電解水制氫，是一種理想的可持續(xù)制氫方法[1] 電解水反應，包括析氧(OER)電極反應和析氫(HER)電極反應[2] 在OER和HER的電子轉移步驟和反應中間體吸/脫附步驟中都存在過程勢壘，因此需要一定的過電位(ηOER或ηHER)以克服較高的過程勢壘形成的動力學障礙[3,4] 為了提高電解水制氫的能量轉換效率，須使用具有催化活性的電極材料來降低OER和HER的反應過電位 貴金屬基材料具有極高的電解水催化活性，RuO2和Pt/C分別是OER和HER催化材料的典型代表[5] 但是，貴金屬基材料價格昂貴，需要尋找可代替的非貴金屬材料

金屬Co在HER過程中屬于中過電位金屬，常用作OER過程的催化電極材料[6] 當Co與B結合時Co的d帶收縮使能量中心靠近Fermi能級 因此，Co-B合金或Co的硼化物具有比純Co更高的電解水催化活性[7] 在Co-B二元組成的基礎上引入Ni，可提高其OER催化活性[8] Co與Ni的協(xié)同作用使Co-Ni-B具有與Pt/C接近的HER催化活性，因此在Co-B中添加適當?shù)漠惙N元素可進一步提高其電解水催化活性[9] 可將W基材料應用于電解水[10,11,12,13]，將W引入Co-B顯著影響其電子結構[14]

常見的催化材料呈粉末狀，制備電極時須使用粘結劑將其涂覆到基底電極表面 粘結劑使電極的內阻增加和掩蔽催化活性位點 同時，在工作過程中粘結劑的附著力較小容易使催化材料脫落 因此，業(yè)界提出使用“自支撐電極” 將催化活性物質原位沉積在具有宏觀物理形態(tài)、自身化學性質穩(wěn)定的導電載體上，形成穩(wěn)固的自支撐催化電極[15] 碳布(CC)材料的導電性和化學穩(wěn)定性高、質量輕、柔韌性好，可作為自支撐型導電載體[16] 本文應用簡易的濕化學還原過程在CC上原位沉積非晶態(tài)Co-W-B物質形成自支撐的Co-W-B/CC復合電極材料，作為電解水過程中的陰極和陽極，研究其電解水催化活性和催化穩(wěn)定性

1 實驗方法1.1 Co-W-B/CC復合電極材料的制備

通過濕化學還原將Co-W-B催化活性物質沉積在1cm×1cm的碳布(CC)上 先在濃硫酸和濃硝酸(VH2SO4:VHNO3=3:1)的混合液中對CC進行活化處理，分別用去離子水、無水乙醇超聲清洗后自然干燥

配制溶液A：將CoCl2·6H2O和Na2WO4·2H2O溶入去離子水，控制[Co2+]+[WO42-]=0.3 mol/L，用氨水調節(jié)pH值為10 配制溶液B：將NaBH4和NaOH溶入去離子水，NaBH4為1.0 mol/L，NaOH為0.25 mol/L 將一定體積的溶液A置入冰水浴，然后將活化處理過的CC浸泡在溶液A中并施加超聲波震蕩 另取相同體積的溶液B逐滴加入到溶液A中，然后維持反應60 min 最后，將得到的Co-W-B/CC樣品取出，用去離子水洗凈后放入60℃的真空干燥箱中進行干燥

通過改變溶液A中WO42-的摩爾濃度占比(χW%=[WO42-]/([WO42-]+[Co2+])×100%，χW%=0%、χW%=25%、33%、50%、66%、75%)調節(jié)Co-W-B中的Co、W元素比例 將不同Co、W元素比例的樣品記為Co-χW-B/CC(χW%=0%時，即Co-B/CC) 用增重法測得不同Co-W-B在CC表面的沉積量均為~0.38 mg/cm2 作為對比，將0.38 mg的商用20wt.% Pt/C和RuO2用傳統(tǒng)的涂覆方式分別負載在1 cm×1 cm的CC表面

