Among various kinds of hydrogen storage system, sodium borohydride $(NaBH_4)$ is considered as a promising candidate because of high hydrogen storage capacity of 10.8wt.%, long and safe hydrogen storage, environmentally benign and recyclable reaction product. $NaBH_4$ with high hydrogen storage capacity can be used as hydrogen supply system for PEMFC and direct borohydride liquid fuel cell(DBFC) as power source for portable electronic devices.
Part I. Hydrogen storage and production from hydrolysis of $NaBH_4$
$NaBH_4$ in aqueous alkaline solutions of pH>11 produces hydrogen gas
and water-soluble, sodium borate, $NaBO_2$, in contact with the selected catalyst.
$NaBH_4(aq) + 2H_2O(l) → 4H_2(g) + NaBO_2(s) + Q (217 kJ)$ (1)
This hydrogen generation from NaBH4 strongly depends on the preparation of $NaBH_4$ alkaline solution and the pasted catalyst used.
The hydrolysis of $NaBH_4$ depends on pH of alkaline solution and $NaBH_4$ concentration. Considering the maximum H2 generation rate and the storage time of $H_2$, the optimum pH of alkaline solution is found to be 13. With increasing $NaBH_4$ concentration, maximum $H_2$ generation rate increases and reaches a maximum in the range 12-20wt.% of $NaBH_4$. At higher weight percent of $NaBH_4$, maximum $H_2$ generation rate decreases due to catalytic site blocking by over-saturation of by-product, $NaBO_2$, on catalyst. Therefore, it is concluded that $NaBH_4$ concentration is an important variable on $H_2$ generation from $NaBH_4$. As binding agent of pasted catalyst, SBR emulsion is more effective than PTFE on hydrogen production of $NaBH_4$ and it results from its hydrophilic property to be smoothly infiltrated the $NaBH_4$ solutions into the reactive sites of pasted catalyst. With a view to optimize the amount of SBR as binder, studies were carried out on the $H_2$ generation rates related with the amount of binder. As shown in Fig. 4, the $H_2$ generation rate decreases with increasing SBR fraction. However, we also found that the binder with less than 20wt.% of catalyst shows a poor durability due to the separation of active material from substrate during several operating times. Therefore, it was concluded that the optimum amount of SBR as binder is 20 wt.% of catalyst material.
All kinds of catalysts developed in our experiments such as $LaNi_{4.5}Al_{0.5}$, $HF-Mg_2Ni$, Co, Filamentary Ni, and Spherical Ni show similar $H_2$ generation behavior and these characteristics of very fast initial hydrogen generation and long maintenance of maximum rate are attractive for application of hydrogen supplying part of PEMFC. While, the maximum $H_2$ generation rate depends on the type of catalyst and Co and Filamentary Ni show higher $H_2$ generation rate than others. It is shown that Co has high intrinsic catalytic activity for hydrolysis of $NaBH_4$, which may be resulted from its superior catalytic effect. Moreover, Filamentary Ni shows a good performance for hydrolysis owing to its large surface area which provides a large number of reactive sites. Therefore it was expected that the nano-sized Co catalyst reveals high $H_2$ generation rate and good performance. Nano sized Co on C by chemical reduction method shows a maximum $H_2$ generation rate of 278ml/min.g-catalyst with an increase of 38.1% over micro-sized Co catalyst and high fuel conversion efficiency of 93.1%. Also the hydrogen gas generated has a high purity of 99.99 % which contains a very small amount of vapor and nitrogen gas. This means that no complicated systems, such as CO remover and heater, are needed to generate a hydrogen gas with high purity for supplying into the PEMFC.
Also in this work, the durability characteristics and the approaches for elucidating the degradation mechanism of a catalyst are extensively studied and reported here. $H_2$ generation rate of filamentary Ni catalyst electrode gently decreases with cycling and 76% of the initial $H_2$ generation rate remains even at 200th cycles. From SEM and BET analyses, the agglomeration of the catalyst was observed and brought about the diminishing of specific surface area of the catalyst electrode with cycling. Therefore, the deterioration of the catalyst with cycling is ascribed to the reduction of reactive sites by a decrease of specific surface area. Also, with cycling, a film not observed prior to cycling, was formed and gradually spreads over the whole surface of the pasted catalyst, eventually covering the reactive sites. From XRD and XPS analyses, it was observed that with cycling, potassium originated from alkaline solution, and sodium and boron accumulated on the surface of pasted catalyst, and finally a film consisting of $Na_2B_4O_7 \cdot 10H_2O$, potassium borate $(KB_xO_y)$ and boron oxide $(B_2O_3)$ was formed. Consequently, a deterioration of the catalyst results from an agglomeration of the catalyst particles and the film formed on the catalyst.
