Hydrogen is considered as a potential energy source for the future because of high energy density and environment friendly reaction products. The successful application of hydrogen energy depends largely on the conception of a safe and convenient method for the hydrogen storage and production.
Sodium borohydride ($NaBH_4$), that is able to store 10.8 wt.% of hydrogen, is very attractive hydrogen storage system since it can produce double amount of its stored hydrogen by a hydrolysis reaction from water. In addition, it stores hydrogen safely in an alkaline solution at room temperatures. For generation of hydrogen from an alkaline $NaBH_4$ solution, suitable catalysts such as Pt and Ru are needed to promote the hydrolysis reaction of $NaBH_4$. Since the cost of such noble metal catalysts as Pt and Ru are very expensive, it is necessary to develop an alternative catalyst with excellent hydrogen generation efficiency at an economical cost.
In this study, Co-P catalysts were produced using electroless deposition, and examined their hydrogen generation properties in alkaline $NaBH_4$ solution. The effects of electroless deposition conditions such as NaOH concentration(pH), temperature of bath and deposition time on microstructures of the Co-P catalysts and their catalytic activities on the hydrolysis of $NaBH_4$ were investigated in order to develop high performance catalysts for hydrogen generation from hydrolysis of $NaBH_4$. Influences of pH, $NaBH_4$ concentration on hydrogen generation properties of Co-P catalysts in alkaline $NaBH_4$ solution were also studied.
Co-P catalysts were produced on Cu sheet with an exposed surface area of 16 ㎠ in 0.1 M $CoCl_2\cdot6H_2O$ + 0.8 M $NaH_2PO_2\cdotH_2O$ + 0.6 M $NH_2CH_2COOH$ solution by electroless deposition. The concentration of NaOH in the Co-P bath and the temperature were varied from 0.5 to 0.9 M and from 50 to 85 ℃. And temperature of Co-P bath was varied from 1 to 30 min. The microstructures of the Co-P catalysts were characterized by scanning electron microscope(SEM), X-ray diffraction(XRD)와 Transmission Electron Microscope(TEM). Hydrogen generation tests of the Co-P catalysts were performed in 50 ml 1 wt.% NaOH + 10 wt.% $NaBH_4$ solution at 30℃. The volume of generated hydrogen gas was measured by a gas flow meter.
From XRD ＆ TEM analysis, it was found that the microstructures of electroless deposited Co-P catalysts were composed of both nano-crystalline Co phase and amorphous Co-P phase. The microstructures and hydrogen generation properties of electroless deposited Co-P catalysts largely depended on NaOH concentration(pH) of Co-P bath. With increasing NaOH concentration from 0.5 M to 0.8 M, the size of nano-crystalline Co decreased from 16.9 nm to 5 nm, and total generated hydrogen volume for 30 min in alkaline $NaBH_4$ solution at 30 ℃ increased from 160 ml to 1850 ml. However, with further increase in NaOH concentration(pH) to 0.9 M, size of nano-crystalline Co increased to 17.7 nm, and total hydrogen generated volume decreased dramatically to 100 ml. It was found that the microstructures and catalytic activities of the electroless deposited Co-P catalysts on hydrolysis of alkaline NaBH4 solution depended on temperature as well as NaOH concentration(pH) of Co-P bath. The Co-P catalysts deposited at 60∼80 ℃ had 4.2∼6 nm Co crystals, and showed very high hydrogen generation rate. However, the Co-P catalyst deposited at 85 ℃ had 8.1 nm Co crystal, and showed comparatively low hydrogen generation rate.
With increasing deposition time from 1 min to 30 min, hydrogen generation rate of Co-P catalysts also increased. This result indicates that with increasing deposition time, both surface area and catalytic activity of Co-P catalyst increase.
As pH of alkaline $NaBH_4$ solution increased, hydrogen generation rate of Co-P catalysts also increased in contrast with Ru and Co catalysts, which showed low hydrogen generation rate at high pH. When $NaBH_4$ concentration increased, hydrogen generation rate decreased due to increasing the solution viscosity.
The Co-P catalyst electroless deposited in the Co-P bath with 0.8 M NaOH at 60℃ showed the best hydrogen generation rate of 4205 ml/min-g-catalyst in 10 wt.% NaOH + 10 wt.% $NaBH_4$ solution at 30 ℃. After 6 cycle in cycle test of 10 h/cycle, the performance of Co-P catalyst decreased to 74 %(3112 ml/min-g -catalyst )due to the separation of Co-P particles from catalyst and oxidation of a catalyst surface. However, the performance didn`t decrease anymore after 6 cycle.