1.2 性能表征和電化學測試

用FESEM(Hitachi SU5000)、EDX(Oxford X-act)分別測試樣品的表面形貌、元素種類、元素含量及分布狀況 用XRD(Rigaku Ultimal Ⅳ)測試樣品的物相和晶體結構 使用電化學工作站(Princeton VersaSTAT4)在25℃下1 mol/L NaOH溶液中對樣品進行電化學測試 在三電極測試體系中，參比電極為Hg/HgO/OH-(1 mol/L)電極，對電極為2 cm2的碳棒電極，所制得的樣品為工作電極，樣品測試面積為1 cm2 本文所有電化學測試結果的電位值，均換算為相對于標準氫電極(RHE) 線性掃描伏安曲線(LSV)的掃描速率為5 mV/s，OER與HER測試過程的掃描電位區(qū)間分別為0.87~2.07Vvs.RHE和0.22~2.62Vvs.RHE；電化學阻抗譜(EIS)的測試頻率范圍為105~10-2 Hz，擾動電位幅值為5 mV

2 結果和討論2.1 Co-W-B/CC復合電極材料的的表面形貌

作為基底材料的CC，由表面光滑、直徑約10 μm的碳纖維構成(圖1a) 在制備過程中χW%=50%的樣品(Co-50W-B/CC)，其表面形貌如圖1b和c所示 可見碳纖維表面被較為致密的沉積物所包覆，這種沉積物由直徑為200~800 nm的胞狀顆粒聚集而成，部分胞狀顆粒之間有空隙，材料的微觀表面高度粗糙 EDX分析(圖1c-inset)結果表明，該沉積物中有Co、W、B、O四種元素(C元素信號來源于CC基底)，其原子分數(shù)分別為44.65%、4.86%、42.62%和7.87% 相應的表面元素分布情況如圖1d所示，表明上述四種元素均勻分布在沉積物中

圖1



圖1CC和Co-50W-B/CC的SEM照片、(c-inset) Co-50W-B/CC的EDX能譜以及Co-50W-B/CC的表面元素分布

Fig.1SEM image of CC (a); SEM images of Co-50W-B/CC (b, c)、(c-inset) EDX energy spectrum of Co-50W-B/CC and EDX-mapping of Co-50W-B/CC (d)

如圖2a所示，當制備過程中隨著χW%值從25%逐漸升至75%沉積物中Co元素含量不斷降低，W元素含量不斷升高，B元素含量(原子分數(shù))則在39.9%~43.6%內波動 由于BH4-難以將WO42-還原為零價W，產物中的W元素一般以其低價態(tài)氧化物的形式存在[14] 因此，EDX分析結果表明，各樣品中均存在O元素且伴隨W元素含量的提高O元素的含量也呈上升趨勢 圖2b給出了CC和不同Co-W-B/CC樣品的XRD衍射圖譜 在CC樣品的圖譜中，23.8°和44°附近的衍射峰分別對應C(JCPD file#08-0451)的(002)及(101)晶面 五種Co-W-B/CC樣品也只在23.8°和44°附近出現(xiàn)衍射峰，短時其強度比CC樣品顯著降低且漫散射信號加強，意味著這些Co-W-B沉積物以非晶狀態(tài)包覆CC表面形成 以往用類似反應制備Co-B或Ni-B粉體材料，其產物多為非晶態(tài)[17,18] 由圖3可見，χW%值過高(χW%≥66%)時Co-W-B沉積物在CC表面分布不均，且沉積物的結構疏松，使其對CC的包覆程度降低 因此，Co-66W-B/CC和Co-75W-B/CC樣品的C衍射峰強度明顯高于其他樣品

圖2



圖2χW%值([WO42-]/([WO42-]+[Co2+])摩爾百分比)對Co-W-B催化活性物質中元素含量的影響和 CC以及不同Co-W-B/CC樣品的XRD衍射圖譜

