow battery

Keywords: vanadium redox flow battery, layer-by-layer deposition, carbon electrodes, carbon nanotubes, graphite felt

Layer-by-Layer (LbL) deposition of multi-walled carbon nanotubes (MWCNT) was used to modify porous electrodes for vanadium redox flow batteries. Preliminary studies performed over glassy carbon electrodes (GCEs) showed that assemblies obtained with MWCNT displayed better mechanical stability than those prepared with other carbon nanoforms. The LbL process involved successive deposition of a positively-charged polymer (in our case polyethylenimine, PEI) and carbon nanotubes modified in such a way to get their surface negatively-charged, and it was found that the use of MWCNT dispersed with poly(acrylic acid) (PAA) allowed a faster and more homogenous film growth than -COOH functionalized SWCNT. The thickness of the MWCNT assembly deposited onto a GCE strongly influenced the electrochemistry of VO 2+ /VO2 + species. The thicker the film, the better the electrochemical reversibility resulted. An electron transfer rate constant k0 of 5x10 -4 cm s -1 was determined for the electrode prepared with one hundred bi-layers and treated at 650 °C in inert atmosphere. After optimizing the process on GCEs, the LbL technique was applied to the modification of graphite felt electrodes (GFEs). GF electrodes modified with ten bilayers of PEI-PAA(MWCNT) were tested in a vanadium redox flow battery, showing a net decrease in the charge and discharge overpotentials with respect to the unmodified graphite felt. Lower overpotentials for charge-discharge resulted in higher energy and voltage efficiencies for the battery with modified GF electrodes. Furthermore, cyclability of the system was tested and no fading was observed in the battery performance after one hundred cycles.

Introduction

The demand for renewable energy, e.g. electricity produced from sun or wind, is actually increasing significantly with the goal to become less dependent from fossil reserves or nuclear plants. The major drawback of renewable energies is their discontinuous production, varying dramatically with time, which makes the estimation of production unreliable. Moreover, the periods of production do not necessarily fit the periods of consumption [START_REF] Yang | Electrochemical energy storage for green grid[END_REF]. Therefore, the development of renewable energy must be associated to the development of suitable battery technologies displaying low cost, long life-cycle and low maintenance [START_REF] Yang | Electrochemical energy storage for green grid[END_REF][START_REF] Dunn | Electrical energy storage for the grid: a battery of choices[END_REF]. Redox-flow batteries are promising devices to exhibit such features [START_REF] Skyllas-Kazacos | Vanadium redox flow batteries (VRBs) for medium-and large-scale energy storage[END_REF][START_REF] Kear | Development of the all-vanadium redox flow battery for energy storage: a review of technological, financial and policy aspects[END_REF].

Redox-flow batteries (RFB) have the characteristic to dissociate the reservoirs for storage of anolyte and catholyte from the electrochemical cell, allowing rapid charging and discharging of the redox species [START_REF] Palacín | Recent advances in rechargeable battery materials: a chemist's perspective[END_REF][START_REF] Ponce De León | Redox flow cells for energy conversion[END_REF][START_REF] Leung | Progress in redox flow batteries, remaining challenges and their applications in energy storage[END_REF][START_REF] Skyllas-Kazacos | Progress in Flow Battery Research and Development[END_REF]. Main components of the battery are the ion-permeable membrane and the electrodes, these latter being the focus of the present study. Different redox species can be involved in RFB. All-vanadium batteries, the most-successful commercial approach to date, involve only vanadium species in both catholyte and anolyte [START_REF] Skyllas-Kazacos | Vanadium redox flow batteries (VRBs) for medium-and large-scale energy storage[END_REF][START_REF] Kear | Development of the all-vanadium redox flow battery for energy storage: a review of technological, financial and policy aspects[END_REF]. It allows minimizing the risk of irreversible mixing of electrolyte in case of defects/cracks or undesired permeability in the membrane separator. Two redox couples are involved in all-vanadium batteries, respectively V(III)/V(II) and V(V)/V(IV) [START_REF] Skyllas-Kazacos | Progress in Flow Battery Research and Development[END_REF][START_REF] Minke | Materials, system designs and modelling approaches in technoeconomic assessment of all-vanadium redox flow batteries -A review[END_REF][START_REF] Ulaganathan | Recent Advancements in All-Vanadium Redox Flow Batteries[END_REF][START_REF] Park | Nanostructured Electrocatalysts for All-Vanadium Redox Flow Batteries[END_REF][START_REF] Choi | A review of vanadium electrolytes for vanadium redox flow batteries[END_REF]. Studies dealing with the optimization of the electrode material for improving battery performance usually focus on increasing the electron transfer rate of the redox reactions, especially those involved in the anode compartment V(V)/V(IV) which is the kinetic limitation. Such improvement can be reached by increasing the electrode surface area and/or by chemical surface modification [START_REF] Chakrabarti | Application of carbon materials in redox flow batteries[END_REF][14][START_REF] Wu | Boosting vanadium flow battery performance by Nitrogen-doped carbon nanospheres electrocatalyst[END_REF][START_REF] Kim | High electrocatalytic performance of N and O atomic co-functionalized carbon electrodes for vanadium redox flow battery[END_REF]. In this respect, carbon materials have been widely used for RFB optimization [START_REF] Chakrabarti | Application of carbon materials in redox flow batteries[END_REF][14][START_REF] Wu | Boosting vanadium flow battery performance by Nitrogen-doped carbon nanospheres electrocatalyst[END_REF][START_REF] Kim | High electrocatalytic performance of N and O atomic co-functionalized carbon electrodes for vanadium redox flow battery[END_REF][START_REF] Di Blasi | Investigation of several graphite-based electrodes for vanadium redox flow cell[END_REF][START_REF] Rychcik | Evaluation of electrode materials for vanadium redox cell[END_REF].

Carbon clothes or graphite felt, after activation, display interesting properties for electrochemical conversion of vanadium species [START_REF] Sun | Modification of graphite electrode materials for vanadium redox flow battery application-I. Thermal treatment[END_REF][START_REF] Sun | Chemical modification of graphite electrode materials for vanadium redox flow battery application-part II. Acid treatments[END_REF][START_REF] Sun | Chemical modification and electrochemical behaviour of graphite fibre in acidic vanadium solution[END_REF][START_REF] Flox | Highly electrocatalytic flexible nanofiber for improved vanadium-based redox flow battery cathode electrodes[END_REF]. Additional modifications can be implemented, like metal [START_REF] Sun | Chemical modification and electrochemical behaviour of graphite fibre in acidic vanadium solution[END_REF][START_REF] Wang | Investigation of Ir-modified carbon felt as the positive electrode of an all-vanadium redox flow battery[END_REF][START_REF] Li | Bismuth Nanoparticle Decorating Graphite Felt as a High-Performance Electrode for an All-Vanadium Redox Flow Battery[END_REF], metal oxide deposition [START_REF] Busacca | Electrochemical performance investigation of electrospun urchin-like V2O3-CNF composite nanostructure for vanadium redox flow battery[END_REF][START_REF] Di Blasi | Synthesis of flexible electrodes based on electrospun carbon nanofibers with Mn3O4 nanoparticles for vanadium redox flow battery application[END_REF][START_REF] Ma | Electrospun lignin-derived carbon nanofiber mats surface-decorated with MnO2 nanowhiskers as binder-free supercapacitor electrodes with high performance[END_REF] or incorporation of heteroatoms such as oxygen, nitrogen or phosphorous [14- [START_REF] Kim | High electrocatalytic performance of N and O atomic co-functionalized carbon electrodes for vanadium redox flow battery[END_REF][START_REF] Shi | Nitrogen-Doped Graphene:Effects of nitrogen species on the properties of the vanadium redox flow battery[END_REF][START_REF] Wang | Nitrogen-Doped Carbon Nanotube/Graphite Felts as Advanced Electrode Materials for Vanadium Redox Flow Batteries[END_REF][START_REF] Yang | Electrocatalytic activity of nitrogen-doped CNT graphite felt hybrid for all-vanadium redox flow batteries[END_REF][START_REF] He | Phosphorus Doped Multi-Walled Carbon Nanotubes: An Excellent Electrocatalyst for the VO 2+ /VO 2 + Redox Reaction[END_REF][START_REF] Noh | Chelating functional group attached to carbon nanotubes prepared for performance enhancement of vanadium redox flow battery[END_REF][START_REF] Kim | A new strategy for integrating abundant oxygen functional groups into carbon felt electrode for vanadium redox flow batteries[END_REF].