Part II. Development of Direct Borohydride Liquid Fuel Cell
Aqueous $NaBH_4$ alkaline solution can be used as direct liquid fuel to fuel cell instead of hydrogen gas. Recently, aqueous solution containing $NaBH_4$ has been considered as a type of fuel due to its high energy density and environment friendliness. $NaBH_4$ oxidizes in aqueous alkaline media, to $BO_2^-$ and water and produces eight electrons.
$BH_4^- + 8OH^- BO_2^- + 6H_2O + 8e^- E^o = 1.24 V$
With the oxidation of $NaBH_4$ at the anode, the atmospheric oxygen is supplied to the cathode leading to a reduction reaction at the interface between the cathode and the aqueous electrolyte, and, thus, electrons are consumed.
$2O_2 + 4H_2O + 8e^- 8OH^- E^o = 0.40 V$
Overall cell reaction is as follows:
$BH_4^- + 2O_2 BO_2^- + 2H_2O E^o = 1.64 V$
The theoretical capacity and specific energy of direct borohydride liquid fuel cells (DBFC), assuming a 1.64V cell potential, are 5558Ah/kg-NaBH4 and 9296 Wh/kg-$NaBH_4$, respectively. Therefore, DBFC system has bright prospects as power source for portable devices.
In the part II, air-breathing DBFC stack of new design, which can suppress a contact between cathode and NaBH4 to decrease mixed potential at the cathode and directly supply air to the cathode, has been fabricated for the first time. Comparing to the performance of existing DBFC, this stack of new design significantly improves the cell performance. Optimizing pH of alkaline solution and $NaBH_4$ concentration in liquid fuel should lead to high power density of DBFC. It is found that the performance of the air-breathing DBFC using a alkaline solution of pH 14 is superior to that with pH 13. This result could be explained on the basis of increasing the anodic reaction due to higher KOH concentration of supporting solution at the anode. In other words, more OH- ions from the liquid fuel arrive at the anode in order to promote the anodic reaction. Therefore, a KOH solution with pH 14 was introduced as a supporting solution in our experiments. With the optimization of $NaBH_4$ concentration, maximum power density is found $14.2mW/ ㎠$ at 4wt.% of $NaBH_4$. Upon increasing $NaBH_4$ concentration, maximum power density and O.C.V.(open circuit voltage) gradually decrease and this behaviors are ascribed to the crossover of $NaBH_4$ from anode to cathode, which creates a mixed potential at the cathode by the $NaBH_4$ oxidation. From the investigation of effective anode catalyst on $NaBH_4$ oxidation, it is found that the Au on C catalyst has the highest catalytic activity on $NaBH_4$ oxidation and reaches to maximum power density of 35.86mW/mg and $60.1mW/ ㎠$. It has been found that Au is and effective anodic catalyst for $NaBH_4$ oxidation but not for $NaBH_4$ hydrolysis while Pt is a good catalyst for both $NaBH_4$ oxidation and hydrolysis, hence it is concluded that Au is more effective catalyst for $NaBH_4$ oxidation than Pt.
Part. I. $NaBH_4$ 알칼라인 용액을 이용한 수소저장 및 발생에 관한 연구 $NaBH_4$ 알칼라인 용액은 안정하게 오랫동안 수소를 저장할 수 있다는 장점을 가진다. 그리고 원하는 시간과 장소에서 촉매 반응을 통한 물 $(H_2O)$ 과의 가수분해(Hydrolysis)를 통하여 4mol의 수소와 $NaBO_2$ 를 발생시킨다. 고성능의 촉매개발은 $NaBH_4$ 알칼라인 용액으로부터의 수소발생에 있어서 핵심기술의 하나이다. 본 연구에서는 Ni과 Co를 촉매로 선택하였다. 촉매의 종류뿐만 아니라 $NaBH_4$ 수소발생특성은 $NaBH_4$ 알칼라인 용액의 제조조건의 최적화, 즉 pH와 $NaBH_4$ 농도가 수소발생특성에 미치는 영향을 고찰함으로써 조건을 최적화하는 과정도 매우 중요하다. Chemical reduction 법으로 제조된 nano-sized Co 촉매는 acetylene black 위에 ~ 5nm의 Co를 전착시킨 Co on C 형태이다. Co on C 촉매의 최대발생속도는 278ml/min.g-catalyst로 micro-sized Co 촉매에 비하여 38.1% 향상된 결과이며, 촉매의 가격을 고려하여 계산하였을 경우 500.9ml/min.$로서 동일한 입자 크기의 Pt on C 촉매의 16.8ml/min.$와 비교하여 우수한 촉매임을 확인하였다. 또한 Co on C 촉매는 93.1%의 높은 연료이용효율(Fuel conversion efficiency)을 보이며, 발생한 수소가스의 purity를 분석한 결과 99.99%의 고순도의 수소를 발생시키는 것으로 확인하였다. 가스발생에 사용하는 촉매는 수명특성(Durability)을 측정한 결과 사이클 진행에 따라 촉매극 표면에 형성된 $Na_2B_4O_7?10H_2O$, unidentified potassium borate $(KB_xO_y)$, boron oxide $(B_2O_3)$ 상으로 구성된 film이 퇴화의 첫 번째 원인이며, 사이클 진행에 따라 반응열을 통한 촉매의 agglomeration이 일어나서 촉매극의 비표면적이 감소하여 반응면적이 감소하는 현상이 촉매퇴화의 두 번째 원인이라는 것을 확인하였다.