Fig.2Effect of the χW% ([WO42-]/([WO42-]+[Co2+]) mole percentage) on the content of elements in Co-W-B (a) and XRD patterns of CC and Co-W-B/CC samples(b)

圖3



圖3Co-66W-B/CC和Co-75W-B/CC的SEM照片

Fig.3SEM image of Co-66W-B/CC (a) and Co-75W-B/CC (b)

2.2 Co-W-B/CC復合電極材料的電催化OER、HER活性及其機制

目前工業(yè)上實施的電解水工藝大都在堿性環(huán)境中進行[19]，因此本文在1 mol/L NaOH溶液中分析Co-W-B/CC材料的電解水催化活性，包括陽極反應的OER過程和陰極反應的HER過程

五種Co-W-B/CC樣品OER過程的線性掃描伏安曲線(LSV)，如圖4a所示 隨著制備過程中χW%值的提高樣品的OER活性呈現(xiàn)先升后降的變化趨勢，其中Co-50W-B/CC的OER活性最高 與CC、Co-B/CC和RuO2樣品的LSV曲線(圖4b)比較，Co-W-B/CC樣品的OER活性均顯著高于CC和Co-B/CC，其中Co-50W-B/CC的OER活性與RuO2接近，甚至在較低的電位區(qū)間超越了RuO2 陽極電流密度為10 mA/cm2時，不同樣品的OER過電位(ηOER-10)分別為ηOER-10,CC=0.548 V、ηOER-10,Co-B/CC=0.462 V、ηOER-10,RuO2=0.432 V、ηOER-10,Co-25W-B/CC=0.438 V、ηOER-10,Co-33W-B/CC=0.406 V、ηOER-10,Co-50W-B/CC=0.394 V、ηOER-10,Co-66W-B/CC=0.441 V、ηOER-10,Co-75W-B/CC=0.457 V

圖4



圖4不同Co-W-B/CC樣品的LSV曲線(嵌入圖為1.50~1.75VvsRHE電位范圍的局部放大圖)、Co-50W-B/CC、CC、Co-B/CC和RuO2樣品的LSV曲線、 Co-50W-B/CC、CC、Co-B/CC和RuO2樣品的Tafel斜率、不同Co-W-B/CC樣品和Co-50W-B/CC、Co-B/CC和RuO2樣品的EIS曲線以及不同樣品的Cdl-OER值和j0-real-OER值

Fig.4OER process:LSV curves of Co-W-B/CC samples (the inset figure is the enlarged view of 1.50~1.75VvsRHE) (a)， LSV curves of Co-50W-B/CC, CC, Co-B/CC and RuO2 (b)， Tafel slopes of Co-50W-B/CC, CC, Co-B/CC and RuO2 (c)， EIS plots of Co-W-B/CC samples (d)， EIS plots of Co-50W-B/CC, CC, Co-B/CC and RuO2 (e)，and Cdl-OER and j0-real-OER of samples (f)

Tafel斜率(Tafel曲線的線性極化區(qū)斜率)也能在一定程度上反映電極材料的催化活性[20] Tafel斜率越低則電極反應速率(反應電流密度)隨過電位增加而提升的幅度越大，意味著催化活性越高[21] 根據(jù)圖4b計算出相應樣品的Tafel斜率，畫在圖4c中 可以看出，Co-50W-B/CC的Tafel斜率為96.8 mV/dec，比CC(144.5 mV/dec)和Co-B/CC(131.1 mV/dec)大幅降低，只比RuO2(94.7 mV/dec)高約2.2%，再次說明Co-50W-B/CC的OER活性與RuO2接近 此外，相近的Tafel斜率還表明，Co-50W-B/CC與RuO2兩者表面發(fā)生OER過程的反應動力學機制類似[22]