It is already known that carbon nanomaterials, i.e. graphite [START_REF] Sun | Modification of graphite electrode materials for vanadium redox flow battery application-I. Thermal treatment[END_REF][START_REF] Sun | Chemical modification of graphite electrode materials for vanadium redox flow battery application-part II. Acid treatments[END_REF][START_REF] Sun | Chemical modification and electrochemical behaviour of graphite fibre in acidic vanadium solution[END_REF], carbon nanotube [START_REF] Li | Modified multiwalled carbon nanotubes as an electrode reaction catalyst for an all vanadium redox flow battery[END_REF][START_REF] Li | Multi-walled carbon nanotubes used as an electrode reaction catalyst for /VO2+ for a vanadium redox flow battery[END_REF][START_REF] Li | The electrochemical catalytic activity of single-walled carbon nanotubes towards VO2+/VO2+ and V3+/V2+ redox pairs for an all vanadium redox flow battery[END_REF][START_REF] Wei | Carbon felt supported carbon nanotubes catalysts composite electrode for vanadium redox flow battery application[END_REF][START_REF] Jelyani | Treated carbon felt as electrode material in vanadium redox flow batteries: a study of the use of carbon nanotubes as electrocatalyst[END_REF][START_REF] Mustafa | Effects of carbonaceous impurities on the electrochemical activity of multiwalled carbon nanotube electrodes for vanadium redox flow batteries[END_REF], graphene [START_REF] Shi | Nitrogen-Doped Graphene:Effects of nitrogen species on the properties of the vanadium redox flow battery[END_REF][START_REF] Jin | Identifying the active site in nitrogen-doped graphene for the VO2+/VO2(+) redox reaction[END_REF][START_REF] Flox | Active nano-CuPt3 electrocatalyst supported on graphene for enhancing reactions at the cathode in all-vanadium redox flow batteries[END_REF] or graphene oxide [START_REF] González | Thermally reduced graphite and graphene oxides in VRFBs[END_REF][START_REF] González | Graphite oxide-based graphene materials as positive electrodes in vanadium redox flow batteries[END_REF][START_REF] Li | Reduced graphene oxide with tunable C/O ratio and its activity towards vanadium redox pairs for an all vanadium redox flow battery[END_REF], are likely to improve the electrochemical reversibility of vanadium species redox reactions. The developed electroactive surface area and the presence of functionalities such as hydroxyl and carboxyl groups [START_REF] Noh | Chelating functional group attached to carbon nanotubes prepared for performance enhancement of vanadium redox flow battery[END_REF][START_REF] Li | Reduced graphene oxide with tunable C/O ratio and its activity towards vanadium redox pairs for an all vanadium redox flow battery[END_REF] or the sp2 carbon content on the surface are important parameters that can improve the performance of the battery [START_REF] Melke | Carbon materials for the positive electrode in all-vanadium redox flow batteries[END_REF]. Nevertheless, carbon or graphite felt electrodes displaying a large porous volume that facilitates mass transport of the electrolytes into the electrochemical cell are still a key component in redox flow batteries [14,[START_REF] Wang | Nitrogen-Doped Carbon Nanotube/Graphite Felts as Advanced Electrode Materials for Vanadium Redox Flow Batteries[END_REF][START_REF] Wei | Carbon felt supported carbon nanotubes catalysts composite electrode for vanadium redox flow battery application[END_REF][START_REF] Zhou | Critical transport issues for improving the performance of aqueous redox flow batteries[END_REF].

Previous reports in the literature described first attempts to get the combination of the graphite felt with carbon nanotubes [START_REF] Yang | Electrocatalytic activity of nitrogen-doped CNT graphite felt hybrid for all-vanadium redox flow batteries[END_REF][START_REF] Li | Modified multiwalled carbon nanotubes as an electrode reaction catalyst for an all vanadium redox flow battery[END_REF][START_REF] Li | Multi-walled carbon nanotubes used as an electrode reaction catalyst for /VO2+ for a vanadium redox flow battery[END_REF][START_REF] Li | The electrochemical catalytic activity of single-walled carbon nanotubes towards VO2+/VO2+ and V3+/V2+ redox pairs for an all vanadium redox flow battery[END_REF][START_REF] Jelyani | Treated carbon felt as electrode material in vanadium redox flow batteries: a study of the use of carbon nanotubes as electrocatalyst[END_REF][START_REF] Mustafa | Effects of carbonaceous impurities on the electrochemical activity of multiwalled carbon nanotube electrodes for vanadium redox flow batteries[END_REF][START_REF] Munaiah | Comparative Electrocatalytic Performance of Single-Walled and Multiwalled Carbon Nanotubes for Zinc Bromine Redox Flow Batteries[END_REF] or reduced graphene [START_REF] González | Thermally reduced graphite and graphene oxides in VRFBs[END_REF][START_REF] Li | Reduced graphene oxide with tunable C/O ratio and its activity towards vanadium redox pairs for an all vanadium redox flow battery[END_REF]. The method usually involved impregnation of the graphite felt with carbon nanostructures and further drying of the material. One issue here is to reach homogeneous deposition of the carbon nanomaterial over the whole surface of the graphite felt fibers while maintaining its internal porosity in order to benefit from the electrocatalytic properties of these materials without losing the mass transport properties of the felt.

Layer-by-layer (LbL) assembly is a process that utilizes different attractions between deposited species to create thin films of controlled thickness and properties [START_REF] Bertrand | Ultrathin polymer coatings by complexation of polyelectrolytes at interfaces: suitable materials, structure and properties[END_REF][START_REF] Decher | Multilayer Thin Films-Sequential Assembly of Nanocomposite Materials[END_REF]. These films are most often created by alternately exposing positively and negatively charged constituents (solid particles or organic polymers), from dilute aqueous suspensions or solutions, to a charged or polar substrate. Each pair of deposited species (consisting of two individual constituents) is referred to as a bi-layer (BL) and can range in thickness from few nm to more than 100 nm thick depending on chemistry, molecular weight of the component, temperature, counter-ion, ionic strength, and pH [START_REF] Zhang | Interaction of Strong Polyelectrolytes with Surface-Attached Polyelectrolyte Brushes-Polymer Brushes as Substrates for the Layer-by-Layer Deposition of Polyelectrolytes[END_REF][START_REF] Mermut | Effects of Charge Density and Counterions on the Assembly of Polyelectrolyte Multilayers[END_REF][START_REF] Mcaloney | Atomic Force Microscopy Studies of Salt Effects on Polyelectrolyte Multilayer Film Morphology[END_REF][START_REF] Sui | Effect of Molecular Weight on the Construction of Polyelectrolyte Multilayers: Stripping versus Sticking[END_REF].

This process can be used to create functional thin films offering a wide range of applications including optical-to-electric power conversion, magnetic sensing and lithium battery electrolytes [START_REF] Hua | Ultrathin Cantilevers Based on Polymer-Ceramic Nanocomposite Assembled through Layer-by-Layer Adsorption[END_REF][START_REF] Liang | Covalent Layer-by-Layer Assembly of Conjugated Polymers and CdSe Nanoparticles: Multilayer Structure and Photovoltaic Properties[END_REF][START_REF] Delongchamp | Highly ion conductive poly(ethylene oxide)based solid polymer electrolytes from hydrogen bonding layer-by-layer assembly[END_REF]. The LbL process has many advantages over other coating methods, including ease of processing, precise control of thickness and composition of the assembly, and applicability to deposition of rather uniform coatings on surfaces of complex morphologies. For example, LbL was already used by Grunlan and co-workers to prepare conductive carbon-based composite films [START_REF] Jason Jan | Carbon black thin films with tunable resistance and optical transparency[END_REF][START_REF] Walton | Deposition and patterning of conductive carbon black thin films[END_REF] and [carbon nanotubes -polyelectrolyte]-based sensors [START_REF] Kumar | Fine control of carbon nanotubes-polyelectrolyte sensors sensitivity by electrostatic layer by layer assembly (eLbL) for the detection of volatile organic compounds (VOC)[END_REF].

In the present study, we have evaluated the interest of the LbL assembly method to modify carbon-based electrodes with carbon nanomaterials (nanoparticles, nanotubes) that could be used in vanadium redox flow batteries. From preliminary experiments on flat carbon plates, it rapidly came out that the best results were obtained with multi-walled carbon nanotubes (MWCNT) and polymers. This most promising approach was thus exploited to grow layers of MWCNT on the inner surface of a highly porous graphite felt electrode in a controlled manner. Optimization was made step-by-step, on the basis of systematic deposition on flat glassy carbon surface, and after that on the graphite felt electrodes (composed of graphitized carbon fibers). The purpose of this work is to demonstrate the influence of the assembly parameters and the carbon layer characteristics on the electrochemistry of VO 2+ /VO2 + and the related battery performance. Special focus was given to provide a simple method likely to be scalable on large electrode surfaces for possible implementation in operational redox flow batteries.

Experimental section

Reagents

Multi-walled carbon nanotubes (MWCNT, length: 5 µm, diameter: 6-9 nm) were purchased from Sigma-Aldrich. Single-walled carbon nanotubes functionalized with carboxyl groups (SWCNT-COOH) were obtained from Nanolab (MA, USA). Carbon black powder (Ketjenblack-EC 600 JD) was kindly provided by Akzo Chemie, Netherlands. Poly(ethylene imine) (PEI) 50 % (w/v) water solution, poly(acrylic acid) (PAA) 35 % wt. solution in water, sulfuric acid 98 % and vanadium(III) chloride 97 % (VCl3) were purchased from Sigma-Aldrich and vanadium(IV) oxide sulfate hydrate 99.9 % (VOSO4•xH2O) by Alfa Aesar. Glassy carbon plates (SIGRADUR G-plates 1 × 12 × 20 mm) were obtained from HTW Hochtemperatur-Werkstoffe GmbH and graphite felt (GFD 4.6 E.A.) from SGL Group (properties, Table S1).

All solutions were prepared with high purity water (18 MΩ cm) from a Purelab Option water purification system (Elga).

Preparation of solutions and suspensions for LbL deposition

The PEI solution (1 mg mL -1 ) was prepared by dilution of a commercial aqueous solution of PEI (50 % (w/v)), which was then used to form the positively charged layers.

Negatively charged layers were prepared from suspensions of either MWCNT or carbon black powder (CB) in PAA, or using a suspension of SWCNT-COOH. To prepare the PAA-based suspensions, the commercial polymeric solution (35 % wt. in water) was diluted to obtain 100 mL of solution containing 1 mg mL -1 PAA, to which 100 mg of MWCNT or CB were added and dispersed by ultrasonication for 15 min. The suspension of functionalized nanotubes was prepared by dispersion of 100 mg of SWCNT-COOH in 100 mL of water under ultrasonication for 15 min. In case of multiple uses, all suspensions have been sonicated again for 15 min prior to use.

Electrode pre-treatment and Layer-by-Layer deposition

The LbL process involves the adsorption of a first layer of a positively charged material (PEI) onto the electrode surface, a washing step, and the deposition of a negatively charged overlayer (PAA) driven by the favorable electrostatic interactions with the underlying positively charged component, and further washing with water. This allows the sequential growth of multiple bi-layers by alternating the deposition of PEI and PAA.

To optimize the LbL deposition several approaches were tested in the GCE. First attempts were made to disperse various amounts of carbon nanotubes (MWCNT) in a PEI solution [START_REF] González | Graphite oxide-based graphene materials as positive electrodes in vanadium redox flow batteries[END_REF][START_REF] Govindan | Improved Electrochemical Preparation of Nano Porous-Graphene Oxide Edge Like Electrode for Cerium / Vanadium Redox Flow Batteries[END_REF], but it was rapidly observed that the resulting PEI(MWCNT) suspensions were not stable. PEI was thus used alone to deposit the positively charged layers and to enable a better control of the film growth process.