Part II. $NaBH_4$ 액체연료를 이용한 직접액체연료전지
$NaBH_4$ 알칼라인 용액은 가수분해를 이용한 발생장치로서의 역할 뿐만이 아니라, $NaBH_4$ 의 산화반응(Oxidation)을 통한 전자발생을 이용하여 연료전지(Fuel Cell)의 연료극(anode) 연료로서 이용할 수 있다. $NaBH_4$ 알칼라인 용액은 수소를 대신하여 연료극에 직접 공급되며, 이러한 $NaBH_4$ 액체연료를 이용하는 연료전지가 Direct Borohydride Liquid Fuel Cell(DBFC)이다. DBFC는 이론전압이 1.64V로 매우 높아서 high power density에 유리하며, 10.8wt.%H의 높은 수소저장밀도로 에너지밀도가 매우 높으며, 탄소(carbon)가 포함되지 않아서 CO 발생을 통한 촉매의 퇴화가 일어나지 않는 다는 장점을 가지고 있다. 새로운 디자인(NaBH4와 공기극의 접촉을 억제, Air의 공기극으로의 직접 공급)의 DBFC 설계가 우선적으로 이루어져야 한다. 또한 아직까지 $NaBH_4$ 산화반응에 효과적인 연료극 개발에 관한 체계적인 연구가 이루어진바가 없기 때문에 $NaBH_4$ 산화반응에 효과적인 촉매로 예상되는 Ni, Co, Au, Pt 촉매와 $ZrCr_{0.8}Ni_{1.2}$ 수소저장합금을 사용하여 우수한 DBFC 연료극을 개발하고자 하였다.
새로운 다자인의 DBFC를 설계하여 성능을 측정한 결과 동일한 조건하에서 수행한 기존 DBFC 성능에 비하여 최대출력(maximum power density) $12.9mW/ ㎠$ 로 140% 향상된 결과를 확인하였다. 다양한 촉매물질에 대한 연료극 성능을 평가한 결과 Au on C 과 Pt on C 촉매가 다른 촉매에 비하여 수백% 높은 최대출력을 보이고 있으며, 특히 Au on C 촉매가 가장 우수한 연료극 촉매물질임을 확인하였으며, 이 때 성능은 35.86mW/mg와 $60.1mW/ ㎠$ 이었다. Au on C 촉매가 Pt on C 촉매에 비하여 우수한 성능을 나타내는 원인은 수소발생특성이 Pt on C 촉매의 60% 정도이기 때문에 수소발생에 의한 연료의 loss가 감소하여 산화반응이 보다 활발하기 때문임을 확인하였다. $NaBH_4$ 액체연료의 $NaBH_4$ 농도 증가는 DBFC의 출력 감소뿐만 아니라 O.C.V.(open circuit voltage)의 감소도 야기시킨다. 이와 같은 $NaBH_4$ 의 농도증가에 따른 출력 및 O.C.V.의 감소는 $NaBH_4$ 의 연료극에서 공기극으로 cross-over 현상에 의한 것임을 확인하였다. 또한 연료극 촉매의 형태로 carbon supported 촉매가 Unsupported 촉매에 비해 성능이 우수하였으며, 이러한 원인은 CO 흡착 분석결과 기하학적 원인에 의한 반응면적의 증가에 기인한 것으로 관찰되었다.