무전해 도금 용액의 NaOH농도(pH), 온도 및 도금시간이 Co-P 촉매의 미세구조 및 알칼리 $NaBH_4$ 용액 내에서의 수소발생특성에 미치는 영향을 조사하여, $NaBH_4$ 가수분해반응에 높은 촉매활성도를 가지는 촉매 개발을 수행하였다. 또한 알칼리 $NaBH_4$ 용액의 pH 및 $NaBH_4$ 농도가 Co-P 촉매의 수소발생특성에 미치는 영향을 분석하였으며, Co-P 촉매의 수명특성을 조사하였다.
1. 무전해 도금된 Co-P 촉매는 비정질 Co-P와 나노-결정질 Co로 이루어졌다.
2. 무전해 Co-P 도금용액의 NaOH 농도가 0.5 M에서 0.8 M로 증가함에 따라 제조된 Co-P 촉매의 나노-결정질 Co의 크기는 감소하였으며, 수소발생속도는 크게 향상되었다. 그러나 도금 용액의 NaOH농도를 더욱 증가시킨 0.9 M에서는 나노-결정질 Co의 크기가 급격히 증가하였고, 수소발생속도는 크게 감소하였다.
2. 0.8 M NaOH 농도인 무전해 Co-P 도금용액의 온도가 60~80 ℃에서 제조된 Co-P 촉매의 나노-결정질 Co의 크기는 4.2~6 nm로 매우 작았으며, 수소발생속도는 매우 높았다. 그러나 85 ℃에서 제조된 Co-P 촉매는 나노-결정질 Co의 크기가 증가하였으며, 수소발생속도는 비교적 감소하였다.
3. 60 ℃, 0.8M NaOH 농도의 무전해 Co-P 도금용액에서 도금시간을 1~30 min으로 증가시키며 도금을 실행한 결과, 제조된 Co-P 촉매의 수소발생량은 도금 시간이 증가함에 따라 증가하였다. 이러한 원인은 도금된 촉매양의 증가에 따른 촉매의 표면적의 증가와 Co-P 촉매의 P 농도의 감소에 의한 촉매 활동도 증가의 복합적인 영향으로 사료된다.
4. 알칼리 $NaBH_4$ 용액의 pH가 증가함에 따라 Co-P 촉매의 수소발생속도는 증가하였으며, 이와 같은 특성은 높은 pH에서 $NaBH_4$ 자기가수분해 반응을 억제하여 안정한 수소의 저장이 가능하게 할 뿐만 아니라, 촉매 침지 시 수소발생속도 또한 높은 장점을 가진다. 알칼리 $NaBH_4$ 용액의 $NaBH_4$ 농도가 증가함에 따라 촉매의 수소발생속도는 감소하였으며, 이는 $NaBH_4$ 가 증가함에 따른 용액의 점도(viscosity) 증가로 사료된다.
5. 알칼리 $NaBH_4$ 용액의 온도가 증가함에 따라 Co-P 촉매의 수소발생속도는 증가하였다. 60 ℃, 0.8 M NaOH 농도의 무전해도금 용액에서 제조된 Co-P 촉매를 10 wt% NaOH + 10 wt% $NaBH_4$ 용액에서 활성화 에너지를 측정한 결과, 60.19 kJ/mol로 측정되었으며, 이는 Ru 촉매의 활성화 에너지인 56 kJ/mol에 비해 큰 차이가 없는 것으로부터 Co-P 촉매 또한 Ru와 같이 알칼리 $NaBH_4$ 용액의 가수분해 반응에 높은 촉매 활성도를 가지는 것으로 확인되었다.
6. 최적화된 무전해 도금 조건인 60 ℃, 0.8 M NaOH 농도에서 제조된 Co-P 촉매는 30 ℃, 10 wt% NaOH + 10 wt% NaBH4 용액에서 4205 ml $min^{-1} g^{-1}$ -촉매의 가장 높은 수소발생속도를 보였다. 그리고 사이클 시험(10 h/ cycle)을 수행한 결과, 1 사이클에서 6 사이클까지 촉매성능이 사이클이 증가함에 따라 감소하였다. 이 후, 10 사이클까지 3000 ml $min^{-1} g^{-1}$ -촉매 이상의 수소발생속도를 유지하는 것으로 확인되었다.