For the deposition of the carbon nanomaterials-based negatively charged layer, different systems were investigated: a suspension containing only SWCNT functionalized with carboxyl groups (SWCNT-COOH); carbon black particles dispersed in poly(acrylic acid) (PAA(CB)); and finally, multi-walled carbon nanotubes dispersed in PAA solution, leading to negatively charged particles (PAA(MWCNT)) as described in the literature [START_REF] González | Graphite oxide-based graphene materials as positive electrodes in vanadium redox flow batteries[END_REF][START_REF] Govindan | Improved Electrochemical Preparation of Nano Porous-Graphene Oxide Edge Like Electrode for Cerium / Vanadium Redox Flow Batteries[END_REF]. Glassy carbon electrodes (GCE) were cleaned and polished using 600 and 1200 grit papers, and then using aqueous slurries of alumina powders (50 nm) on a polishing micro-cloth. These experiments were performed by means of an automatic 3 axis moving system made in the laboratory. The system was programmed to introduce the GCE, alternately in the three suspensions/solutions (PEI suspension, carbon nanomaterial suspension, and water -washing solution). The protocol was repeated up to reach a defined number of bi-layers on the electrode surface and samples were kept in each cup for 1 min. From these experiments with GCE, it was concluded that the most promising system resulted the one based on the assembly of bi-layers made of PEI and PAA(MWCNT), and therefore this protocol was employed to obtain functionalized GF electrodes.

In the case of graphite felts (GF), these electrodes were activated by thermal treatment at 400 °C for 30 hours under air atmosphere before impregnation in order to improve their wettability and conductivity, according to the procedure found in literature [START_REF] Sun | Modification of graphite electrode materials for vanadium redox flow battery application-I. Thermal treatment[END_REF]. The principle of modification was basically the same as for the GCE, however longer contact times between the material and the suspensions/solution were necessary and an additional convection was applied, either by forcing the suspensions/solutions to flow through the felt or by mechanical stirring. The contact time between the GF electrodes and the suspensions was 20 min and it was repeated 10 times. After deposition and washing with water, the samples were slowly dried under air atmosphere by heating from room temperature to 80 °C for 5 hours in an oven. The deposits were then thermally treated at 650 °C under nitrogen atmosphere, for 1 h (GCE) or 2 h (graphite felt electrodes).

Physicochemical Characterization and Battery test

Samples morphology was characterized by scanning electron microscopy (SEM, Hitachi FEG S4800). Measurements of the thickness for the layers deposited on glassy carbon were conducted using the profilometry function of a home-built scanning electrochemical microscope (SECM) operating with a shear force control of the tip-to-sample distance [START_REF] Etienne | SECM-based automate equipped with a shearforce detection for the characterization of large and complex samples[END_REF]). Xray photoelectron spectroscopy (XPS) analysis on graphite felt samples was performed with a KRATOS AXIS Ultra DLD.

Cyclic voltammetry experiments were performed with a SP 150 Biologic potentiostat (Grenoble, France) in a three-electrode cell containing a 0.15 M VOSO4 solution in H2SO4 3 M as electrolyte, the modified/unmodified carbon material (glassy carbon or graphite felt) as working electrode, an Ag/AgCl as reference electrode and a carbon rod as counter-electrode.

The quantitative analysis of cyclic voltammetric curves recorded at various scan rates (from 10 to 200 mV s -1 ) allows to estimate the electron transfer rate constant, k0, on the basis of Randles-Sevcik (Equation S1) and Butler-Volmer equations (Equations S2 and S3) [START_REF] Bard | Electrochemical methods: fundamentals and applications[END_REF][START_REF] Sum | Skyllas-Kazacos, A study of the V(II)/V(III) redox couple for redox flow cell applications[END_REF].

Battery tests and Electrochemical Impedance Spectroscopy (EIS) were performed using a home-designed redox flow battery (Figure S1) with cavities for electrodes of 0.4 cm × 2 cm × 7 cm (14 cm 2 of apparent surface area). Both activated graphite felt and MWCNT functionalized graphite felt were tested as electrodes. For EIS measurements, the system was immersed in 3 M H2SO4, and the cell was closed. The measurements were carried out at open circuit potential (OCP) in a frequency range from 0.1 Hz to 100 kHz with a sinusoidal potential amplitude of 10 mV.

In the case of charge-discharge experiments the system was fed with the electrolyte from two recipients containing 20 mL a 50/50 stoichiometric mixture of V(III) and V(IV) at 1.5 M prepared by diluting the appropriated amounts of VCl3 and VOSO4 in 3 M H2SO4 which was recirculated at a flow rate of 20 mL min -1 . Nafion® 115 (purchased from Ion Power, Germany) was used as the ion-exchange membrane. Polarization curves were obtained up to 1.2 V at a scan rate of 5 mV s -1 before charge-discharge experiments. Then several charge-discharge cycles at several current densities (20, 50 and 80 mA cm -2 ) were performed, followed by one hundred charge-discharge cycles at 20 mA cm -2 in order to test the cyclability of the system.

Results and discussion

Physico-chemical characterization.

SEM micrographs were recorded respectively before and after modification, to confirm the successful deposition of PEI-PAA(MWCNT) bi-layers in the felt electrode. Figure 1 depicts a view of one fiber of the felt (A) before modification and (B) after coverage with 10 PEI-PAA(MWCNT) bi-layers and pyrolysis. These images show successful deposition of MWCNT in the form of a quite uniform layer onto the carbon fiber (Figure 1B). The quantity of deposited materials was however found larger at the periphery of the felt (Figure 1C) than on the fibers situated in the center of the electrode (Figure 1D). Even if the thickness of the MWCNT layer is difficult to estimate, the defects/cracks induced during SEM sample preparation enable ones to conclude that deposits onto the felt surface are thinner (< 1 µm) than on GCE (1.7 µm for 10 bi-layers, Figure S3). Deposition of MWCNT was thus less effective on graphite felt than on the glassy carbon, because of the restricted motion of the slurries used for LbL deposition into the porous carbon electrode.

XPS analyses were performed on the pristine graphite felt (curve a, Figure 2A), air activated graphite felt at 400 °C for 30 hours (curve b, Figure 2A), and functionalized graphite felt after deposition of 10 bi-layers of PEI-PAA(MWCNT) and pyrolysis (curve c, Figure 2A).

The corresponding surface chemical compositions are presented in Table 1 (deconvolution of

C1s signal is provided in Figure S4 of the Supplementary Material). In agreement with the literature [START_REF] Sun | Modification of graphite electrode materials for vanadium redox flow battery application-I. Thermal treatment[END_REF], the oxygen content at the surface of the graphite felt was increased (from 3.0 to 5.7 %) after thermal activation, due to the formation of oxidized groups on the carbon surface.

The deposition of 10 bi-layers of MWCNT, and their subsequent pyrolysis at 650 °C, results also in slightly higher oxygen content (6.0 %) and the presence of an additional nitrogen contribution (2.0 %) is also evidenced, originated from PEIs moieties. A magnification of the N1s signal on the sample modified with MWCNT is shown in Figure 2B. These two signals can be attributed to pyridinic N (398.2 eV) and pyrrolic N (400.1 eV) and no graphitic N (401.3 eV) was detected, in good agreement with results reported elsewhere [START_REF] Wang | Nitrogen-Doped Carbon Nanotube/Graphite Felts as Advanced Electrode Materials for Vanadium Redox Flow Batteries[END_REF]. The carbon type was also dramatically modified by the deposition of MWCNT as illustrated by the variation of the sp 2 content (Table 1), from 62.5% for the thermally activated graphite felt to more than 73 % after MWCNT deposition. This is consistent with the fact that the sp 2 -C content correlates with the conductivity of the carbon, as electrons are transported along the delocalized p-orbitals [START_REF] Melke | Carbon materials for the positive electrode in all-vanadium redox flow batteries[END_REF], contributing to explain better performance of the MWCNT modified felt electrodes.

Electrochemistry of vanadium species on GCE electrodes.

After characterizing the surface chemistry and morphology of materials, their electrocatalytic performance for vanadium electrochemistry was evaluated by means of Cyclic Voltammetry. When comparing results for un-modified and modified GCEs, it can be observed that in the absence of MWCNT, the electrochemical response was characterized by a low electron transfer rate as evidenced by a peak potential difference close to 0.4 V (curve a, Figure 3A). The number of bi-layers has also a strong influence on the electrochemical behavior of the electrode after their pyrolysis (Figure 3). The deposition of 5 and 10 bi-layers (BL) led to a dramatic decrease of Ep, i.e., down to 178 mV for 10 BL (curve b, Figure 3A). A further increase on the number of BL led then to a continuous, yet slower, decrease of Ep values (Figure 3B), down to 110 mV for 100 BL (curve c, Figure 3A). Thus, it was decided that 10 BL were proper for the GFE functionalization, since a further deposition could lead to restrictions in the mobility of the electrolyte along the porosity of the material.

It is also important to remark the needing of the thermal treatment, since a poor electrochemical response was observed in the presence of 0.15 M VO 2+ in 3 M H2SO4 after Lb-L deposition (curve a, Figure 4A). The presence of polyelectrolytes will reduce the conductivity of glassy carbon surface so it is necessary to treat it thermally in order to graphitize and stabilize the film. Treatments have been performed under nitrogen atmosphere at various temperatures ranging from 450 to 850 °C for 1 hour. As illustrated for the film treated at 650 °C (see curve b in Figure 4A), well defined redox peaks can be obtained for the VO 2+ /VO2 + redox system. Actually, one can qualitatively analyze the efficiency of the thermal treatment on the electron transfer kinetics via a measurement of the anodic-to-cathodic peak potentials separation (Ep). As shown on Figure 4B, the lowest Ep value was obtained for 650 °C, which is thus the optimal temperature ensuring the effective graphitization of the organic polymers, ensuring thereby an efficient electron transfer between vanadyl species and the electrode.

