Supporting Information One-Step Facile Synthesis of Cobalt Phosphides for Hydrogen Evolution Reaction Catalyst in Acidic and Alkaline Medium Afriyanti Sumboja, a Tao An, a Hai Yang Goh, b Mechthild Lübke, a,c Dougal Peter Howard, c Yijie Xu, a,c Albertus Denny Handoko, a Yun Zong a* and Zhaolin Liu a* a Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis #08-03, Singapore 138634 b School of Applied Science, Temasek Polytechnic, 21 Tampines Avenue 1, Singapore 529757 c Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK *E-mail: y-zong@imre.a-star.edu.sg; zl-liu@imre.a-star.edu.sg S-1
1. Thermogravimetric analysis of cobalt (II) acetate tetrahydrate Figure S1. TGA of cobalt (II) acetate tetrahydrate. Under the performed TGA condition, cobalt (II) acetate tetrahydrate undergoes a four-step decomposition reaction. Steps I and II comprise of the loss of free water and coordinated water molecules ( 4H 2 O) to form anhydrous cobalt(ii) acetate, Co(CH 3 COO) 2. Step III consists of the decomposition of anhydrous cobalt acetate Co(CH 3 COO) 2 to basic cobalt acetate, Co(OH) x (CH 3 COO) 2-X. Decomposition of basic cobalt acetate to form cobalt oxide, CoO is shown in step IV. The decomposition of basic cobalt acetate proceeds according this reactions. 1 Co(OH)(CH 3 COO) ½ CoO + ½ CoCO 3 + ½ H 2 O + 0.5CH 3 COCH 3 CoCO 3 CoO + CO 2 + H 2 O S-2
2. Thermogravimetric analysis of cobalt (II) acetylacetonate Figure S2. TGA of cobalt (II) acetylacetonate. Under the performed TGA condition, cobalt (II) acetylacetonate undergoes a two-step decomposition reaction. The decomposition Co(C 5 H 7 O 2 ) might have resulted in the formation of various forms of CoO (Cobalt (II) Oxide, Cobalt (II,III) Oxide, Cobalt (III) Oxide) and metallic cobalt as the resulting products from the decomposition reaction. It would have also resulted in the release of acetate and carbon dioxide gas. 3. Characterisation of the commercial cobalt salts Figure S3. SEM images of (a) Cobalt (II) acetate tetrahydrate and (b) cobalt cobalt (II) acetylacetonate. S-3
4. Heat-treatment of cobalt salts precursors in N 2 Heat treatment of cobalt salt in inert N 2 environment in absence of NaH 2 PO 2 H 2 O yields primarily Co 3 O 4 (along with significant amount of unreacted cobalt (II) acetate tetrahyrate precursor) for Co-P I precursor (Figure S3a) and CoO for CoP-II precursor (Figure S4b). Figure S4. XRD data of heat treated (a) cobalt (II) acetate tetrahydrate and (b) cobalt (II) acetylacetonate in N 2 atmosphere without the presence of sodium hypophosphite. 5. HER performance of the cobalt phosphides in neutral medium Figure S5. HER performance of Co-P I and Co-P II in 0.2 M phosphate buffer solution: (a) LSV curve at a scan rate of 5 mv s -1 and rotating speed of 1600 and (b) Stability under a constant current of 10 ma cm -2 for over 13 hours. Despite the good stability, due to the different HER mechanism and the lack of mobile ions in neutral condition, Co-P I and Co-P II have large overpotentials of 429 mv and 467 mv in 0.2 M phosphate-buffered saline (PBS) solution, respectively S-4
6. Electrocatalytic Surface Area (ECSA) Cyclic Voltammograms (CV) curves gathered from CV scans conducted at 0.1 potential rangeat 6 scan rates (2 mv s -1, 5 mv s -1, 10 mv s -1, 50 mv s -1, 100 mv s -1, 200 mv s -1 )in acidic and alkaline electrolytes to obtain the double layer capacitance (C dl ). ECSA was calculated according from the following equation: 2-3 ECSA = C dl / C s Where C s is general specific capacitance. The general specific capacitance for a flat surface is generally found to be in the range of 20-60 µf cm -2. 4-6 40 µf cm -2 per cm 2 ECSA was used as C s in this report. 2-3, 7 Figure S6. Cyclic voltammograms and the corresponding double layer capacitance of (a, b) Co-P I and (c, d) Co-P II in 0.5 M H 2 SO 4. S-5