在陽極過電位為0.6 V的條件下測定了不同樣品的電化學阻抗譜(EIS)，如圖4d和4e所示 EIS曲線均有兩個容抗弧，表明各樣品表面的電極過程有兩個時間常數(shù)，其中弦長較小的高頻容抗弧與電極表面的微觀孔隙有關，弦長較大的中-低頻容抗弧則與電荷轉移過程相關[9] 圖4d中的插圖為表征電極過程的等效電路模型，其中Rs為溶液電阻，R1為孔隙電阻，Rct為電荷轉移電阻，CPE1和CPE2為分別代表孔隙電容和雙電層電容的恒相位角元件 根據(jù)對Rct值的比較，可判斷電極表面法拉第過程的難易程度[23] 根據(jù)等效電路模型對EIS數(shù)據(jù)進行擬合，得出OER過程的Co-50W-B/CC的Rct值為2.24 Ω/cm2，幾近于RuO2 (2.17 Ω/cm2)，Co-B/CC (8.87 Ω/cm2)、Co-25W-B/CC (3.86 Ω/cm2)、Co-33W-B/CC (3.18 Ω/cm2)、Co-66W-B/CC (4.36 Ω/cm2)和Co-75W-B/CC(5.92 Ω/cm2)五種樣品的Rct值則相對較高 這些結果表明，Co-50W-B/CC可有效促進OER過程中的界面電子轉移，加速反應的進行

經EIS數(shù)據(jù)擬合得到的OER過程的雙電層電容(Cdl-OER)，反映電極材料表面催化活性位點的多少[24] 根據(jù)圖4a和b中的LSV曲線并結合Tafel關系式，可推算出不同樣品OER過程的表觀交換電流密度(j0-OER) 而OER過程的表觀交換電流密度與其雙電層電容的比值(j0-real-OER=j0-OER/Cdl-OER)，則是表征電極材料OER本征催化活性的直觀參量[25] 各樣品的Cdl-OER和j0-real-OER的計算結果，如圖4f所示 RuO2是典型的高活性OER催化材料，其j0-real-OER值遠高于其他樣品，但其Cdl-OER值卻最小 其原因是，在電極的制備過程中RuO2粉體的團聚和粘結劑掩蔽了部分催化活性位點 在CC表面自主沉積形成的Co-W-B/CC和Co-B/CC樣品中，Co-50W-B/CC的Cdl-OER值和j0-real-OER值均最大 據(jù)此分析，Co-50W-B/CC的OER活性與RuO2接近，可能與兩個因素有關 其一是，非晶結構造成活性原子配位高度不飽和，且其微觀表面高度粗糙不存在粘結劑，使之產生并暴露出更多的催化活性位點，提高了OER反應的發(fā)生幾率[25] 其二是，由于靜電吸引OH-易于與Co-W-B中呈氧化態(tài)的Wδ+發(fā)生配位 電負性較強的B原子促使配位的OH-發(fā)生去質子化而形成過氧化物中間體(*OOH)，進而促進OER反應的進行[26]，而χW%=50%的樣品(Co-50W-B/CC)其元素比例可能更利于這一情況的發(fā)生