Higher temperature led to the degradation of the MWCNT assembly, while a lower temperature was not sufficient to convert the polymer from the film into carbon. For instance, a treatment at 850 °C in these conditions led to the complete degradation of the MWCNT layer and no electrochemical reaction could be observed (see curve c in Figure 4A).

The critical influence of MWCNT on the electrochemical response to vanadium species has been largely discussed in the literature [START_REF] Wang | Nitrogen-Doped Carbon Nanotube/Graphite Felts as Advanced Electrode Materials for Vanadium Redox Flow Batteries[END_REF][START_REF] Li | Modified multiwalled carbon nanotubes as an electrode reaction catalyst for an all vanadium redox flow battery[END_REF][START_REF] Li | Multi-walled carbon nanotubes used as an electrode reaction catalyst for /VO2+ for a vanadium redox flow battery[END_REF][START_REF] Wei | Carbon felt supported carbon nanotubes catalysts composite electrode for vanadium redox flow battery application[END_REF], however we only found one work resulting in quantitative analysis of this electrochemical reaction [START_REF] Friedl | Electron transfer kinetics of the -Reaction on multi-walled carbon nanotubes[END_REF]. The quantitative analysis of cyclic voltammetric curves recorded at various scan rates allows to estimate the electron transfer rate constant, k0, on the basis of Randles-Sevcik and Butler-Volmer equations [START_REF] Bard | Electrochemical methods: fundamentals and applications[END_REF][START_REF] Sum | Skyllas-Kazacos, A study of the V(II)/V(III) redox couple for redox flow cell applications[END_REF].

A value of k0 = 5x10 -4 cm s -1 was obtained for the sample prepared with 100 BL, from CV curves recorded at various scan rates and corresponding plots of peak currents, ip, versus square root of scan rate and ln(ip) versus potential (see details in Supplementary Material, Figures S5-7 and associated discussion). This value is in the same range of magnitude as or higher than kinetic data reported for electrochemistry of vanadium species with different carbon materials [START_REF] Chakrabarti | Application of carbon materials in redox flow batteries[END_REF][START_REF] Wu | Acceleration of the redox kinetics of VO2+/VO2 + and V3+/V2+ couples on carbon paper[END_REF][START_REF] Yamamura | Electron-Transfer Kinetics of Np3 + ∕ Np4 + , NpO2 + ∕ NpO22 + , V2 + ∕ V3 + , and VO2 + ∕ VO2 + at Carbon Electrodes[END_REF][START_REF] Wang | Temperature-related reaction kinetics of the vanadium( iv )/( v ) redox couple in acidic solutions[END_REF]. All these results point out the interest of applying LbL assembly for the fabrication of active electrode surface likely to be used in vanadium redox flow battery application.

Electrochemistry of vanadium species on GFE electrodes.

After demonstrating the improvement on the VO 2+ /VO2 + electrochemistry by the LbL functionalization, as well as, optimizing the pyrolysis temperature and the number of BL for GCEs modification with PEI-PAA(MWCNT), the cyclic voltammograms were obtained for GF based electrodes. When results for air activated graphite felt are compared with the one for functionalized graphite felt, a considerable improvement on electrode performance can be observed (Figure 5). The graphite felt electrode activated at 400 °C for 30 hours under air atmosphere showed a peak separation E=0.38V between oxidation and reduction signals (curve a), while for the one modified with 10 bi-layers of MWCNT, this potential separation decreased down to 0.26 V. Moreover, the cathodic current recorded on reversal scan increased significantly from -51 mA to -86 mA (curve b). All these features indicate that the modification of the graphite felt with MWCNT contributes to enhance the electron transfer rates of the VO 2+ /VO2 + redox couple, and therefore, a better performance of these materials as electrodes for the battery is expected. Consistently with observation made on GCE (Figure 4B), a thermal at 750 °C led to a degradation of the electrochemical performances, as it is illustrated in curve c of Figure 5 that shows that the peak separation was increased up to E=0.68 V.

Vanadium redox flow battery.

The functionalized material was finally tested in a battery setup designed in our laboratory (see Figure S1 in the Supplementary Material). We first performed impedance characterization of the battery (Figure 6A) in the presence of activated graphite felt (curve a) and in the presence of graphite felt modified by MWCNT in 3 M H2SO4. The fitting results are provided in Supplementary Materials (Figure S8, Table S2). At high frequency, the DC resistance of these two systems was not significantly different, indicating that the incorporation of MWCNT did not significantly alter the electronic conductivity of the graphite felt. As compared with the unmodified graphite felt, the MWCNT loaded felt showed much lower Rct values, suggesting that the electroactive surface area of the felt was enhanced. Besides, a capacitive element was observed in the MWCNT-loaded felt, suggesting that the LbL modification might make the graphite felt suitable as capacitive material.

The influence of the MWCNT on the electrochemical response was found critical in terms of improving the battery performance. Figure 6B shows the polarization curves in the presence of 1.5 M V(III)/V(IV) electrolyte with batteries filled with (a) the original graphite felt electrode or (b) the MWCNT-modified one. The current started to increase in the presence of MWCNT at much lower potential than in their absence, the difference being larger than 800 mV at current densities below 10 mA cm -2 .

Charge-discharge experiments demonstrated the effectiveness of the LbL functionalization of GF. As depicted in the previous section, the potential for the V(IV) oxidation reaction to occur was lower in the case of MWCNT functionalized sample, as well as the difference between the anodic and the cathodic peak potential (Figure 5). This is clearly reflected in the charge-discharge experiments (Figure 7) performed at low current density tested (20 mA cm -2 ). With activated graphite felt, the initial charging potential was closed to 1.4 V (curve a of Figure 7A). The process started at much lower potential with MWCNT modified graphite felt, between 0.8 and 0.9 V (curve b of Figure 7A). This difference is ascribed not only to the more efficient oxidation of V(IV) to V(V), but also to the faster reduction of the vanadium species on the modified electrode than on the unmodified electrode. After 1 hours, the potential of both batteries became similar, but after several cycles, the advantage of the modified graphite felt becomes obvious (Figure 7A). Figure 7B reports the efficiencies of the two batteries, with graphite felt (dashed line) or with MWCNT modified graphite felt (continuous). The major difference is observed in the voltage efficiency (blue lines) that is improved by more than five percent in the presence of MWCNT. A small difference in coulombic efficiency (green lines) was also observed in these conditions, leading to an improvement of the energy efficiency (red lines).

Figure 8A reports the long stability of the modified electrode while charging/discharging at 20, 50 and 80 mA cm -2 . The battery was first tested during 39 cycles, showing an energy efficiency of 83 % at 20 mA cm -2 , 73 % at 50 mA cm -2 and 62 at 80 mA cm -2 . At cycle 39, a polarization curve was performed and the experiment was reinitialized. This polarization curve reported in Figure 6B (curve c) is showing that the current was lower after 39 cycles than at the very beginning of the experiment (curve b of Figure 6B), but still significantly higher than the current measured with graphite felt in the battery cell (curve a of Figure 6B). This decrease in the current measured in the polarization curve could be due to small degradation of the electrode or by the state of charge of the electrolyte. The chargingdischarging cycles were then reinitialized, with using slightly lower limits for the charge and the discharge processes (Figure 7A), leading to stable energy efficiency of 85 % at 20 mA cm - 2 over 100 cycles.

Finally, we compared the efficiency of the battery with activated GF as electrodes over more than 100 cycles, at 20, 50 and 80 mA cm -2 (Figure S9). The coulombic efficiencies were similar for both electrodes, however, the lower potential needed to start the charging for the functionalized electrode and the shorter difference between charging and discharging onset potentials led to much higher energy voltage efficiencies for this electrode compared to activated GF (Figure S9B). This difference increases as the current density does being nearly 8 % at 20 mA cm -2 , more than 20 % at 50 mA cm -2 , and around 30 % at 80 mA cm -2 (Figure S9B). These results confirmed that the LbL protocol depicted in this paper is an excellent procedure to obtain functionalized GF based electrodes.

Conclusions

Layer-by-layer assembly can be applied for modification of carbon electrodes with MWCNT in order to enhance the electron transfer rates of redox species involved in allvanadium redox flow batteries. The polyelectrolyte used to assemble MWCNT needs to be converted into carbon by pyrolysis under nitrogen atmosphere prior to use. The temperature of treatment strongly influenced the electrochemical behavior of vanadium species and an optimum at 650 °C was found. Increasing the number of bi-layers improved significantly the reversibility of VO 2+ /VO2 + redox couple, especially up to 10 BLs. An electron transfer rate constant of 5x10 -4 cm s -1 was determined in the best conditions. After optimizing the conditions on a system where the functionalization process was faster (glassy carbon electrodes), the method was successfully applied to the modification of porous graphite felt electrodes. XPS analysis after MWCNT deposition shows a significant increase in the sp 2 carbon proportion on the surface and the presence of O and N groups from MWCNT, polyethyleneimine and polyacrylic acid.

Modified graphite felt electrodes showed a better electrochemical performance on vanadium electrolyte, needing a lower potential for the oxidation reaction of V(IV) to V(V) and lowering the difference between the cathodic and the anodic peaks as well as increasing their intensities. This was also reflected in the battery tests; modified GF electrodes presented higher energy and voltage efficiencies and this performance was not lost after several charge-discharge cycles. Therefore, this work demonstrates the benefits of layer-by-layer functionalization of graphite felt electrodes with PEI-PAA(MWCNT) for all-vanadium redox flow batteries.

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure 2. (A) XPS spectra of (a) the pristine graphite felt, (b) the activated graphite felt and (c)

Figure 3 .

 3 Figure 3. (A) Cyclic voltammograms recorded at 5 mV•s -1 in presence of solution containing

Figure 4 .

 4 Figure 4. (A) Cyclic voltammograms recorded in the same conditions as in Fig. 3 using GCE

Figure 5 .

 5 Figure 5. (A) Cyclic voltammograms recorded in the same conditions as in Fig. 3, using (a) an

Figure 6 .

 6 Figure 6. (A) Electrochemical Impedance Spectrocopy characterization in 3 M H2SO4 of the

Figure 7 .