Figure S7. Cyclic voltammograms and the corresponding double layer capacitance of (a, b) Co-P I and (c, d) Co-P II in 1 M KOH. 7. Durability tests of the cobalts phosphides in acidic and alkaline medium Figure S8. Stability of the cobalt phosphides as HER catalyst in different electrolyte media under a constant current of 10 ma cm -2 for over 44 hours: (a) acidic and (b) alkaline. Noticeable colour changes of electrolyte are observed after stability tests in acidic electrolyte, suggesting that both catalysts are prone to dissolution in acidic electrolyte. S-6
8. XPS analysis of Co-P I and Co-P II after stability test in alkaline electrolyte Figure S9. (a) Co 2p and (b) P 2p spectra of Co-P I after 44 h chronoamperometry test in 1.0 M KOH. (c) Co 2p and (d) P 2p spectra of Co-P II after 44 h chronoamperometry test in 1.0 M KOH. Lower amount of cobalt phosphides were detected in both samples after stability test in comparison to the as-synthesized samples, suggesting the loss of cobalt phosphides during the 44 h stability test contributed to the degradation of samples. S-7
9. SEM and TEM images of the cobalt phosphides after 44 h of chronoamperometry test Figure S10. SEM image of (a) as-synthesized Co-P I and after 44 hours chronoamperometry test in (b) 1.0 M KOH and (c) 0.5 M H2SO4. SEM image of (d) as-synthesized Co-P II and after 44 hours chronoamperometry test in (e) 1.0 M KOH and (f) 0.5 M H2SO4. Figure S11. TEM image of (a) as-synthesized Co-P I and after 44 hours chronoamperometry test in (b) 1.0 M KOH and (c) 0.5 M H2SO4. TEM image of (d) as-synthesized Co-P II and after 44 hours chronoamperometry test in (e) 1.0 M KOH and (f) 0.5 M H2SO4. S-8
Table S1. Performance of transition metal phosphides as HER catalyst in acidic medium which were tested using similar testing condition. Overpotential Tafel slope Catalyst at 10 ma cm -2 Reference (mv dec -1 ) (mv) CoP urchin like 105 46 CoP nanowires 110 54 CoPnanosheets 164 61 CoP/Co 2 P/Co nanoparticles 160 56 This work CoP/Co 2 P/Co nanoparticles 169 65 This work CoPmicroparticles 202 86 CoP nanoparticles 203 81 CoP nanoparticles 221 87 CoP microspheres 226 76 CoP/graphene 121 50 CoP/CNT 122 54 FeP/C 112 58 FeP/graphene 123 50 Ni 2 P nanoparticles 346 70 MoP nanoparticles 246 60 MoP nanoparticles 225 65 WP 2 microparticles 161 57 8 9 9 10 7 9 11 10 11 12 13 14 14 15 16 S-9
Table S2. Performance of transition metal phosphides as HER catalyst in alkaline medium which were tested using similar testing condition. Overpotential Tafel slope Catalyst at 10 ma cm -2 Reference (mv dec -1 ) (mv) CoP 2 nanoparticles 115 75 O 2 incorporated Co 2 P 160 61.1 CoP/Co 2 P/Co nanoparticles 175 84 This work CoP/Co 2 P/Co nanoparticles 188 97 This work Co 2 Pnanorods 247 86 CoP/rGO 150 38 FeP 2 nanowires 189 67 FeP nanowires 194 75 MoP/rGO 140 72 MoP nanoparticles 276 105 WP 2 microparticles 153 60 NiCoP hollow nanocubes 150 60.6 17 18 18 4 19 19 15 15 16 1 Table S3. Co/P ratio of cobalt phosphides before and after stability test Sample *Co/P ratio Co-P I 0.40 Co-P I after 44 h chronoamperometry test in 1.0 M KOH 0.48 Co-P II 1.20 Co-P II after 44 h chronoamperometry test in 1.0 M KOH 1.35 *Ratio of the non-oxidized P and Co extracted from XPS analysis S-10
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