針對電解水過程的陰極反應，各樣品HER過程的LSV曲線，如圖5a和b所示，選擇Pt/C作為HER過程的性能對照 Pt/C的HER活性最高，陰極電流密度為-10 mA/cm2時的HER過電位(ηHER-10)為0.031 V，而CC幾乎無HER活性 Co-W-B/CC樣品的HER活性均高于Co-B/CC，其中Co-50W-B/CC的HER活性相對最高 -10 mA/cm2時不同Co-W-B/CC樣品和Co-B/CC的析氫過電位分別為ηHER-10,Co-B/CC=0.229 V、ηHER-10,Co-25W-B/CC = 0.149 V、ηHER-10,Co-33W-B/CC = 0.132 V、ηHER-10,Co-50W-B/CC = 0.094 V、ηHER-10,Co-66W-B/CC = 0.137 V、ηHER-10,Co-75W-B/CC = 0.153 V 圖5c給出了Co-50W-B/CC、CC、Co-B/CC和Pt/C四種樣品HER過程的Tafel斜率 Pt/C的Tafel斜率為31.9 mV/dec，與以往諸多報道的數(shù)值極為接近[27,28]，其表面的HER反應動力學過程受Tafel步驟控制 Co-50W-B/CC (117.4 mV/dec)和Co-B/CC(156.2 mV/dec)的Tafel斜率接近于120 mV/dec，表明兩者表面的HER反應動力學過程受Volmer步驟控制[29] W的外圍電子層排布為5d46s2，有1個空的和4個半滿的5d軌道，易于接受氫原子的吸附，從而促進HER過程中Volmer反應的進行 因此，Co-50W-B/CC的Tafel斜率比Co-B/CC下降了38.8 mV/dec 有必要指出，W含量過高時氫原子的脫附變得困難，將抑制后續(xù)的H2析出，進而阻礙HER反應的進行 因此，Co-W-B中的W含量應控制在一定的合理范圍內

圖5



圖5不同Co-W-B/CC樣品的LSV曲線(嵌入圖為-0.10~-0.30VvsRHE電位范圍的局部放大圖)、Co-50W-B/CC、CC、Co-B/CC和Pt/C樣品的LSV曲線、Co-50W-B/CC、CC、Co-B/CC和Pt/C樣品的Tafel斜率、不同Co-W-B/CC樣品的EIS曲線、Co-50W-B/CC、Co-B/CC和Pt/C樣品的EIS曲線以及不同樣品的Cdl-HER值和j0-real-HER值

Fig.5HER process: LSV curves of Co-W-B/CC samples (the inset figure is the enlarged view of -0.10~-0.30VvsRHE) (a), LSV curves of Co-50W-B/CC, CC, Co-B/CC and Pt/C (b), Tafel slopes of Co-50W-B/CC, CC, Co-B/CC and Pt/C (c), EIS plots of Co-W-B/CC samples (d), EIS plots of Co-50W-B/CC, CC, Co-B/CC and RuO2 (e) and Cdl-HER and j0-real-HER of samples (f)

在0.1 V陰極過電位下不同樣品的EIS曲線，如圖5d和e所示 與OER過程類似，HER過程的EIS曲線也都有兩個容抗弧，也用等效電路模型對EIS數(shù)據(jù)進行擬合 在HER過程中Co-50W-B/CC的Rct值為6.46 Ω/cm2，低于Co-B/CC(28.78 Ω/cm2)、Co-25W-B/CC(19.53 Ω/cm2)、Co-33W-B/CC(13.49 Ω/cm2)、Co-66W-B/CC (14.23 Ω/cm2)和Co-75W-B/CC (25.22 Ω/cm2)五種樣品，僅比Pt/C(3.46 Ω/cm2)高 因此，在HER過程中Co-50W-B/CC表面也能發(fā)生極快的界面電子轉移，使H+還原 各樣品在HER過程中的Cdl-HER和j0-real-HER(HER過程的表觀交換電流密度j0-HER與其雙電層電容的比值)的計算結果，如圖5f所示 在HER過程中Co-50W-B/CC的Cdl-HER值也最大，其j0-real-HER值也只比Pt/C低 換言之，在所測試的樣品中，Co-50W-B/CC有最多的HER催化活性位點和只次于Pt/C的HER本征催化活性