 7 Figure 7. (A) Charge-discharge experiments at 20 mA cm -2 with (a) graphite felt and (b)

Figure 8 .

 8 Figure 8. (A) Charge-discharge experiments at 20, 50 and 80 mA cm -2 using MWCNT

Figure 2 Binding

 2 Figure 2

Table 1 -

 1 Atomic concentration and carbon type (relative amounts expressed in %) of various graphite felt samples, as obtained from XPS analyses.

	Sample	Atomic concentration (%) C O N	Carbon type (%) C sp 2 C sp 3	C-O
	(a) Pristine GF	97.0	3.0	-	68.1	22.6	9.4
	(b) Activated GF	94.3	5.7	-	62.5	27.2	10.3
	(c) Activated GF modified with 10
	PEI-PAA(MWCNT) bi-layers and	92.0	6.0	2.0	73.3	19.3	7.4
	pyrolysed at 650 °C under N2

Acknowledgements

This work has been financially supported by the UE-FEDER Interreg IV-A program (RFB-Solar project). We acknowledge Lisa Salsi for SEM imaging (SCMEM, Université de Lorraine), Aurélien Renard for XPS measurements and Jean-Paul Moulin for the help in construction of the different accessories for layer-by-layer deposition and the battery cells.


Isabelle Sancho

Isabelle Sancho Encapsulating

Yi Yulgok'

Yi Yulgok

Anne Cheng

    Encapsulating Neo-Confucianism and the Great Learning in 16th century Korea: Yi Yulgok's Outline of the Sagely Learning, Sŏnghak chipyo (1575)

which still stands as a noteworthy achievement of the Korean reappraisal of Neo-Confucianism: the Outline of the Sagely Learning, Sŏnghak chibyo 聖學輯要, written and presented to king Sŏnjo 宣祖 (1568-1608) in 1575 by Yulgok 栗谷, Yi I 李珥 (1536-1584), a major icon of Korean Neo-Confucianism and national history. 3 It was later used as the reference for Classics Mat lectures (kyŏngyŏn 經筵) in the Chosŏn court as from 1680, and before the modern era was officially reprinted in 1744, 1749 and 1759. This renowned text, considered as being a genuine Korean Classic,4 might nowadays also be regarded as encapsulating many characteristics of the Korean reception of Neo-Confucianism.

Which Neo-Confucianism? Which Great Learning?

Popular Korean scholarship generally argues that the Neo-Confucianism introduced into Korea was Song-China orthodox Cheng/Zhu Learning, mainly referred to in Korea as xinglixue/sŏngnihak 性理學 or Daoxue/Tohak 道學. Such a position is certainly not wrong as far as the course of Chosŏn history as a whole is concerned, but should nevertheless be strongly nuanced when considering the first half of Chosŏn -as has been done by John Duncan for example. 5 It is in fact rather biased and tends to overlook the importance of the Yuan dynasty in both Chinese and Korean history. From a historical standpoint, the reception of Neo-Confucianism in Korea is much indebted to what might tentatively be called the Yuan version of Song Neo-Confucianism, and which has been studied extensively by William Theodore de Bary. 6 Generally speaking, Yuan Neo-Confucianism could be summed up as having been created by Xu Heng 許衡 (1209-1281), Kubilai's teacher, while its main feature is a strong form of didacticism meant to 'sinicize' the Mongol dynasty. Xu Heng was the spiritual heir of Zhen Dexiu 眞德秀 (1178-1235) of the Southern Song period, who had formalized the so-called Learning of the Sovereign (dixue/chehak 帝學), centered on one text, the Great Learning, and one notion, 'straightening one's heart' (zhengxin/chŏngsim 正心).

Zhen's most famous work is certainly his exegesis of the Great Learning, the Extended Meaning of the Great Learning (Daxue yanyi 大學衍義). This work was one of the most important texts read, in Mongol translations, by Mongol emperors, their counselors and part of the bureaucracy, from the 1320's onwards in Yuan China. It thus enjoyed the status of a canonical text, without however being part of the orthodox curriculum.

According to Chi Tuhwan 7 the Daxue yanyi most probably reached Korea in the fourteenth century. It particularly caught the attention of the reformist scholars of late Koryŏ.

Two allusions to the text can be found in the main primary sources concerning Koryŏwritten and compiled in early Chosŏn. The Koryŏsa 高麗史 and the Koryŏsa chŏryo 高麗史 節要 both record that Yun T'aek 尹澤 (1289-1370) urged king Kongmin 恭愍 (r. 1352-1374) to read the Daxue yanyi during a Classics Mat lecture in 1357. 8 The Koryŏsa also states that Yun Sojong 尹紹宗 (1345-1393) exhorted king Kongyang 恭讓 (r. 1389-1392) to give priority to the study of the Daxue yanyi instead of the Zhenguan zhengyao 貞觀政要. 9 What's more, evidence suggests that the future king T'aejo 太祖 (r.1392-1398) and his son T'aejong 太宗 (r.1400-1418) used to read the text in their youth. 10 It can therefore be reasonably argued that, just like in the Mongol court, Zhen Dexiu's work was considered during the transition from Koryŏ to Chosŏn as a major reference for the education of Korean monarchs.

The Great Learning in early Chosŏn

After the new dynasty was founded, the Yi kings continued to use the Daxue yanyi as , vol.10 (1993), Hallim taehakkyo taedong kojŏn yŏnguso 翰林大學校 東古典 究所, p.341. 8 Cf. Koryŏsa, kwŏn 106, yŏlchŏn 列傳 19, yunhae yunt'aek 尹諧尹澤, and Koryŏsa chŏryo, kwŏn 26, Kongmin wang 1 恭愍王一, the 6th year of the reign (1357). 9 Cf. Koryŏsa,kwŏn 120,yŏlchŏn 列傳 33,quoted in Chi Tuhwan,op.cit.,note 4 p.340. 10 Cf. their respective biographies in the Annals of Chosŏn, the Chosŏn wangjo sillok 朝鮮王祖實錄. 11 This research was carried out in an unpublished doctoral dissertation: Isabelle SANCHO, Chŏngsim 正心 ("rendre droit son coeur"), une notion cardinale de l'interprétation coréenne du néo-confucianisme. Étude centrée sur Yulgok Yi I (1536-1584), Inalco, Paris, 2006. Xi's exegesis, they were either rather general didactic materials for beginners, or free personal commentaries on the Neo-Confucian corpus or the Confucian Classics taken as a whole.

Kings

Length of reign

Daxue

Secondly, because of this strong bias towards Zhen Dexiu's work, Korean scholars, who were in majority high officials, mainly used the Great Learning during Classics Mat lectures or when they were discussing political matters. For them, the Great Learning dealt with statecraft and kingship. To sum up, from the fourteenth to the sixteenth century, the Great Learning was basically read at court in Zhen Dexiu's version, which was used as the reference book for a state ideology patterned on Yuan Neo-Confucian orthodoxy.

What was at stake?

We can therefore say that the Daxue yanyi and the Daxue's paradigm were part of repeated efforts to build up a strong state under the new Yi dynasty. its so-called 'Supplement', the Daxue yanyibu 大學衍義補 19 -were regularly part of the cultural items circulating between Ming China and Chosŏn Korea through diplomatic missions. 20 In so doing, the Chosŏn and Ming courts in all likelihood asserted their common ideological grounds. In Korea, these texts were particularly studied and highlighted by strong kings who used Neo-Confucian ideology to reinforce their personal authority. For T'aejong,

Sejong 世宗 (1419-1450) and Chungjong 中宗 (1506-1544) for instance, texts related to Zhen Dexiu's version of the Great Learning were subject to royal patronage. They regularly ordered official printings 21 as well as précis or didactic commentaries on them. 22 They also publicly gave selected extracts to their high officials and to the royal family and in-laws. 23 They were thus simply acting -or even performing -like the ideal wise rulers praised by the Great Learning, that is to say the supreme educators of the state.

However, resorting to the Great Learning was also part of the strategies used by Korean scholar-officials to secure and reinforce their own position in this apparently monarch-centered political scene. By arguing that Confucian kingship was the basis for an ideal state and society, Korean scholar-officials paradoxically developed a very specific rhetoric about the balance of power, in which they portrayed themselves as being equal to kings. It is worth underlining that one main difference between Korean and Chinese exegeses 19 See below. 20 In 1401 (T'aejong 1, 12/9), emissaries sent to the Ming court brought back, among other books, the Daxue yanyi. In 1403 and 1406 (T'aejong 3, 10/27; T'aejong 6, 12/22), Chinese emissaries who came on a diplomatic mission presented the king with an edition of the Daxue yanyi. In 1494 (Sŏngjong 25, 1/7), emissaries sent to China, amongst whom An Ch'im 安琛 (1445-1515), brought back an edition of the Daxye yanyibu by Qiu Jun. 21 In 1411 (T'aejong 11, 5/20), the king gave orders to print the Daxue yanyi. This edition was presented to the throne a year later, in 1412 (T'aejong 12, 10/1). In 1472 (Sŏngjong 3, 4/16), the king gave orders to print the Taehak yŏnŭi chimnyak 大學衍義輯略 just presented by Yi Sŏkhyŏng and alii. In 1494 (Sŏngjong 25, 1/7), the king gave orders to print the Daxue yanyibu. 22 In 1409 (T'aejong 9, 9/4), the king rebuked Kim Kwa 金科 who was in charge of writing a summary of the Daxue yanyi for the crown prince. In 1409 (T'aejong 9, 9/17), the king commanded a summary of the Daxue yanyi to be written and ordered the civil servants to learn it by heart. In 1451 (Munjong 1, 7/12), the king commanded a summary of the Daxue yanyi to be written in order to circulate it amongst the civil servants and the royal family. 23 In 1394 (T'aejo 3, 5/23), the king ordered the civil servants to study the Great learning. In 1403 (T'aejong 3, 5/21), the king commanded the preface and the address to the throne of the Daxue yanyi to be set on a folding screen in order to display it in one of his studies. In 1408 (T'aejong 8, 10/1), the king gave the crown prince one volume of the Daxue yanyi. In 1409 (T'aejong 9, 3/25), the king commanded the Daxue yanyi to be explained to the crown prince during lessons. In 1411 (T'aejong 11, 12/15), the king commanded excerpts of the Daxue yanyi to be written on the palace walls for the edification of all of his ministers. In 1418 (T'aejong 18, 6/29), the king ordered the crown prince Yangnyŏng taegun 讓寧大君, then in exile in Kwangju because of his scandalous behavior, to read the Analects and the Daxue yanyi. In 1424 (Sejong 6, 2/14), the king gave orders to print and circulate about fifty copies of the Daxue yanyi among his ministers. In 1434 (Sejong 16, 9/5), the king gave orders to circulate the Daxue yanyi among his ministers and the royal family. In 1451 (Munjong 1, 7/12), the king ordered the scholars-officials as well as the royal family to study the Daxye yanyi. In 1516 (Chungjong 12, 8/30), the king declared that he would like to further encourage the study of both the Elementary learning and Great learning throughout the country. In 1533 (Chungjong 28, 4/13), the king commanded ten copies of the Great Learning to be circulated in the royal apartments.