2.3 Co-W-B/CC復合電極材料的電催化穩(wěn)定性和全解水活性

圖6a和b給出了Co-50W-B/CC在1 mol/L NaOH溶液中分別經歷1000次OER循環(huán)測試和1000次HER循環(huán)測試結果，可見測試前后的LSV曲線均十分接近 以電流密度絕對值為50 mA/cm2時的OER過電位和HER過電位為例，分別循環(huán)1000次后，ηOER較首次增加10 mV、ηHER較首次增加6 mV，增幅分別為5.7%和2.4% 分別在30 mA/cm2(OER過程)和-30 mA/cm2(HER過程)的電流密度下恒電流極化24 h的V-t曲線，如圖6c所示，可見極化過程中的陽極電位和陰極電位均整體保持穩(wěn)定 這表明，無論在OER過程還是HER過程，Co-50W-B/CC都表現(xiàn)出良好的電催化穩(wěn)定性

圖6



圖6Co-50W-B/CC在OER過程中循環(huán)1000次前后的LSV曲線、Co-50W-B/CC在HER過程中循環(huán)1000次前后的LSV曲線、Co-50W-B/CC在30 mA/cm2和-30 mA/cm2下的V-t曲線以及Co-50W-B/CC(+)??Co-50W-B/C(-)與RuO2(+)??Pt/C(-)的全解水活性比較

Fig.6LSV curves of Co-50W-B/CC before and after 1000 potential cycles in OER process (a), LSV curves of Co-50W-B/CC before and after 1000 potential cycles in HER process (b), V-t plots of Co-50W-B/CC at 30 mA/cm2 and -30 mA/cm2 (c) and comparison for the electrocatalytic water splitting activity of Co-50W-B/CC(+)??Co-50W-B/CC(-) and RuO2(+)??Pt/C(-) (d)

在恒電流電解水工藝的操作中，其全解水過程的槽電壓為Ecell=E0OER +ηOER+E0HER+ηHER[30] 因此，根據(jù)OER和HER過程的LSV曲線可以計算出Co-50W-B/CC同時作為陽極和陰極時(Co-50W-B/CC(+)??Co-50W-B/CC(-))特定電解電流密度(注：LSV曲線中電流密度的+/-號表示其為陽極或陰極反應，全解水過程的電解電流密度取其絕對值)下的Ecell，并與RuO2和Pt/C分別作為陽極和陰極時(RuO2(+)??Pt/C(-))相應的Ecell進行比較，結果如圖6d所示 在較低的電解電流密度下Co-50W-B/CC(+)??Co-50W-B/CC(-)的Ecell值與RuO2(+)??Pt/C(-)的十分接近 往往將光伏發(fā)電設備與電解水設備相結合，通過電解水來儲存光伏發(fā)電設備所產生的電能，這類設備的電解電流密度約為10 mA/cm2[31] 電解電流密度為10 mA/cm2時，Co-50W-B/CC(+)??Co-50W-B/CC(-)的Ecell值為1.718 V，僅比RuO2(+)??Pt/C(-)高出0.026 V

3 結論

(1) 通過濕化學還原在CC表面沉積非晶Co-W-B催化活性物質，可制備一種自支撐的Co-W-B/CC復合電極材料 這種材料在堿性環(huán)境(1 mol/L NaOH)中表現(xiàn)出理想的電解水催化活性 [WO42-]/([WO42-]+[Co2+])比值為50%的Co-50W-B/CC樣品的催化活性最高：10 mA/cm2時的OER過電位為0.394 V，-10 mA/cm2時的HER過電位為0.094 V

(2) 使用Co-50W-B/CC同時作為陽極和陰極進行全解水時，在10 mA/cm2電解電流密度下Co-50W-B/CC(+)??Co-50W-B/CC(-)的Ecell值為1.718 V，只比RuO2(+)??Pt/C(-)高0.026 V

(3) 本征催化活性和電化學活性面積的提高，使Co-50W-B/CC樣品在較低電流密度下具有與貴金屬基材料接近的催化活性 Co-50W-B/CC樣品在OER和HER過程都具有良好的催化穩(wěn)定性

參考文獻

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CoOx-carbon nanotubes hybrids integrated on carbon cloth as a new generation of 3D porous hydrogen evolution promoters

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2017