of the Daxue and the Daxue yanyi lies in the recurring and insistent stress in Korea on 'rectifying the king's mind' (zheng junxin/chŏng kunsim 正君心) -the core idea of the Learning of the Sovereign (dixue/chehak 帝學). Generally speaking, from the fourteenth to the sixteenth century, Korean scholars understood this idea as being deeply connected to the good-hearted interaction between kings and Confucian scholar-officials, indispensable for extricating themselves from a rather conflicting relationship. 24 These conflicts were notably illustrated by the infamously traumatic series of 'literati purges, ' sahwa 士禍 (1498, 1504, 1519, 1545, 1547). The locus classicus, 'Rectifying the king's mind, which enables one to rectify the court's mind and civilize the country' taken from the Hanshu 漢書, 25 was basically understood as being the motto, and even the raison d'être, of Korean Neo-Confucian scholarofficials. Therefore, although according to dixue/chehak ideology the civilizing process (jiaohua/kyohwa 教化) was theoretically in the hands of the king, the scholar-officials believed the aristocracy, that is to say themselves, to be the real driving force of the project.

The crème de la crème of these elites happened to sometimes fulfill that duty, guided by the very text of the Great Learning, seeing to the implementation of correct kingship while holding important positions at court. Some of them also tried to work for the people's sake when they lived as retired scholars in the countryside. However, to put things in a nutshell, by hammering home the idea that only correct interplay between the king and his elites could provide Korean kingship with moral legitimacy, Chosŏn scholar-officials actually legitimated their own social status and political predominance.

A significant difference with China

A display different strategies at work in the philosophical exegeses of high officials addressing themselves to kings who were also their younger pupils. 36 Yi Sŏkhyŏng and Yulgok's works are circumstantial texts, designed for strong didactic use and not necessarily meant to be transmitted to posterity. They stem from the actual practice of counseling and educating kings in daily audiences and Classics Mat lectures, illustrating the very close relationship between the kings and their Neo-Confucian advisors. So it can be reasonably argued that these two texts reflect a strong particularity of the Korean reappraisal of Neo-Confucianism: the focus on a collegial power as well as the close relationship between kings and ministers. However, the Taehak yŏnŭi chimnyak, the purpose of which was to revise and adapt Zhen Dexiu's work to the Korean context, was quickly criticized and considered as being outdated. On the contrary, the Sŏnghak chibyo is still regarded nowadays as one of the most important works of Korean Neo-Confucianism. One reason is certainly that, beyond differences related to their respective historical contexts, these two works had radically different scopes and goals. In the peritexts of the Sŏnghak chibyo, Yulgok is rather explicit about his aim. He notably gives in his preface a negative definition of his project by criticizing Zhen Dexiu himself. According to him, the Daxue yanyi is too wordy and one loses sight of the essentials:

According to Your humble servant, the Way is subtle and shapeless; it takes a visible shape thanks to written works. The Four Books and the Six Classics have completely brought it to light. If one relied on these writings to search for the Way, nothing concerning the Principle would be concealed anymore. Yet one cannot help feeling concerned that the whole amount of these texts represents a vast ocean spread out as far as the eye can see where the guiding principles are difficult to find. Previous upright men have extolled the virtues of the Great Learning to establish rules and principles. The hundreds of thousands of lessons and recommendations by the Sages and Worthies don't go beyond this text that expounds the method of finding guiding principles. Sir Zhen, Xishan, has extended and enlarged it in his Extended Meaning [of the Great Learning]. He selected many quotations from the Classics and their commentaries and he compiled excerpts from diverse historical books. He has clearly shown that, concerning the 'root of Learning' as well as the 'order of governance,' there are steps to follow, and that full importance should be accorded to he who embodies the royal function. These are most certainly the guidelines left by emperors and kings for entering the path to Learning. However his work is divided into so many rolls and his phraseology makes one drip with such heavy sweat that it finally looks like a tedious list of facts, which is contrary to the very principle of practice-oriented Learning. The work is indubitably appealing but it fails at attaining perfection. 37

The main characteristics of Yi Yulgok's Outline of the Sagely Learning

Contrary to Zhen Dexiu and Yi Sŏkhyŏng who both focused on historical examples to discuss and explain the Great Learning (thus giving equal importance to the study of history and that of the Classics), Yulgok used in his Outline of the Sagely learning quotations taken from the Classics and the Confucian corpus. He only alludes to history when discussing the general aim of his work in the final and most important chapter, "The Transmission of the Way by the Sages and Worthies" (Sŏnghyŏn Tot'ong 聖賢道統). Therefore, whereas the Daxue yanyi and the Taehak yŏnŭi chimnyak were basically summaries of the Learning of the Sovereign and may be regarded as being teaching materials used by scholar-officials to inculcate monarchs with a set of very detailed prescriptive rules, the Sŏnghak chibyo is rather a systematic outline of 'Sagely Learning', shengxue/sŏnghak 聖學, understood as being 'practice-oriented Learning' (sirhak 實學). Learning which was close to Yulgok's heart was that of 'the Sages and Worthies,' Yao and Shun. In that sense, Yulgok was different from the majority of the scholar-officials of his time, as has been underlined by JaHyun Kim Haboush. 38 In the late sixteenth century, the majority of the most influential scholar-officials tended to reinforce their own positions vis-à-vis the ruler. Yulgok emphasized on the contrary the role of the sovereign, who had to be the only person actually 'transmitting the Way'. 39 Interestingly, for him the Way had no longer been transmitted since Zhu Xi in China (Zhen Dexiu is not part of that transmission) and the torch could be handed on to a Korean ruler, provided that he showed himself capable of 'Sagely learning.' In this both demanding and noble path, the ruler had to be ably assisted by good ministers -that is to say, by true 37 op.cit.,p.15. 39 Cf. Sŏnghak chibyo 5, Sŏnghyŏn Tot'ong 聖賢道統. 謙 (1536-1587), the younger brother of Insun wanghu 仁順王后 (1532-1575), related to Yulgok's family on his paternal grand-mother's side and the leader of the Sŏin 西人 faction.

Cf. Preface to the Outline of Sagely Learning

: 臣按。道妙無形。文以形道。四書六經。旣明且備。因文 求道。理無不現。第患全書浩渺。難以領要。先正表章大學。以立規模。聖賢千謨萬訓。皆不外此。此 是領要之法。西山眞氏推廣是書。以爲衍義。博引經傳。兼援史籍。爲學之本。爲治之序。粲然有條。 而歸重於人主之身。誠帝王入道之指南也。但卷帙太多。文辭汗漫。似紀事之書。非實學之體。信美而 未能盡善焉。 38 Cf. Kim Haboush
The consequences of removing these important and influential men were numerous. From then on, Yulgok was irremediably considered as being involved in this phenomenon, which sullied the Chosŏn court for centuries, right up until the dawn of the modern era. Yulgok's role in the development of factional strife has been largely debated in past and present times alike, but it is generally acknowledged today that he did play a role in debates concerning this burning topic. 41 Besides, his premature death in his early forties might also be ascribed to the consequences of the harassment he had been continuously subjected to from 1572 to 1584.

What is worth noticing here is that the many memorials and letters he sent to the throne on the topic of factionalism from 1572 onwards all called for a policy of appeasement. Moreover, his address to the throne, which opens the Sŏnghak chibyo, brings to the forefront his deep concerns about the situation at court, as well as in the country as a whole, in the 1570's. By presenting this Outline of Sagely Learning, Yulgok meant to exhort the young king Sŏnjo, who had already showed signs of distrust towards his ministers and scholar-officials in general, to trust worthy people once again. For Yulgok, embodying the royal function meant becoming a wise king capable of listening to upright ministers. Collegial power, supposed to spring from the good-hearted interplay between a king and his ministers (which, as explained above, was thought to be the main message of the Learning of the Sovereign), was the only way of appeasing the political instability caused by widespread suspicion among the elites ruling the country. As a result of this, beyond its undeniable universalistic and philosophical significance, Yulgok's masterpiece on the Great learning has also to be understood in the light of the historical context of late sixteenth-century Chosŏn Korea. Besides, his use of the Great Learning was most certainly marked by the historical context of his times, that is to say the beginning of intense factionalism in the late sixteenth century. In the midst of political turmoil, he therefore resorted (as his predecessors had) to the philosophy of the Great Learning, in order to find a solution for the balance of power in the Korean monarchical system. However, his Sŏnghak chibyo cannot be reduced to a simple continuation of certain dominant tendencies of the Korean reception of Great Learning philosophy, which focused on Zhen Dexiu's rigorist interpretation. Indeed, Yulgok fundamentally intended to go back to a broader Cheng/Zhu understanding of the Great Learning, in order to enhance the education of kings, but also Confucian learning in general.

Conclusion

His focus on the cultivation of vital energy in self-cultivation, for example, which reminds us more of Zhang Zai's ideas than Cheng/Zhu teachings, is a good illustration of a rather cosmological approach to ethics and politics. Such a comprehensive viewpoint, systematized in a well-organized and convincing work like the Sŏnghak chibyo, most certainly paved the way for a more holistic approach to Confucianism in Korea. This point might also explain the important symbolic status given to Yulgok in the national memory of contemporary Korea. In both North and South Korea, his stature as a philosopher and statesman has been widely acknowledged. His life and thought are commonly believed to be emblematic of something specific to Korean history, and even its mentality. To sum up -and maybe to use some more cautious words -his Sŏnghak chibyo can most probably be understood as evidence of that sincere faith in the Great Learning, understood as the paradigmatic text of (Neo-)Confucianism, that generations of Korean scholar-officials seem to have had, earnestly seeking to put it into practice in their own lives.

  an unchallenged model. Detailed research of the study of the Great Learning during the Classics Mat lectures mentioned in the Annals of Chosŏn, for the period going from the fourteenth to the sixteenth century, clearly shows that Zhen Dexiu's work was considered the unchallenged reference work on Great Learning philosophy in the early Chosŏn court. 11 7 Cf. CHI Tuhwan 지두환, Chosŏn chŏngi taehak yŏnŭi yihae kwajŏng « 朝鮮前期 "大學衍義" 이해과정 » (« The Process of Assimilation of the Extended Meaning of the Great Learning at the Beginning of Chosŏn »), taedong kojŏn yŏngu 泰東古典 究

  Yi Yulgok's Sŏnghak chibyo might be regarded nowadays as being both the continuation of previous tendencies and the dawn of a new era in Korean intellectual history. While Neo-Confucianism became, in the late sixteenth century, a significant part of the 41 Cf. HWANG Unyong 黃雲龍, Yulgok Yi I wa tangnon 栗谷李珥와 黨論 ("Yulgok Yi I and the Discourse on Factional Strife"), Tonga nonch'ong 東亞論叢 23, Tonga University, 1986 ; and CHŎNG Manjo 鄭萬祚, Chosŏn sidae chobungdangnon ŭi chŏngae wa kŭ sŏnggyŏk 朝 鮮 時 代 朝 朋 黨 論 의 展 開 와 그 性 格 (« The Development and Characteristics of the Discourse on Court Factionalism in the Chosŏn Period. »), in KIM Sòngmu 金成茂 (ed.), Chosŏn hugi tangjaeng ŭi chonghapjŏk kŏmt'o 朝鮮後期黨爭의 綜合的 檢討 ("A Synthetic Examination of Factional Strife at the end of the Chosŏn Period"), Hanguk chŏngsin munhwa yŏnguwŏn, 1992.distinctive identity and culture of an elite working hard to implement its ideals at both central and local levels, textual exegesis of the Great Learning became paradoxically more idealistic and moral. It might tentatively be said that, maybe for the first time in Korean history, Confucianism became a holistic Weltanschauung, rather than just a vague reservoir of rhetorical devices used for political strategies in the balance of power between the monarchy and the aristocracy.It is indisputable that Yulgok was an heir of the ideologues of the early Chosŏn State, who took the Yuan version of Song Neo-Confucianism as a possible model for Korea.

  大學The above table also shows that the Daxue zhangju 大學章句 (which, because of its inclusion in the Four Books, is still the orthodox version of the Great Learning) was never studied, with the noteworthy exception of the preface by Zhu Xi studied once during Chungjong's reign. Generally speaking, the two above-mentioned texts by Zhu Xi (Daxue

					Daxue	Daxue	Daxue	Daxue	Total of
					zhangju	huowen	yanyi	yanyibu	occurrences
					大學章	大學或	大學衍	大學衍義
					句	問	義	補
	T'aejo		6 years				11
	太祖		and
	2 months zhangju and Daxue huowen) were seldom ever read. Moreover, the founder of the new
	Chŏngjong 定宗 dynasty, T'aejo, studied the Daxue yanyi a great deal. In fact, he seems to have only studied 2 years and 2 3
	months this text about the Great Learning. Related to this point, one can notice that it was only
	T'aejong 太宗 starting from Sejong's reign that the Great Learning itself had been studied during Classics 17 years and 2 7
	10 months Mat lectures at court. Chungjong was, proportionally, the king who most studied all texts
	Sejong related to the Great Learning and its exegeses. He was notably the only one who read, for a 31 years 3 13
	世宗 number of years, the Daxue yanyibu. As for Tanjong and Injong, they never studied any text and 6 months
	Munjong concerning the Great Learning. Lastly, in spite of a rather long reign, Sejo read the Great 2 years 2 7
	文宗 Learning only once, without studying the Daxue yanyi at all. and 3 months
	Tanjong	3 years As for the scholarship of the beginnings of Chosŏn, a quick glimpse at various
	端宗 Collected writings of literati (munjip 文集) from the same period allows us to make two and 2 months
	Sejo 世祖 important statements. Firstly, Koreans exhaustively studied Zhen Dexiu's Daxue yanyi rather 13 years and 1
	3 months than the canonical text, as formalized in the Four Books. In fact, they seldom ever wrote
	Yejong 睿宗 philosophical commentaries on it. A few works on the Great Learning and its various Chinese 1 year and 2 months 1 1
	Sŏngjong exegeses were, however, written by some scholars, as for example: the Taehak samgang 25 years 6 1 14
	成宗 p'almokcham 大學三綱八目箴 by Yu Sungjo 柳崇祖 (1452-1512), 12 the Taehak changgu and 1
	month 12 years poyu 大學章句補遺 by Yi Ŏnjŏk 李彥迪 (1491-1553), 13 the Taehakto 大學圖 and Yŏnsan kun 7
	燕山君 Taehakkyŏng iljangyŏnŭi 大學經一腸演義 by Pak Yŏng 朴英 (1471-1540), 14 the Taehak and 6 months
	Chungjong	38 years	11	1		23	12
	中宗 12 Also sometimes called Taehakkangmokcham 大學綱目箴, this work was printed in 1511 on the order of king and 2 months (preface only) Injong Chungjong 中宗 (r. 1506-1544). It consists of a compilation of prescriptions based on the Three Principles (san 9 months gangling/sam kangnyŏng 三綱領) and Eight Stages (ba tiaomu/p'al chomok 八條目) drawn from the Great 仁宗 Learning by Zhu Xi. At the end of the book, there is a text with the title Sŏngniyŏnwŏnch'waryo 庄理淵源撮要,
	Myŏngjong explaining the viewpoints of Zhou Dunyi and Zhu Xi on the Book of Changes. The whole work aims at showing 22 years 3 5
	the essential principles of the correct behavior expected of a sage king. 明宗 Sŏnjo 宣祖 40 years 5 4 13 This text is said to have been printed for the first time at a rather late moment, in 1794. In the oldest remaining 12 edition, one can in fact find a preface written by king Chŏngjo 正祖 (r. 1777-1800), as well as a sixteenth-and century postface attributed to No Susin 盧守愼 (1515-1590). Yi Ŏnjŏk is supposed to have written this 7 months commentary to correct Zhu Xi's exegesis on the Great Learning in order to go back to the original meaning of
	the text. In Yi Ŏnjŏk's commentary, the Great Learning is divided into one canonical part and nine
	Total commentaries (and not ten, like in Zhu Xi's reading), and a few changes appear in the interpretation of some of 216 years 32 1 7 103 12
	these commentaries. At the end of the book, one can find a summary, called Soktaehakhongmun 續大學或問,
	Percentage		20,65% 0,65%	4,52%	66,45% 7,74%	100%

explaining the six main differences opposing Yi Ŏnjŏk to Zhu Xi.

14

The main characteristic of the diagram called Taehakto 大學圖, located in kwŏn 2 of the collected works of Pak Yŏng (Songdang chip 松堂集), is that the author distinguishes the basic principles of the Great Learning according to what he calls 'practice by inference' (ch'uhaeng 推行) on the one hand, and 'practice by knowledge' chirŭi 大 學 質 疑 by Yi Tŏkhong 李 德 弘 (1541-1596),

15

the Taehak tongja mundap 大學童子問答 by Cho Hoik 曺好益 (1546-1609),

16

and the Taehak chilsŏ 大學疾書 by Yi Ik 李瀷 (1579-1624). 17 Yet one cannot know precisely to what extent these works were read and circulated. What's more, with the noteworthy exception of Yi Ŏnjŏk's revision of Zhu

  Part of the collected works of Yi Tŏkhong, the Kanjaejip 艮齋集, it was printed for the first time in 1766 and was later supplemented, in a 1829 edition, by a series of questions and answers between Yi Tŏkhong and Yi Hwang.16 In 1609 a certain Kim Hyŏn 金鉉 is said to have paid a visit to Cho Ho'ik, asking for a commentary on the Great Learning. Cho Ho'ik then started writing down what he used to teach his pupils, but passed away that very same year, before completing the work. The first printed edition of the text is unknown. In this work, Cho Hoik remains faithful to Zhu Xi's commentaries.

18

It is thus not surprising that the main use of the Daxue yanyi in Korea intended first and foremost to strengthen royal authority inside and outside the peninsula, in a context marked by diplomatic struggles with the concomitantly founded Ming dynasty. It is worth noticing that Zhen Dexiu's work -and (chihaeng 知行) on the other. As for the Taehakkyŏng iljangyŏnŭi 大學經一腸演義, it is an explanation following the diagram which deals only with the first paragraph of the Great Learning. Pak Yŏng quotes and discusses a few commentaries by Zhu Xi, Zhen Dexiu, Yang Shi, Shao Yong and a few other Chinese scholars, before expounding his own ideas.

15

This text was written by Yi Tŏkhong, one of the disciples of To'egye 退溪, Yi Hwang 李滉 (1501-1570), within the context of a large exegetical work on the Four Books, the Book of Changes, the Classic of the Mind (Xinjing 心經) by Zhen Dexiu, known as Simkyŏng chirŭi 心經質疑.

17

In this text, which is part of the Sŏnhojilsŏ 星湖疾書, a compilation of Yi Ik's commentaries on the Three Classics and the Four Books, one can see the author's political reflections based upon the Great Learning. It is worth noting that the Taehak chilsŏ was not included in the 1922 edition of the complete works of Yi Ik, the Sŏngho chŏnjip 星湖全集.

18

On this topic, see in this volume the article by Kim Daeyeol, "King Chǒngjo's (r. 1776-1800) reading of the Great Learning and his political perspective: powerful monarch and active government."

  consequence of this search for self-definition and empowerment by scholar-officials in the first centuries of Chosŏn may explain another significant difference with China in the use of the Great Learning at a later period. In Ming China, Zhen Dexiu's work inspired Qiu Jun 丘濬 (1420-1495) to write a monumental Supplement, the Daxue yanyibu 大學衍義補. 't discuss at all the second part of the Great Learning's paradigm, 'ordering the state' (zhiguo/ch'iguk 治國). In the first half of Chosŏn, both the Daxue yanyi and the Daxue yanyibu were studied at court, as has been explained above. Yet a controversy took place in the fifteenth and sixteenth centuries to determine which text was to be given priority in the royal curriculum. This reminds us that scholar-officials were mainly concerned with keeping a hand on the royal curriculum and censoring kings through Classics Mat lectures.26 Selecting texts for these lectures also became an important bone of contention among opposing factions at court -a phenomenon that might explain why Korean factionalism plaguing the dynasty from the late sixteenth century onwards combined political opposition with philosophical, or at least ideological, antagonism. Unlike the authoritarian Ming and Qing empires, during which the official exegesis of the Learning of the Sovereign focused on the Great Learning, It appears for example in his civil service examination essays 29 and numerous memoranda, among which his famous Questions and Answers from the Eastern Lake (Tongho mundap 東湖問答) written with a few colleagues during his26 On the topic of the royal curriculum and the education of Yi crown princes, see KIM HABOUSH JaHyun, "The Education of the Yi Crown Prince: A Study in Confucian Pedagogy", in Wm. Theodore de BARY and JaHyun KIM HABOUSH (eds), The Rise of Neo-Confucianism in Korea, Columbia University Press, New York, 1985.27 A workshop was held on the topic at the Collège de France in June 2011. It was organized by the Chair of Modern Chinese History under the direction of Pr. Pierre-Etienne Will. sabbatical leave' (sagadoksŏ 賜暇讀書) at the Toksŏdang 讀書堂 in 1569.30 In these texts Yulgok appealed, as his predecessors had before him, to the basic ideas of the philosophy of the Great Learning and the Learning of the Sovereign, to discuss various political matters of the day. Besides, the formal structure of his Kyŏngmong yogyŏl 擊蒙要訣, a short book explaining the essentials of Confucian learning, was patterned upon the Great Learning scheme.31 In this handbook for beginners written when he was momentarily retired in 1577, Yulgok sticks to the Great Learning paradigm whereby socio-political behavior is fundamentally rooted in self-cultivation, 32 contrary to the scheme chosen by Zhu Xi and Liu Qingzhi 劉清之 (1130-1195) for the Elementary Learning (Xiaoxue/Sohak 小學). Lastly, his biographies and official and personal correspondence 33 provide a number of recurrent examples of his constant attempts to put the Great Learning's ideal into practice in his own family and personal life. For instance, to implement the notion of 'regulating one's family' (qi Sŏnghak chibyo. Contrary to other Korean commentaries or summaries, it might be said that these two works were written in the framework of the Learning of the Sovereign, in the sense that they mirror the duties expected of skilled and faithful ministers in an ideal Confucian monarchy. Furthermore, they illustrate shifts in the Korean understanding of Zhen Dexiu's work in the fifteenth and sixteenth centuries, for they 30 Cf. Ibid, kwŏn 15, 雜著 2.31 For the first bilingual translation in a Western language with an introduction and annotations, see SANCHO Isabelle, Principes essentiels pour éduquer les jeunes gens, Bibliothèque Chinoise, Les Belles Lettres, Paris, 2011. Cf. Ibid, kwŏn as well as Haengjang 行狀 by Kim Changsaeng 金長生 (1548-1631) and Yŏnbo 年譜 by Song Siyŏl 宋時烈 (1607-1689). 34 Some of them have been listed above.35 On this text, see in this volume the article by John B. Duncan, "The Taehakyŏnŭichimnyak and the Politics of Confucian Learning in the Early Chosŏn."

	This gigantic administrative textbook, overflowing with technical details, figures and numbers,
	deals with public finances, military organization, transportation, hydraulic control, taxes, and
	so forth. The aim was to compensate for the shortcomings of Zhen Dexiu's work, which

24

Cf. SANCHO, op.cit., chapter 2.

25

Cf. Hanshu, juan 56, biography 26, Dong Zhongshu zhuan 董仲舒傳: 正心以正朝廷, 正朝廷以正百官, 正百官以正萬民. didnthus giving birth to a multiplicity of rather technical administrative textbooks bearing a strong legal and formalist aspect, like the Daxue yanyibu, 27 in Chosŏn Korea officially sanctioned exegetical works on the Daxue tended rather to remain moral and philosophical albeit aiming at improving 'practice-oriented' political reflection. 28

Yulgok and the Great Learning

The beginning of this latter phenomenon might be traced back to Yi Yulgok's masterpiece, the Sŏnghak chibyo 聖學輯要, which tended to replace Chinese works in Classics Mat lectures from the seventeenth century onwards. Before moving on to a general overview of this work, let us not forget that Yulgok's interest in the paradigm of the Great Learning, and especially the notion of 'rectifying the king's mind,' pervaded most of his writings throughout his life and career.

28

However, in both Chinese and Korean cases, there was certainly some philosophical and philological exegesis of the Great Learning, especially amongst scholars of the so-called kaozhengxue 考證學 and sirhak 實學. 29 Cf. 死生鬼神策, 神仙策, 祈禱策, 壽夭策, 醫藥策, 天道人事策, 化策, etc. in Yulgok chŏnsŏ 栗谷全書 (« The Complete Works of Yulgok »), 7 vol., Hanguk chŏngsin munhwa yŏnguwŏn 漢國精神文化研究院, Seoul, 1987.

'qijia/ che kiga 齊其家) he wrote and circulated a short text in vernacular Korean for his own family, the Tonggŏ kyesa 同居戒辭. It would therefore not be an overstatement to say that the Great Learning was the cornerstone of his life and thought.

What's new in the Sŏnghak chibyo?

To my knowledge, nine Korean scholars wrote at least one commentary specifically dedicated to the Great Learning from the fourteenth to the sixteenth century. 34 However, only two texts pertain to our discussion: the Taehak yŏnŭi chimnyak 大學衍義輯略 of Yi Sŏkhyŏng 35 and Yulgok's 32 Cf. Daxue, canonical part : 自天子以至於庶人, 壹是皆以修身為本..

33

His official portrait is printed on the 5,000 KRW bank notes in contemporary South Korea. This imaginary portrait was made in 1965 on the presidential order of PakChŏnghŭi (1917Chŏnghŭi ( -1979)). The model was a 14 th generation descendant of Yulgok, Yi Chaenŭng from the Tŏksu Yissi 德水李氏.

The Sŏnghak chibyo is among the '100 Classics' that have been selected and will be translated into English in the few years to come through the project led by the UCLA and the AKS (Academy of Korean Studies).

Cf. DUNCAN John B., The Origins of Chosŏn Dynasty, University of Washington Press, Seattle and London, 2000.

Cf. de BARY Wm.Theodore, Neo-Confucian Orthodoxy and the Learning of the Mind-and-Heart, Columbia University Press, New York, 1981, and The Message of the Mind in Neo-Confucianism, Columbia U.P., New York, 1989.

In 1472 when king Sŏngjong was presented the Taehak yŏnŭi chimnyak, he was only fifteen years old, and Yi Sŏkhyŏng used to be his preceptor from 1466 to 1468. As for king Sŏnjo, he was twenty-one years old when Yulgok presented the Sŏnghak chibyo in 1575.

Confucian masters. Yulgok thus reasserted the core idea of the Korean reception of Neo-Confucianism: good interaction between ruler and minister in order to bring about an ideal society or, in other words, to practice the Way.

Concerning this point, let us not forget that Zhen Dexiu and Yi Sŏkhyŏng didn't elaborate upon the 'ordering the state' part of the Great Learning's paradigm in their respective works, considering that the proper cultivation of the self would 'naturally' expand into the correct governance of men. Another characteristic of Yulgok's Sŏnghak chibyo is his equally discussing both parts of the Daxue's paradigm: self-cultivation (xiuji/sugi 修己) and ordering the state (zhiguo/ch'iguk 治國). For Yulgok, there is no solution of continuity between these two parts that are constantly interacting in the same vital and moral process. As can be seen in the very organization of the work, Yulgok elaborated a new exegesis of the Great Learning, emphasizing the dynamic role of material force or vital energy, Qi/Ki 氣.

Here is a reproduction of the diagram of the Sŏnghak chibyo that Yulgok added at the end of his "Introductory explanation," pŏmnye 凡例.

The most striking feature of this diagrammatic presentation, thought-provoking in many ways, is most certainly the axial position of the step of 'nurturing vital energy,' yanggi 養氣, in the process of self-cultivation. More precisely, this cultivation of vital energy, which must be understood as an equivalent of the very notion of heart/mind in Yulgok's philosophy, enables a twofold goal of self-cultivation: firstly, 'rectifying one's household' (chŏngga 正家), and secondly, 'implementing [good] governance' (wuijŏng 為政). Yulgok thus emphasizes the cultivation of the mind/heart that could be summarized in two of the eight steps of the Great Learning : chengyi/sŏngŭi 誠意 and zhengxin/chŏngsim 正心. This shows that he was in the same interpretative vein as his Korean predecessors influenced by Zhen Dexiu and the Yuan version of Song Neo-Confucianism. However, he suggested a more dynamic reading of the Great Learning pattern. Playing with words, we could say that he breathed renewed vital energy into the prescriptive scheme of the Great Learning, understood as the proper way of living as a true human being. Governing is then only part of that ethical path that should be followed by any man worthy of the name.

A final remark on the historical context